MICROWAVE AND OPTICAL COMMUNICATIONS

MALLA REDDY INSTITUTE OF TECHNOLOGY & SCIENCE

(SPONSORED BY MALLA REDDY EDUCATIONAL SOCIETY)
Permanently Affiliated to JNTUH & Approved by AICTE, New Delhi
NBA Accredited Institution, An ISO 9001:2015 Certified,
Approved by UK Accreditation CentreGranted Status of 2(f) & 12(b) under UGC Act. 1956, Govt. of India.

MICROWAVE AND OPTICAL COMMUNICATIONS

COURSE FILE

DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING

(2022-2023)

Faculty In-Charge HOD-ECE

CH.RAJASHEKAR Dr.R.Prabhakar
 MICROWAVE AND OPTICAL COMMUNICATIONS

COURSE FILE
SUBJECT: MICROWAVE AND OPTICAL COMMUNICATIONS

ACADEMIC YEAR: 2022-2023.

REGULATION: R18

NAME OF THE FACULTY: C H . R A J A S H E K A R

DEPARTMENT: ECE

YEAR & SECTION: IV ECE A,B,C

S
INDEX

COURSE FILE

S.NO TOPIC PAGE NO

1. PEO’S, PO’S ,PSO’S 2

2 Syllabus Copy 4

3 Class Time table & Individual Time table 5

4 Student Roll List 6

5 Lesson Plan 15

Unit Wise Lecture Notes

Notes of Units 19

Assignment Questions 201

6 Short and long answer question with BloomsTaxonomy 202

Beyond the syllabus topics and notes
209
Objective Questions
223
PPT’S/NPTEL VIDEOS/any other

Previous University Question Papers to practice 229

9 Sample Internal Examination Question papers with key 230

10 Course Attendance Register

1
 MICROWAVE AND OPTICAL COMMUNICATIONS

1.PEO’S, PO’S, PSO’S

PROGRAM EDUCATIONAL OBJECTIVES:

PEO1: To excel in different fields of electronics and communication as well as in
multidisciplinary areas. This can lead to a new era in developing a good electronic product.
PEO2: To increase the ability and confidence among the students to solve any problem in their
profession by applying mathematical, scientific and engineering methods in a better and
efficient way.
PEO3: To provide a good academic environment to the students which can lead to excellence,
and stress upon the importance of teamwork and good leadership qualities, written et hical
codes and guide lines for lifelong learning needed for a successful professional career.
PEO4: To provide student with a solid foundation to students in all areas like mathematics,
science and engineering fundamentals required to solve engineering pr oblems, and also to
pursue higher studies.
PEO5: To expose the student to the state of art technology so that the student would be in a
position to take up any assignment after his graduation.

PROGRAM OUTCOMES:-
Engineering knowledge: Apply the knowledge of mathematics, science, engineering fundamentals,and an
engineering specialization to the solution of complex engineering problems.
Problem analysis: Identify, formulate, review research literature, and analyze complex engineeringproblems
reaching substantiated conclusions using first principles of mathematics, natural sciences, and engineering
sciences.
Design/development of solutions: Design solutions for complex engineering problems and designsystem
components or processes that meet the specified needs with appropriate consideration for the public health
and safety, and the cultural, societal, and environmental considerations.
Conduct investigations of complex problems: Use research-based knowledge and research methods
including design of experiments, analysis and interpretation of data, and synthesis of theinformation to
provide valid conclusions.
Modern tool usage: Create, select, and apply appropriate techniques, resources, and modern engineering
and IT tools including prediction and modeling to complex engineering activities with anunderstanding of the
limitations.
The engineer and society: Apply reasoning informed by the contextual knowledge to assess societal,health,
safety, legal and cultural issues and the consequent responsibilities relevant to the professional engineering
practice.
Environment and sustainability: Understand the impact of the professional engineering solutions in
societal and environmental contexts, and demonstrate the knowledge of, and need for sustainable
development.
Ethics: Apply ethical principles and commit to professional ethics and responsibilities and norms of the
engineering practice.
Individual and team work: Function effectively as an individual, and as a member or leader indiverse teams,
and in multidisciplinary settings.
Communication: Communicate effectively on complex engineering activities with the engineering
community and with society at large, such as, being able to comprehend and write effective reportsand design
2
 MICROWAVE AND OPTICAL COMMUNICATIONS
documentation, make effective presentations, and give and receive clear instructions.
Project management and finance: Demonstrate knowledge and understanding of the engineering and
management principles and apply these to one’s own work, as a member and leader in a team, tomanage
projects and in multidisciplinary environments.

Life-long learning: Recognize the need for, and have the preparation and ability to engage inindependent
and life-long learning in the broadest context of technological change.

PROGRAM SPECIFIC OUTCOMES:

PSO1: The ability to absorb and apply fundamental knowledge of core Electronics
and Communication Engineering subjects in the analysis, design, and development of various
types of integrated electronic systems as well as to interpret and synthesize the experimental
data leading to valid conclusions.
PSO2: Competence in using electronic modern IT tools (both software and hardware)
for the design and analysis of complex electronic systems in furtherance to research activities.
PSO3: Excellent adaptability to changing work environment, good interpersonal skills
as a leader in a team in appreciation of professional ethics and societal responsibilities.

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 MICROWAVE AND OPTICAL COMMUNICATIONS

EC701PC: MICROWAVE AND OPTICAL COMMUNICATIONS (PC)

B.Tech. IV Year I Semester L T P C

3 0 0 3
Prerequisite: Antennas and Propagation

Course Objectives:
 To get familiarized with microwave frequency bands, their applications and to understand the
limitations and losses of conventional tubes at these frequencies.
 To distinguish between different types of microwave tubes, their structures and principles of
microwave power generation.
 To impart the knowledge of Scattering Matrix, its formulation and utility, and establish the S-
Matrix for various types of microwave junctions.
 Understand the utility of Optical Fibres in Communications.

Course Outcomes: Upon completing this course, the student will be able to
 Known power generation at microwave frequencies and derive the performance characteristics.
 realize the need for solid state microwave sources and understand the principles of solid state
devices.
 distinguish between the different types of waveguide and ferrite components, and select proper
components for engineering applications
 understand the utility of S-parameters in microwave component design and learn the
measurement procedure of various microwave parameters.
 Uunderstand the mechanism of light propagation through Optical Fibres.

UNIT - I
Microwave Tubes: Limitations and Losses of conventional Tubes at Microwave
Frequencies, Microwave Tubes – O Type and M Type Classifications, O-type Tubes: 2
Cavity Klystrons – Structure, Reentrant Cavities, Velocity Modulation Process and
Applegate Diagram, Bunching Process and Small Signal Theory – Expressions for O/P
Power and Efficiency. Reflex Klystrons – Structure, Velocity Modulation and Applegate
Diagram, Mathematical Theory of Bunching, Power Output, Efficiency, Oscillating Modes
and O/P Characteristics.
Helix TWTs: Types and Characteristics of Slow Wave Structures; Structure of TWT and
Amplification Process (qualitative treatment), Suppression of Oscillations, Gain
Considerations.

UNIT - II
M-Type Tubes:
Introduction, Cross-field Effects, Magnetrons – Different Types, Cylindrical Traveling Wave
Magnetron
– Hull Cut-off and Hartree Conditions, Modes of Resonance and PI-Mode Operation,
Separation of PI-Mode, o/p characteristics,
Microwave Solid State Devices: Introduction, Classification, Applications. TEDs –
Introduction, GunnDiodes – Principle, RWH Theory, Characteristics, Modes of Operation
- Gunn Oscillation Modes, Principle of operation of IMPATT and TRAPATT Devices.

UNIT - III
4
 MICROWAVE AND OPTICAL COMMUNICATIONS
Waveguide Components: Coupling Mechanisms – Probe, Loop, Aperture types.
Waveguide Discontinuities – Waveguide Windows, Tuning Screws and Posts, Matched
Loads. Waveguide Attenuators – Different Types, Resistive Card and Rotary Vane
Attenuators; Waveguide Phase Shifters
– Types, Dielectric and Rotary Vane Phase Shifters, Waveguide Multiport Junctions - E
plane and H plane Tees. Ferrites– Composition and Characteristics, Faraday Rotation,
Ferrite Components – Gyrator, Isolator,

UNIT - IV
Scattering matrix: Scattering Matrix Properties, Directional Couplers – 2 Hole, Bethe
Hole, [s] matrix of Magic Tee and Circulator.

Microwave Measurements: Description of Microwave Bench – Different Blocks and

their Features, Errors and Precautions, Measurement of Attenuation, Frequency.
Standing Wave Measurements, measurement of Low and High VSWR, Cavity Q,
Impedance Measurements.

UNIT - V
Optical Fiber Transmission Media: Optical Fiber types, Light Propagation, Optical
fiber Configurations, Optical fiber classifications, Losses in Optical Fiber cables, Light
Sources, Optical Sources, Light Detectors, LASERS, WDM Concepts, Optical Fiber System
link budget.

TEXT BOOKS:
1. Microwave Devices and Circuits – Samuel Y. Liao, Pearson, 3rd Edition, 2003.
2. Electronic Communications Systems- Wayne Tomasi, Pearson, 5th Edition

REFERENCE BOOKS:
1. Optical Fiber Communication – Gerd Keiser, TMH, 4th Ed., 2008.
2. Microwave Engineering - David M. Pozar, John Wiley & Sons (Asia) Pvt Ltd., 1989, 3r ed., 2011
Reprint.
3. Microwave Engineering - G.S. Raghuvanshi, Cengage Learning India Pvt. Ltd., 2012.
4. Electronic Communication System – George Kennedy, 6th Ed., McGrawHill.

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 MICROWAVE AND OPTICAL COMMUNICATIONS

3.Class Time Table & Individual Time Table

III-I ECE-A,B,C

Individual Time Table

9.15- 10.15- 11.15- 12.15-1.15 1.15-2.00 2.00- 3.00-

10.15 11.15 12.15 3.00 4.00

MON MWOC(A) MWOC(B) LUNC MWOC LAB (A) IST BATCH

H
TUES MWOC(B) MWOC(C) MWOC LAB (B) IST BATCH

WED MWOC(C) MWOC(B) MWOC(A) MWOC LAB (C) IST BATCH

THUR MWOC(A) MWOC(C) COOS(B) MWOC LAB (A) IIND

BATCH
FRI COOS(A) COOS(C) MWOC LAB (A) IIND
BATCH
SAT COOS(B) COOS(A) MWOC LAB (A) IIND
BATCH

6
 MICROWAVE AND OPTICAL COMMUNICATIONS

4.Students Roll List ECE-A

SNo H.T.NO NAME OF THE STUDENT SNo H.T.NO NAME OF THE STUDENT
1 19S11A04C3 AKHIL K 30 19S11A04F5 SATHVIK V

2 19S11A04C4 AKSHAYA NADENDLA 31 19S11A04F6 SHIRISHA GAJAM

3 19S11A04C5 APOORVA DEEKONDA 32 19S11A04F7 SRIDHAR G

4 19S11A04C6 ASHISH M 33 19S11A04F8 SRILATHA DESHAVENI

5 19S11A04C7 BALAJI M 34 19S11A04F9 SRINIVAS C B S

6 19S11A04C8 CHANDANA PRIYA KANDHAROJU 35 19S11A04G3 TRISHIKA B

7 19S11A04C9 DIVYA VUNIYALA 36 19S11A04G4 VIDHYA A

8 19S11A04D0 GANESH CHETKURI 37 19S11A04G5 VINAY KUMAR BONALA

9 19S11A04D1 HEMANTH URUTURU 38 19S11A04G7 SHAIK MOHAMMAD AKRAM

10 19S11A04D2 HITESH K R 39 19S11A04G8 SHIVA KUMAR REDDY PALLE

11 19S11A04D3 JAGADISH YERROLU 40 20S15A0401 ANIL AITIPAMULA

12 19S11A04D4 JASWANTH SAI YADAV PAMIDI 41 20S15A0402 ANUSHA MANCHIKATLA

13 19S11A04D5 MADHU N 42 20S15A0403 ARCHANA SURABOYINA

14 19S11A04D6 MANOHAR VADLA 43 20S15A0404 ARUN PIDUGU

15 19S11A04D8 MANSI SAINI 44 20S15A0405 AVIKSHITH YEDLA

16 19S11A04D9 NAGA VENKATA SATYANARAYANA V 45 20S15A0406 BHARATH KALYAN ALLARAM

17 19S11A04E0 NAVEEN CH 46 20S15A0407 BHASKAR D B V S

18 19S11A04E1 NIKHIL KUMAR THALLAPALLY 47 20S15A0408 GANAPATHI NAGAVATH

19 19S11A04E2 PADMAHARI M 48 20S15A0409 GOWTHAMI MATTIPALLI

20 19S11A04E3 PRAVEEN KUMAR A 49 20S15A0410 KRISHNAVENI

21 19S11A04E4 RAGHAVENDRA REDDY C 50 20S15A0412 LEELA VINODINI GORLI

22 19S11A04E6 RAJA KIRAN NYALAKANTI 51 20S15A0413 MANOJ KUMAR KALYANAPU

23 19S11A04E7 RAMYA ADEPU 52 20S15A0414 NITHISH MARELLA

24 19S11A04E8 RAMYA K 53 20S15A0415 PRABHAS NERELLA

25 19S11A04E9 SABITHA NALLAGANTI 54 20S15A0416 PRANADEEP POTHUNURI

26 19S11A04F0 SAI GYANESHWAR N 55 20S15A0417 PRANAYA BHUPALA

27 19S11A04F2 SAI PRASAD R 56 20S15A0418 PRASHANTH KUMAR POTTAVARTHI

28 19S11A04F3 SAI ROHITH M 57 20S15A0419 PRAVALIKA K

29 19S11A04F4 SATHISH NALAPARAJU 58 20S15A0420 RAJA SHEKER REDDY BOKKA

7
 MICROWAVE AND OPTICAL COMMUNICATIONS
ECE-B
SNo H.T.NO NAME OF THE STUDENT SNo H.T.NO NAME OF THE STUDENT
1 19S11A0461 ABHHINANDU K G 34 19S11A0497 SAI SANGAMESH GUPTA VATTAMVAR

2 19S11A0462 AKHIL MARYADA 35 19S11A0498 SAI SUMANTH CHARY VADLA

3 19S11A0463 AKHIL VIKKURTHI 36 19S11A0499 SAMAD REDDY GADILA

4 19S11A0464 AMULYA K 37 19S11A04A0 SANMAPRIYA SURAKASULA

5 19S11A0465 ARAVIND REDDY SINGIDI 38 19S11A04A1 SHAIK ABUTALHA

6 19S11A0466 BHARGAV REDDY M 39 19S11A04A2 SHAILESH PALA

7 19S11A0467 BHASKAR NAGA SAI N 40 19S11A04A3 SHEKAR BAGANNAGARI

8 19S11A0468 DHEERAJ KUMAR S 41 19S11A04A4 SHIVA SAI KUMAR B

9 19S11A0469 ESHWAR TEJA VALABOJU 42 19S11A04A5 SOWMYA NALLAGORLA

10 19S11A0470 HARIKRISHNA ANUMULA 43 19S11A04A6 SRAVAN KUMAR JAJAM

11 19S11A0471 HEMA SRI YERRA 44 19S11A04A7 SRISHNA GONE

12 19S11A0472 KARTHIK KUMAR KASULAVADHA 45 19S11A04A8 SRIVIDHYA MAMIDI

13 19S11A0473 KARTHIK MUMMADI 46 19S11A04A9 TARUN CHANDRA T

14 19S11A0474 KETHAN PATEL 47 19S11A04B0 TARUN KUMAR GUMMALA

15 19S11A0475 KOWSHIK C 48 19S11A04B1 THARUN MIDDE

16 19S11A0476 KRISHNA MUSTIPALLY 49 19S11A04B3 UTHAM KUMAR REDDY BADDAM

17 19S11A0477 LIKHITH REDDY UPPELA 50 19S11A04B4 UTTEJ KYATHAM

18 19S11A0478 MADHURYA AKKINAGUNTA 51 19S11A04B5 VAMSHIKRISHNA KOTHI

19 19S11A0479 MANIDEEP THATIPELLY 52 19S11A04B6 VARSHINI PAPANKA

20 19S11A0480 MANVITH REDDY NAREDLA 53 19S11A04B7 VASAVI MANDEPUDI

21 19S11A0481 MD RAASHID ALI 54 19S11A04B8 VIJAYA BADETI

22 19S11A0482 NARASIMHA RAJU B 55 19S11A04B9 VIVEK THALLA

23 19S11A0483 NARENDRA DIKONDA 56 19S11A04C0 YASHWANTH KOTLA

24 19S11A0484 NAVYASRI RAYUDU 57 20S15A0421 RAMYA MADUPU

25 19S11A0486 NIREEKSHAN H 58 20S15A0422 SAI ADITHYA CHATLA

26 19S11A0487 NITHIN CHOWDARY 59 20S15A0423 SAI DHANUSH GUGULOTHU

27 19S11A0488 PAVAN S 60 20S15A0424 SAI PRAKASH GAJULA

28 19S11A0489 PRAVEEN KOLAGANI 61 20S15A0425 SAI TEJA KARNE

29 19S11A0491 RAMYA PATLOLLA 62 20S15A0426 SARA SUSHANK

30 19S11A0492 REDDY PAVAN KALYAN CH 63 20S15A0427 SHIVA KUMAR KALPAGURI

31 19S11A0493 RUCHITHA ASKANI 64 20S15A0428 SRI RAM MANIKANTA PALLA

32 19S11A0494 RUCHITHA THALLA 65 20S15A0429 SUJITH YAMSANI

33 19S11A0495 SAHITHI KURA 66 20S15A0430 VAMSHI GOPAGANI

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 MICROWAVE AND OPTICAL COMMUNICATIONS
SNo H.T.NO NAME OF THE STUDENT SNo H.T.NO NAME OF THE STUDENT

1 19S11A0401 ABHILASH POLAM 36 19S11A0436 RISHIKETHAN REDDY MAMIDI

2 19S11A0402 ABHISHEK REDDY KAITHI 37 19S11A0437 RUCHITHA SHANAGONDA

3 19S11A0403 AJENDER REDDY BADDAM 38 19S11A0438 SAI KUMAR REDDY GURRALA

4 19S11A0404 AKSHITHA PITTALA 39 19S11A0439 SAI NIKHIL KOTHA

5 19S11A0405 ANKAL REDDY PATHAKOTTU 40 19S11A0440 SAI PRASAD RAJA BAI

6 19S11A0406 ANUSHA YERVA 41 19S11A0441 SAI PRIYA KONNERU

7 19S11A0407 ANVESH REDDY GADDAM 42 19S11A0442 SAI SUMANTH PEDDAGOLLA

8 19S11A0408 AVINASH KOLLI 43 19S11A0443 SAI TEJA GANDHAM

9 19S11A0409 BHARGAVA KRISHNA 44 19S11A0444 SAIRAM KURA

CHENNUPALLY
10 19S11A0410 BHARGAVI PAILA 45 19S11A0445 SANDHYA RANI DONGALA

11 19S11A0411 BHARGAVI TEKMAL 46 19S11A0446 SANGEETHA KUMBAM

12 19S11A0412 BHAVIKA MANNEKUNTA 47 19S11A0447 SANJEEVINI SOMAYAJI CHEEPALLI

13 19S11A0413 DEEPTHI RAYALA 48 19S11A0448 SATHISH KUMAR BATTA

14 19S11A0414 DIVYATEJA BONGANI 49 19S11A0449 SATYANARAYANA KAVITI

15 19S11A0415 DURGA PRASAD MADDALA 50 19S11A0450 SHARATH KUMAR NARSINGA

16 19S11A0416 GOPI RAVULA 51 19S11A0451 SHIVA KUMAR K

17 19S11A0417 HANUMAN DUKHIYA 52 19S11A0452 SHREEYA CHEERLA

18 19S11A0418 KEERTHANA THONUPUNURI 53 19S11A0453 SOUMYA PATHI

19 19S11A0419 LAHARI MOTHE 54 19S11A0454 SRICHARAN REDDY PONDUGULA

20 19S11A0420 MANASA GUNDRA 55 19S11A0455 SWAPNA GAJJALA

21 19S11A0421 MANASA JANGALA 56 19S11A0456 UMA NAGALLA

22 19S11A0422 MANIKANTA YARA 57 19S11A0457 VAMSHI PANJALA

23 19S11A0423 MOHAN REDDY PININTI 58 19S11A0458 VENU KUMAR GOUD EDIGA

24 19S11A0424 NAGA VENKATA SAI KUMAR BONU 59 19S11A0459 VISHNU SAI P

25 19S11A0425 NAGU GOUD AITAGONI 60 19S11A0460 YASHWANTH GADE

26 19S11A0426 NANDINI KITIKE 61 17S11A04B2 THARUNDEEP RAKAM

27 19S11A0427 NARESH REDDY NAGIREDDY 62 18S11A0404 ANIL BADAVATH

28 19S11A0428 NIKHIL REDDY ETIKYALA 63 18S11A0421 GURUNATH AMGOTH

29 19S11A0429 NIRANJAN REDDY MANNE 64 18S11A0429 NAGARJUNA DEVA

30 19S11A0430 POOJITHA MUCHARLA 65 18S11A0434 RAJKUMAR BADE

31 19S11A0431 PRANITHA PALADUGU 66 18S11A0436 RAVITEJA GANJI

32 19S11A0432 RAKESH GUDI 67 18S11A0457 VIJAY TEJA KANDU

33 19S11A0433 RASHMITHA ALIMI 68 18S11A0459 VYSHNAV KUMAR REDDY KUMMETHA

34 19S11A0434 RAVI KUMAR GUNDEBOINA 69 18S11A04A2 SAI KRISHNA THALLAPELLI

9
 MICROWAVE AND OPTICAL COMMUNICATIONS
35 19S11A0435 RENUSRI SREERAM

10
 MICROWAVE AND OPTICAL COMMUNICATIONS

LESSON PLAN:
Contact Date of
Course Academic

Learnin

(CLOs)
CourseTitle Year Branches Periods commenceme
Code Year
/Week nt of Semester
MICROWAVE AND

g
16EC7L
OPTICAL IV ECE 5 2022-23
01
COMMUNICATIONS
COURSE OUTCOMES
After completion of the course students are able to
Summarize about different types of modes in wave guides and how to decrease the
1 transmission and power losses, different types of microwave solid state devices and their
applications (K2)
2 Describe the knowledge about how these microwaves are generated transmitted, amplified and
finally measured using Passive devices.(K1,K2)
Explain the fundamentals, advantages ,Ray theory transmission in Optical Communication and
3 effect of dispersion of the signal, types of fiber materials, different losses in fibers
(K2,K3,K4)
4 Observe the knowledge about Optical transmitters, receivers and estimation of link and
power budget analysis. .(K1,K2)
Text
Out Comes / Topic Book / Contact Delivery
UNIT Topics/Activity
Bloom’s Level s No. Referenc Hour Method
e
UNIT-1: WAVEGUIDES
1.1 Microwave Spectrum, Bands and T1, T2 1
Applications of Microwaves
CO1: Summarize 1.2 Rectangular Waveguides – TE/TM T1, T3 1
about different mode analysis
types of modes in 1.3 Expressions for Fields T1, T3 1
wave guides and 1.4 Characteristic Equation and Cut- T1, T3 1
how to decrease the off Frequencies
transmission and 1.5 Dominant and Degenerate Modes T1, T3 1
I power losses, 1.6 Sketches of TE and TM mode T1, T3 1
different types of fields in the cross-section
microwave solid 1.7 Mode Characteristics – Phase and T1,T3, 1
state devices and Group Velocities R1,R2
their 1.8 Wavelengths and Impedance T1,T3, 1
applications (K2) Relations R1,R2
1.9 Power Transmission and Power T1,T3, 1 Chalk &
Losses in Rectangular wave guide R1,R2 Talk,Smart
1.10 Impossibility of TEM mode. T1, T3, 1 Class,
T3 ,R2 PPT
10 Tutorial
Total
UNIT-2: MICROWAVE ACTIVE DEVICES
II
2.1 Transferred Electron T1, T2 Chalk &
2
Devices: Gunn Diode-
11
 MICROWAVE AND OPTICAL COMMUNICATIONS
Principle, Two Valley Model Talk, Smart
Theory/RWH Theory, Class,
2.2 Characteristics and Modes of T1, T2 PPT
1
operation. Tutorial, &
2.3 Avalanche Transit Time T1, T2 Case Study
Devices: IMPATT Diode-
CO2: Describe the Principle of Operation and 1
knowledge about Characteristics, related
how these expressions
microwaves are 2.4 TRAPATT Diode- Principle of T1,
generated Operation and Characteristics, T2,R1 1
transmitted, related expressions
2.5 IMPATT Diode , TRAPATT T1,
amplified and 1
Diode -Problems T2,R1
finally measured
2.6 Two Cavity Klystron Amplifier T1, T2,
using Passive – Power and Efficiency R1, R2 1
devices. (K1,K2) considerations
2.7 Reflex Klystron Oscillators – T1, T2,
Modes and Efficiency R1, R2 1
considerations
2.8 Magnetrons T1, T2,
1
R1, R2
2.9 TWT T1,T2 1
Total 10
UNIT – 3: MICROWAVE PASSIVE DEVICES
3.1 Waveguide Corners, Bends, T1,
1
Twists, T3,R2
CO2: Describe the 3.2 Scattering Parameters and T1, T3
knowledge about Matrix,
how these 3.3 Scattering parameters of Wave T1,T3,
1
microwaves are Guide Tees: E-Plane R2
generated 3.4 H-Plane T1,T2,
1
transmitted, R1,R2
amplified and finally 3.5 E & H Plane T1,T2,
1
measured using R1,R2 Chalk &
Passive 3.6 Hybrid Rings (Rat-Race) T1, T2,
III 1 Talk,
devices.(K1,K2) R1,R2
3.7 Directional Coupler: Single T1,T3, PPT
2 Tutorial,
hole R2
, Smart Class
3.8 Directional Coupler:Multi hole T1,T3, 2
R1,R2

3.9 Fixed and Variable Attenuators T1,T3, 1

R2,R3
3.10 Ferrite Devices: Gyrator, T1,T3,
1
R2,R3
3.11 Isolator T1,T3,
1
R2,R3

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 MICROWAVE AND OPTICAL COMMUNICATIONS
3.12 Circulator T1,T3, 1
R1,R2

Total 13

UNIT - 4 MICROWAVE MEASUREMENTS

4.1 Description Microwave Bench, T1,
Different Blocks and their T2.R1, 1
Features, Precautions, R2
4.2 Frequency Meter T2.R1 1
4.3 Slotted line section, T2.R1 1
CO2: Describe the 4.4 Measurement of Attenuation,T2.R1 1
Chalk &
4.5 Measurement of Frequency T1,
knowledge about 1 Talk,
T3.R1,
how these PPT, Smart
4.6 Measurement of Power, T1,
IV microwaves are T3.R1, 1 Class
generated R2 Tutorial,
transmitted, 4.7 Measurement of VSWR, T1, Active
amplified and T3.R1, 1 Learning &
finally measured R2 Case Study
using Passive 4.8 Measurement of Cavity Q T1,
devices. (K1,K2) T3.R1, 1
R2
4.9 Measurement of Impedance. T1,
T3.R1, 1
R2
Total 9
UNIT – 5: OPTICAL FIBERS AND DEVICES

5.1 Propagation of light - Optical T1,

fiber structures, T3.R1, 1
CO3: Explain the
R2
fundamentals,
5.2 Acceptance angle, Numerical T1,
advantages ,Ray
aperture, Attenuation, T2.R1, 1
theory R2
transmission in 5.3 Absorption losses T1,
Optical T3.R1, 1 Chalk &
V Communication R2 Talk, Smart
and effect of 5.4 Scattering losses T1, Class,
dispersion of the T3.R1, 1
PPT
signal, types of R2
5.5 Dispersion – Radiation losses T1, Tutorial
fiber materials,
different losses in T3.R1, 1
fibers (K2,K3,K4) R2
5.6 Splicing Technique T1,
T3.R1, 1
R2
5.7 Optical Fiber connector, T1, 1

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 MICROWAVE AND OPTICAL COMMUNICATIONS
T3.R1,
R2
5.8 Connector types T1,
T3.R1, 1
R2
5.9 single mode fiber connector T1,
T3.R1, 1
R2
Total 9
UNIT – 5 : OPTICAL NETWORKS
6.1 Optical Source - LED, ILD T1,
characteristics. T3.R1, 1
R2
6.2 Optical detectors – PIN and T1, T3,
APD characteristics. 1
T3,R1
6.3 Optical transmitters and T1, T3,
receivers, T3,R1, 1
CO4: Observe
knowledge about R2
Optical 6.4 System block diagram T1, T3, Chalk &
transmitters, T3,R1, 1 Talk,
V
receivers and R2 PPT
estimation of 6.5 point to point link T1, Tutorial,
link and power T2.R1, 1 Smart Class,
budget R2 Active
analysis.(K1,K2) Learning &
6.6 link design T1,
Case Study
T2.R1, 1
R2
6.7 power budget analysis T1,
T2.R1, 1
R2
6.8 WDM- DWDM T2 2
Content
beyond
Syllabus Applications of Microwave-Microwave Oven, Fundamentals of RF Engineering
(if
needed)
Total 9
CUMULATIVE PROPOSED PERIODS 60

Text Books:
S.No. AUTHORS, BOOK TITLE, EDITION, PUBLISHER, YEAR OF PUBLICATION
1. Samuel Y. Liao, Microwave Devices and Circuits –PHI, 3rd Edition, 1994.
2. M.Kulkarni, Microwave and Radar Engineering- Umesh Publications 4th Edition, 2010.

3. Gerd Keiser, “Optical Fiber Communications”, the McGraw Hill Companies, 4th Edition, 2008.
Reference Books:
S.No. AUTHORS, BOOK TITLE, EDITION, PUBLISHER, YEAR OF PUBLICATION

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 MICROWAVE AND OPTICAL COMMUNICATIONS
1. Annapurna Das, Sisir K Das, “Microwave Engineering”, 2nd edition, 2006, Tata McGraw Hill.
2. John. M. Senior, “Optical Fiber Communications Principles and Practice”, Second Edition,
PHI, 1992.
Web Details
1. https://www.microwaves101.com/encyclopedias/waveguide-primer

2. http://www.tallguide.com/Waveguidelinearity.html

3. https://www.tutorialspoint.com/microwave_engineering

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 MICROWAVE AND OPTICAL COMMUNICATIONS
6.UNIT WISE LECTURE NOTES
a) Notes of Units

MICROWAVE AND OPTICAL COMMUNICATIONS

IV B. Tech I semester (JNTUH-R18)

ELECTRONICS & COMMUNICATION

ENGINEERING

16
 MICROWAVE AND OPTICAL COMMUNICATIONS

MALLA REDDY COLLEGE OF ENGINEERING &

TECHNOLOGY
IV Year B. Tech ECE – I Sem L T/P/D C 5 -/-/- 4

(R15A0421) MICROWAVE ENGINEERINGOBJECTIVES

1. To analyze micro-wave circuits incorporating hollow, dielectric and planar waveguides,

transmissionlines, filters and other passive components, active devices.
2. To Use S-parameter terminology to describe circuits.
3. To explain how microwave devices and circuits are characterized in terms of their “S”
Parameters.
4. To give students an understanding of microwave transmission lines.
5. To Use microwave components such as isolators, Couplers, Circulators, Tees, Gyrators etc..
6. To give students an understanding of basic microwave devices (both amplifiers and oscillators).
7. To expose the students to the basic methods of microwave measurements.

UNIT I:
Waveguides & Resonators: Introduction, Microwave spectrum and bands, applications of
Microwaves, Rectangular Waveguides-Solution of Wave Equation in Rectangular
Coordinates, TE/TM mode analysis, Expressions for fields, Cutoff frequencies, filter
characteristics, dominant and degenerate modes, sketchesof TE and TM mode fields in the
cross-section, Mode characteristics - Phase and Group velocities, wavelengths and
impedance relations, Rectangular Waveguides – Power Transmission and Power Losses,
Impossibility of TEM Modes ,Micro strip Lines-Introduction,Z0 Relations, losses, Q-factor,
Cavity resonators-introduction, Rectangular and cylindrical cavities, dominant modes and
resonant frequencies, Q-factor and coupling coefficients, Illustrative Problems.

UNIT II:
Waveguide Components-I: Scattering Matrix - Significance, Formulation and properties,
Wave guide multiport junctions - E plane and H plane Tees, Magic Tee,2-hole Directional
coupler, S Matrix calculations for E plane and H plane Tees, Magic Tee, Directional
coupler, Coupling mechanisms -Probe, Loop, Aperture types, Wave guide discontinuities -
Waveguide Windows, tuning screws and posts,Irises,Transitions,Twists,Bends,Corners and
matched loads, Illustrative Problems.
Waveguide Components-II: Ferrites composition and characteristics, Faraday rotation,
Ferrite components - Gyrator, Isolator, Circulator.

UNIT III:
Linear beam Tubes: Limitations and losses of conventional tubes at microwave
frequencies, Classification of Microwave tubes, O type tubes - 2 cavity klystrons-structure,
Reentrant cavities,velocity modulation process and Applegate diagram, bunching process
and small signal theory Expressions for o/p power and efficiency, Reflex Klystrons-structure,
Velocity Modulation, Applegate diagram, mathematical theory of bunching, power output,
efficiency, oscillating modes and o/p characteristics, Effect of Repeller Voltage on Power o/p,

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 MICROWAVE AND OPTICAL COMMUNICATIONS
Significance, types and characteristics of slow wave structures, structure of TWT and
amplification process (qualitative treatment), Suppression of oscillations, Gain
considerations.

Malla Reddy College of Engineering and Technology (MRCET) - Autonomous 153

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 MICROWAVE AND OPTICAL COMMUNICATIONS

UNIT IV:
Cross-field Tubes: Introduction, Cross field effects, Magnetrons-different types,
cylindrical travellingwave magnetron-Hull cutoff and Hartree conditions, modes of
resonance and PI-mode operation, separation of PI-mode, O/P characteristics.
Microwave Semiconductor Devices: Introduction to Microwave semiconductor devices, classification,
applications, Transfer Electronic Devices, Gunn diode - principles, RWH theory, Characteristics, Basic modes of
operation - Gunn oscillation modes, LSA Mode, Introduction to Avalanche Transit time devices (brief treatment
only), Illustrative Problems.

UNIT V:
Microwave Measurements: Description of Microwave Bench – Different Blocks and their Features, Precautions;
Waveguide Attenuators – Resistive Card, Rotary Vane types; Waveguide Phase Shifters – Dielectric, Rotary Vane
types. Microwave Power Measurement – Bolometer Method. Measurement of Attenuation, Frequency, VSWR,
Cavity Q. Impedance Measurements.

TEXT BOOKS:
8. Microwave Devices and Circuits – Samuel Y. Liao, PHI, 3rd Edition,1994.
9. Microwave and Radar Engineering- M.Kulkarni, Umesh Publications,1998.

REFERENCES :
1. Foundations for Microwave Engineering – R.E. Collin, IEEE Press, John Wiley, 2nd Edition, 2002.
10. Microwave Circuits and Passive Devices – M.L. Sisodia and G.S.Raghuvanshi, Wiley Eastern Ltd., New Age
InternationalPublishers Ltd., 1995.
11. Microwave Engineering Passive Circuits – Peter A. Rizzi, PHI, 1999.
12. Electronic and Radio Engineering – F.E. Terman, McGraw-Hill, 4th ed., 1955.
13. Elements of Microwave Engineering – R. Chatterjee, Affiliated East-West Press Pvt. Ltd., New Delhi,1988.

OUTCOMES
14. Understand the significance of microwaves and microwave transmission lines
15. Analyze the characteristics of microwave tubes and compare them
16. Be able to list and explain the various microwave solid state devices
17. Can set up a microwave bench for measuring microwave parameters

19
MICROWAVE AND OPTICAL COMMUNICATIONS

20
 MICROWAVE AND OPTICAL COMMUNICATIONS

UNIT- I
MICROWAVE TRANSMISSION LINES-I

INTRODUCITON
Microwaves are electromagnetic waves with frequencies between 300MHz (0.3GHz) and 300GHz in the
electromagnetic spectrum.

Radio waves are electromagnetic waves within the frequencies 30KHz - 300GHz, and include microwaves.
Microwaves are at the higher frequency end of the radio wave band and low frequency radio waves are at the lower
frequency end.
Mobile phones, phone mast antennas (base stations), DECT cordless phones, Wi-Fi,WLAN, WiMAX and Bluetooth
have carrier wave frequencies within the microwave band of the electromagnetic spectrum, and are pulsed/modulated.
Most Wi-Fi computers in schools use 2.45GHz (carrier wave), the same frequency as microwave ovens. Information
about the frequencies can be found in Wi-Fi exposures and guidelines.
It is worth noting that the electromagnetic spectrum is divided into different bands based on frequency. But the
biological effects of electromagnetic radiation do not necessarily fit into these artificial divisions.
A waveguide consists of a hollow metallic tube of either rectangular or circular cross section used to
guide electromagnetic wave. Rectangular waveguide is most commonly used as waveguide. waveguides
are used at frequencies in the microwave range.
At microwave frequencies ( above 1GHz to 100 GHz ) the losses in the two line transmission system
will be very high and hence it cannot be used at those frequencies . hence microwave signals are
propagated throughthe waveguides in order to minimize the losses.

Properties and characteristics of waveguide:

1. The conducting walls of the guide confine the electromagnetic fields and thereby
guide the electromagnetic wave through multiple reflections .
2. when the waves travel longitudinally down the guide, the plane waves are reflected
from wall to wall .the process results in a component of either electric or magnetic
fields in the direction of propagation of the resultant wave.
3. TEM waves cannot propagate through the waveguide since it requires an axial
conductor for axial current flow .
4. when the wavelength inside the waveguide differs from that outside the guide, the

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 MICROWAVE AND OPTICAL COMMUNICATIONS
velocity of wave propagation inside the waveguide must also be different fromthat
through free space.

5. if one end of the waveguide is closed using a shorting plate and allowed a wave to
propagate from other end, then there will be complete reflection of the waves

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 MICROWAVE AND OPTICAL COMMUNICATIONS
resulting in standing waves.

APPLICATION OF MAXWELLS EQUATIONS TO THE RECTANGULAR

WAVEGUIDE:

Let us consider waves propagating along Oz but with restrictions in the x and/or y
directions. The wave is now no longer necessarily transverse. The wave

equation can be written as

In the present case this becomes

and similarly for .electric field.

There are three kinds of solution possible

Boundary conditions:
We assume the guides to be perfect conductors so = 0 inside the guides.
Hence, the continuity of Et at a boundary implies that Et = 0 in the wave guide at the
boundary.

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 MICROWAVE AND OPTICAL COMMUNICATIONS

En is not necessarily zero in the wave guide at the boundary as there may be surface
charges onthe conducting walls (the solution given below implies that there are such
charges)

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 MICROWAVE AND OPTICAL COMMUNICATIONS
It follows from Maxwell's equation that because = 0, is also zero inside the conductor
(the time dependence of is exp(-iTt)). The continuity of Hn implies that Hn = 0 at the
boundary.

There are currents induced in the guides but for perfect conductors these can be only surface
currents. Hence, there is no continuity for Ht. This is to be contrasted with the boundary
conditionused for waves reflecting off conducting surfaces with finite conductivity.

The standard geometry for a rectangular wave guide is given fig 1. A wave can be guided
by two parallel planes for which case we let the planes at x = 0, a extend to y = ±4.

TE Modes: By definition, Ez = 0 and we start from

as the wave equation in Cartesian coordinates permits the use of the separation of variables.
TM Modes: By definition, Hz = 0 and we start from

It is customary in wave guides to use the longitudinal field strength as the reference. For

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 MICROWAVE AND OPTICAL COMMUNICATIONS
the parallel plate wave guide there is no y dependence so just set Y
=

TE modes
Using the above form for the solution of the wave equation, the wave equation can be rewritten as

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 MICROWAVE AND OPTICAL COMMUNICATIONS

the minus signs being chosen so that we get the oscillatory solutions needed to fit the
boundaryconditions.

Now apply the boundary conditions to determine the restrictions on Hz. At x

= 0, a:Ey = 0 and H x = 0 (Ez is zero everywhere)

For the following Griffith's writes down all the Maxwell equations specialized to
propagationalong 0z. I will write just those needed for the specific task and motivate the
choice.

We need to relate Ey, Hx to the reference Hz. Hence, we use the y component of ME2
(which has 2 H fields and 1 E field)

The first term is ikzHx which is zero at the boundary.

The absence of an arbitrary constant upon integration is justified below. At y

= 0, b:Ex = 0 and Hy = 0 and we now use the x component of ME2

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MICROWAVE AND OPTICAL COMMUNICATIONS
As the second term is proportional Hy we get

28
 MICROWAVE AND OPTICAL COMMUNICATIONS

However, m = n = 0 is not allowed for the following reason.

When m = n = 0, Hz is constant across the waveguide for any xy plane. Consider the integral
version of Faraday's law for a path that lies in such a plane and encircles the wave guide
but in the metal walls.

As E = 0 in the conducting walls and the time dependence of is given by exp(-iTt) this
equation requires that . We need only evaluate the integral over the guide as = 0 in the walls.

For constant Bz this gives Bzab = 0. So Bz = 0 as is Hz. However, as we have chosen Ez =

0 this implies a TEM wave which cannot occur inside a hollow waveguide. Adding an
arbitrary constant would give a solution like

which is not a solution to the wave equation ... try it. It also equivalent to adding a solution
with either m = 0 or n = 0 which is a solution with a different

Cut off frequency

This restriction leads to a minimum value for k. In order to get propagation kz2 >
0. Consequently

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 MICROWAVE AND OPTICAL COMMUNICATIONS
Suppose a > b then the minimum frequency is cB/a and for a limited range of T (dependent on a
and b) this solution (m = 1, n = 0, or TE10) is the only one possible.
Away from the boundaries

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 MICROWAVE AND OPTICAL COMMUNICATIONS

where Hzx means that cos k xx has been replaced by sin kxx.

We need another relation between Ey and either Hx or Hz, which must come from the other
Maxwell equation (ME1). We have to decide which component of ME1 to use. If we choose
the z component, the equation involves Ex and Ey, introducing another unknown field (Ex).
However, the x component involves Ey and Ez. As Ez = 0, this gives the required relation.

Substituting in the above gives

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 MICROWAVE AND OPTICAL COMMUNICATIONS

TM modes
The boundary conditions are easier to apply as it is Ez itself that is zero at the boundaries.

Consequently, the solution is readily found to be

Note that the lowest TM mode is due to the fact that Ez . 0. Otherwise, along with Hz = 0,
the solution is a TEM mode which is forbidden. The details are not given here as the TM
wave between parallel plates is an assignment problem.
It can be shown that for ohmic losses in the conducting walls the TM modes are more
attenuated than the TE modes.

Rectangular Waveguide:
• Let us consider a rectangular waveguide with interior dimensions are a x b,
• Waveguide can support TE and TM modes.
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MICROWAVE AND OPTICAL COMMUNICATIONS
– In TE modes, the electric field is transverse to the direction of
propagation.
– In TM modes, the magnetic field that is transverse and an electric field
component is in the propagation direction.

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 MICROWAVE AND OPTICAL COMMUNICATIONS
• The order of the mode refers to the field configuration in the guide, and is given
by m and n integer subscripts, TEmn and TMmn.
– The m subscript corresponds to the number of half-wave variations of the
field in the x direction, and
– The n subscript is the number of half-wave variations in the y direction.
• A particular mode is only supported above its cutoff frequency. The cutoff
frequency is given by

Rectangular Waveguide

e Location of mod

2 2 2 2
fcm = 1 m n = c  m   n 
n 2  a     
 a  b 
  b  2 r r

34
 o o
MICROWAVE AND OPTICAL COMMUNICATIONS
1 1 1 1 c
u= = = =
mme
e

o r o r

35
 MICROWAVE AND OPTICAL COMMUNICATIONS

We can achieve a qualitative understanding of wave propagation in waveguide by

considering the wave to be a superposition of a pair of TEM waves.

Let us consider a TEM wave propagating in the z direction. Figure shows the wave fronts;
bold lines indicating constant phase at the maximum value of the field (+Eo), and lighter
lines indicating constant phase at the minimum value (- Eo).

The waves propagate at a velocity uu, where the u subscript indicates media unbounded by
guide walls. In air, uu = c.

Since we know E = 0 on a perfect conductor, we can replace the horizontal lines of zero field with
perfect conducting walls. Now, u+ and u- are reflected off the walls as they propagate along the guide.

The distance separating adjacent zero-field lines in Figure (b), or separating the conducting walls in

36
 MICROWAVE AND OPTICAL COMMUNICATIONS
Figure (a), is given as the dimension a in Figure (b).
The distance a is determined by the angle q and by the distance between wavefront peaks, or the
wavelength l. For a given wave velocity uu, the frequency is f = uu/l.

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 MICROWAVE AND OPTICAL COMMUNICATIONS
If we fix the wall separation at a, and change the frequency, we must then also change the angle q if
we are to maintain a propagating wave. Figure (b) shows wave fronts for the u+ wave.

The edge of a +Eo wave front (point A) will line up with the edge of a –Eo front (point B), and the
two fronts must be l/2 apart for the m = 1 mode.

For any value of m, we can write by simple trigonometry

sinq 2
= a

u
l=
s
i
n
q
=
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 MICROWAVE AND OPTICAL COMMUNICATIONS
u

a m f

The waveguide can support propagation as long as the wavelength is smaller than a critical
value, lc, that occurs at q = 90°, or
2a uu
l = =
c
m fc

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 MICROWAVE AND OPTICAL COMMUNICATIONS

Where fc is the cutoff frequency for the propagating mode.

We can relate the angle  to the operating frequency and the cutoff frequency by

sin  

 f
c
f

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 MICROWAVE AND OPTICAL COMMUNICATIONS

The time tAC it takes for the wavefront to move from A to C (a distance lAC) is

Distance from A to C lAC m  2

tAC  
Wavefront Velocity uu uu
A constant phase point moves along the wall from A to D. Calling this phase
velocity up, and given the distance lAD is

m2
l AD
cos
Then the time tAD to travel from A to D is

l
t  AD  m  2
AD up cos up

Since the times taAnD

d tAC must be equal, we have

p
u
u
The Wave velocity is given by

c
o
s
q
1 1
uu   

1

1
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 MICROWAVE AND OPTICAL COMMUNICATIONS
c  o o

The Phase velocity is given by
u p  uu
cos
q
2 2 2
uG  uu cos
The Group velocity is given
byThe phase constant is
given by

42
 MICROWAVE AND OPTICAL COMMUNICATIONS

  u 1 

 fc


2

f
The guide wavelength is given by
u


The ratio of the transverse electric field to the transverse magnetic field for a propagating
mode at a particularfrequency is the waveguide impedance.

For a TE mode, the wave impedance is

For a TM mode, the wave impedance is

43
MICROWAVE AND OPTICAL COMMUNICATIONS

General Wave Behaviors:

The wave behavior in a waveguide can be determined by

44
MICROWAVE AND OPTICAL COMMUNICATIONS

45
 MICROWAVE AND OPTICAL COMMUNICATIONS
2 2
 E k E 0
2 2
 H 0
k H
where k
Then applying on the z-component 2


2


c
2 2 2 2
 E   E  E k E 0
2
E

  2
z z 


z
z
zz
2 2 k
x y E
2 z
z
0
Solving by methodof Separation of Variables :
Ez (x, y, z)  X (x)Y ( y)Z (z)
fromwhere we obtain :
''
X'' Z Y'' 2
   k
X Y Z

X '' Y Z ''
''
  
X Y 
k
Z

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 MICROWAVE AND OPTICAL COMMUNICATIONS
2  2
 k2
x  k y  k
2

which resultsin the expressions :

X '' 2
 k X 0
x
Y '' 2
k Y  0
y
2
Z ''   Z 0

From Faraday and Ampere Laws we can find the remaining four components
g  j

Ex   E
h
2 h2
z

x

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 MICROWAVE AND OPTICAL COMMUNICATIONS

Modes of propagation:
From the above equations we can conclude:
 TEM (Ez=Hz=0) can’t propagate.

 TE (Ez=0) transverse electric

 In TE mode, the electric lines of flux are perpendicular to the axis of
thewaveguide

 TM (Hz=0) transverse magnetic, Ez exists

 In TM mode, the magnetic lines of flux are perpendicular to the axis of
thewaveguide.
 HE hybrid modes in which all components exists.

TM Mode:
E


m



n


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 MICROWAVE AND OPTICAL COMMUNICATIONS
 jz
z Eo sin x ye
 a  
si
n




b

H z 0
E     m  mx n  y 
 E  z

z sin e
E  x 2 Eo cos

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 MICROWAVE AND OPTICAL COMMUNICATIONS

 The m and n represent the mode of propagation and indicates the

number ofvariations of the field in the x and y directions

TM Cutoff:

 The cutoff frequency 2


occurs when n
2

 m 
2
When
 c    
 a  b


or fc 
 

No propagation, everything is attenuated

2 2
 m  
n



50
 MICROWAVE AND OPTICAL COMMUNICATIONS
2
 n 
P
r
o
p
a
g
a
t
i
o
n
:
2
 m

2
W   
h

Cutoff e
n

 The cutoff frequency is the frequency below which attenuation occurs and
abovewhich propagation takes place. (High Pass)
u'
fc mn 
2

The phase constant becomes

2
 n 2   ' 1  f c
    
 a  b  f 

Phase velocity and impedance

 The phase velocity is defined as

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 MICROWAVE AND OPTICAL COMMUNICATIONS
2 up
up  
' f
 intrinsic impedance of the mode is
E E 2
TM 
x
 y '  f c
1  
Hy Hx f

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 MICROWAVE AND OPTICAL COMMUNICATIONS

Microstrip transmission line is a kind of "high grade" printed circuit construction, consisting of a track of copper
or other conductor on an insulating substrate. There is a "backplane" on the other side of the insulating substrate,
formed from similar conductor.There is a "hot" conductor which is the track on the top, and a "return" conductor
which is the backplane on the bottom. Microstrip is therefore a variant of 2-wire transmission line.

If one solves the electromagnetic equations to find the field distributions, one finds very nearly a
completely TEM (transverse electromagnetic) pattern. This means that there are only a few
regions in which there is a component of electric or magnetic field in the direction of wave
propagation.

The field pattern is commonly referred to as a Quasi TEM pattern. Under some conditions one has
to take account of the effects due to longitudinal fields. An example is geometrical dispersion,
where different wave frequencies travel at different phase velocities, and the group and phase
velocities are different.

The quasi TEM pattern arises because of the interface between the dielectric substrate and the
surrounding air. The electric field lines have a discontinuity in direction at the interface. The
boundary conditions for electric field are that the normal component (ie the component at right
angles to the surface) of the electric field times the dielectric constant is continuous across the
boundary; thus in the dielectric which may have dielectric constant 10, the electric field suddenly
drops to 1/10 of its value in air. On the other hand, the tangential component (parallel to the
interface) of the electric field is continuous across the boundary. In general then we observe a
sudden change of direction of electric field lines at the interface, which gives rise to a longitudinal
magnetic field component from the second Maxwell's equation, curl E = - dB/dt.

Since some of the electric energy is stored in the air and some in the dielectric, the effective
dielectric constant for the waves on the transmission line will lie somewhere between that of the
air and that of the dielectric. Typically the effective dielectric constant will be 50-85% of the
substrate dielectric constant.

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 MICROWAVE AND OPTICAL COMMUNICATIONS

SUBSTRATE MATERIALS:
Important qualities of the dielectric substrate include

 The microwave dielectric constant

 The frequency dependence of this dielectric constant which gives rise to
"materialdispersion" in which the wave velocity is frequency-dependent
 The surface finish and flatness
 The dielectric loss tangent, or imaginary part of the dielectric constant,
which setsthe dielectric loss

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 MICROWAVE AND OPTICAL COMMUNICATIONS
 The cost
 The thermal expansion and conductivity
 The dimensional stability with time
 The surface adhesion properties for the conductor coatings
 The manufacturability (ease of cutting, shaping, and drilling)
 The porosity (for high vacuum applications we don't want a substrate
whichcontinually "out gasses" when pumped)

Types of substrate include plastics, sintered ceramics, glasses, and single crystal substrates (single
crystals may have anisotropic dielectric constants; "anisotropic" means they are different along
the different crystal directions with respect to the crystalline axes.)

Common substrate materials

 Plastics are cheap, easily manufacturability, have good surface adhesion,

but have poor microwave dielectric properties when compared with
other choices.They have poor dimensional stability, large thermal
expansion coefficients, and poor thermal conductivity.
o Dielectric constant: 2.2 (fast substrate) or 10.4 (slow substrate)
o Loss tangent 1/1000 (fast substrate) 3/1000 (slow substrate)
o Surface roughness about 6 microns (electroplated)
o Low thermal conductivity, 3/1000 watts per cm sq per degree
 Ceramics are rigid and hard; they are difficult to shape, cut, and drill; they
comein various purity grades and prices each having domains of application;
they have low microwave loss and are reasonably non- dispersive; they have
excellent thermal properties, including good dimensional stability and high
thermal conductivity; they also have very high dielectric strength. They cost
more than plastics. In principle the size is not limited.
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MICROWAVE AND OPTICAL COMMUNICATIONS
o Dielectric constant 8-10 (depending on purity) so slow substrate
o Loss tangent 1/10,000 to 1/1,000 depending on purity
o Surface roughness at best 1/20 micron

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 MICROWAVE AND OPTICAL COMMUNICATIONS
o High thermal conductivity, 0.3 watts per sq cm per degree K
 Single crystal sapphire is used for demanding applications; it is very hard,
needsorientation for the desired dielectric properties which are
anisotropic;

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is very expensive, can only be made in small sheets; has high dielectric
constant so is used for very compact circuits at high frequencies; has low
dielectric loss; has excellent thermal properties and surface polish.
o Dielectric constant 9.4 to 11.6 depending on crystal orientation
(slowsubstrate)
o Loss tangent 5/100,000
o Surface roughness 1/100 micron
o High thermal conductivity 0.4 watts per sq cm per degree K
 Single crystal Gallium Arsenide (GaAs) and Silicon (Si) are both used
formonolithic microwave integrated circuits (MMICs).
o Dealing with GaAs first we have.....
 Dielectric constant 13 (slow substrate)
 Loss tangent 6/10,000 (high resistivity GaAs)
 Surface roughness 1/40 micron
 Thermal conductivity 0.3 watts per sq cm per degree K (high)

GaAs is expensive and piezoelectric; acoustic modes can propagate in the substrate and can couple
to the electromagnetic waves on the conductors.

The dielectric strength of ceramics and of single crystals far exceeds the strength of plastics, and
so the power handling abilities are correspondingly higher, and the breakdown of high Q filter
structures correspondinglyless of a problem.

It is also a good idea to have a high dielectric constant substrate and a slow wave propagation
velocity; this reduces the radiation loss from the circuits. However at the higher frequencies the
circuits get impossible small, which restricts the power handling capability.

Stripline is a conductor sandwiched by dielectric between a pair of ground planes, much like a
coax cable would look after you ran it over with your small-manhood indicating SUV (let's not go
there. ) In practice, strip

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line is usually made by etching circuitry on a substrate that has a ground plane on the opposite
face, then addinga second substrate (which is metalized on only one surface) on top to achieve
the second ground plane. Strip line is most often a "soft-board" technology, but using low-
temperature co-fired ceramics (LTCC), ceramic stripline circuits are also possible.

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Transmission lines on either of the interior metal layers behave very nearly like "classic" stripline,
the slight asymmetry is not a problem. Excellent "broadside" couplers can be made by running
transmission lines parallel to each other on the two surfaces.

Other variants of the stripline are offset strip line and suspended air stripline (SAS).

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For stripline and offset stripline, because all of the fields are constrained to the same dielectric,
the effective dielectric constant is equal to the relative dielectric constant of the chosen dielectric
material. For suspended stripline, you will have to calculate the effective dielectric constant, but
if it is "mostly air", the effective dielectric constant will be close to 1.

Advantages and disadvantages of stripline:

Stripline is a TEM (transverse electromagnetic) transmission line media, like coax. This means
that it is non- dispersive, and has no cutoff frequency. Whatever circuits you can make on
microstrip (which is quasi-TEM), you can do better using stripline, unless you run into fabrication
or size constraints. Stripline filters and couplers always offer better bandwidth than their
counterparts in microstrip.

Another advantage of stripline is that fantastic isolation between adjacent traces can be achieved
(as opposed to microstrip). The best isolation results when a picket-fence of vias surrounds each
transmission line, spaced at less than 1/4 wavelength. Stripline can be used to route RF signals
across each other quite easily when offset stripline is used.

Disadvantages of stripline are two: first, it is much harder (and more expensive) to fabricate than
microstrip. Lumped-element and active components either have to be buried between the ground
planes (generally a tricky proposition), or transitions to microstrip must be employed as needed
to get the components onto the top of the board.

The second disadvantage of stripline is that because of the second ground plane, the strip widths
are much narrower for a given impedance (such as 50 ohms) and board thickness than for
microstrip. A common reaction to problems with microstrip circuits is to attempt to convert them
to stripline. Chances are you'll end up with a board thickness that is four times that of your
microstrip board to get equivalent transmission line loss. That means you'll need forty mils thick
strip line to replace ten mil thick micro strip! This is one of the reasons that soft-board
manufacturers offer so many thicknesses.
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Stripline equations

A simplified equation for characteristic impedance of stripline is given as:

COPLANAR STRIP LINES

A coplanar strip line consisting of two strip conductors each of width separated by a distance "s",
mounted on a single dielectric substrate, with one conducting strip grounded. Since both the strips
are on one side of the substrate unlike the parallel strip lines, connection of shunt elements is very
easy. This is an added advantage in the manufacture of microwave integrated circuits (MICs).
Because of this, reliability increases.

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The characteristic impedance of the coplanar strip line is given by

P = average power flowing through the coplanar strip

SHIELDED STRIP LINES

The configuration of strip line consisting of a thin conducting strip of width "w" much greater than
its thickness "t". This strip line is placed at the centre surrounded by a low-loss dielectric substrate
of thickness "b", between two ground plates as shown. The mode of propagation is TEM
(transverse electro-magnetic) wave where the electric field lines are perpendicular to the strip
and concentrated at the centre of the strip. Fringing field lines also exist at the edges .When the
dimension 'b' is less than half wavelength, the field cannot propagate in transverse direction and
is attenuated exponentially. The energy will be confined to the line cross-section provided a> 5b.
The commonly used dielectrics are teflon,polyolefine, polystyrene etc., and the operating
frequency range extends from 100 MHz to 30 GHz.

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LOSSES IN STRIP LINES:

For low-loss dielectric substrate, the attenuation factor in the strip line arises from conductor losses and is
given by

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The attenuation constant of a microstrip line depends on frequency of operation, electrical

properties ofsubstrate and the conductors and the geometry of mounting of strip on the
dielectric.
When the dielectric substrate of dielectric constant is purely non-magnetic then three types of
losses occur inmicrostrip lines . they are
1. Dielectric losses in substrate
1.Ohmic losses in strip conductor and ground
plane1.Radiation loss

Dielectric losses in substrate:

All dielectric materials possess some conductivity but it will be small , but when it is not negligible,
then the displacement current density leads the conduction current density by 90 degrees,
introducing loss tangent for a lossy dielectric .

1. Ohmic losses in strip conductor and ground plane

In a microstrip line the major contribution to losses at micro frequencies is from finite
conductivity ofmicrostrip conductor placed on a low loss dielectric substrate. Due to current
flowing through the strip, there will be ohmic losses and hence attenuation of the microwave
signal takes place. The current distribution in the transverse palne is fairly uniform with
minimum value at the central axis and shooting up to maximum valuesat the edge of the strip.

2. Radiation losses:
At microwave frequencies , the microstrip line acts as an antenna radiating a small amount of
power resulting in radiation losses. This loss depends on the thickness of the substrate, the
characteristic impedance Z, effective dielectric constant and the frequency of operation.

For low-loss dielectric substrate, the attenuation factor in the strip line arises from conductor losses and is given
by

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Advantages and disadvantages of Planar Transmission Lines over Co- axial Lines:
Advantages:-
The advantages of planar transmission lines are
(a) very small size and hence low weight
(b) can be easily mounted on a metallic body including substrate.
(c) increased reliability
(d) cost is reduced due to small size
(e) series and shunt maintaining of components is possible
(f) the characteristic impedance Zo is easily controlled by defining the dimensions
of theline in a single plane
(g) by changing the dimensions of the line in one plane only, it is possible to
achieveaccurate passive circuit design

Disadvantages:-
The disadvantages of planar transmission lines are
(a) low power handling capability due to small size
(b) The microstrip, slot and coplanar lines tend to radiate power resulting in
radiationlosses
(c) low Q-factor

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UNIT- II
MICROWAVE WAVEGUIDE COMPONENTS AND APPLICATIONS

INTRODUCITON
WAVE GUIDE CORNERS , BENDS AND TWISTS:
The waveguide corner, bend, and twist are shown in figure below, these waveguide
components are normallyused to change the direction of the guide through an arbitrary angle.
In order to minimize reflections from the discontinuities, it is desirable to have the mean
length L betweencontinuities equal to an odd number of quarter wave
lengths. That is,

where n = 0, 1, 2, 3, ... , and Ag is the wavelength in the waveguide. If the mean length L is an odd
number of quarter wavelengths, the reflected waves from both ends of the waveguide section
are completely canceled. Forthe waveguide bend, the minimum radius of curvature for a small
reflection is given by Southworth as

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DIRECTIONAL COUPLERS:
A directional coupler is a four-port waveguide junction as shown below. It Consists of a
primary waveguide 1-2 and a secondary waveguide 3-4. When all Ports are terminated in their
characteristic impedances, there is free transmission of the waves without reflection, between
port 1 and port 2, and there is no transmission of power between port I and port 3 or between
port 2 and port 4 because no coupling exists between these two pairs of ports. The degree of
coupling between port 1 and port4 and between port 2 and port 3 depends on the structure of
the coupler.
The characteristics of a directional coupler can be expressed in terms of its Coupling factor and
its directivity.
Assuming that the wave is propagating from portto port2 in the primary line, the coupling
factor and thedirectivity are defined,

where PI = power input to port I

P3 =power output from port 3
P4 = poweroutput from port 4

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It should be noted that port 2, port 3, and port 4 are terminated in their characteristic impedances.
The coupling factor is a measure of the ratio of power levels in the primary and secondary lines.
Hence if the coupling factor is known, a fraction of power measured at port 4 may be used to
determine the power input at port 1 .

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This significance is desirable for microwave power measurements because no disturbance, which
may be caused by the power measurements, occurs in the primary line. The directivity is a
measure of how well the forward traveling wave in the primary waveguide couples only to a
specific port of the secondary waveguide ideal directional coupler should have infinite directivity.
In other words, the power at port 3 must be zero because port 2 and portA are perfectly matched.
Actually well-designed directional couplers have a directivityof only 30 to 35 dB.

Several types of directional couplers exist, such as a two-hole direct couler, four-hole directional
coupler, reverse-coupling directional coupler , and Bethe- hole directional coupler the very
commonly used two-hole directional coupler is described here.

TWO HOLE DIRECTIONAL COUPLERS:

A two hole directional coupler with traveling wave propagating in it is illustrated . the spacing
between the centers of two holes is

A fraction of the wave energy entered into port 1 passes through the holes and is radiated into the
secondary guide as he holes act as slot antennas. The forward waves in the secondary guide are
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in same phase , regardless of the hole space and are added at port 4. the backward waves in the
secondary guide are out of phase and are cancelled in port 3.

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CIRCUALTORS AND ISOLATORS:

Both microwave circulators and isolators are non reciprocal transmission devices that use the
property of Faraday rotation in the ferrite material. A non reciprocal phase shifter consists of thin
slab of ferrite placed in a rectangular waveguide at a point where the dc magnetic field of the
incident wave mode is circularly polarized. When a piece of ferrite is affected by a dc magnetic
field the ferrite exhibits Faraday rotation. It does so because the ferrite is nonlinear material
and its permeability is an asymmetric tensor.

MICROWAVE CIRCULATORS:
A microwave circulator is a multiport waveguide junction in which the wave can flow only from
the nth port to the (n + I)th port in one direction Although there is no restriction on the number
of ports, the four-port microwave circulator is the most common. One type of four-port microwave
circulator is a combination of two 3-dB side hole directional couplers and a rectangular waveguide
with two non reciprocal phase shifters.

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The operating principle of a typical microwave circulator can be analyzed with the aid of Fig shown above .Each
of the two 3-dB couplers in the circulator introduces a phase shift of 90°, and each of the two phase shifters
produces a certain amount of phase change in a certain direction as indicated. When a wave is incident to port
1,the wave is split into two components by coupler I. The wave in the primary guide arrives at port 2 with a
relative phase' change of 180°. The second wave propagates through the two couplers and the secondary guide
and arrives at port 2 with a relative phase shift of 180°. Since the two waves reaching port 2 are in phase, the
power transmission is obtained from port 1 to port 2. However, the wave propagates through the primary guide,
phase shifter, and coupler 2 and arrives at port 4 with a phase change of 270°. The wave travels through coupler
1and the secondary guide, and it arrives at port 4 with a phase shift of 90°. Since the two waves reaching port
4 are out of phaseby 180°, the power transmission from port 1 to port 4 is zero. In general, the differential
propagation constants in the two directions of propagation in a waveguide containing ferrite phase shifters
should be

where m and n are any integers, including zeros. A similar analysis shows that a wave incident
to port 2 emerges at port 3 and so on. As a result, the sequence of power flow is designated as
1 ~ 2 ~ 3 ~ 4 ~ 1. Many types of microwave circulators are in use today. However, their
principles of operation remain the same. .A four-port circulator is constructed by the use of two
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magic tees and a phase shifter. The phase shifter produces a phase shift of 180°.

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A perfectly matched, lossless, and nonreciprocal four-port circulator has an S matrix of the form

Using the properties of S parameters the S-matrix is

MICROWAVE ISOLATORS:
An isolator is a nonreciprocal transmission device that is used to isolate one component from
reflections of other components in the transmission line. An ideal isolator completely absorbs the
power for propagation in one direction and provides lossless transmission in the opposite
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direction. Thus the isolator is usually called uniline.

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Isolators are generally used to improve the frequency stability of microwave generators, such as
klystrons and magnetrons, in which the reflection from the load affects the generating frequency.
In such cases, the isolator placed between the generator and load prevents the reflected power
from the unmatched load from returning to the generator. As a result, the isolator maintains the

frequency stability of the generator.

Isolators can be constructed in many ways. They can be made by terminating ports 3 and 4 of a four-port
circulator with matchedloads. On the other hand, isolators can be made by inserting a ferrite rod along the axis
of a rectangular waveguide as shown below.
The isolator here is a Faraday-rotation isolator. Its operating principle can be explained as follows
. The input resistive card is in the y-z plane, and the output resistive card is displaced 45° with
respect to the input card. Thedc magnetic field, which is applied longitudinally to the ferrite rod,
rotates the wave plane of polarization by 45°. The degrees of rotation depend on the length and
diameter of the rod and on the applied de magnetic field. An input TEIO dominant mode is incident
to the left end of the isolator. Since the TEIO mode wave is perpendicular to the input resistive
card, the wave passes through the ferrite rod without attenuation. The wave in the ferrite rod
section is rotated clockwise by 45° and is normal to the output resistive card. As a result of
rotation, the wave arrives at the output.

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end without attenuation at all. On the contrary, a reflected wave from the output end is similarly
rotated clockwise 45° by the ferrite rod. However, since the reflected wave is parallel to the input
resistive card, the wave is thereby absorbed by the input card. The typical performance of these
isolators is about 1-dB insertion loss in forward transmission and about 20- to 30-dB isolation in
reverse attenuation.

WAVE GUIDE TEE JUNCTIONS:

A waveguide Tee is formed when three waveguides are interconnected in the form of
English alphabet T and thus waveguide tee is 3-port junction. The waveguide tees are
used to connects a branch or section of waveguide in series or parallel with the main
waveguide transmission line either for splitting or combining power in a waveguide
system.

There are basically 2 types of tees namely

1.H- plane Tee junction

2.E-plane Tee junction
A combination of these two tee junctions is called a hybrid tee or “ Magic Tee”.

E-plane Tee(series tee):

An E-plane tee is a waveguide tee in which the axis of its side arm is parallel to the E
field of themain guide . if the collinear arms are symmetric about the side arm.
If the E-plane tee is perfectly matched with the aid of screw tuners at the junction ,
the diagonalcomponents of the scattering matrix are zero because there will be no
reflection.
When the waves are fed into side arm, the waves appearing at port 1 and port 2 of
the collineararm will be in opposite phase and in same magnitude.

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H-plane tee: (shunt tee)

An H-plane tee is a waveguide tee in which the axis of its side arm is shunting the E
field orparallel to the H-field of the main guide.

If two input waves are fed into port 1 and port 2 of the collinear arm, the output
wave at port 3will be in phase and additive .
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If the input is fed into port 3, the wave will split equally into port 1 and port 2 in
phase and insame magnitude .

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Magic Tee ( Hybrid Tees )

A magic tee is a combination of E-plane and H-plane tee. The characteristics of magic tee are:

1. If two waves of equal magnitude and same phase are fed into port 1 and port 2 the
output will be zero at port 3 and additive at port 4.
2. If a wave is fed into port 4 it will be divided equally between port 1 and port 2 of
the collinear arms and will not appear at port 3.
3. If a wave is fed into port 3 , it will produce an output of equal magnitude and
opposite phase at port 1 and port 2. the output at port 4 is zero.
4. If a wave is fed into one of the collinear arms at port 1 and port 2, it will not appear
in the other collinear arm at port 2 or 1 because the E-arm causes a phase delay while
H armcauses a phase advance.

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DIRECTIONAL COUPLERS:
A directional coupler is a four-port waveguide junction as shown below. It Consists of a
primary waveguide 1-2 and a secondary waveguide 3-4. When all Ports are terminated
in their characteristic impedances, there is free transmission of the waves without
reflection, between port 1 and port 2, andthere is no transmission of power between port
I and port 3 or between port 2 and port 4 because no coupling exists between these two
pairs of ports. The degree of coupling between port 1 and port4 and between port 2 and
port 3 depends on the structure of the coupler.
The characteristics of a directional coupler can be expressed in terms of its Coupling
factor and its directivity. Assuming that the wave is propagating from port to port2 in the
primary line, the couplingfactor and the directivity are defined,

where PI = power input to

port IP3 = power output

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from port 3 P4 = power

output from port 4

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It should be noted that port 2, port 3, and port 4 are terminated in their characteristic
impedances. The coupling factor is a measure of the ratio of power levels in the
primary and secondary lines. Hence if the coupling factor is known, a fraction of
power measured at port 4 may be used to determine the power input at port 1.
This significance is desirable for microwave power measurements because no
disturbance, which may be caused by the power measurements, occurs in the primary
line. The directivity is a measure of how well the forward traveling wave in the
primary waveguide couples only to a specific port of the secondary waveguide ideal
directional coupler should have infinite directivity. In other words, the power at port 3
must be zero because port 2 and portA are perfectly matched. Actually well- designed
directional couplers have a directivity of only 30 to 35 dB.Several types of directional
couplers exist, such as a two-hole direct couler, four-hole directional coupler, reverse-
coupling directional coupler , and Bethe-hole directional coupler the very commonly
used two-hole directional coupler is described here.

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TWO HOLE DIRECTIONAL COUPLERS:

A two hole directional coupler with traveling wave propagating in it is illustrated . the spacing
between the centers of two holes is

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A fraction of the wave energy entered into port 1 passes through the holes and is radiated into the
secondary guide as he holes act as slot antennas. The forward waves in the secondary guide are
in same phase , regardless of the hole space and are added at port 4. the backward waves in the
secondary guide are out of phase and are cancelled in port 3.

S-matrix for Directional coupler:

The following characteristics arc observed in an ideal Directional Coupler:
1. Since the directional coupler is a 4-portjunction, the order or (S I matrix is 4 x 4
givcnby

2. Microwave power fed into port (I) cannot comc out of port (3) as port (3) is the
backport. Therefore the scattering co-efficient S13 is zero...'

3. Because of the symmetry of the junction, an input power at port (2) cannot
couple toport (4) as port (4) is the back-port for port (2)

4. Let us assume that port (3) and (4) are perfectly matched to the junction so that

Then, the remaining two ports will be "automatically" matched to the junction

From the symmetric property of ISI matrix, we have

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With the above characteristic values for S-parameters, the matrix of (5.125)

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becomes
From unitary property of equation we have

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UNIT-3 MICROWAVE
TUBES
Limitations and Losses of conventional tubes at microwave frequencies

Following are the limitations of conventional active devices like transistors or tubes at microwave frequencies

1) Interelectrode capacitance.
What is interelectrode capacitance?

Vacuum has a dielectric constant of 1. As the elements of the triodes are made of metal and are
separated by adielectric, capacitance exists between them. This capacitance is interelectrode
capacitance.

The capacitance between the plate and grid is Cpg. The grid-to-cathode capacitance is Cgk. The total
capacitanceacross the tube is Cpk.

Now, we know that the capacitive reactance is given by

So as the input frequency increases, the effective grid to cathode impedance decreases due to decrease in
reactance ofinterelectrode capacitance. At higher frequencies (greater than 100MHz) it becomes so small that
signal is short circuited with the tube. Also, gain of the device reduces significantly.

This effect can be minimized by taking smaller (reducing the area) electrodes and by increasing distance
betweenthem (i.e. reducing capacitance because C=epsilon*A/d) therefore by increasing reactance.

2) Lead inductance.

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Lead or stray inductance are effectively in parallel within the device with the interelectrode capacitance.
Inductivereactance is given by:

XL=2 π f L

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As the frequency increases, the effective reactance of the circuit also increases. This effect raise the frequency
limit tothe device. The inductance of cathode lead is common to both grid and plate circuits. This provides a
path for degenerative feedback which reduces the overall efficiency of the circuit.

3) Transit time
Transit time is the time required for electrons to travel from the cathode to the plate. At low frequency, the
transit time is very negligible. But, however at higher frequencies, transit time becomes an appreciable
portion of a signalcycle which results in decrease in efficiency of device.

4) Gain bandwidth product

Gain bandwidth product is independent of frequency. So for a given tube higher gain can be only obtained
at theexpense of narrower bandwidth.

5) Skin effect
This effect is introduced at higher frequencies. Due to it, the current flows from the small sectional area to the
surfaceof the device. Also at higher frequencies, resistance of conductor increases due to which the there are
losses.

R=ρl( √f)

6) Dielectric loss
Dielectric material is generally different silicon plastic encapsulation materials used in microwave devices. At
higherfrequencies the losses due to these materials are also prominent.

Microwave Tubes:
1. Linear beam tubes (O-type)-Dc magnetic field is in parallel with
the dcelectric field.
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2. Crossed-field tubes (M-type)-Dc electric field and the dc magnetic

fieldare perpendicular to each other.

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Cavity Klystrons

In microwave region, performs the functions of generates, receives and amplifies signals

Configurations:
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1.Reflex – low power microwave oscillator

2.Multicavity – low power microwave amplifier

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a) Reflex Klystron
-Has a reflector and one cavity as a resonator
-Reflex action of electron beam

Performance:
- Frequency range: 2-200 GHz
- BW: ± 30 MHz for _VR: ±10 V
- Power o/p: 10mW – 2.5W
- used as microwave source in lab,microwave transmitter
- frequency modulation and amplitude modulation

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Mechanism of oscillation
_ The electron passing through the cavity gap
–experience the RF field

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_ velocity modulated

a: Electrons which encountered the positive half cycle of the RF field in the cavity gap will be

acceleratedb: Electrons which encountered zero RF field will pass with unchanged
original velocity

c: Electrons which encountered the negative half cycle will be retarded and entering the
repellerspace.

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Multicavity Klystron

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Two cavity Klystron Amplifier

_ Assumption for RF amplification
_ Transit time in the cavity gap is very small compared to the period of the input RF signal cycle
_ Input RF signal amplitude is very small compared to the dc beam voltage
_ The cathode, anode, cavity grids and collectors are all parallel
_ No space charge or debunching take place at the bunch point
_ The RF fields are totally confined in the cavity gaps, zero in the drift space
_ Electrons leave the cathode with zero initial velocity

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Traveling wave tube (TWT)

Travelling Wave Tube Amplifier:
_ High gain > 40 dB
_ Low NF < 10 dB
_ Wide Band > Octave
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_ Frequency range:0.3 – 50 GHz

_ Contains electron gun, RF interaction circuit, electron beam focusing magnet, collector
_ Amplify a weak RF input signal many thousands of times

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a) Electron gun
_ To get as much electron current flowing into as small a region as possible without distortion or fuzzy edges

Sources of electrons for the beam- 6 elements:

 gun shells
 heater
 cathode
 control grid
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 focus electrode
 anode

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b) RF interaction circuit
_ Interaction structures : helix, ring bar, ringloop, coupled cavity
_ RF circuit – complex trade off analysis, based on many interlocking parameters
_ Low power level : helix
_ Medium power level : ring loop, ring bar
_ Power level & frequency increased: RF losses on the circuit become more appreciate able.

c) Electron beam focusing

_ A magnetic field – to hold the electron beam together as it travels through the
interactionstructure of the tube
_ The beam tends to disperse or spread out as a result of the natural repulsive
forcesbetween electrons.
_ Methods of magnetic focusing
_ Solenoid magnetic structure
_ Permanent magnet
_ Periodic permanent magnet (PPM)
_ Radial magnet PPM

d) The collector
_ To dissipate the electrons in the form of heat as they emerge from the slow wave structure
_ Accomplished by thermal conduction to a colder outside surface – the heat is absorbed by circulated air or a
liquid

1. Gain compression
_ the amount of gain decrease from the small signal condition (normally 6dB)

2. Beam Voltage
_ the voltage between the cathode and the RF structure

3. Synchronous Voltage
_ the beam voltage necessary to obtain the greatest interaction between the electrons in the electron beam
and the RFwave on the circuit

4. Gain
_ the ratio of RF output power to RF input power (dB)

5. Phase Characteristic
_ Phase shift – the phase of output signal relative to the input signal
_ Phase sensitivity – the rate of phase change with a specific operating parameter.

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UNIT- IV

TRANSFER ELECTRON DEVICES

INTRODUCTION:
The application of two-terminal semiconductor devices at microwave frequencies has been
increased usage during the past decades. The CW, average, and peak power outputs of these
devices at higher microwave frequencies are much larger than those obtainable with the best
power transistor. The common characteristic of all active two-terminal solid-state devices is their
negative resistance. The real part of their impedance is negative over a range of frequencies. In a
positive resistance the current through the resistance and the voltage across it are in phase. The
voltage drop across a positive resistance is positive and a power of (12 R) is dissipated in the
resistance.
In a negative resistance, however, the current and voltage are out of phase by 180°. The voltage
drop across a negative resistance is negative, and a power of (-I!R) is generated by the power
supply associated with the negative resistance. In positive resistances absorb. power (passive
devices), whereas negative resistances generate power (active devices). In this chapter the
transferred electron devices(TEDs) are analyzed.
The differences between microwave transistors and transferred electron devices (TEDs) are
fundamental.Transistors operate with either junctions or gates, but TED
res baulk de vices having
no junctions or gates. The majority of transistors are fabricated from elemental semiconductors,
such as silicon or germanium, whereas 1tDs are fabricated from compound semiconductors, such
as gallium arsenide (r.As),indium phosphide (lnP), or cadmium telluride (CdTe). Transistors
operate As "warm" electrons whose energy is not much greater than the thermal energy
0.026eVat room temperature) of electrons in the semiconductors.

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GUNN EFFECT DIODES – GaAs diode

Gunn effect are named after J. B. Gunn who is 1963 discovered a periodic fluctuation of current
passing through the n- type gallium arsenide . when the applied voltage exceeded a certain critical
value.

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Shockley in 1954 suggested that the two terminal negative resistance devices using
semiconductors hadadvantages over transistors at high frequencies.

In 1961 , Ridley and Watkins described a new method for obtaining negative differential mobility
in semiconductors. The principle involved is to heat carriers in a light mass , low mobility , higher
energy sub band when they have a high temperature.

Finally Kroemer stated that the origin of the negative differential mobility is Ridley Watkins
Hilsum’s mechanism of electron transfer into the valleys that occur in conduction bands.

Gunn effect:
The below figure shows the diagram of a uniform n-type GaAs diode with ohmic contacts at the
end surfaces. Gunn stated that “ Above some critical voltage , corresponding to an electric field
of 2000 to 4000 Volts/cm, the current in every specimen became a fluctuating function of time.

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Gunn Diodes

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Single piece of GaAs or Inp and contains no junctions Exhibits

negativedifferential resistance

Applications:
low-noise local oscillators for mixers (2 to 140 GHz). Low-power
transmitters andwide band tunable sources
Continuous-wave (CW) power levels of up to several hundred mill watts can be obtained in
the X-, Ku-, andKa-bands. A power output of 30 mW can be achieved from commercially
available devices at 94 GHz.

Higher power can be achieved by combining several devices in a power combiner.

Gunn oscillators exhibit very low dc-to-RF efficiency of 1 to 4%.

Gunn also discovered that the threshold electric field Eili varied with the length and type of
material. He developed an elaborate capacitive probe for plotting the electric field distribution
within a specimen of n-typeGaAs of length L = 210 JLIll and cross-sectional area 3.5 x 10-3
cm2 with a low-field resistance of 16 n.
Current instabilities occurred at specimen voltages above 59 V, which means that the threshold
field is

RIDLEY WATKINS AND HILSUM THEORY:

Many explanations have been offered for the Gunn effect. In 1964 Kroemer [6] suggested that

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Gunn' sobservations were in complete agreement with the Ridley- Watkins-Hilsum (RWH)
theory.

Differential Negative Resistance:

The fundamental concept of the Ridley-Watkins-Hilsum (RWH) theory is the differential negative
resistance developed in a bulk solid-state III-V compound when either a voltage (or electric field)
or a current is applied tothe terminals of the sample. There are two modes of negative-resistance
devices: voltage- controlled and current controlled Modes.

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In the voltage-controlled mode the current density can be multivalued, whereas in the current-
controlled mode the voltage can be mu1tivalued. The major effect of the appearance of a
differential negative-resistance region in the current density field curve is to render the sample
electrically unstable. As a result, the initially homogeneous sample becomes electrically
heterogeneous in an attempt to reach stability. In the voltage- controlled negative-resistance
mode high-field domains are formed, separating two low-field regions. The interfaces
separating low and high-field domains lie along equi potentials; thus they are in planes
perpendicularto the current direction.

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Expressed mathematically, the negative resistance of the sample at a particular region is

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If an electric field Eo (or voltage Vo) is applied to the sample, for example, the current density
10is generated.As the applied field (or voltage) is increased to E1 (or V2), the current density
is decreased to 12. When the field (or voltage) is decr~ to £. (or VI), the current density is
increased to 1, . These phenomena of the voltage controlled negative resistance are shown in
Fig. 7-2-3(a). Similarly, for the current controlled mode, the negative-resistance profile is as
shown below.

TWO VALLEY MODEL THEORY:

Kroemer proposed a negative mass microwave amplifier in 1958 [lO] and 1959 [II]. According to
the energy band theory of the n -type GaAs, a high-mobility lower valley is separated by an
energy of 0.36 eV from a low-mobility upper valley

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Electron densities in the lower and upper valleys remain the same under an Equilibrium
condition. When the applied electric field is lower than the electric field of the lower valley (E <
Ee), no electrons will transfer to theupper valley.

When the applied electric field is higher than that of the lower valley and lower than that of
the uppervalley (Ee < E < Eu)), electrons will begin to transfer to the upper valley.

when the applied electric field is higher than that of the upper valley (Eu < E), all electrons will
transfer to theupper valley.

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When a sufficiently high field E is applied to the specimen, electrons are accelerated and their
effective temperature rises above the lattice temperature also increases. Thus electron density/I
and are both functions of electric field E.

MODES OF OPERATION OF GUNN DIODE:

A gunn diode can operate in four modes:

1. Gunn oscillation mode

2. stable amplification mode
3. LSA oscillation mode
4. Bias circuit oscillation mode

Gunn oscillation mode: This mode is defined in the region where the product of frequency
multiplied bylength is about 107 cm/s and the product of doping multiplied by length is greater
than 1012/cm2.In this region the device is unstable because of the cyclic formation of either the
accumulation layer or the high field domain.
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When the device is operated is a relatively high Q cavity and coupled properly to the
load, the domain Iquenched or delayed before nucleating.

2.Stable amplification mode: This mode is defined in the region where the product of
frequencytimes length is about 107 cmls and the product of doping times length is between

l011and 1O12/cm2
3. LSA oscillation mode: This mode is defined in the region where the product of frequency times length

is above 107 cmls and the quotient of doping divided by frequency is between 2 x 104 and 2 x 105.
4. Bias-circuit oscillation mode: This mode occurs only when there is either Gunn or LSA oscillation. and
it isusually at the region where the product of frequency times length is too small to appear in the figure.
When a bulk diode is biased to threshold. the average current suddenly drops as Gunn oscillation begins.

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The drop in current at the threshold can lead to oscillations in the bias circuit that are
typically 1 kHz to 100MHz .

Delayed domain mode (106 cm/s < fL < 107 cm/s). When the transit time is Chosen so that the
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domain iscollected while E < Eth as shown in Fig. 7-3-4(b), a new domain cannot form until the
field rises above threshold again. In this case, the oscillation period is greater than the transit

time-that is, To > T,. This

delayed mode is also called inhibited mode. The efficiency of this mode is about 20%.

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Quenched domain mode (fL > 2 X 107 cm/s).
If the bias field drops below the sustaining field Es during the negative half-cycle as shown ,the
domain collapses before it reaches the anode. When the bias field swings back above threshold ,a
new domain is nucleated and the process repeats. Therefore the oscillations occur at the
frequency of the resonant circuit ratherthan at the transit-time frequency, It has been found that
the resonant frequency of the circuit is several timesthe transit-time frequency, since one dipole
does not have enough time to readjust and absorb the voltage of the other dipoles . Theoretically,
the efficiency of quenched domain oscillators can reach 13%

LSA MODE
When the frequency is very high, the domains do not have sufficient time to form While the field
is above threshold. As a result, most of the domains are maintained In the negative conductance
state during a large fraction of the voltage cycle. Any Accumulation of electrons near the cathode
has time to collapse while the signal is Below threshold. Thus the LSA mode is .the simplest mode
of operation.

AVALANCHE TRANSIT TIEM DEVICES:

READ DIODE:
Read diode was the first proposed avalanche diode. The basic operating principles of IMPATT
diode can be easily understood by first understanding the operation of read diode.
The basic read diode consists of four layers namely n+ p I p+ layers. The plus superscript refers
to very high doping levels and ‘i’ denotes intrinsic layer. A large reverse bias is applied across
diode . the avalanche multiplication occurs in the thin “p” region which is also called the high field
region or avalanche region.

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The holes generated during the avalanche process drift through the intrinsic region while moving
towards p+ contact. The region between n+ p junction and the i-p+ junction is known as space
charge region.
When this diode is reverse biased and placed inside an inductive microwave cavity microwave
oscillations are produced due to the resonant action of the capacitive impedance of the diode and
cavity inductance. The dc bias power is converted into microwave power by that read diode
oscillator.

Avalanche multiplication occurs when the applied reverse bias voltage is greater then the
breakdown voltage sothat the space charge region extends from n+p junction through the p and I
regions, to the i to p+ junction.

IMPATT DIODE:

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lmpatt diodes are manufactured AND OPTICAL
having different forms such COMMUNICATIONS
as n+pip+, p+nin+, p+nn+ abrupt
junction and p+ i n+ diode configuration. The material used for manufacture of these modes are
either Germanium, Silicon, Gallium Arsenide (GaAs) or Indium Phosphide (In P).
Out of these materials, highest efficiency, higher operating frequency and lower noise is obtained
with GaAs. But the disadvantage with GaAs is complex fabrication process and hence higher cost.
The figure below shows a reverse biased n+ pi p+ diode with electric field variation, doping
concentration versus distance plot, the microwave voltage swing and the current variation.

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PRINICPLE OF OPERATION:
When a reverse bias voltage exceeding the breakdown voltage is applied, a high electric field
appears across then+ p junction. This high field intensity imparts sufficient energy to the valence
electrons to raise themselves into the conduction band. This results avalanche multiplication of
hole-electron pairs. With suitable doping profile design, it is possible to make electric field to have
a very sharp peak in the close vicinity of the junction resulting in "impact avalanche
multiplication". This is a cumulative process resulting in rapid increase of carrier density. To
prevent the diode from burning, a constant bias source is used to maintain average current at safe
limit 10, The diode current is contributed by the conduction electrons which move to the n+ region
and the associated holes which drift through the steady field and a.c. field.· The diode ~wings into
and out of avalanche conditions under the influence of that reverse bias steady field and the a.c.
field.

Due to the drift time of holes being' small, carriers drift to the end contacts before the a.c. voltage
swings the diode out of the avalanche Due to building up of oscillations, the a.c. field takes energy
from the applied bias lid the oscillations at microwave frequencies are sustained across the diode.
Due to this a.c. field, the hole current grows exponentially to a maximum and again decays
exponentially to Zero.
During this hole drifting process, a constant electron current is induced in the external Circuit
which starts flowing when hole current reaches its peak and continues for half cycle
Corresponding to negative swing of the
a.c. voltage as shown in figure Thus a 180 degrees Phase shift between the external current and
a.c. microwave voltage provides a negative Resistance for sustained oscillations.

The resonator is usually tuned to this frequency so that the IMPATI diodes provide a High power
continuous wave (CW) and pulsed microwave signals.

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Applications of IMPATT Diodes

(i) Used in the final power stage of solid state microwave
transmitters forcommunication purpose.
(ii) Used in the transmitter of TV system.
(iii) Used in FDM/TDM systems.
(iv) Used as a microwave source in laboratory for measurement purposes.

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TRAPATT DIODE:
Silicon is usually used for the manufacture of TRAPATT diodes and have a configuration of p+ n n+ as shown
.The p-N junction is reverse biased beyond the breakdown region, so that the current density is
larger. This decreases the electric field in the space charge region and increases the carrier transit
time. Due to this, the frequency of operation gets lowered to less than 10 GHz. But the efficiency
gets increased due to low power dissipation.
Inside a co-axial resonator, the TRAPATT diode is normally mounted at a point where maximum
RF voltage swing is obtained. When the combined dc bias and RF voltage exceeds breakdown
voltage, avalanche occurs and a plasma of holes and electrons are generated which gets trapped.
When the external circuit current flows, the voltage rises and the trapped plasma gets released
producing current pulse across the drift space. The total transit time is the sum of the drift time
and the delay introduced by the release of the trapped plasma. Due to this longer transit time,
the operating frequency is limited to 10 GHz. Because the current pulse is associated with low
voltage, the power dissipation is low resulting in higher efficiency.
The disadvantages of TRAPATT are high noise figure and generation of strong harmonics due to
short durationof the current pulse.
TRAPATT diode finds application in S-band pulsed transmitters for pulsed array radar systems.

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The electric field is expressed as

BARITT DIODE ( Barrier injection transmit time devices ):

BARITT devices are an improved version of IMPATT devices. IMPATT devices employ impact
ionization techniques which is too noisy. Hence in order to achieve low noise figures, impact
ionization is avoided in BARRITT devices. The minority injection is provided by punch-through of
the intermediate region (depletion region). The process is basically of lower noise than impact
ionization responsible for current injection in an IMPATT. The negative resistance is obtained on
account of the drift of the injected holes to the collector end ofthe aterial.

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The construction of a BARITT device consisting of emitter, base, intermediate or drift or depleted region and
collector. An essential requirement for the BARITT device is therefore that the intermediate drift region be
entirely depleted to cause punch through to the emitter-base junction without causing avalanche breakdown of
the base-collector junction.

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The parasitic should be kept as low as possible. The equivalent circuit depends on the type of
encapsulation and mounting make. For many applications, there should be a large capacitance
variation, small value of minimum capacitance and series resistance Rs' Operation is normally
limited to f/l0 [25 GHz for Si and 90 GHz for GaAs]. Frequency of operation beyond (f /10) leads
to increase in R, decrease in efficiency and increase in noise.

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UNIT- IV

ATTENUATORS:
In order to control power levels in a microwave system by partially absorbing the transmitted
microwave signal, attenuators are employed. Resistive films (dielectric glass slab coated with
aquadag) are used in the design of both fixed and variable attenuators. A co-axial fixed attenuator
uses the dielectric lossy material inside the centre conductor of the co-axial line to absorb some
of the centre conductor microwave power propagating through it dielectric rod decides the
amount of attenuation introduced. The microwave power absorbed by the lossy material is
dissipated as heat.

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Figure 5.8 shows a flap attenuator which is also a variable attenuator. A semi- circular flap made
of lossy dielectric is made to descend into the longitudinal slot cut at the centre of the top wall of
rectangular waveguide. When the flap is completely outside the slot, then the attenuation is zero
and when it is completely

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inside, the attenuation is maximum. A maximum direction of 90 dB attenuation is possible with
this attenuator with a VSWR of 1.05. The dielectric slab can be properly shaped according to
convenience to get a linear variation of attenuation within the depth of insertion.
A precision type variable attenuator consists of a rectangular to circular transition (ReT), a piece
of circular waveguide (CW) and a circular-to-rectangular transition

PHASE SHIFTERS:
A microwave phase shifter is a two port device which produces a variable shift in phase of the
incoming microwave signal. A lossless dielectric slab when placed inside the rectangular
waveguide produces a phase shift.
PRECISION PHASE SHIFTER

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The rotary type of precision phase shifter is shown in figure 5.12 which consists of a circular
waveguide containing a lossless dielectric plate of length 2l called "half- wave section", a section
of rectangular-to-circular transition containing a lossless dielectric plate of length l, called
"quarter-wave section", oriented at an angle of 45° to the broader wall of the rectangular
waveguide and a circular-to-rectangular transition again containing a

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lossless dielectric plate of same length 1 (quarter wave section) oriented at an angle 45°. The
incident TEIO mode becomes TEll mode in circular waveguide section. The half-wave section
produces a phase shift equal to twice that produced by the quarter wave section. The dielectric
plates are tapered at both ends to reduce reflections due to discontinuity.

When TEIO mode is propagated through the input rectangular waveguide of the rectangular to
circular transition, then it is converted into TEll in the circular waveguide section. Let E; be the
maximum electric field strength of this mode which is resolved into components, EI parallel to the
plate and E2 perpendicular to El as shown in figure 5.12 (b). After propagation through the plate
these components are given by

The length I is adjusted such that these two components E1 and Ez have equal amplitude but
differing in phase by = 90°.

The quarter wave sections convert a linearly polarized TEll wave into a circularly polarized wave
and vice- versa. After emerging out of the half-wave section, the electric field components parallel
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and perpendicular to the half-wave plate are given by

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After emerging out of the half-wave section, the field components E3 and E4 as given by equations
(5.19) and (5.20), may again be resolved into two TEll mQdes, polarized parallel and
perpendicular to the output quarterwave plate. At the output end of this quarterwave plate, the
field components parallel and perpendicular to the quarter wave plate, by referring to figure 5.12
(d), can be expressed as

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Comparison of equation (5.21) and (5.22) yields that the components Es and E6 are
identical in both magnitude and phase and the resultant electric field strength at the
output is given by

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UNIT-V
OPTICAL FIBER
Introduction

Fiber-optic communication is a method of transmitting information from one place to

another by sending pulses of light through an optical fiber. The light forms an
electromagnetic carrier wave that is modulated to carry information. Fiber is preferred
over electrical cabling when high bandwidth, long distance, or immunity to
electromagnetic interference are required. This type of communication can transmit
voice, video, and telemetry through local area networks, computer networks, or across
long distances.

Optical fiber is used by many telecommunications companies to transmit telephone

signals, Internet communication, and cable television signals. Researchers at Bell Labs
have reached internet speeds of over 100 peta bit ×kilometer per second using fiber-optic
communication. The process of communicating using fiber-optics involves the following
basic steps:

1. Creating the optical signal involving the use of a transmitter, usually from an
electricalsignal

2. Relaying the signal along the fiber, ensuring that the signal does not become too
distortedor weak

3. Receiving the optical signal

4. Converting it into an electrical signal

Historical Development
First developed in the 1970s, fiber-optics have revolutionized the telecommunications
industry and have played a major role in the advent of the Information Age. Because
of its advantages over electrical transmission, optical fibers have largely replaced
copper wire communications in core networks in the developed world.

In 1880 Alexander Graham Bell and his assistant Charles Sumner Tainter created a
very early precursor to fiber-optic communications, the Photophone, at Bell's
newly established Volta Laboratory in Washington, D.C. Bell considered it his most
important invention. The device allowed for the transmission of sound on a beam of
light.On June 3, 1880, Bell conducted the world's first wireless telephone transmission
between two buildings, some 213 meters apart. Due to its use of an atmospheric
transmission medium, the Photophone would not prove practical until advances in laser
and optical fiber technologies permitted the secure transport of light. The Photophone's
first practical use came in military communication systems many decades later.
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In 1954 Harold Hopkins and Narinder Singh Kapany showed that rolled fiber glass
allowed light to be transmitted. Initially it was considered that the light can traverse in
only straight medium. Jun-ichi Nishizawa, a Japanese scientist at Tohoku University,
proposed the use of optical fibers for communications in 1963. Nishizawa invented
the PIN diode and the static induction transistor, both of which contributed to the

development of optical fiber communications.

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In 1966 Charles K. Kao and George Hockham at STC Laboratories (STL) showed that
the losses of 1,000 dB/km in existing glass (compared to 5–10 dB/km in coaxial cable)
were due to contaminants which could potentially be removed.

Optical fiber was successfully developed in 1970 by Corning Glass Works, with
attenuation low enough for communication purposes (about 20 dB/km) and at the
same time GaAs semiconductor lasers were developed that were compact and therefore
suitable for transmitting light through fiber optic cables for long distances. In 1973,
Optelecom, Inc., co-founded by the inventor of the laser, Gordon Gould, received a contract
from APA for the first optical communication systems. Developed for Army Missile
Command in Huntsville, Alabama, it was a laser on the ground and a spout of optical fiber
played out by missile to transmit a modulated signal over five kilometers.

After a period of research starting from 1975, the first commercial fiber-optic
communications system was developed which operated at a wavelength around 0.8 μm
and used GaAs semiconductor lasers. This first-generation system operated at a bit rate
of 45 Mbit/s with repeater spacing of up to 10 km. Soon on 22 April 1977, General
Telephone and Electronics sent the first live telephone traffic through fiber optics at a 6
Mbit/s throughput in Long Beach, California.

In October 1973, Corning Glass signed a development contract with CSELT and Pirelli
aimed to test fiber optics in an urban environment: in September 1977, the second cable
in this test series, named COS-2, was experimentally deployed in two lines (9 km) in
Turin,for the first time in a big city, at a speed of 140 Mbit/s.

The second generation of fiber-optic communication was developed for commercial use
inthe early 1980s, operated at 1.3 μm and used InGaAsP semiconductor lasers. These
early systems were initially limited by multi mode fiber dispersion, and in 1981 the single-
mode fiber was revealed to greatly improve system performance, however practical
connectors capable of working with single mode fiber proved difficult to develop.
Canadian service provider SaskTel had completed construction of what was then the
world's longest commercial fiber optic network, which covered 3,268 km (2,031 mi) and
linked 52 communities. By 1987, these systems were operating at bit rates of up to 1.7
Gb/s with repeater spacing up to 50 km (31 mi). The first transatlantic telephone cable to
use optical fiber was TAT-8, based on Desurvire optimised laser amplification technology.
It went into operation in 1988.

Third-generation fiber-optic systems operated at 1.55 μm and had losses of about 0.2
dB/km. This development was spurred by the discovery of Indium gallium arsenide and
the development of the Indium Gallium Arsenide photodiode by Pearsall. Engineers
overcame earlier difficulties with pulse- spreading at that wavelength using conventional
InGaAsP semiconductor lasers. Scientists overcame this difficulty by using dispersion-
shifted fibers designed to have minimal dispersion at 1.55 μm or by limiting the laser
spectrum to a single longitudinal mode. These developments eventually allowed third-
generation systems to operate commercially at 2.5 Gbit/s with repeater spacing in excess
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The fourth generation of fiber-optic communication systems used optical amplification to
reduce the need for repeaters and wavelength-division multiplexing to increase data
capacity. These two improvements caused a revolution that resulted in the doubling of
system capacity every six months starting in 1992 until a bit rate of 10 Tb/s was reached
by 2001. In 2006 a bit-rate of 14 Tbit/s was reached over a single 160 km (99 mi) line
using optical amplifiers.

The focus of development for the fifth generation of fiber-optic communications is on

extending the wavelength range over which a WDM system can operate. The
conventional wavelength window, known as the C band, covers the wavelength range
1.53–1.57 μm, and dry fiber has a low-loss window promising an extension of that
range to 1.30–1.65 μm. Other developments include the concept of "optical solutions",
pulses that preserve their shape by counteracting the effects of dispersion with the
nonlinear effects of the fiber by using pulses of a specific shape.

In the late 1990s through 2000, industry promoters, and research companies such as KMI,
andRHK predicted massive increases in demand for communications bandwidth due to
increased use of the Internet, and commercialization of various bandwidth-intensive
consumer services, such as video on demand. Internet protocol data traffic was
increasing exponentially, at a faster rate than integrated circuit complexity had
increased under Moore's Law. From the bust of the dot-com bubble through 2006,
however, the main trend in the industry has been consolidation of firms and off
shoring of manufacturing to reduce costs.

Advantages of Fiber Optic Transmission

Optical fibers have largely replaced copper wire communications in core networks in the
developed world, because of its advantages over electrical transmission. Here are the
main advantages of fiber optic transmission.

Extremely High Bandwidth: No other cable-based data transmission medium offers the
bandwidth that fiber does. The volume of data that fiber optic cables transmit per unit
time is far great than copper cables.

Longer Distance: in fiber optic transmission, optical cables are capable of providing low
power loss, which enables signals can be transmitted to a longer distance than copper
cables.

Resistance to Electromagnetic Interference: in practical cable deployment, it’s

inevitableto meet environments like power substations, heating, ventilating and
other industrial sources of interference. However, fiber has a very low rate of bit error
(10 EXP- 13), as a result of fiber being so resistant to electromagnetic interference. Fiber
optic transmission is virtually noise free.

Low Security Risk: the growth of the fiber optic communication market is mainly driven
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by increasing awareness about data security concerns and use of the alternative raw

material. Data or signals are transmitted via light in fiber optic transmission. Therefore
there is no way to detect the data being transmitted by "listening in" to the
electromagnetic energy "leaking" through the cable, which ensures the absolute security
of information.

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Small Size: fiber optic cable has a very small diameter. For instance, the cable diameter
of a single OM3 multimode fiber is about 2mm, which is smaller than that of coaxial
copper cable. Small size saves mere space in fiber optic transmission.

Light Weight: fiber optic cables are made of glass or plastic, and they are thinner
thancopper cables. These make them lighter and easy to install.

Easy to Accommodate Increasing Bandwidth: with the use of fiber optic cable, new
equipment can be added to existing cable infrastructure. Because optical cable can
provide vastly expanded capacity over the originally laid cable and WDM
(wavelength division multiplexing) technology, including CWDM and DWDM, enables
fiber cables the ability to accommodate more bandwidth.

Disadvantages of Fiber Optic Transmission

Though fiber optic transmission brings lots of convenience, its disadvantages also
cannot be ignored.

Fragility: usually optical fiber cables are made of glass, which lends to they are more
fragile than electrical wires. In addition, glass can be affected by various chemicals
includinghydrogen gas (a problem in underwater cables), making them need more cares
when deployed underground.

Difficult to Install: it’s not easy to splice fiber optic cable. And if you bend them too much,
they will break. And fiber cable is highly susceptible to becoming cut or damaged during
installation or construction activities. All these make it difficult to install.

Attenuation & Dispersion: as transmission distance getting longer, light will be

attenuated and dispersed, which requires extra optical components like EDFA to be
added.

Cost is Higher Than Copper Cable: despite the fact that fiber optic installation costs are
dropping by as much as 60% a year, installing fiber optic cabling is still relatively
higher than copper cables. Because copper cable installation does not need extra care like
fiber cables. However, optical fiber is still moving into the local loop, and through
technologies such as FTTx (fiber to the home, premises, etc.) and PONs (passive optical
networks), enabling subscriber and end user broadband access.

Special Equipment Is Often Required: to ensure the quality of fiber optic transmission,
some special equipment is needed. For example, equipment such as OTDR (optical time-
domain reflectometry) is required and expensive, specialized optical test equipment such
as optical probes and power meter are needed at most fiber endpoints to properly
provide testing of optical fiber.

Applications of Optical Fiber Communications

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Fiber optic cables find many uses in a wide variety of industries and applications.
Someuses of fiber optic cables include:

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Medical -Used as light guides, imaging tools and also as lasers for surgeries

Defense/Government-Used as hydrophones for seismic waves and SONAR , as

wiring inaircraft, submarines and other vehicles and also for field networking

Data Storage- Used for data transmission

Telecommunications- Fiber is laid and used for transmitting and receiving purposes

Networking- Used to connect users and servers in a variety of network settings and
helpincrease the speed and accuracy of data transmission

Industrial/Commercial- Used for imaging in hard to reach areas, as wiring where EMI
is anissue, as sensory devices to make temperature, pressure and other measurements,
and as wiring in automobiles and in industrial settings.

Broadcast/CATV-Broadcast/cable companies are using fiber optic cables for wiring

CATV, HDTV, internet, video on- demand and other applications. Fiber optic cables are
used for lighting and imaging and as sensors to measure and monitor a vast array of
variables. Fiber optic cables are also used in research and development and testing across
all the above mentioned industries

The optical fibers have many applications. Some of them are

as follows
 Used in telephone systems

 Used in sub-marine cable networks

 Used in data link for computer networks, CATV Systems

 Used in CCTV surveillance cameras

 Used for connecting fire, police, and other emergency services.

 Used in hospitals, schools, and traffic management systems.

 They have many industrial uses and also used for in heavy duty constructions.

Block Diagram of Optical Fiber Communication System

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Block Diagram of Optical Fiber Communication System

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Message origin:
Generally message origin is from a transducer that converts a non-electrical message
into an electrical signal. Common examples include microphones for converting sound
wavesinto currents and video (TV) cameras for converting images into current. For data
transfer between computers, the message is already in electrical form.

Modulator:
The modulator has two main functions.

1) It converts the electrical message into proper format.

2) It impresses this signal onto the wave generated by the carrier source.

Two distinct categories of modulation are used i.e. analog modulation and digital modulation.

Carrier source:
Carrier source generates the wave on which the information is transmitted. This wave is
called the carrier. For fiber optic system, a laser diode (LD) or a light emitting diode (LED)
isused. They can be called as optic oscillators, they provide stable, single frequency waves

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withsufficient power for long distance propagation.

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Channel coupler:
Coupler feeds the power into information channel. For an atmospheric optic system, the
channel coupler is a lens used for collimating the light emitted by the source and directing
thislight towards the receiver. The coupler must efficiently transfer the modulated
light beamfrom the source to the optic fiber. The channel coupler design is an important
part of fiber system because of possibility of high losses.

Information channel:
The information channel is the path between the transmitter and receiver. In fiber
optic communications, a glass or plastic fiber is the channel. Desirable characteristics of
the information channel include low attenuation and large light acceptance cone angle.
Optical amplifiers boost the power levels of weak signals. Amplifiers are needed in very
long links to provide sufficient power to the receiver. Repeaters can be used only for
digital systems. They convert weak and distorted optical signals to electrical ones and
then regenerate the original digital pulse trains for further transmission.

Another important property of the information channel is the propagation time of the
waves travelling along it. A signal propagating along a fiber normally contains a range of
fiber optic frequencies and divides its power along several ray paths. This results in a
distortion of the propagation signal. In a digital system, this distortion appears as a
spreading and deforming of the pulses. The spreading is so great that adjacent pulses
begin to overlap and become unrecognizable as separate bits of information.

Optical detector:
The information begin transmitted is detected by detector. In the fiber system the optic
wave is converted into an electric current by a photodetector. The current developed by
the detector is proportional to the power in the incident optic wave. Detector output
current contains the transmitted information. This detector output is then filtered to
remove the constant bias and then amplified. The important properties of photodetectors
are small size, economy, long life, low power consumption, high sensitivity to optic
signals and fast response to quick variations in the optic power. Signal processing includes
filtering, amplification. Proper filtering maximizes the ratio of signal to unwanted
power. For a digital system decisioncircuit is an additional block. The bit error rate
(BER) should be very small for quality communications.

Signal processing:
Signal processing includes filtering, amplification. Proper filtering maximizes the ratio of
signal to unwanted power. For a digital syst5em decision circuit is an additional block.
The bit error rate (BER) should be very small for quality communications.

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Message output:
The electrical form of the message emerging from the signal processor is transformed
into a sound wave or visual image. Sometimes these signals are directly usable when
computers or other machines are connected through a fiber system.

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Electromagnetic Spectrum
The radio waves and light are electromagnetic waves. The rate at which they alternate in
polarity is called their frequency (f) measured in hertz (Hz). The speed of electromagnetic
wave (c) in free space is approximately 3 x 108 m/sec. The distance travelled during each
cycle is called as wavelength (λ)

In fiber optics, it is more convenient to use the wavelength of light instead of the
frequency with light frequencies; wavelength is often stated in microns or nanometers.

1 micron (µ) = 1 Micrometre (1 x 10-6) ;1 nano (n) = 10-9 meter

Fiber optics uses visible and infrared light. Infrared light covers a fairly wide range of
wavelengths and is generally used for all fiber optic communications. Visible light is
normally used for very short range transmission using a plastic fiber

Electromagnetic Spectrum

Optical Fiber Waveguides

In free space light ravels as its maximum possible speed i.e. 3 x 10 8 m/s or 186 x 103
miles/sec. When light travels through a material it exhibits certain behavior explained
by laws of reflection, refraction. An optical wave guide is a structure that "guides" a light
wave by constraining it to travel along a certain desired path. If the transverse
dimensions of the guide are much larger than the wavelength of the guided light, that
explain how the optical waveguide works using geometrical optics and total internal
reflection.

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A wave guide traps light by surrounding a guiding region, called the core, made from a
material with index of refraction ncore, with a material called the cladding, made from a
material with index of refraction ncladding <ncore. Light entering is trapped as long as sinθ >
ncladding/ncore.

Light can be guided by planar or rectangular wave guides, or by optical fibers. An optical
fiber consists of three concentric elements, the core, the cladding and the outer coating,
often called the buffer. The core is usually made of glass or plastic. The core is the light-
carrying portion of the fiber. The cladding surrounds the core. The cladding is made of
a material with a slightly lower index of refraction than the core. This difference in the
indices causes total internal reflection to occur at the core-cladding boundary along the
length of the fiber. Light is transmitted down the fiber and does not escape through the
sides of the fiber.

Fiber Optic Core: the inner light-carrying member with a high index of refraction.

Cladding: the middle layer, which serves to confine the light to the core. It has a lower
indexof refraction.

Buffer: The outer layer, which serves as a "shock absorber" to protect the core and
claddingfrom damage. The coating usually comprises one or more coats of a plastic
material to protect the fiber from the physical environment.

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Light injected into the fiber optic core and striking the core-to-cladding interface at an
angle greater than the critical angle is reflected back into the core. Since the angles of
incidence and reflection are equal, the light ray continues to zigzag down the length of the
fiber. The light is trapped within the core. Light striking the interface at less than the
critical angle passes into the cladding and is lost.

Fibers for which the refractive index of the core is a constant and the index changes
abruptlyat the core-cladding interface are called
step-index fibers.Step-index fibers are available with core diameters of 100 mm to 1000
mm. They are well suited to applications requiring high-power densities, such as delivering
laser power for medical and industrial applications.

Multimode step-index fibers trap light with many different entrance angles, each mode in
a step-index multimode fiber is associated with a different entrance angle. Each mode
therefore travels along a different path through the fiber. Different propagating modes
have different velocities. As an optical pulse travels down a multimode fiber, the pulse
begins to spread. Pulses that enter well separated from each other will eventually
overlap each other. This limits the distance over which the fiber can transport data.
Multimode step-index fibers are not well suited for data transport and communications.

In a multimode graded-index fiber the core has an index of refraction that decreases as the
radial distance from the center of the core increases. As a result, the light travels faster
near the edge of the core than near the center. Different modes therefore travel in
curved pathswith nearly equal travel times. This greatly reduces the spreading of optical
pulses.

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A single mode fiber only allows light to propagate down its center and there are no longer
different velocities for different modes. A single mode fiber is much thinner than a
multimode fiber and can no longer be analyzed using geometrical optics. Typical core
diameters are between 5 mm and 10 mm.

When laser light is coupled into a fiber, the distribution of the light emerging from the
other end reveals if the fiber is a multimode or single mode fiber.

Optical fibers are used widely in the medical field for diagnoses and treatment. Optical
fibers can be bundled into flexible strands, which can be inserted into blood vessels, lungs
and other parts of the body. An Endoscope is a medical tool carrying two bundles of optic
fibers inside one long tube. One bundle directs light at the tissue being tested, while the
other bundle carries light reflected from the tissue, producing a detailed image.
Endoscopes can be designed to look at regions of the human body, such as the knees,
or other joints in the body

In a step-index fiber in the ray approximation, the ray propagating along the axis of the
fiber has the shortest route, while the ray incident at the critical angle has the longest
route. Determine the difference in travel time (in ns/km) for the modes defined by those
two rays fora fiber with ncore = 1.5 and ncladding = 1.485.

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Solution:

If a ray propagating along the axis of the fiber travels a distance d, then a ray incident
at thecritical angle θc travels a distance L = d/sinθc.

The respective travel times are td = dncore/c and tL =

dncore/(sinθc c).sinθc = ncladding/ncore.

θc = 81.9 deg.

For d = 1000 m , td = 5000 ns and tL =5050.51 ns.

The difference in travel time is therefore 50.51

ns/km.Ray theory

The phenomenon of splitting of white light into its constituents is known as dispersion.
The concepts of reflection and refraction of light are based on a theory known as Ray
theory or geometric optics, where light waves are considered as waves and represented
with simple geometric lines or rays.

The basic laws of ray theory/geometric optics

 In a homogeneous medium, light rays are straight lines.

 Light may be absorbed or reflected.

 Reflected ray lies in the plane of incidence and angle of incidence will be equal to
theangle of reflection.

 At the boundary between two media of different refractive indices, the refracted ray
will liein the plane of incidence. Snell’s Law will give the relationship between the
angles of incidence and refraction.

Reflection depends on the type of surface on which light is incident. An essential

condition for reflection to occur with glossy surfaces is that the angle made by the
incident ray of light with the normal at the point of contact should be equal to the angle of
reflection with that normal. The images produced from this reflection have different
properties according to the shape of the surface. For example, for a flat mirror, the image
produced is upright, has the same size as that of the object and is equally distanced
from the surface ofthe mirror as the real object. However, the properties of a parabolic
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mirror are different and so on. AND OPTICAL COMMUNICATIONS

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Refraction is the bending of light in a particular medium due to the speed of light in
thatmedium. The speed of light in any medium can be given by

The refractive index for vacuum and air is 1.0 for water it is 1.3 and for glass refractive
indexis 1.5. Here n is the refractive index of that medium. When a ray of light is incident at
the interface of two media with different refractive indices, it will bend either towards
or away from the normal depending on the refractive indices of the media. According
to Snell’s law, refraction can be represented as

= refractive index of first medium

= angle of incidence, n2= refractive index of second medium

= angle of refraction

For , is always greater than . Or to put it in different words, light moving

from a medium of high refractive index (glass) to a medium of lower refractive index (air)
will move away from the normal.

Total internal reflection

To consider the propagation of light within an optical fiber utilizing the ray theory
model it is necessary to take account of the refractive index of the dielectric medium.
Optical materials are characterized by their index of refraction, referred to as n. The
refractive index of a medium is defined as the ratio of the velocity of light in a vacuum to
the velocity of light in the medium.

When a beam of light passes from one material to another with a different index of
refraction,the beam is bent (or refracted) at the interface.
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where nI and nR are the indices of refraction of the materials through which the beam is
refracted and I and R are the angles of incidence and refraction of the beam. If the angle
of incidence is greater than the critical angle for the interface (typically about 82° for
optical fibers), the light is reflected back into the incident medium without loss by a
process knownas total internal reflection .

Figure Total Internal Reflection allows light to remain inside the core of the fiber

Refraction is described by Snell’s law:

A ray of light travels more slowly in an optically dense medium than in one that is less
dense, and the refractive index gives a measure of this effect. When a ray is incident on the
interface between two dielectrics of differing refractive indices (e.g. glass–air), refraction
occurs, as illustrated in Figure . It may be observed that the ray approaching the interface
is propagating in a dielectric of refractive index n and is at an angle φ to the normal at the
surface of the interface.

If the dielectric on the other side of the interface has a refractive index n which is less than
n1, then the refraction is such that the ray path in this lower index medium is at an angle
to the normal, where is greater than . The angles of incidence and refraction are
related to each other and to the refractive indices of the dielectrics by Snell’s law of
refraction, which states that:

It may also be observed in Figure that a small amount of light is reflected back into the
originating dielectric medium (partial internal reflection). As n is greater than n, the
angle of refraction is always greater than the angle of incidence. Thus when the angle of
refraction is 90° and the refracted ray emerges parallel to the interface between the
dielectrics, the angle of incidence must be less than 90°.

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Figure Light rays incident on a high to low refractive index

This is the limiting case of refraction and the angle of incidence is now known as the critical
angle φc, as shown in Figure. The value of the critical angle is given by

At angles of incidence greater than the critical angle the light is reflected back into the
originating dielectric medium (total internal reflection) with high efficiency (around
99.9%). Hence, it may be observed in Figure that total internal reflection occurs at the inter-
face between two dielectrics of differing refractive indices when light is incident on the
dielectric oflower index from the dielectric of higher index, and the angle of incidence of
the ray exceeds the critical value. This is the mechanism by which light at a sufficiently
shallow angle (less than90° ) may be considered to propagate down an optical fiber with
low loss.

Figure Transmission of a light ray in a perfect optical fiber

The above figure illustrates the transmission of a light ray in an optical fiber via a series
of total internal reflections at the interface of the silica core and the slightly lower
refractive index silica cladding. The ray has an angle of incidence φ at the interface which
is greater than the critical angle and is reflected at the same angle to the normal. The light
ray shown in Figure is known as a meridional ray as it passes through the axis of the fiber
core. This type ofray is the simplest to describe and is generally used when illustrating
the fundamental transmission properties of optical fibers.
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It must also be noted that the light transmission illustrated in Figure assumes a perfect
fiber, and that any discontinuities or imperfections at the core–cladding interface would
probably result in refraction rather than total internal reflection, with the subsequent
loss of the light ray into the cladding.

Critical Angle
When the angle of incidence is progressively increased, there will be progressive increase
of refractive angle. At some condition the refractive angle becomes 90o to the normal.
When this happens the refracted light ray travels along the interface. The angle of
incidence at the point at which the refractive angle becomes 90 o is called the critical
angle. The critical angle is defined as the minimum angle of incidence at which the ray
o
strikes the interface of two media and causes an angle of refraction equal to 90 . Figure
shows critical angle refraction. When the angle of refraction is 90 degree to the normal
the refracted ray is parallel to the interface between the two media. Using Snell’s law

It is important to know about this property because reflection is also possible even if the
surfaces are not reflective. If the angle of incidence is greater than the critical angle for a
given setting, the resulting type of reflection is called Total Internal Reflection, and it
is the basis of Optical Fiber Communication.

Acceptance angle
In an optical fiber, a light ray undergoes its first refraction at the air-core interface. The
angle at which this refraction occurs is crucial because this particular angle will
dictate whether the subsequent internal reflections will follow the principle of Total
Internal Reflection. This angle, at which the light ray first encounters the core of an optical
fiber is called Acceptance angle.
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The objective is to have greater than the critical angle for this particular setting. As you
can notice, depends on the orientation of the refracted ray at the input of the optical
fiber. This in turn depends on , the acceptance angle. The acceptance angle can be
calculatedwith the help of the formula below.

Numerical Aperture
Numerical Aperture is a characteristic of any optical system. For example, photo-detector,
optical fiber, lenses etc. are all optical systems. Numerical aperture is the ability of the
optical system to collect the entire light incident on it, in one area. The blue cone is known
as the cone of acceptance. As you can see it is dependent on the Acceptance Angle of the
optical fiber. Light waves within the acceptance cone can be collected in a small area
which can then be sent into the optical fiber (Source).

Numerical aperture (NA), shown in above Figure, is the measure of maximum angle at
which light rays will enter and be conducted down the fiber. This is represented by the
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following equation:

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Skew rays: In a multimode optical fiber, a bound ray that travels in a helical path along the
fiber and thus (a) is not parallel to the fiber axis, (b) does not lie in a meridional plane,
and (c) does not intersect the fiber axis is known as a Skew Ray.

Figure, view (a), provides an angled view and view (b) provides a front view.

1. Skew rays are rays that travel through an optical fiber without passing through its axis.

2. A possible path of propagation of skew rays is shown in figure.

3. Skew rays are those rays which follow helical path but they are not confined to a single
plane. Skew rays are not confined to a particular plane so they cannot be tracked easily.
Analyzing the meridional rays is sufficient for the purpose of result, rather than skew rays,
because skew rays lead to greater power loss.

4. Skew rays propagate without passing through the center axis of the fiber. The
acceptance angle for skew rays is larger than the acceptance angle of meridional rays.

5. Skew rays are often used in the calculation of light acceptance in an optical fiber. The
addition of skew rays increases the amount of light capacity of a fiber. In large NA fibers,
the increase may be significant.

6. The addition of skew rays also increases the amount of loss in a fiber. Skew rays tend
to propagate near the edge of the fiber core. A large portion of the number of skew rays
that are trapped in the fiber core are considered to be leaky rays.

7. Leaky rays are predicted to be totally reflected at the core-cladding boundary.

However, these rays are partially refracted because of the curved nature of the fiber
boundary. Mode theory is also used to describe this type of leaky ray loss.

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Cylindrical fiber
1. Modes

When light is guided down a fiber (as microwaves are guided down a waveguide), phase
shifts occur at every reflective boundary. There is a finite discrete number of paths down
the optical fiber (known as modes) that produce constructive (in phase and therefore
additive) phase shifts that reinforce the transmission. Because each mode occurs at a
different angle to the fiber axis as the beam travels along the length, each one travels a
different length through the fiber from the input to the output. Only one mode, the zero-
order mode, travels the length of the fiber without reflections from the sidewalls. This is
known as a single-mode fiber. The actual number of modes that can be propagated in a
given optical fiber is determined by the wavelength of light and the diameter and index
of refraction of the core of the fiber.

The exact solution of Maxwell’s equations for a cylindrical homogeneous core dielectric
waveguide* involves much algebra and yields a complex result. Although the presentation
of this mathematics is beyond the scope of this text, it is useful to consider the resulting
modal fields. In common with the planar guide TE (where Ez = 0) and TM (where Hz = 0)
modes are obtained within the dielectric cylinder. The cylindrical waveguide, however,
is bounded in two dimensions rather than one. Thus two integers, l and m, are necessary
in order to specify the modes, in contrast to the single integer (m) required for the planar
guide.

For the cylindrical waveguide, therefore refer to TElm and TMlm modes. These modes
correspond to meridional rays traveling within the fiber. However, hybrid modes
where Ez and Hz are nonzero also occur within the cylindrical waveguide.

These modes, which result from skew ray propagation within the fiber, are designated
HElm and EHlm depending upon whether the components of H or E make the larger
contribution to the transverse (to the fiber axis) field. Thus an exact description of the
modal fields in a step index fiber proves somewhat complicated.

Fortunately, the analysis may be simplified when considering optical fibers for
communication purposes. These fibers satisfy the weakly guiding approximation where
the relative index difference Δ1. This corresponds to small grazing angles θ. In fact is
usually less than 0.03 (3%) for optical communications fibers. For weakly guiding
structures with dominant forward propagation, mode theory gives dominant transverse
field components. Hence approximate solutions for the full set of HE, EH, TE and TM
modes may be given by two linearly polarized components.

These linearly polarized (LP) modes are not exact modes of the fiber except for the
fundamental (lowest order) mode. However, as in weakly guiding fibers is very small, then
HE– EH mode pairs occur which have almost identical propagation constants. Such modes
are said to be degenerate. The superposition of these degenerating modes characterized by
a common propagation constant correspond to particular LP modes regardless of their HE,
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EH, TE or TM field configurations. This linear combination of degenerate modes obtained
from the exact solution produces a useful simplification in the analysis of weakly guiding
fibers.

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The relationship between the traditional HE, EH, TE and TM mode designations and the
LPlm mode designations is shown in Table. The mode subscripts l and m are related
to the electric field intensity profile for a particular LP mode. There are in general 2l field
maxima around the circumference of the fiber core and m field maxima along a radius
vector. Furthermore, it may be observed from Table 1.1 that the notation for labeling
the HE and EH modes has changed from that specified for the exact solution in the
cylindrical waveguide mentioned previously.

2. Mode coupling
Thus, so far the propagation aspects of perfect dielectric waveguides were considered.
However, waveguide perturbations such as deviations of the fiber axis from straightness,
variations in the core diameter, irregularities at the core–cladding interface and refractive
index variations may change the propagation characteristics of the fiber. These will have
the effect of coupling energy traveling in one mode to another depending on the specific
perturbation. Ray theory aids the understanding of this phenomenon, as shown in Figure
which illustrates two types of perturbation. It may be observed that in both cases the ray
nolonger maintains the same angle with the axis. In electromagnetic wave theory this
corresponds to a change in the propagating mode for the light. Thus individual modes do
not normally propagate throughout the length of the fiber without large energy transfers
toadjacent modes, even when the fiber is exceptionally good quality and is not strained or
bent by its surroundings. This mode conversion is known as mode coupling or mixing. It
is usuallyanalyzed using coupled mode equations which can be obtained directly from
Maxwell’s equations.

Figure Ray theory illustrations showing two of the possible fiber perturbations which
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givemode coupling: (a) irregularity at the core–cladding interface; (b) fiber bend

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3. Step index fibers
The optical fiber considered in the preceding sections with a core of constant refractive
index n1 and a cladding of a slightly lower refractive index n2is known as step index fiber.
This is because the refractive index profile for this type of fiber makes a step change at the
core– cladding interface, as indicated in Figure which illustrates the two major types of
step index fiber.

Figure shows a multimode step index fiber with a core diameter of around 50µm or
greater, which is large enough to allow the propagation of many modes within the fiber
core. This is illustrated in Figure by the many different possible ray paths through the
fiber. Figure shows a single-mode or monomode step index fiber which allows the
propagation of only one transverse electromagnetic mode (typically HE11), and hence
the core diameter must be ofthe order of 2 to 10µm. The propagation of a single mode is
illustrated in Figure as corresponding to a single ray path only (usually shown as the axial
ray) through the fiber. The single-mode step index fiber has the distinct advantage of low
intermodal dispersion (broadening of transmitted light pulses), as only one mode is
transmitted, whereas with multimode step index fiber considerable dispersion may occur
due to the differing group velocities of the propagating modes. This in turn restricts the
maximum bandwidth attainable with multimode step index fibers, especially when com-
pared with single-mode fibers.

The refractive index profile may be defined as

Figure Refractive index profile and ray transmission in step index a) multimode b)
singlemode

However, for lower bandwidth applications multimode fibers have several

advantages oversingle- mode fibers. These are:

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a) The use of spatially incoherent optical sources (e.g. most light-emitting diodes)
whichcannot be efficiently coupled to single-mode fibers.

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b) Larger numerical apertures, as well as core diameters, facilitating easier coupling to
optical sources

c) Lower tolerance requirements on fiber connectors

Multimode step index fibers allow the propagation of a finite number of guided modes
along the channel. The number of guided modes is dependent upon the physical
parameters (i.e. relative refractive index difference, core radius) of the fiber and the
wavelengths of thetransmitted light which are included in the normalized frequency V
for the fiber.

Mode propagation does not entirely cease below cutoff. Modes may propagate as unguided
or leaky modes which can travel considerable distances along the fiber. Nevertheless, it is
the guided modes which are of paramount importance in optical fiber communications
as theseare confined to the fiber over its full length. The total number of guided modes or
mode volume Ms for a step index fiber is related to the V value for the fiber by the
approximate expression that allows an estimate of the number of guided modes
propagating in a particular multimode step index fiber.

4. Graded index fibers

Graded index fibers do not have a constant refractive index in the core* but a decreasing
core index n(r) with radial distance from a maximum value ofn1 at the axis to a constant
value n2 beyond the core radius a in the cladding. This index variation may be represented
as:

where is the relative refractive index difference and α is the profile parameter which
gives the characteristic refractive index profile of the fiber core. Equation which is a
convenient method of expressing the refractive index profile of the fiber core as a variation
of α, allows representation of the step index profile when α = ∞, a parabolic profile when
α = 2 and a triangular profile when α = 1. This range of refractive index profiles is
illustrated in Figure. The graded index profiles which at present produce the best results
for multimode optical propagation have a near parabolic refractive index profile core with
~~2. Fibers with such core index profiles are well established and consequently when
the term ‘graded index’ is usedwithout qualification it usually refers to a fiber with this
profile.

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Figure Refractive index profile and ray transmission in multimode graded index

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Where, r = Radial distance from fiber axis, a = Core radius, n1= Refractive index of core, n2 =
Refractive index of cladding, α = Shape of index profile.

Profile parameter α determines the characteristic refractive index profile of fiber core. For
this reason in this section, consider the waveguiding properties of graded index fiber with
aparabolic refractive index profile core. A multimode graded index fiber with a parabolic
index profile core is illustrated in Figure. It may be observed that the meridional rays
shown appear to follow curved paths through the fiber core. Using the concepts of
geometric optics, the gradual decrease in refractive index from the center of the core
creates many refractions of the rays as they are effectively incident on a large number
or high to low index interfaces. This mechanism is illustrated in Figure where a ray is
shown to be gradually curved, with an ever- increasing angle of incidence, until the
conditions for total internal reflection are met, and the ray travels back towards the core
axis, again being continuously refracted.

Figure An expanded ray diagram showing refraction

Multimode graded index fibers exhibit far less intermodal dispersion than multimode step
index fibers due to their refractive index profile. Although many different modes are
excited inthe graded index fiber, the different group velocities of the modes tend to be
normalized by theindex grading. Again considering ray theory, the rays traveling close to
the fiber axis have shorter paths when compared with rays which travel.

However, the near axial rays are transmitted through a region of higher refractive
index and therefore travel with a lower velocity than the more extreme rays. This
compensatesfor the shorter path lengths and reduces dispersion in the fiber. A similar
situation exists for skew rays which follow longer helical paths, as illustrated in Figure.
These travel for the most part in the lower index region at greater speeds, thus giving the
same mechanism of mode transit time equalization. Hence, multi- mode graded index
fibers with parabolic or near- parabolic index profile cores have trans- mission
bandwidths which may be orders of magnitude greater than multimode step index fiber
bandwidths.

Consequently, although they are not capable of the bandwidths attain- able with single-
mode fibers, such multimode graded index fibers have the advantage of large core
diameters (greater than 30 µm) coupled with bandwidths suitable for long- distance

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communication. ANDfor
The parameters defined OPTICAL COMMUNICATIONS
step index fibers (i.e. NA, Δ, V ) may be applied
to graded index fibers and give a comparison between the two fiber types.

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However, it must be noted that for graded index fibers the situation is more complicated
since the numerical aperture is a function of the radial distance from the fiber axis. Graded
index fibers, therefore, accept less light than corresponding step index fibers with the
same relative refractive index difference.

Single-mode fiber
The advantage of the propagation of a single mode within an optical fiber is that the signal
dispersion caused by the delay differences between different modes in a multimode fiber
may be avoided. Multimode step index fibers do not lend themselves to the propagation of
a single mode due to the difficulties of maintaining single-mode operation within the fiber
when mode conversion (i.e. coupling) to other guided modes takes place at both input
mismatches and fiber imperfections. Hence, for the transmission of a single mode the fiber
must be designed to allow propagation of only one mode, while all other modes are
attenuated by leakage or absorption. Following the preceding discussion of multimode
fibers, this may be achieved through choice of a suitable normalized frequency for the
fiber. For single-mode operation, only the fundamental LP01 mode can exist. Hence the
limit of single-mode operation depends on the lower limit of guided propagation for
the LP11 mode. The cutoff normalized frequency for the LP11 mode in step index fibers
occurs at Vc = 2.405. Thussingle-mode propagation of the LP01 mode in step index fibers
is possible over the range:

As there is no cutoff for the fundamental mode. It must be noted that there are in fact two
modes with orthogonal polarization over this range, and the term single-mode applies to
propagation of light of a particular polarization. Also, it is apparent that the normalized
frequency for the fiber may be adjusted to within the range given in Equation by reduction
of the core radius.

1. Cutoff wavelength
It may be noted that single-mode operation only occurs above a theoretical cutoff wavelength
λc given by:

Where Vc- Cut off normalized frequency.

Dividing above equation by

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 MICROWAVE AND OPTICAL COMMUNICATIONS
Thus for step index fiber where Vc=2.405, the cut-off wavelength is given by

An effective cutoff wavelength has been defined by the ITU-T which is obtained from a 2
m length of fiber containing a single 14 cm radius loop. This definition was produced
becausethe first higher order LP11 mode is strongly affected by fiber length and curvature
near cutoff. Recommended cutoff wavelength values for primary coated fiber range from
1.1 to 1.28 µm for single-mode fiber designed for operation in the 1.3µm wavelength
region in order to avoid modal noise and dispersion problems. Moreover, practical
transmission systems are generally operated close to the effective cutoff wave- length in
order to enhance the fundamental mode confinement, but sufficiently distant from cutoff
so that no power is transmitted in the second- order LP11 mode.

2. Mode-field diameter and spot size

Many properties of the fundamental mode are determined by the radial extent of its
electromagnetic field including losses at launching and jointing, micro bend losses,
waveguidedispersion and the width of the radiation pattern. Therefore, the MFD is an
important parameter for characterizing single-mode fiber properties which takes into
account the wavelength-dependent field penetration into the fiber cladding. In this context
it is a better measure of the functional properties of single- mode fiber than the core
diameter. For step index and graded (near parabolic profile) single-mode fibers operating
near the cutoff wavelength λc, the field is well approximated by a Gaussian distribution. In
this case the MFD is generally taken as the distance between the opposite 1/e = 0.37 field
amplitude points and the power 1/e2 = 0.135 points in relation to the corresponding
values on the fiber axis. Another parameter which is directly related to the MFD of a
single-mode fiber is the spot size (or mode-field radius) ω0. Hence MFD = 2ω 0, where ω0
is the nominal half width of the input excitation.

The MFD can therefore be regarded as the single- mode analog of the fiber core diameter
in multimode fibers. However, for many refractive index profiles and at typical operating
wavelengths the MFD is slightly larger than the single-mode fiber core diameter. Often, for
real fibers and those with arbitrary refractive index profiles, the radial field distribution is
not strictly Gaussian and hence alternative techniques have been proposed. However, the
problem of defining the MFD and spot size for non-Gaussian field distributions is difficult
one and at least eight definitions exist.

3. Effective refractive index

The rate of change of phase of the fundamental LP01 mode propagating along a straight
fiber is determined by the phase propagation constant. It is directly related to the
wavelength of the LP01 mode λ01 by the factor 2π, since β gives the increase in phase

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 MICROWAVE
angle per unit length. Hence: AND OPTICAL COMMUNICATIONS

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 MICROWAVE AND OPTICAL COMMUNICATIONS

Morever, it is convenient to define an effective refractive index for single mode fiber,
sometimes referred to as a phase index or normalized phase change coefficient n eff by the
ratio of the propagation constant of the fundamental mode to that of the vaccum
propagation constant.

Hence, the wavelength of the fundamental mode is smaller than the vaccum wave by the
factor 1/ neff ,where

It should be noted that the fundamental mode propagates in a medium with a refractive
index n(r) which is dependent on the distance r from the fiber axis. The effective refractive
index cantherefore be considered as an average over the refractive index of this medium.
Within a normally clad fiber, not depressed-cladded fibers, at long wavelengths (i.e. small
V values) the MFD is large compared to the core diameter and hence the electric field
extends far into the cladding region. In this case the propagation constant β will be
approximately equal to n2k (i.e. the cladding wave number) and the effective index will
be similar to the refractive index of the cladding n2. Physically, most of the power is
transmitted in the cladding material. At short wavelengths, however, the field is
concentrated in the core region and the propagation constant β approximates to the
maximum wave number nlk. Following thisdiscussion, and as indicated previously, then
the propagation constant in single-mode fiber varies over the interval n2k< β <n1k. Hence,
the effective refractive index will vary over the range n2<neff<n1.

4. Group delay and mode delay factor

The transit time or group delay τg for a light pulse propagating along a unit length of fiber
is the inverse of the group velocity, υg

The group index of a uniform plane wave propagating in a homogenous medium has been
identified as

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 MICROWAVE AND OPTICAL COMMUNICATIONS
However, for a single mode fiber, it is usual to define an effective group index by

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 MICROWAVE AND OPTICAL COMMUNICATIONS

Hence, where υg is considered to be the group velocity of the fundamental fiber mode.
Hence,the specific group delay of the fundamental fiber mode becomes:

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 MICROWAVE AND OPTICAL COMMUNICATIONS
APPLICATIONS

Examples of Typical Application of Fiber Optic Mode in SCADA application

Examples of Application of optical fiber in Dentistry

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MICROWAVE AND OPTICAL COMMUNICATIONS

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 MICROWAVE AND OPTICAL COMMUNICATIONS

Code No: 127FE

R15
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY
HYDERABAD
B.Tech IV Year I Semester Examinations, May/June
- 2019 MICROWAVE ENGINEERING
(Electronics and Communication Engineering)
Time: 3 Hours Max. Marks: 75

Note: This question paper contains two parts A and B.

Part A is compulsory which carries 25 marks. Answer all questions
in Part A.Part B consists of 5 Units. Answer any one full question from
each unit. Each question carries 10 marks and may have a, b, c as sub
questions.

PART- A
(25
1.a) What modes are the dominant modes in TE and TM Marks)
waveguides. [2]
b) Define effective permittivity of Microstip line. [3]
c) Define Q factor of Circular waveguides. [2]
d) Compare probe and loop connections. [3]
e) What are the reentrant cavities? [2]
f) How Microwave tubes are classified? [3]
g) What is strapping in Magnetron? [2]
h) How cross-field concept is used to produce oscillations in
Magnetron? [3]
i) What type of slot is used in Microwave bench? [2]
j) What are the properties of S-matrix? [3]

(50
PART-B Marks)

2.a) What are the applications of Microwave

frequencies?
b) Derive the equation for impedance of
Microstrip line.

c) Prove the cutoff frequency of Rectangular waveguide in TM and TE

3
 MICROWAVE AND OPTICAL COMMUNICATIONS

d) modes is same. [10]

OR
3.a) Determine the equations of Fields of Rectangular waveguide in TM
mode starting fromMaxwell’s equations.
b) Explain the power loss in waveguides with suitable equations. [5+5]

4.a) What are the different types of Phase shifters? Explain them with neat
diagrams.
b) Draw the structure diagram of H-plane Tee and explain its
characteristics. [5+5]
OR
5.a) Explain how Ferrites are used for isolators? Explain any one of such
circuit.
b) What are the waveguide windows? How these are used in Microwave
circuits? [5+5]

6. Explain the bunching process of two cavity klystron amplifier with

Applegate diagramand also derive the equations for power efficiency.
[10
]
OR
7.a) What are the different oscillating modes in TWT and explain them.
b) Compare the performance of TWT with Klystron amplifier. [6+4]

8. Explain the electron bunching process in Cylindrical Magnetron with neat

diagrams andderive the Hartee condition. [10]
OR
9.a) Draw the characteristics of Gunn diode and explain how negative resistance
region isobtained?
b) What are the applications of Gunn diode? [6+4]

10. What are the characteristics of two hole direction coupler and derive the S-matrix of
it.
[10]
OR
11. Explain how to measure the VSWR of a given load at microwave frequencies with
neatblock diagram. [10]

4
 MICROWAVE AND OPTICAL COMMUNICATIONS

Code No: 157CM R18

JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
B. Tech IV Year I Semester Examinations, February/March - 2022MICROWAVE AND
OPTICAL COMMUNICATIONS
(Electronics and Communication Engineering)
Time: 3 Hours Max. Marks: 75
Answer any Five Questions All Questions
Carry Equal Marks
---

1.a) Explain in detail the operation of Reflex Klystron and derive equation for its efficiency.
b) What is Velocity modulation? How is it different from normal modulation? Explain how
velocity modulation is utilized in Klystron amplifier. [8+7]

2.a) Explain the operation of TWT and derive its gain. Give its characteristics and applications.
b) What is a Gunn Diode? Explain how it works as a Oscillator and also discuss about the characteristic curve.
[8+7]

3.a) Explain the operation of magnetron and derive its Hull Cutoff Voltage equation.
b) Explain the operation of IMPATT Diode and explain its characteristics curve. [7+8]

4.a) Discuss the design of Waveguide terminations.

b) With a neat diagram explain in detail about H-plane tee and determine its S-matrix.
[8+7]

5.a) What are ferrites? How they are useful in microwaves? Explain faradays rotation.
b) Explain the design and working principle of a Gyrator. [8+7]

6.a) Explain the operation of Magic Tee. Describe how it can be used in constructing a Circulator and a
Duplexer.
b) Discuss in detail the operation of a 2-hole directional coupler, Calculate the coupling factor if the power
in the primary waveguide is 65mw and the power delivered to the directional coupler is 7mw.
[8+7]

7.a) With a neat block diagram of typical microwave bench, explain the functionality of each block.
b) Define an optical fiber. Explain in detail different types of optical fibers with neatsketches. [8+7]

8.a) Explain P-I-N photo detector with neat sketch.

b) Briefly explain the types of losses occur in optical fiber. [7+8]

--ooOoo--

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MICROWAVE AND OPTICAL COMMUNICATIONS

245