EXCEL ENGINEERING COLLEGE

(Autonomous)
Electronics and Communication Engineering
VII
EC8751- OPTICAL COMMUNICATION
Regulations 2017
Question Bank
UNIT –4 (INTRODUCTION TO OPTICAL FIBERS)
PART- A
Q.No Questions Marks CO BL
1 What is V number of a fibre? 2 CO1 R
State the term optical fibre. 2
2 CO1 R
3 Distinguish between TE and TM waves 2 CO1 U
4 Define guided waves 2 CO1 R
5 Infer the concept of numerical aperture 2 CO1 U
6 Define refractive index difference 2 CO1 R
7 List any two raw materials used for glass fiber. 2 CO1 R
8 What is total Internal reflection? 2 CO1 R
9 Define cutoff wavelength. 2 CO1 R
10 State the PCS fiber. 2 CO1 R
11 What is acceptance angle? 2 CO1 R
A silica optical fiber with a large core diameter has a core refractive index of 1.5 and a 2
12 CO1 U
cladding refractive index of 1.47.Determine the acceptance angle in air for the fiber.
13 What is a TEM wave or principal wave? 2 CO1 R
14 State the relation between phase velocity and group velocity. 2 CO1 R
15 List any four applications of optical fibres. 2 CO1 R
16 Why is single mode fibre used in communication? 2 CO1 R
17 State the 8/25 dimension 2 CO1 R
A step index fiber has the normalized frequency of 26.6 at 1300nm. If the core radius is
18 2 CO1 R
25μm, find the numerical aperture.
19 What is meant by refractive index? 2 CO1 R
20 Define Group Velocity 2 CO1 R
 PART- B

Q.No Questions Marks CO BL

1 Describe the fiber optical fibre communication system with a neat diagram 16 CO2 R
2 Derive the equation for the fundamental laws and definition of optics. 16 CO2 U
(i) Explain fiber optic modes and configurations 8
3 CO2 U
(ii) Discuss about mode analysis in optical fiber 8
Describe the working operation of fiber fabrication techniques with a neat diagram
4. 16 CO2 R

Enumerate the fiber optics materials with examples

5. 16 CO2 R

Illustrate the Plasma chemical vapor deposition method with a neat diagram.
6 16 CO2 U

(Note:*Blooms Level (R – Remember, U – Understand, AP – Apply, AZ – Analyze, E – Evaluate, C – Create)

PART A- Blooms Level : Remember, Understand, Apply
PART B- Blooms Level: Understand, Apply, Analyze, Evaluate(if possible)
Marks: 16 Marks, 8+8 Marks, 10+6 Marks)

Subject In charge Course Coordinator HOD IQAC

(Name & Signature) (Name & Signature)
 EXCEL ENGINEERING COLLEGE
(Autonomous)
Electronics and Communication Engineering
VII
EC8751- OPTICAL COMMUNICATION
Regulations 2017
Question Bank with Answers
UNIT – 4 (INTRODUCTION TO OPTICAL FIBERS)
PART- A
Q.No Questions and Answers
What is called V numver of a fibre?
 Normalized frequency or V number is a dimensionless parameter.
1
 It is expressed as V= 2πa (NA)/λ.

State the term optical fibre.

 Optical fibre is a transparent and light transmission cables.
2
 Electro magnetic wave is transmitting in the form of zig zag path.

Distinguish between TE and TM waves

TE TM
 Electric field strength E is  Magnetic field strength is
3 entirely transverse. entirely transverse.
 It has z component of  It has z component of electric
magnetic field (Hz). field (Ez).
 It has no z component of  It has no z component of
electric field (Ez). magnetic field (Hz).
Define guided waves.
 The electromagnetic waves that are guided
4  Over conducting / dielectric surface are
called guided waves.
 Examples of guided waves are: parallel wires and transmission lines
Infer the concept of numerical aperture
 Numerical Aperture (NA) of the fiber is the light collecting efficiency of the fiber
5  Measure of the amount of light rays that can be accepted by the fiber
 It is equal to the sine of acceptance angle θa NA=nsinø= (n12-n22)1/2
Where n1 and n2 are the refractive indices of core and cladding respectively.
Define refractive index difference.
6  The relative refractive index difference is the ratio of the refractive index difference between
core and cladding and refractive index of core.Δ = (n1-n2)/2n1

List any two raw materials used for glass fiber.

7
 Metal oxides and halides
Elucidate Total Internal reflection
8  The angle of incidence is greater than the critical angle.
 Øi>Øc
Define cutoff wavelength.
9  The frequency at which the wave motion ceases.
  The cut-off wavelength is the wavelength at which an optical fiber becomes single-mode

State the PCS fiber.

10  Plastic Clad Fiber
 Commercial Plastic clad silica (PCS) fibers consist of a pure silica core, a soft silicone cladding.
What is acceptance angle?
11  It is the light collecting efficiency of the fibre.
 θa = sin−1 √𝑛1 2 − 𝑛2 2
A silica optical fiber with a large core diameter has a core refractive index of 1.5 and a cladding refractive
index of 1.47.Determine the acceptance angle in air for the fiber.
n1= 1.5
n2= 1.47
Formula:
θa = sin−1 √𝑛1 2 − 𝑛2 2
12
Solution:
θa = sin−1 (n12-n22)1/2=θa = sin−1 √1.52 − 1.472

θa = 17.36◦

What is a TEM wave or principal wave?

 The Transverse Electromagnetic (TEM) waves are waves in which both electric and magnetic fields
13 are transverse entirely but have no components of Ez and Hz.

 It is also called the principal wave.

State the relation between phase velocity and group velocity.

 Phase velocity (vp) is defined as the velocity of propagation of equiphase surfaces along a guide.It is
14 given by,vp = ω/β
 Group velocity (vg) is defined as the velocity with which the energy propagates along a guide. It is
given by,vg = dω/dβ

List any four applications of optical fibers.

15  Large band width, low attenuation, low weight and high secure.

Why is single mode fibre used in communication?

16  It has no intermodal dispersion
 bandwidth is more.
State the 8/25 dimension.
17  Core diameter has 8 µm
 Cladding diameter has 25µm.
A step index fiber has the normalized frequency of 26.6 at 1300nm. If the core radius is 25μm,find the
numerical aperture.
Solution:
18 V= 2πa (NA)/λ
NA=V λ/2πa =(26.6x1300x10-9)/ (2x3.14x25x10-6)
NA=0.22
 What is meant by refractive index?.
 The refractive index (or index of refraction) ‘n’ is defined as the ratio of the velocity of light
in vacuum to the velocity of light in the medium.
19  n=c/v
c = speed of light in free space.
v = speed of light in a given material.

Define –Group Velocity.

 Group of waves with closely similar frequencies

 This wave packet does not travel at the phase velocity of individual
20
Vg given by
 Vg =𝝏𝝎/𝝏𝜷
where ω is the angular frequency and β is the propagation constant.

PART- B
Q.No Questions and Answers
1 Describe the fibre optical fibre communication system with a neat diagram
GENERAL OPTICAL FIBRE COMMUNICATION SYSTEM:

 The basic components of a practical optical communication link consist of optical transmitter,optical
repeater, optical receiver, optical fiber waveguide, connector, splice, splitter, optical amplifier, among
others.
 The information source provides an electrical signal usually received from a message signal.
 The electrical signal from the information source is fed to a transmitter
 Comprising an electrical stage that drives an optical source to produce modulation of light wave.
 The message signal may be in a continuous analog form or in the form of digital pulses representing
bits 1s and 0s.
 The message signal is used to modulate the intensity of the optical source
 An electrical drive circuit is acting as modulator.

Block Diagram:

Fig.. Optical fibre Communication system

Explanation
 The function of the optical source in a way is to provide electrical-to-optical (E/O) conversion.
 This is usually achieved with the help of an LED or ILD.
 The modulated lightwave output from the optical source is coupled to the transmission medium
consisting of optical fiber cable.
 The fiber cable contains a group of optical fibers which are generally long thin strands
 Typically of the order of 100 μm diameter).
  An optical fiber consists of two coaxial solid cylinders mode of slightly different refractive index.
 The inner solid cylinder called the core has a higher refractive index as compared to the outer cylinder
known as cladding.
 The optical signal propagates through the fiber by total internal reflection.
 As the optical signal propagates down the fiber length, it gets attenuated due to absorption, scattering,
etc
 The weak distorted optical signal is received by the receiver at the destination.
 The key component of the receiver is an optical detector.
 The signal is then processed by an electrical receiver
 The output is finally sent to the destination.
 Finally the reproduced signal is received at the destination.
2 Derive the equation for the fundamental laws and definition of optics.

BASIC OPTICAL LAWS AND DEFINITINS:

Snell’s Law.

 The refracted ray traveling in the other dielectric would make an angle of refractive f 2 with the
normal, where Ø2 is greater than Ø1.
 The angle of incidence and angle of refraction are governed by Snell’s law, given by

----------(1)

---------(2)

 As the angle of incidence Ø1 is increased, the refracted ray bends more and more away from the
normal resulting in an increase in the value of Ø2.
 At a certain value of Ø1, the angle of refraction Ø2 becomes 90° and the refracted ray emerges
parallel to the interface between the dielectric.
 .If the angle of incidence is more the critical value Øc, the entire light energy is reflected back in
the denser medium.

 The limiting value of the angle of incidence Ø1 in the denser medium for which the refracted ray
grazes the interface between the two media is called the critical angle, given by

-----=(3)
Then,the critical angle is given by

-----(4)

TOTAL INTERNAL REFLECTION:

 An important optical parameter of a material is its refractive index (or index of refraction).
 In free space, light travels with a velocity c = 3 × 108 m / s. When light enters a dielectric
medium, the wave travels with a velocity, v which is less than c.
 The speed with which light travels in the medium is a characteristic of the medium
  The ratio of the velocity of light in vacuum to that is the material is defined as the refractive
index of the material given by

------(5)
where c = v,  being the wavelength and v the frequency of the light wave in free space.
 The value of the refractive index for free space or air is 1.00, water is 1.33 and pure silica glass is 1.5.
 When a light ray encounters a boundary separating two different media, a portion of the incident light is
reflected back into the first medium
 The rest is bent and transmitted into the second medium.
 The refraction (or bending) of light ray at the interface results from the difference of the speed of light is
the two media which are having different refractive indices.
 The light is assumed to travel from a denser to a rarer medium (n2 < n1). This type of reflection is
referred to as internal reflection.
 when light traveling in medium is reflected off an optically denser medium, the reflection is called
external reflection..

Fig.Illustrating the refraction for light travelling from denser to rarer medium

3 (i) Explain the fibre optic modes and configurations

OPTICAL MODES AND CONFIGURATIONS

 The light is guided down a fiber, 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 path

 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

 The wavelength of light and the diameter and index of refraction of the core of the fiber
  These linearly polarized (LP) modes are not exact modes of the fiber except for the fundamental
(lowest order) mode

 Weakly guiding fibers are very small, then HE– EH mode pairs

 Such modes are said to be degenerate

 The super positions of these degenerating modes characterized by a common propagation constant

 Correspond to particular LP modes regardless of their HE, 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

 The relationship between the traditional HE, EH, TE and TM mode designations and the LPlm mode

(ii) Discuss about mode analysis in optical fibre

MODE ANALYSIS FOR OPTICAL PROPAGATION IN FIBRES

Electromagnetic Waves:
 The analysis is based on the particular nature of light that treats light
 An electromagnetic wave Maxwell’s equations to explain its propagation though dielectric waveguides.
 An electromagnetic wave comprises two fields
 Both the electric and the magnetic fields are vectors having a direction and a magnitude
 The two fields are orthogonal to each other and moves with the speed of light
 An arbitrarily assuming that the electric field is oriented along the x-axis, and the magnetic field along
the y-axis
 Under this condition the direction of propagation of light will be along the z-direction
 The electric field vector always oscillates along the x-direction (vertically polarized) and thus the plane
wave corresponding to the electric field is linearly polarized with polarization vector
 The magnetic field in the vertical plane and the electric field in the horizontal plane

Fig. Electromagnetic wave propagation in the z direction

 Polarization refers to orientation of the electromagnetic field with respect to some plane or boundary
towards which the wave advances
 The field vector is oriented in the vertical, horizontal, or somewhere in between the two extremes
 The general state of polarization can be determined by expressing the electric field of a linearly
polarized wave mathematically as
 where, the electric field vector has a generalised form
4 Describe the working operation of fibre fabrication technique with a neat diagram

FIBER FABRICATION TECHNIQUE

 Fiber fabrication techniques are different for glass and plastic fibers
 This process is useful for fabrication of various techniques available for making all glass fibers
 Two basic techniques are present for fabrication of glass fiber
 One is based on traditional glass making approach where fibers are directly drawn from molten glass
 The other method involves the so-called preform which is a prefabricated clear solid glass rod or tube
 The preforms are typically of the order of 1–10 cm in diameter and 1 min length
FIBER PULLING FROM A PREFORM
 High quality glass fibers are generally fabricated by pulling long strands a preform using a fiber drawing
tower
 The fiber drawing machine is an apparatus which is typically several meters high
Working process
 The preform is heated close to the melting point using a drawing furnace fixed
 The fiber from a single preform in this process can have lengths of several kilometers
 During the pulling process, the fiber diameter is held constant
 By automatically adjusting the pulling speed of the take-up drum
 The speed of the take-up drum determines how fast the fibers are drawn
 This in turn also determines the thickness of the fiber produced in the process

Block Diagram:

Fig.A schematic of a fibre drawing method:

Enumerate the fibre optics materials with examples

FIBER MATERIALS
 Optical fibers are long, thin and flexible strands of optically transparent materials and work as optical
5 waveguides.
 The principal materials used for making optical fibers are generally based on some form of glass or
plastic material or a combination of both.
  Fused silica (amorphous silicon dioxide, SiO2) is most widely used material for fabrication of high
quality optical fibers.
 Silica glass exhibits the following properties which make it especially attractive for making optical
fibers:
 Silica has a good optical transparency in the near infrared (NIR) wavelength region ranging from 0.85
mm to 1.65 mm. High quality silica glass exhibits lowest attenuation of 0.2 dB/km around 1.5 μm
wavelength.
 Long strands of fibers can be drawn from molten silica at reasonably high temperatures.
 Silica-based fibers can be spliced and cleaved without much of practical difficulties.
 A silica fiber has an extremely high mechanical strength
 The mechanical strength of a fiber can be further improved with a suitable polymer jacket.
 Silica is chemically very stable and does not react with most of the chemicals. Silica fibers can, thus,
be used even in abusive environment.
Glass Fibers
 Glass is a non-crystalline solid (NCS) (or amorphous) solid
 Glass in general is a hard substance, usually brittle and transparent, composed chiefly of silicates and
an alkali fused at high temperature
 Glass is obtained by fusing mixtures of elements, metal oxides, halides, sulfides, tellurides or selenide.
Table.1 Glass forming substances

 The dopants are generally introduced during the formation of preforms

Fig.Variation of refractive index of silica with percentage of dopants

Fluoride Fibers
 Fluoride optical fibers are based on fluoride glasses, e.g., fluoroaluminate or fluorozirconate glasses.
Table. Composition of silica based fibre
 Among the various halide glass ZBLAN glass (ZrF4-BaF2-LaF3-AlF3-NaF) is most extensively investigated

Table. Constituents of ZBLAN glass with molecular percentage

Active Glass Fibers

 Optical fibers used as channel in a guided optical communication
 These properties can be subsequently exploited to obtain amplification, phase retardation and other
non-linear behavior of light
Plastic Optical Fiber (POF)
 Plastic optical fibers are manufactured from a variety of polymers commonly referred to as plastic
materials such as polystyrene, polycarbonates, and polymethylmethacrylate (PMMA). Plastic Clad
Silica (PCS) Fiber
 Plastic clad silica (PCS) is a compromise between high performance silica fiber.
 It consists of a core made of silica glass and cladding made of a compatible polymer of lower
refractive index.
 Commercial Plastic clad silica (PCS) fibers consist of a pure silica core, a soft silicone cladding, and a
protective jacket.

Table. Specifications of a typical commercial PCS fiber

6 Illustrate the Plasma chemical vapor deposition method with a neat diagram.

 Plasma chemical vapor deposition (PCVD) is similar to MCVD deposition inside a silica tube discussed
in the previous section
 The method uses a non-isothermal low pressure plasma to initiate gas phase reaction for deposition of
doped and undoped silica without involving sintering process separately
 The silica tube is preheated at temperatures in the range of 1000–1200°C in order to reduce
 mechanical stress during the growth of glass film inside the tube
 A moving microwave resonator operating at 2.45 GHz generates plasma inside the tube and activates
the chemical reaction
 The preform is subsequently loaded on a fiber drawing tower for drawing fibers
 A modified method with particularly high precision is plasma impulse chemical vapor deposition
(PICVD)
 Short microwave pulses are used in place of CW microwave source
 Another modified form of PCVD is so-called plasma-enhanced chemical vapor deposition (PECVD)
which operates at atmospheric pressure with fairly high deposition rate
 The preforms for multimode fibers, particularly for large core fibers, are often fabricated using plasma
outside deposition (POD)
 The core can then be made of pure silica, without any dopant

Fig. PCVD set up

 PCVD the water content in the performs can be greatly reduced

 Strong loss peak at 1400 nm can be greatly reduced (1ppb of OH– ion can give rise to a loss of 1
dB/km)
 Material purity, precision and flexibility of refractive index control, the mechanical strength of the
fabricated fibers