(a) In an optical fiber, which of the following statements is correct?

✔ II. The acceptance angle is the minimum angle made between the axis of the fiber
and the incident ray at the air-fiber interface.

(b) Convert the power level 25 μW into dBm.

✔ III. 13.98 dBm

(c) In Mie scattering process, the scattered light propagates in

✔ II. Backward direction

(d) In a graded index fiber (GIF), the value of the profile parameter for a triangular
refractive index profile will be equal to

✔ III. 3

(e) ____ are the non-linear scattering types.

✔ III. Stimulated Raman Scattering

(f) The quantum efficiency of an LED is defined as a ratio of ____

✔ III. Number of photons emitted out / Number of photons injected in

(g) The first three bytes in the SONET frame are

✔ I. Transport overhead

(h) The densities of electrons and holes are same in

✔ III. A p-n junction in equilibrium

(i) A step-index fiber has a core with a refractive index of 1.50 and a cladding with a
refractive index of 1.46. Its numerical aperture is:

✔ II. 0.344

Formula for Numerical Aperture (NA):

(j) A fiber which is referred to as a non-dispersive shifted fiber is:

✔ IV. Non-zero dispersion shifted fiber

Explanation:

• A non-zero dispersion shifted fiber (NZDSF) is designed to have a small,

controlled amount of dispersion to avoid non-linear effects, making it ideal for
high-speed optical communication.

• It is different from dispersion-shifted fiber (DSF), which has zero dispersion at

1550 nm but is prone to non-linearities.

Q 2 (a) What is an optical fiber? Explain with suitable diagram. Define the terms
acceptance angle. critical angle and numerical aperture.

What is an Optical Fiber?

An optical fiber is a flexible, transparent fiber made of glass or plastic, which transmits
light signals over long distances with minimal loss. It is widely used in
telecommunications, medical imaging, and networking applications.

Structure of Optical Fiber

An optical fiber consists of three main parts:

1. Core: The central region where light propagates. It has a higher refractive index.

2. Cladding: Surrounds the core and has a lower refractive index to ensure total
internal reflection.
 3. Protective Coating (Buffer): Provides mechanical strength and protects the
fiber from damage.

Diagram of Optical Fiber

Key Terms Related to Optical Fiber

1. Acceptance Angle (θₐ):

o It is the maximum angle at which light can enter the fiber and still be
guided through total internal reflection.

o Light entering at angles greater than the acceptance angle will not be
confined within the core.

2. Critical Angle (θc):

o The minimum angle of incidence at the core-cladding interface where

total internal reflection occurs.

o It is given by: θc=sin⁡−1(ncladdingncore)\theta_c = \sin^{-1} \left(

\frac{n_{\text{cladding}}}{n_{\text{core}}} \right)
Q2b. Already solved

Q3 a Draw the optical fiber attenuation vs. wavelength curve, show the three optical
windows and explain in brief.

Optical Fiber Attenuation vs. Wavelength Curve

Attenuation in optical fiber refers to the loss of optical power as the signal propagates
through the fiber. It is measured in dB/km and varies with wavelength. The attenuation
vs. wavelength curve shows different loss mechanisms in optical fiber.

Attenuation vs. Wavelength Graph

The curve consists of three main regions:

1. High loss at short wavelengths (~400–800 nm) due to Rayleigh scattering.

2. Low-loss regions around 850 nm, 1310 nm, and 1550 nm, known as optical
windows.

3. Absorption peaks due to impurities such as OH⁻ ions (water molecules).

Three Optical Windows

1. First Window (850 nm)

o Used in early fiber optic systems.

o Higher attenuation (~3 dB/km).

o Mostly used in multimode fiber.

2. Second Window (1310 nm)

o Lower attenuation (~0.35 dB/km).

o Minimal dispersion.

o Widely used in metro networks.

3. Third Window (1550 nm)

o Lowest attenuation (~0.2 dB/km).

o Used in long-haul communication (submarine cables, high-speed

internet).

o Higher dispersion, but managed using dispersion-shifted fibers.

Q3 (b) Explain in brief about the Rayleigh and Mie scattering losses in an optical fiber.

Rayleigh and Mie Scattering Losses in Optical Fiber

Scattering losses occur in optical fibers due to microscopic variations in the fiber
material, leading to the scattering of light waves. The two primary types of scattering
losses are Rayleigh scattering and Mie scattering.

1. Rayleigh Scattering Loss

Cause:

• Caused by microscopic density fluctuations in the fiber core during

manufacturing.

• These fluctuations are smaller than the wavelength of light.

• The scattered light intensity is inversely proportional to the fourth power of the
wavelength (∝ 1/λ⁴).

Effect on Optical Fiber:

 • Significant in the visible and near-infrared regions (400–1600 nm).

• Dominates fiber loss in the 850 nm, 1310 nm, and 1550 nm windows.

• Higher loss at shorter wavelengths (e.g., 850 nm suffers more than 1550 nm).

Formula for Rayleigh Scattering Loss:

2. Mie Scattering Loss

Cause:

• Occurs due to larger imperfections in the fiber, such as:

o Air bubbles

o Core-cladding interface irregularities

o Contaminants in the fiber material

Effect on Optical Fiber:

• More prominent in multimode fibers.

• Scattered light propagates in all directions.

• Affects fiber performance at longer wavelengths (>1600 nm).

Minimization:

• Improve fiber manufacturing to reduce large-scale imperfections.

• Use single-mode fibers to minimize scattering.

Comparison of Rayleigh and Mie Scattering Losses

Conclusion

• Rayleigh scattering is the main intrinsic loss mechanism in optical fibers,

especially at shorter wavelengths.

• Mie scattering is an extrinsic loss that occurs due to fiber imperfections and
can be reduced by improving manufacturing techniques.

4a. Discuss about different network topologies used in optical network.

Network Topologies in Optical Networks

Optical networks use different topologies to manage and route data efficiently. The
main topologies used are:

1. Point-to-Point Topology

o Direct fiber connection between two nodes.

o Provides high-speed, low-latency communication.

o Used in long-distance fiber connections.

2. Bus Topology

o All nodes share a common optical fiber link.

o Simple and cost-effective but prone to single-point failure.

o Suitable for local area networks (LANs).

3. Star Topology

o All nodes are connected to a central optical switch/hub.

o Easy to manage and allows efficient traffic control.

o Used in fiber-to-the-home (FTTH) and corporate networks.

4. Ring Topology
 o Nodes are connected in a closed-loop fiber ring.

o Supports bidirectional transmission, providing fault tolerance.

o Used in SONET/SDH networks for high reliability.

5. Mesh Topology

o Every node is connected to multiple other nodes.

o Offers high fault tolerance and redundancy.

o Used in core networks and data centers.

6. Hybrid Topology

o Combination of two or more topologies.

o Used in metropolitan area networks (MANs) and large-scale optical

systems.

(b) Wavelength Division Multiplexing (WDM)

Definition:
Wavelength Division Multiplexing (WDM) is a technique that allows multiple optical
signals with different wavelengths (colors) to be transmitted through a single optical
fiber, increasing bandwidth and efficiency.

Types of WDM:

1. CWDM (Coarse Wavelength Division Multiplexing)

o Uses fewer channels with wider spacing (20 nm).

o Supports up to 18 wavelengths from 1270 nm to 1610 nm.

o Used in short to medium-distance optical networks.

2. DWDM (Dense Wavelength Division Multiplexing)

o Uses more channels with narrow spacing (0.8 nm or less).

o Supports up to 160 wavelengths in the C-band (1530–1565 nm) and L-

band (1565–1625 nm).

o Used in long-haul and high-capacity networks.

Advantages of WDM:
Increases fiber capacity without laying more cables.
Supports multiple communication channels simultaneously.
Enables high-speed internet and data transmission. Efficient for
telecommunications and data centers.

Q 4b. Explain in brief about the wavelength division multiplexing (WDM).

Wavelength Division Multiplexing (WDM) – A Brief Explanation

Definition:
Wavelength Division Multiplexing (WDM) is an optical communication technology that
enables the transmission of multiple data streams over a single optical fiber by using
different wavelengths (colors) of laser light.

Working Principle:

• Each signal is assigned a unique wavelength (λ) within the fiber’s bandwidth.

• A multiplexer (MUX) combines multiple signals into a single fiber.

• At the receiver end, a demultiplexer (DEMUX) separates the wavelengths and

directs them to their respective channels.

Types of WDM:

1. Coarse Wavelength Division Multiplexing (CWDM)

o Uses fewer wavelengths (up to 18).

o Wavelength spacing: 20 nm.

o Supports data rates up to 10 Gbps per channel.

o Cost-effective for short to medium-distance networks (e.g.,

metropolitan networks).

2. Dense Wavelength Division Multiplexing (DWDM)

o Uses more wavelengths (up to 160).

o Wavelength spacing: 0.8 nm or less.

o Supports data rates up to 400 Gbps per channel.

o Used in long-haul and high-speed optical networks (e.g., submarine

cables, backbone internet).

Advantages of WDM:
 High Bandwidth Utilization – Transmits multiple signals without interference.
Cost-Effective – Increases fiber capacity without laying more cables.
Scalability – Can support growing data demands.
Efficient for Long-Distance Communication – Used in submarine and national
fiber networks.

Applications of WDM:

• Telecommunications and Internet backbone (fiber-optic networks).

• Data centers and cloud computing networks.

• Broadcasting and video transmission.

• Submarine fiber-optic cables for international communication.

Q.5 (a Explain the concepts of population inversion in LASER? Differentiate between

direct and indirect bandgap semi-conductor?

What is Population Inversion?

Population inversion is a condition in which the number of atoms or electrons in a

higher energy state (excited state) is greater than those in a lower energy state
(ground state). This is a necessary condition for LASER (Light Amplification by
Stimulated Emission of Radiation) action.

Process of Achieving Population Inversion:

1. Absorption of Energy – Atoms in the ground state absorb external energy

(electrical, optical, or thermal) and move to a higher energy level.

2. Metastable State – Some atoms remain in an intermediate state longer than

usual, increasing the number of excited atoms.

3. Stimulated Emission – When a photon of the same energy interacts with an

excited atom, it releases another photon, leading to coherent light
amplification.

Conditions for Population Inversion:

• Pumping Mechanism – External energy source to excite atoms.

• Metastable State – Ensures longer stay in excited state.

• Optical Cavity – Mirrors to reflect photons and enhance stimulated emission.

Difference Between Direct and Indirect Bandgap Semiconductors

Conclusion: Direct bandgap semiconductors are crucial for light-emitting devices like
LEDs and LASERs, whereas indirect bandgap semiconductors are widely used in
microelectronics and processors.

5b. Explain in detail about double heterojunction LED dipole.

Double Heterojunction LED (Light Emitting Diode) Dipole

1. Introduction to Double Heterojunction LED

A double heterojunction (DH) LED is an advanced light-emitting diode structure that

enhances efficiency and brightness. It consists of two heterojunctions, which are
interfaces between different semiconductor materials with distinct bandgaps. These
heterojunctions help in better carrier confinement and efficient light emission.

2. Structure of Double Heterojunction LED

A DH-LED consists of three layers:

1. P-type Semiconductor (Wide Bandgap)

2. Active Layer (Narrow Bandgap, Light-Emitting Region)

3. N-type Semiconductor (Wide Bandgap)

Common Material Combination:

• GaAs/AlGaAs (Gallium Arsenide/Aluminum Gallium Arsenide)

• InP/InGaAsP (Indium Phosphide/Indium Gallium Arsenide Phosphide)

Why Double Heterojunction?

• The active layer (narrow bandgap) is sandwiched between the wide-bandgap p-

type and n-type semiconductors.
 • This ensures better carrier confinement, reducing leakage and increasing
recombination efficiency.

3. Working Principle of Double Heterojunction LED

Carrier Injection:

• When a forward bias is applied, electrons from the n-region and holes from the
p-region are injected into the active layer.

Carrier Confinement:

• The heterojunctions confine electrons and holes within the narrow bandgap
active layer, leading to high recombination rates.

Efficient Light Emission:

• As electrons and holes recombine in the active layer, photons (light) are emitted
with high efficiency.

4. Advantages of Double Heterojunction LED

✔ High Efficiency – Better carrier confinement reduces losses.

✔ High Brightness – More recombination means higher light output.
✔ Reduced Threshold Current – Less power is needed to achieve efficient emission.
✔ Faster Response Time – Suitable for optical communication applications.

5. Applications of DH-LEDs

Optical Fiber Communication

LED Displays
Biomedical Devices
High-Speed Data Transmission

Conclusion

The double heterojunction LED dipole structure significantly improves LED

performance by enhancing carrier confinement, reducing power losses, and
increasing light efficiency. This technology is widely used in modern optoelectronics,
making LEDs brighter, faster, and more efficient.
Q6 a. Explain the working principle of distributed feedback lasers? Differentiate it with
distributed brag reflector laser.

Distributed Feedback (DFB) Lasers

1. Introduction

A Distributed Feedback (DFB) Laser is a semiconductor laser that incorporates a

periodic grating structure within the active region to provide optical feedback. This
ensures single-wavelength operation, which is essential for optical communication
systems.

2. Working Principle of DFB Lasers

The key working mechanism of a DFB laser relies on Bragg diffraction and optical
feedback:

Periodic Grating for Wavelength Selection:

• The grating structure within the active region acts as a distributed reflector
instead of using conventional mirrors.

• This grating provides selective feedback for a specific wavelength while

suppressing other modes.

Single Longitudinal Mode (SLM) Operation:

• The periodic variations in the refractive index cause Bragg reflection at a

particular wavelength, allowing only a single, stable wavelength to oscillate.

• This is essential for long-distance optical fiber communication since multiple

modes cause signal distortion.

Gain and Feedback Interaction:

• The gain medium (active region) provides optical amplification, and the grating
ensures the desired mode is reinforced while unwanted modes are
suppressed.

3. Difference Between DFB Laser and Distributed Bragg Reflector (DBR) Laser
4. Applications of DFB and DBR Lasers

DFB Laser Applications:

• Optical fiber communication (due to single-mode operation)

• High-speed data transmission

• Spectroscopy

DBR Laser Applications:

• Tunable laser applications

• Optical sensing

• LiDAR systems

Conclusion

While DFB lasers offer single-mode stability with distributed feedback within the
cavity, DBR lasers use external Bragg reflectors for wavelength selection, making them
more tunable but less mode-stable.

Q6 b. Discuss material and wave guide dispersion mechanics with necessary

mathematical expressions?

Material and Waveguide Dispersion Mechanics

Dispersion in optical fibers leads to pulse broadening, which limits the bandwidth and
transmission distance. The two primary types of dispersion are:
 1. Material Dispersion

2. Waveguide Dispersion

Both affect the group velocity of light traveling through the fiber.

1. Material Dispersion

Definition

Material dispersion occurs due to the wavelength-dependent refractive index of the

fiber core material. This results in different phase velocities for different wavelengths
of light, causing pulse spreading.

Mathematical Expression

The refractive index n(λ) of the core varies with wavelength λ, leading to changes in the
group velocity:

Key Points

✔ Material dispersion is dominant in multimode fibers and affects high-speed optical

communication.
✔ It is significant in silica fibers in the range of 800–1600 nm, with minimal dispersion
near 1.3 µm (zero-dispersion wavelength).

2. Waveguide Dispersion

Definition

Waveguide dispersion arises due to the geometrical structure of the fiber, where light
distribution between the core and cladding depends on wavelength. Unlike material
dispersion, it is caused by the fiber’s physical design, not the material itself.

Mathematical Expression
Waveguide dispersion is given by:

Key Points

✔ Waveguide dispersion is significant in single-mode fibers where mode confinement

affects the propagation speed.
✔ It is engineered in dispersion-shifted fibers (DSF) to optimize transmission
performance at 1550 nm.

Conclusion

Material dispersion depends on the fiber material’s refractive index variation with
wavelength, while waveguide dispersion arises from fiber geometry.
Both dispersions must be optimized in fiber design to minimize pulse broadening
for long-distance communication.
Dispersion-compensating fibers (DCF) are used to counteract total dispersion in
modern optical networks.

Q7. Distinguish between multimode step index and graded index fibers. What is the
difference between multimode and single-mode fiber?

Distinguish Between Multimode Step Index and Graded Index Fibers

Difference Between Multimode and Single-Mode Fiber

7 b. A graded index fiber with a parabolic profile supports the propagation of 700 guided
modes. The fiber has a relative refractive index difference of 2%, a core refractive index
of 1.45 and a core diameter of 75_µm. Calculate the wavelength of light propagating in
the fiber. Further, estimate the maximum diameter of the fiber core which can give
single-mode operation at the same wavelength.

Given Data:

• Number of guided modes, M = 700

• Relative refractive index difference, Δ = 2% = 0.02

• Core refractive index, n₁ = 1.45

• Core diameter, d = 75 µm

• Core radius, a = d/2 = 75/2 = 37.5 µm

Step 1: Calculate the V-Number (Normalized Frequency)

The V-number (or normalized frequency) is given by:

Thus, the wavelength of light propagating in the fiber is 1.82 µm.

Step 2: Maximum Core Diameter for Single-Mode Operation

For single-mode operation, the fiber must satisfy the condition:

Final Answers

1. Wavelength of light propagating in the fiber = 1.82 µm

2. Maximum core diameter for single-mode operation = 4.84 µm

Q8. (a) Explain about Avalanche Photodiode.

An Avalanche Photodiode (APD) is a highly sensitive photodetector that operates

under a high reverse bias voltage to achieve internal signal amplification through an
avalanche multiplication process.

Working Principle:

1. When a photon strikes the depletion region, it generates an electron-hole pair.

2. The strong electric field in the depletion region accelerates these charge carriers,
leading to impact ionization.

3. This ionization generates additional charge carriers, leading to an avalanche

effect.

4. The avalanche multiplication significantly increases the photocurrent, making

APDs more sensitive than standard photodiodes.

Advantages of APD:

• Higher sensitivity compared to PIN photodiodes.

• Faster response time.

 • Suitable for low-light applications, such as optical fiber communication and
LiDAR.

Disadvantages of APD:

• Requires a high operating voltage.

• Generates higher noise due to avalanche multiplication.

• More expensive compared to conventional photodiodes.

Thus, the required incident optical power is 3.6 µW.

9. Applications of Optical Fiber Communication Systems

1. Military Applications

Optical fiber plays a crucial role in military communication systems due to its high
security, reliability, and resistance to electromagnetic interference.

• Secure Communication: Optical fibers are difficult to tap, making them ideal for
confidential data transmission.

• Guidance Systems: Used in missile guidance and sensor applications.

• Radar Systems: Optical fiber enhances signal processing in radar technology.

• Remote Sensing: Helps in monitoring battlefield conditions and enemy

movements.

2. Civil Applications

Optical fibers are widely used in civil infrastructure for efficient communication and
data transmission.

• Telecommunication Networks: Used in telephone lines, internet services, and

cable TV.
 • Traffic Management: Fiber optic sensors help in intelligent traffic control
systems.

• Medical Applications: Used in endoscopy, laser surgery, and biomedical

sensors.

• Smart Cities: Supports high-speed data transmission in urban planning and

smart lighting systems.

3. Consumer Applications

Optical fiber technology has transformed everyday life with fast and reliable
connectivity.

• High-Speed Internet: Enables broadband and fiber-to-the-home (FTTH)

services.

• Home Entertainment: Supports high-definition TV, streaming, and gaming.

• Smart Devices: Used in smart home automation and IoT applications.

• Fiber Optic Lighting: Provides decorative and energy-efficient lighting solutions.

4. Industrial Applications

Industries utilize optical fibers for efficient communication, automation, and safety
monitoring.

• Manufacturing Automation: Used in robotics and automated machinery for

precise control.

• Oil and Gas Industry: Optical fiber sensors monitor temperature and pressure in
pipelines.

• Aerospace and Aviation: Used in aircraft navigation, communication, and

structural monitoring.

• Power Grid Monitoring: Helps in detecting faults and managing power

distribution efficiently.

By Shivam Sinha