1. (a) An optical transmitter transmits 10W power.

Compute its equivalent power in

dBm.
Answer: (iii) 40 dBm
Explanation: dBm = 10 × log₁₀(P in mW) = 10 × log₁₀(10,000) = 40 dBm

(b) The responsivity of the optical power of 0.4µW and photocurrent of 0.294µA is
Answer: (iii) 0.56
Explanation: Responsivity (R) = Photocurrent / Optical power = 0.294µA / 0.4µW = 0.56
A/W

(c) Which of the following fibers has the highest modal dispersion?
Answer: (i) Step-index multimode
Explanation: Step-index multimode fibers have the highest modal dispersion due to
different path lengths taken by different modes.

(d) A fiber-optic cable has a loss of 15 dB/km. The attenuation in a cable 1000 ft long is
Answer: (i) 4.57 dB
Explanation: 1000 ft = 304.8 m = 0.3048 km. Attenuation = 15 dB/km × 0.3048 km = 4.57
dB

(e) Which of the following fibers is suitable for wavelength-division multiplexing of

signals?
Answer: (ii) Dispersion shifted
Explanation: Dispersion-shifted fibers reduce chromatic dispersion at wavelengths
used for WDM.

(f) A fiber which is referred to as non-dispersive shifted fiber is

Answer: (iv) Non-zero dispersion shifted fiber
Explanation: These fibers shift the dispersion minimum slightly away from the
operating wavelength to control non-linear effects.

(g) In an optical communication system, the zero dispersion wavelength is given by

Answer: (iii) 1550 nm
Explanation: The zero dispersion wavelength for most single-mode fibers is around
1550 nm.

(h) A graded index fiber has a parabolic refractive index profile α = 2, the radius of core a
= 25µm, and numerical aperture NA = 0.22, then the total number of guided modes at
wavelength 1310 nm is given by
Answer: (ii) 174
Explanation: The number of guided modes in a graded-index fiber is given by:
(i) Which of the following is being used at the receiver in optical fiber communication?
Answer: (iii) Photo Diode
Explanation: Photodiodes are used to convert optical signals into electrical signals.

(j) Which of the following mechanisms is being used for the generation of light in LASER?
Answer: (iii) Stimulated Emission
Explanation: LASER light is generated through stimulated emission, which amplifies
light coherently.

2 (a) Block Diagram of Fiber Optic Communication System and Functions of Each
Component

Block Diagram:

A simple block diagram of a fiber optic communication system includes the following
components:

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 waves into 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 the 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) is used. They can be called as optic oscillators,
they provide stable, single frequency waves with sufficient power for long distance
propagation.

Channel coupler : • Coupler feeds the power into the information channel. For an
atmospheric optic system, the channel coupler is a lens used for collimating the
light emitted by the source and directing this light towards the receiver. The coupler
must efficiently transfer the modulated light beam from 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 disgital pulse trains for further transmission.

Optical detector :

• The information being transmitted is 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 thn 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 :

• Signal processing includes filtering, amplification. Proper filtering maximizes the

ratio of signal to unwanted power. For a digital system decision circuit is an
additional block. The bit error rate (BER) should be very small for quality
communications. Message output :

• The electrical form of the message emerging from the signal processor are
transformed into a soud wave or visual image. Sometimes these signals are directly
usable when computers or other machines are connected through a fiber system.
Function of Each Component:

1. Information Source:

o Provides the data or signal (e.g., voice, video, or digital data) that needs to
be transmitted.

2. Transmitter (Optical Source):

o Converts electrical signals into optical signals.

o Uses LED (for short distances) or Laser Diodes (for long distances).

3. Optical Fiber (Transmission Medium):

o Transmits the optical signal from the transmitter to the receiver.

o Made of a core, cladding, and protective coating.

4. Optical Receiver (Photo Detector):

o Converts the received optical signal back into an electrical signal.

o Uses photodiodes like PIN or Avalanche Photodiodes (APD).

5. Destination (Output Device):

o The final output of the received signal, which could be an audio speaker,
display, or computer system.

(b) Advantages and Disadvantages of Optical Fiber Communication

Advantages:

1. High Bandwidth: Optical fibers support higher data transmission rates than
copper cables.

2. Low Signal Loss: Optical fibers have minimal attenuation over long distances.

3. Immunity to Electromagnetic Interference (EMI): Unlike copper cables, optical

fibers are not affected by electromagnetic noise.

4. Lightweight and Small Size: Optical fibers are thinner and lighter than
traditional copper cables.

5. Security: Optical communication is more secure because it is difficult to tap

into fiber optic signals.
 6. Long Distance Transmission: Optical fibers can transmit data over several
kilometers without requiring repeaters.

Disadvantages:

1. High Initial Cost: The installation and components of fiber optic systems are
expensive.

2. Fragile: Optical fibers are more delicate compared to copper wires and can
break if bent excessively.

3. Difficult to Splice: Fiber splicing requires precision and special equipment.

4. Requires Special Skills for Installation: Technicians need specialized training

for handling and installing fiber optic cables.

5. Power Source Dependency: Optical fiber networks require electrical power for
optical transmitters and receivers.

Q 3 a What is numerical aperture? Derive an expiration for numerical aperture and

maximum acceptance angle in the case of step-index optical fiber in terms of refractive
indices of core and cladding material.

Definition:

Numerical Aperture (NA) is a dimensionless number that represents the light-gathering

ability of an optical fiber. It determines the range of angles at which light can enter the
fiber and still be guided through total internal reflection.

Mathematical Expression:

The numerical aperture is given by:

where:

• n1n_1n1 = Refractive index of the core

• n2n_2n2 = Refractive index of the cladding

• θa\theta_aθa = Maximum acceptance angle of the fiber

Significance of Numerical Aperture:

1. Higher NA → More light can enter the fiber, improving light collection efficiency.

2. Lower NA → Reduces dispersion, improving signal quality for long-distance

communication.

3. Determines Acceptance Angle → The maximum angle at which light can be

launched into the fiber.

4. Affects Coupling Efficiency → Important in fiber alignment and optical system

design.
2 (b) Find the maximum core diameter for an optical fiber with a core refractive index of
1.5 and cladding index of 1.49, operating at 850 nm, to be single mode. What is the
numerical aperture for this fiber?

Maximum Core Diameter and Numerical Aperture Calculation

Given Data:

Step 2: Find Maximum Core Diameter for Single-Mode Operation

To ensure single-mode operation, the core diameter ddd must be small enough so that
only one mode propagates.
For a step-index fiber, the maximum core diameter can be approximated by the
following condition:

Final Answer:

• Numerical Aperture (NA) = 0.173

• Maximum Core Diameter for Single-Mode Operation = 4.91 μm

3 a What do you mean by attenuation? What are the basic attenuation mechanism? On
what factors these mechanism depends.

Attenuation in Optical Fiber Communication

Definition:
Attenuation refers to the loss of optical power as light propagates through an optical
fiber. It is measured in decibels per kilometer (dB/km) and represents the reduction in
signal strength due to various factors.
Basic Attenuation Mechanisms in Optical Fiber

Attenuation in optical fibers is mainly caused by three mechanisms:

1. Absorption Loss

• Cause: Due to intrinsic properties of fiber material (silica) and impurities (OH⁻
ions, metal ions).

• Types:

o Intrinsic Absorption: Due to the natural vibrations of silica molecules.

o Extrinsic Absorption: Due to impurities like water (OH⁻ ions) or metal

particles.

• Dependency Factors: Fiber material purity, wavelength of light used.

2. Scattering Loss

• Cause: Due to microscopic variations in fiber structure, causing light to scatter.

• Types:

o Rayleigh Scattering: Dominant at lower wavelengths due to random

microscopic density variations in silica.

o Mie Scattering: Due to larger structural defects in fiber.

• Dependency Factors: Fiber composition, fabrication quality, operating

wavelength.

3. Bending Loss

• Cause: When fiber is bent, some light escapes from the core.

• Types:

o Macrobending Loss: Large, gradual bends lead to leakage of light.

o Microbending Loss: Small, microscopic bends cause signal loss due to

uneven stress on the fiber.
 • Dependency Factors: Fiber bending radius, external mechanical stress.

Factors Affecting Attenuation Mechanisms

1. Wavelength of Light:

o Shorter wavelengths suffer more from Rayleigh scattering.

o Longer wavelengths are more affected by absorption losses.

2. Fiber Material & Purity:

o High-purity silica reduces absorption losses.

o Impurities like OH⁻ ions increase loss.

3. Manufacturing Defects:

o Uneven fiber structure increases scattering losses.

4. Bending Radius & Stress:

o Tighter bends lead to higher bending losses.

(iii) Overall Signal Attenuation for a 10 km Optical Link with Splices

• Fiber length = 10 km

• Attenuation per km = 2.00 dB/km

• Number of splices = 1 per km, so for 10 km → 10 splices

• Each splice adds 1 dB loss.

Total attenuation:

5a (a) Dispersion in Optical Fiber & Intramodal Dispersion

Dispersion in Optical Fiber: Dispersion in an optical fiber refers to the spreading of

optical pulses as they travel through the fiber, which limits the bandwidth and data
transmission rate. The main types of dispersion are:

1. Intramodal Dispersion (Chromatic Dispersion) – Occurs within a single mode

of transmission.

2. Intermodal Dispersion – Occurs in multimode fibers where different modes

travel at different speeds.

Intramodal Dispersion: Intramodal dispersion (or chromatic dispersion) is caused by

the dependency of a material’s refractive index on wavelength. It consists of:

1. Material Dispersion: Caused by the wavelength-dependent refractive index of

the core material. It is dominant in silica fibers.

2. Waveguide Dispersion: Arises due to the wavelength-dependent distribution of

light between the core and cladding of the fiber.

Intramodal dispersion is significant in single-mode fibers and can be reduced by

choosing appropriate wavelengths, such as the zero-dispersion wavelength.
Final Answer:

The total first-order dispersion at 1.54 µm is 29.17 ps/nm/km.

Q.6 (a) Principle of LASER Diode & Pumping Techniques

Principle of LASER Diode:

A LASER (Light Amplification by Stimulated Emission of Radiation) diode is a

semiconductor device that produces coherent light through the process of stimulated
emission. The working principle involves the following key steps:

1. Electrons Excitation: When a forward bias voltage is applied, electrons move

from the valence band to the conduction band, creating electron-hole pairs.

2. Spontaneous Emission: Some electrons recombine with holes, emitting

photons. This process is called spontaneous emission.

3. Stimulated Emission: The emitted photons stimulate further electron-hole

recombination, generating more photons of the same phase and wavelength.
 4. Optical Feedback & Amplification: The diode structure includes a resonant
cavity formed by cleaved facets that reflect light back and forth, amplifying the
emitted photons.

5. Laser Output: Once the gain surpasses the losses, a coherent and
monochromatic laser beam is emitted.

Pumping Techniques of LASER Diode:

Pumping refers to the method of exciting electrons to higher energy states. The common
pumping techniques for a LASER diode are:

1. Electrical Pumping:

o Used in semiconductor laser diodes.

o Achieved by applying a forward bias voltage across the junction.

o Most commonly used method in laser diodes.

2. Optical Pumping:

o Used in solid-state and gas lasers.

o External light sources (like flash lamps or another laser) provide the
required energy.

3. Chemical Pumping:

o Used in chemical lasers.

o Chemical reactions generate the required excitation energy.

4. Thermal Pumping:

o Uses high temperatures to excite electrons.

o Less commonly used in modern laser applications.

Q.6 (b) Various Structures of LED

LEDs (Light Emitting Diodes) are semiconductor devices that emit light when a forward
voltage is applied. Different LED structures impact their efficiency, brightness, and
applications.

1. Surface Emitting LED (SLED):

• Simple and widely used LED structure.

• Light is emitted perpendicular to the surface.

• Lower efficiency due to internal reflection.

2. Edge-Emitting LED (ELED):

• Light is emitted from the edge of the semiconductor.

• Has a waveguide structure to direct the light.

• Higher efficiency and better directionality than SLED.

3. Double Heterostructure LED (DH-LED):

• Consists of a p-n junction sandwiched between two layers of different

semiconductor materials.

• Improved efficiency due to better carrier confinement.

• Common in high-brightness LEDs.

4. Quantum Well LED:

• Uses a quantum well structure to enhance electron-hole recombination.

• Highly efficient and used in high-performance applications.

5. Superluminescent LED (SLED):

• A combination of LED and laser diode principles.

• Provides higher brightness and better coherence.

• Used in fiber-optic communication.

Conclusion:

• LASER diodes work based on stimulated emission and require pumping

techniques like electrical or optical pumping.

• LED structures vary, including surface-emitting, edge-emitting, double

heterostructure, quantum well, and superluminescent designs.
Q.7 Explain the working of p-i-n photodiode. Explain the various measures of efficiency
in PIN photodiode.

Working of a p-i-n Photodiode

A p-i-n photodiode is a semiconductor device used for detecting light by converting

photons into electrical current. It consists of three layers:

1. p-region (Positive) – A heavily doped layer that supplies holes.

2. Intrinsic (i-region) – A lightly or undoped region that increases the depletion

width for efficient charge carrier generation.

3. n-region (Negative) – A heavily doped layer that supplies electrons.

Working Principle:

1. Reverse Bias Operation:

o A reverse voltage is applied across the diode, which widens the depletion
region in the intrinsic layer.

2. Photon Absorption:

o When incident light (photons) strikes the intrinsic region, it excites

electrons from the valence band to the conduction band, creating
electron-hole pairs.

3. Charge Carrier Separation:

o Due to the electric field in the depletion region, electrons move toward
the n-region, and holes move toward the p-region.

4. Current Flow:

o The movement of charge carriers generates a photocurrent, which is

proportional to the intensity of incident light.

5. Signal Output:

o The generated current is collected as an electrical signal, used in

applications like optical communication, light sensing, and medical
imaging.

Efficiency Measures in p-i-n Photodiodes

Several factors determine the efficiency of a p-i-n photodiode, including:

1. Quantum Efficiency (η)

 o Definition: It is the ratio of the number of electron-hole pairs generated to
the number of incident photons.

o Formula: η=Number of electron-

hole pairs generatedNumber of incident photons\eta = \frac{\text{Number
of electron-hole pairs generated}}{\text{Number of incident photons}}

3. Dark Current (Id)

o Definition: It is the leakage current flowing through the diode in the

absence of light.

o Lower dark current is desirable to minimize noise in signal detection.

4 Response Time (τ)

o Definition: It is the time taken for the photodiode to respond to an optical

signal.

o Shorter response time enables faster signal processing, crucial for high-
speed communication.

5 Noise Equivalent Power (NEP)

o Definition: It represents the minimum optical power required to generate

a signal equal to the noise level.

o Lower NEP indicates higher sensitivity.

6. Gain (Avalanche Multiplication Factor in APD)

o In Avalanche Photodiodes (APD), additional gain is achieved through

impact ionization.
 o p-i-n photodiodes do not have gain, but APDs use avalanche
multiplication to enhance detection.

Q 8. Q.8 (a) Structure of a SONET Frame and STS-1 Data Rate Calculation

Structure of a SONET Frame

SONET (Synchronous Optical Network) is a standardized digital communication

protocol that is used to transmit multiple digital signals over optical fiber. The
fundamental unit of transmission in SONET is the STS (Synchronous Transport Signal)
frame.

• The basic SONET frame structure is defined for STS-1 (Synchronous Transport
Signal level-1).

• An STS-1 frame consists of 9 rows × 90 columns = 810 bytes.

• The frame is transmitted every 125 µs, resulting in a high-speed digital

transmission.

SONET Frame Components:

1. Transport Overhead (TO):

o Consists of section and line overhead.

o Includes synchronization, framing, error checking, and maintenance

functions.

2. Payload:

o Carries user data or other lower-speed multiplexed signals.

o Uses SPE (Synchronous Payload Envelope) to allow flexible data

transport.

3. Pointer:

o Used to indicate the start of the SPE within the SONET frame.
Calculation of STS-1 Data Rate

The total number of bits in an STS-1 frame can be calculated as:

Q.8 (b) Network Topologies in Optical Fiber Communication

Network topology refers to the way in which network devices and communication
channels are arranged.

Types of Network Topologies in Optical Fiber Communication

1. Bus Topology

o All nodes are connected to a single backbone cable.

o Data travels in both directions, and each node receives all data but only
processes the intended data.

o Commonly used in passive optical networks (PONs).

2. Star Topology

o All nodes are connected to a central hub or switch.

o Data transmission is controlled by the hub, ensuring better reliability.

o Used in active optical networks (AONs).

Comparison of Bus and Star Topologies in Optical Networks

Conclusion

• SONET uses STS frames for high-speed optical transmission, with STS-1
operating at 51.84 Mbps.

• Network topologies like Bus and Star are used in fiber networks, where Bus is
cost-effective but less reliable, while Star is more reliable but requires more
infrastructure.

Q9 (a) Military Applications

Optical fiber plays a crucial role in defense and military operations due to its secure and
reliable communication capabilities:

• Secure Communication: Optical fibers are immune to electromagnetic

interference (EMI), making them ideal for encrypted and highly secure military
communications.

• Radar and Sensing Systems: Used in fiber optic gyroscopes (FOG) for
navigation and guidance in aircraft, submarines, and missiles.

• Remote Monitoring and Surveillance: Optical fiber sensors help detect

pressure, temperature, and structural integrity in military equipment and
installations.

• Underwater and Space Applications: Used in submarine communication and

satellite links due to low signal loss over long distances.

(b) Civil Applications

Optical fibers are widely used in civil infrastructure and everyday communication:
 • Telecommunications: Optical fibers are the backbone of high-speed internet,
telephony, and cable TV services.

• Medical Field: Used in endoscopy, laser surgeries, and biomedical sensors for
diagnostics.

• Transportation Systems: Fiber optics are integrated into smart traffic

management, railway signaling, and highway monitoring systems.

• Aerospace and Aviation: Optical fibers are used in aircraft communication and
in-flight entertainment systems due to their lightweight nature and immunity to
electromagnetic interference.

(c) Consumer and Industrial Applications

Optical fiber technology benefits both consumers and industries in various ways:

• Consumer Electronics: Used in fiber optic audio/video transmission for home

theaters, gaming consoles, and digital displays.

• Industrial Automation: Helps in data transmission for factory automation,

robotics, and process monitoring.

• Oil and Gas Industry: Optical fiber sensors detect pressure and temperature
changes in pipelines and drilling operations.

• Power and Energy Sector: Used in smart grids for efficient power distribution
and monitoring.

• Lighting and Decorations: Fiber optics are used in decorative lighting,

architectural designs, and Christmas lights due to their flexible and safe nature.

By Shivam Sinha