(SEM V) THEORY EXAMINATION 2023-24 OPTICAL COMMUNICATION

Subject Overview: Optical Communication (KEC058)
 

This subject deals with the principles, design, and performance of optical communication systems, focusing on the use of light as a medium for transmitting information through optical fibers.
 

It covers:
 

Light propagation in fibers

Losses and dispersion

Optical sources (LEDs, Lasers)

Optical detectors (PIN and APD photodiodes)

Link design and system performance

This is a core subject for Electronics & Communication Engineering (ECE) students, providing foundational knowledge for careers in telecommunication, fiber-optic networking, and photonics.

Exam Paper Details
 

Course: B.Tech (Semester V)

Subject Code: KEC058

Subject Name: Optical Communication

Maximum Marks: 100

Time: 3 Hours

Paper ID: 310410

Date: 19 January 2024

Note: Attempt all sections. Missing data, if any, can be assumed suitably

Paper Pattern Summary

SECTION A — Short Answer Questions (10 × 2 = 20 Marks)
 

Each question tests fundamentals. Below are explanations and key points to prepare:
 

(a) Mode Field Diameter (MFD):

Defines the effective area over which light propagates in a single-mode fiber.
It affects coupling efficiency, bending losses, and dispersion.

Formula (approx.):

MFD≈2w0\text{MFD} ≈ 2w_0MFD≈2w0​

where w0w_0w0​ is mode radius.
 

(b) Refractive Index (R.I.) Difference in Core and Cladding:

The core has a slightly higher refractive index than the cladding to ensure total internal reflection of light.
This difference (Δ) controls light confinement and numerical aperture (NA).
 

(c) Stimulated Raman Scattering (SRS):

A nonlinear optical effect where high-power light transfers energy to another wavelength, causing signal degradation or amplification (Raman Amplifier principle).
 

(d) Kerr Effect:

The refractive index of the medium changes with applied electric field intensity, leading to intensity-dependent phase modulation in optical communication.
 

(e) Population Inversion:

A state in a laser where more electrons occupy excited energy levels than ground state, enabling stimulated emission and laser generation.
 

(f) Surface Emitting LED vs. Edge Emitting LED:

Surface Emitting: Emits light perpendicular to the surface; simple but low coupling efficiency.

Edge Emitting: Emits light from the edge; higher efficiency and narrower spectral width.
 

(g) Source-to-Fiber Power Launching:

Refers to coupling efficiency between source and fiber. Depends on:

Core diameter

Numerical Aperture (NA)

Alignment and lens coupling
 

(h) Multiplication Factor in Avalanche Photodiodes (APD):

Represents gain due to avalanche effect.

M=Total currentPrimary photocurrentM = \frac{\text{Total current}}{\text{Primary photocurrent}}M=Primary photocurrentTotal current​

Higher M improves sensitivity but increases noise.

(i) Quantum Limit:

The minimum optical power required to detect a signal based on quantum (photon) noise — defines receiver sensitivity.

(j) Receiver Selectivity:

Ability of the optical receiver to distinguish between desired signals and unwanted noise or interference.

SECTION B — Descriptive / Numerical Questions (Any 3 × 10 = 30 Marks)

(a) Phase and Group Velocity:
 

Phase Velocity (Vp): Speed of a wavefront. Vp=cnV_p = \frac{c}{n}Vp​=nc​

Group Velocity (Vg): Speed of the modulated envelope carrying information.

• Vg=c(n−λdndλ)−1V_g = c \left( n - \lambda \frac{dn}{d\lambda} \right)^{-1}Vg​=c(n−λdλdn​)−1

Derive relation and explain signal distortion due to different velocities.

(b) Linear Scattering Losses:

Rayleigh Scattering: Occurs due to microscopic variations in material density; dominant in silica fibers.

Mie Scattering: Caused by larger irregularities; occurs in multimode fibers.

These lead to attenuation and signal weakening.

(c) Optical Sources and Modulation Bandwidth:

Explain requirements for optical sources:

High efficiency

High modulation speed

Compatible wavelength with fiber

LEDs: Cheap, wide spectral width, low power.
LASERs: High power, narrow spectrum, suitable for long distance.

(d) P–N Junction Photodiode Numerical:

Given:

Quantum efficiency = 65%

Photon energy E=1.5×10−19JE = 1.5 × 10^{-19} JE=1.5×10−19J

Required photocurrent Ip=2.5μAI_p = 2.5 μAIp​=2.5μA

To find:
Wavelength λ=hcEλ = \frac{hc}{E}λ=Ehc​
Incident power P=IphνηeP = \frac{I_p hν}{η e}P=ηeIp​hν​

(e) Short Notes:

Multichannel & Multiplexing Transmission:

TDM, FDM, and WDM increase bandwidth efficiency.

Eye Diagram Features:

Visual tool for analyzing signal quality. Eye opening shows SNR and distortion.

SECTION C — Long Answer / Analytical (Any 1 Part Each = 50 Marks)

Q3(a)

Given:

Relative refractive index = 1%,

Core index = 1.46

Find:

Numerical Aperture (NA):

1. NA=n12ΔNA = n_1 \sqrt{2Δ}NA=n1​2Δ​

Solid Acceptance Angle: θ=sin⁡−1(NA)\theta = \sin^{-1}(NA)θ=sin−1(NA)

Critical Angle: sin⁡−1(n2/n1)\sin^{-1}(n_2 / n_1)sin−1(n2​/n1​)

Q3(b)

Difference between Step Index and Graded Index fiber.
Also compute cutoff wavelength for single-mode operation.

λc=2πaNA2.405λ_c = \frac{2πa NA}{2.405}λc​=2.4052πaNA​

where aaa = core radius.

Q4(a)

Given: Input power = 120 mW, Output = 3 mW, Fiber length = 8 km.
Find:

Overall loss (dB):

1. L=10log⁡10(PiPo)L = 10 \log_{10} \left( \frac{P_i}{P_o} \right)L=10log10​(Po​Pi​​)

Attenuation per km

For 10 km link with 1 dB splices/km, compute total loss and input/output ratio.

Q4(b)

Explain Intermodal Dispersion and derive RMS pulse broadening in multimode step-index fiber.
It occurs due to multiple propagation paths having different velocities.

Q5(a)

Semiconductor Injection Laser:
Explain construction, working, and principles.
Define:

Total Efficiency: ηt=PoptPinputη_t = \frac{P_{opt}}{P_{input}}ηt​=Pinput​Popt​​

External Efficiency: ηext=ΔPoptΔIη_{ext} = \frac{ΔP_{opt}}{ΔI}ηext​=ΔIΔPopt​​

Q5(b)

Heterojunction LED Numerical:
Given:

Radiative = 60 ns

Non-radiative = 100 ns
Compute total recombination time:

1τ=1τr+1τnr\frac{1}{τ} = \frac{1}{τ_r} + \frac{1}{τ_{nr}}τ1​=τr​1​+τnr​1​ 

Q6(a)

APD Photodiode:
Explain construction, working, and temperature effect on gain.
Higher temperature reduces ionization rate → decreases gain.

Q6(b)

P-I-N Photodiode:
Explain structure (P, intrinsic, N layers), working principle, and speed limiting factors:

Junction capacitance

Carrier transit time

Q7(a)

Power Penalty:
Extra optical power required to maintain Bit Error Rate (BER).
Types:

Dispersion penalty

Nonlinear distortion penalty

Receiver sensitivity penalty

Error Control Techniques: Forward Error Correction (FEC), Hamming Code, Reed-Solomon Coding.

Q7(b)

Free Space Optics (FSO):
Wireless optical communication using lasers through atmosphere.
Heterodyne Detection: Combines received light with local oscillator signal to improve SNR.

Important Topics to Revise Before Exam

Fiber structure, NA, and acceptance angle.

Loss mechanisms: scattering, absorption, bending.

Dispersion types: material, waveguide, intermodal.

LED and LASER theory + efficiency formulas.

Photodiodes: PIN and APD operations.

Optical link design and power budgeting.

Multiplexing techniques and signal analysis (Eye Diagram).

Free Space Optics and Power Penalties.

Conclusion
 

The Optical Communication (KEC058) exam evaluates both conceptual understanding and problem-solving skills.

Focus on:

Formulas and derivations

Numerical accuracy

Labeled diagrams

This subject connects theory to modern technologies like fiber-optic internet, laser communications, and photonics, making it essential for any ECE student.