(SEM V) THEORY EXAMINATION 2021-22 OPTICAL COMMUNICATION

OPTICAL COMMUNICATION (KEC-058) – EXAM ANSWERS

SECTION-A (2 Marks Each)

a) Acceptance Angle & Numerical Aperture

Acceptance Angle: Maximum angle at which light can enter the fiber and still propagate.

Numerical Aperture (NA): Light-gathering ability of fiber.

• NA=n12−n22NA = \sqrt{n_1^2 - n_2^2}NA=n12​−n22​​

b) Normalized Frequency (V-Number)

V=2πaλ×NAV = \frac{2\pi a}{\lambda} \times NAV=λ2πa​×NA

Multimode fiber: V > 2.405                     Single mode fiber: V < 2.405

c) Non-linear Scattering Types

Stimulated Brillouin Scattering (SBS)         Stimulated Raman Scattering (SRS)

d) Electrical vs Optical Bandwidth

Electrical bandwidth: Limited by electronic components.

Optical bandwidth: Wider, determined by fiber dispersion and light source.

e) Importance of Double Hetero-junction

Improves efficiency                   Better carrier confinement        Higher optical output power

f) Temperature Effect on Avalanche Gain

As temperature increases → avalanche gain decreases

Due to increased phonon collisions

g) Receiver Sensitivity & Quantum Limit

Receiver Sensitivity: Minimum power required for acceptable BER.

Quantum Limit: Theoretical minimum optical power for detection.

h) Intrinsic & Extrinsic Absorption

Intrinsic: Due to material itself (UV & IR absorption)

Extrinsic: Due to impurities (OH⁻ ions, metal ions)

i) Minimum Gain Condition (Fabry-Perot)

gth=12Lln⁡(1R1R2)g_{th} = \frac{1}{2L} \ln\left(\frac{1}{R_1 R_2}\right)gth​=2L1​ln(R1​R2​1​) 

j) Stimulated Emission

Emission of photons when excited atoms are forced to emit identical photons by incident radiation.

SECTION-B (10 Marks Each – Attempt Any Three)

a) Acceptance Angle & NA Derivation

NA=n12−n22NA = \sqrt{n_1^2 - n_2^2}NA=n12​−n22​​

Given:

n1=1.56n_1 = 1.56n1​=1.56

n2=1.40n_2 = 1.40n2​=1.40

NA=(1.56)2−(1.40)2=0.69NA = \sqrt{(1.56)^2 - (1.40)^2} = 0.69NA=(1.56)2−(1.40)2​=0.69 

b) Attenuation & Power Calculation

Pout=Pin×10−αL/10P_{out} = P_{in} \times 10^{-\alpha L /10}Pout​=Pin​×10−αL/10

Given:

Pin=200µWP_{in} = 200µWPin​=200µW

Attenuation = 0.4 dB/km

Length = 30 km

Pout≈12.6µWP_{out} \approx 12.6µWPout​≈12.6µW 

c) Population Inversion & Threshold Condition

Population inversion occurs when N2>N1N_2 > N_1N2​>N1​

Threshold gain:

gth=αi+αmΓg_{th} = \frac{\alpha_i + \alpha_m}{\Gamma}gth​=Γαi​+αm​​ 

d) Noise in Photodiode

Shot noise                                          Thermal noise

Dark current noise                             Quantum noise: Due to random photon arrival

e) Free Space Optics (FSO)

Wireless optical communication through air                 High data rate       Affected by fog, rain, dust

SECTION-C (10 Marks)

a) Classification of Optical Fibers

By modes: Single-mode, Multi-mode             By index profile: Step index, Graded index

b) Numerical Problem

Given:

Core diameter = 70µm → a = 35µm

Δ = 1.7% = 0.017                   n1=1.48n_1 = 1.48n1​=1.48                λ = 0.85µm

NA=n12ΔNA = n_1 \sqrt{2\Delta}NA=n1​2Δ​ V≈39V \approx 39V≈39

Number of modes:          M=V22≈760M = \frac{V^2}{2} \approx 760M=2V2​≈760 

SECTION-D (10 Marks)

a) Intermodal Dispersion

σs=n1ΔLc\sigma_s = \frac{n_1 \Delta L}{c}σs​=cn1​ΔL​

Delay difference:

ΔT=n1ΔLc\Delta T = \frac{n_1 \Delta L}{c}ΔT=cn1​ΔL​ 

b) Bending Losses               Macrobending                 Microbending

Critical radius:

Rc=3n12λ4π(n12−n22)3/2R_c = \frac{3n_1^2\lambda}{4\pi(n_1^2-n_2^2)^{3/2}}Rc​=4π(n12​−n22​)3/23n12​λ​ 

SECTION-E (10 Marks)

a) Fabry-Perot Cavity

Given:

Length = 5 cm          n = 1.67                λ = 0.65µm

Mode spacing:                                       Δf=c2nL\Delta f = \frac{c}{2nL}Δf=2nLc​ 

b) S-LED vs E-LED

S-LED: Surface emitting, low power      E-LED: Edge emitting, higher efficiency

SECTION-F (10 Marks)

a) PIN Photodiode                               Fast response

Low noise                                               Speed limited by:

Transit time                                             RC time constant

b) Avalanche Photodiode                    Internal gain

High sensitivity                                       Requires high bias voltage

SECTION-G (10 Marks)

a) Eye Diagram

Shows ISI, noise, jitter                             Wide eye opening → good system

b) Power Penalty

Extra power required to maintain BER.