Course: B.Tech (Semester V)
Subject: Optical Communication
Subject Code: KEC-058
Duration: 3 Hours
Total Marks: 100
Instructions: Attempt all sections. Assume suitable data wherever required.
Section A — Short Answer Questions (2 × 10 = 20 Marks)
Answer all questions briefly:
Define Goos–Hänchen shift in optical fiber waveguide.
A silica optical fiber has a core refractive index of 1.50 and cladding index of 1.47. Determine the critical angle at the core–cladding interface.
What are the causes of attenuation in optical communication?
Define polarization of light in optical communication.
A surface-emitting LED launches 190 µW optical power into a multimode fiber with a forward current of 25 mA at 1.5 V. Find the power conversion efficiency.
State advantages of LED light.
When 3×10¹¹ photons (λ = 0.85 µm) are incident on a photodiode and 1.2×10¹¹ electrons are collected, determine quantum efficiency.
Define optical detector in optical communication.
Define quantum limit in optical communication.
What is an eye pattern in optical systems?
Section B — Descriptive Questions (10 × 3 = 30 Marks)
Attempt any three:
For a silica fiber with core index = 1.50 and cladding = 1.47, determine:
(i) Critical angle
(ii) Numerical Aperture (NA)
(iii) Acceptance angle in air.
Explain dispersion and describe intramodal dispersion.
Discuss drawbacks of LEDs compared with lasers. Describe types of LED structures with diagrams.
A Ge PIN photodiode with dimensions 100×50 µm has 55% quantum efficiency at λ = 1.3 µm. Dark current = 8 nA. Calculate Noise Equivalent Power (NEP) and Specific Detectivity (D*), assuming dark current as the dominant noise source.
Explain homodyne and heterodyne detection methods.
Section C — Long Answer / Analytical Questions (10 × 5 = 50 Marks)
Attempt one part from each:
Q3.
(a) Draw and explain the basic block diagram of an optical fiber communication system and describe its advantages.
or
(b) Define graded index fiber. For a 50 µm diameter graded-index fiber with NA = 0.2 and λ = 1 µm, estimate the total number of guided modes.
Q4.
(a) Explain the Kerr effect in optical fibers and describe fiber bending losses.
or
(b) Discuss types of nonlinear scattering losses in optical waveguides.
Q5.
(a) The radiative and non-radiative lifetimes of minority carriers in a double-heterojunction LED are 80 ns and 100 ns, respectively. Find total recombination lifetime and internally generated power for peak λ = 0.87 µm at 40 mA.
or
(b) Describe optical feedback and laser oscillation in waveguides.
Q6.
(a) Explain working principle of PIN photodiode with diagram.
or
(b) Define laser modes and describe threshold condition for oscillation.
Q7.
(a) Explain Free Space Optics (FSO) communication system with applications.
or
(b) Describe multichannel and multiplexing techniques in fiber optics.
