EC-801 Optical Fiber Communication - May 2023 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aDerive an expression for numerical aperture of a step index fiber.
   How to answer

   • Define numerical aperture and acceptance angle for step-index fiber
   • Apply Snell's law at air-core and core-cladding interfaces
   • Derive condition for total internal reflection
   • Obtain expression $NA = \sqrt{n_1^2-n_2^2} = n_1\sqrt{2\Delta}$
2. 7 Marks
   bA fiber has a core radius of 25 mm, core refractive index of 1.48 and relative refractive index difference is 0.01. If the operating wavelength is 0.84 mm, find the value of normalized frequency and the number of guided modes. Determine the number of guided modes if D is reduced to 0.03.
   How to answer

   • Compute NA from $n_1$ and $\Delta$
   • Compute normalized frequency $V = (2\pi a/\lambda)NA$
   • Compute guided modes $M \approx V^2/2$ for step-index
   • Repeat calculation for reduced Delta value
3. 7 Marks
   aDerive an expression to determine the modes propagating in step index fiber.
   How to answer

   • State ray and mode guidance condition in step-index fiber
   • Apply total internal reflection and transverse resonance condition
   • Derive V-number and cutoff condition
   • Obtain expression for number of propagating modes
4. 7 Marks
   bDraw and explain ray theory transmission in an optical communication.
   How to answer

   • Sketch meridional/skew ray paths by total internal reflection
   • Define acceptance cone, NA and critical angle
   • Explain ray theory transmission mechanism
   • Distinguish meridional and skew rays
5. 7 Marks
   aDiscuss in detail the intermodal dispersion with relevant expressions and diagrams.
   How to answer

   • Define intermodal dispersion in multimode fiber
   • Derive delay difference between axial and extreme ray with diagram
   • Give expression for pulse broadening per unit length
   • Discuss effect on bandwidth
6. 7 Marks
   bDiscuss the attenuation encountered in optical fiber communication due to Scattering and Absorption.
   How to answer

   • Define fiber attenuation mechanisms
   • Explain Rayleigh/Mie scattering loss
   • Explain intrinsic and extrinsic absorption loss
   • Give total attenuation expression
7. 7 Marks
   aDerive an expression for the internal optical power level generated in LEDs.
   How to answer

   • Describe LED structure and spontaneous emission
   • Relate injected current to carrier recombination
   • Derive internal quantum efficiency and optical power
   • Obtain expression for internal optical power level
8. 7 Marks
   bExplain about Avalanche photo diode.
   How to answer

   • Define avalanche photodiode structure and principle
   • Explain impact ionization and avalanche multiplication
   • Describe gain, bias and guard-ring features
   • State advantages and applications
9. 7 Marks
   aA ruby LASER containing a crystal length 4 cm with a refractive index of 1.78. The peak emission wavelength from the device is 0.55. Determine the number of longitudinal modes and their frequency separation.
   How to answer

   • Compute cavity resonant frequencies and mode spacing $\Delta \nu = c/2nL$
   • Relate emission linewidth to number of longitudinal modes
   • Calculate frequency separation
   • Estimate number of modes
10. 7 Marks
    bExplain the step involved in splicing the fiber. Discuss the various splicing techniques employed between two fibers.
    How to answer

    • List preparation, alignment and splicing steps
    • Explain fusion splicing technique
    • Explain mechanical/V-groove splicing technique
    • Compare losses and practical considerations
11. 7 Marks
    aExplain the following in respect of digital link:

    1. i)Point to point links
    2. ii)Power penalties
    How to answer

    • Define point-to-point digital link elements
    • Explain link power budget concept
    • Define power penalties and their causes
    • Explain impact on receiver sensitivity
12. 7 Marks
    bCalculate the maximum bit rate that may be achieved on the fiber link length of 50 km without repeaters and using NRZ format. Transmitter rise time is 4 ns. Intermodal rise time is 5 ns. Intramodal rise time 1 ns. Receiver rise time is 2 ns.
    How to answer

    • Compute system rise time from component rise times
    • Relate system bandwidth to rise time
    • Apply NRZ bit-rate criterion
    • Calculate maximum bit rate for 50 km link
13. 7 Marks
    aDiscuss the concept of WDM with neat diagram.
    How to answer

    • Define WDM principle and multiplexing/demultiplexing
    • Draw WDM link block diagram with wavelengths
    • Explain components: MUX, DEMUX, EDFAs
    • State types and applications
14. 7 Marks
    bDerive the expression for EDFA conversion efficiency and amplifier gain.
    How to answer

    • Describe EDFA structure and pumping scheme
    • Write rate equations for erbium system
    • Derive conversion efficiency expression
    • Derive amplifier gain expression
15. 7 Marks
    aExplain the layers of SONET with neat sketches.
    How to answer

    • Define SONET and its layered architecture
    • Sketch section, line and path layers
    • Explain functions of photonic, section, line, path layers
    • State SONET hierarchy and overheads
16. 7 Marks
    bDiscuss the following optical coupler parameters:

    1. i)Splitting ratio
    2. ii)Excess loss
    3. iii)Insertion loss
    4. iv)Cross talk
    How to answer

    • Define optical coupler and its power relations
    • Explain splitting ratio, excess loss definitions and formulas
    • Explain insertion loss and cross talk formulas
    • Give typical values and significance