EC-801 Optical Fiber Communication - May 2022 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
   aElaborate any five differences between a step index and graded index fiber.
   How to answer

   • Define step-index and graded-index fiber with refractive index profile
   • Compare on profile, ray path, modal dispersion, bandwidth, applications
   • Tabulate any five differences
   • Sketch index profiles for both
2. 7 Marks
   bAn optical fiber has a NA of 0.2 and a cladding refractive index of 1.59. Determine the acceptance angle for the fiber in water which has a refractive index of 1.33.
   How to answer

   • State formula for NA and acceptance angle $NA = n_0 \sin Q_{o(max)}$
   • Use given NA = 0.2 and outer medium $n_0 = 1.33$
   • Calculate $Q_{o(max)} = \sin^{-1}(NA/n_0)$
   • Obtain numerical value in degrees
3. 7 Marks
   aWrite an introductory note on normalized frequency. Derive its expression.
   How to answer

   • Define normalized frequency $V$ and its physical significance
   • Relate core radius, NA and wavelength
   • Derive $V = (2\pi a/\lambda) NA$
   • Explain single-mode condition $V<2.405$
4. 7 Marks
   bCalculate the numerical aperture of step index fiber having $n_1 = 1.48$ and $n_2 = 1.46$. What is the maximum entrance angle $Q_o$ (max) for this fiber if the outer medium is air with $n = 1$? Also derive the expression used.
   How to answer

   • Derive $NA = \sqrt{n_1^2-n_2^2}$ and $Q_o(max)=\sin^{-1}(NA/n_0)$ from Snell's law
   • Substitute $n_1=1.48$, $n_2=1.46$ to get NA
   • Calculate maximum entrance angle with $n_0=1$
   • Interpret result for light gathering ability
5. 7 Marks
   aExplain the scattering and bending losses that occur in optical fiber with suitable diagram.
   How to answer

   • Define scattering losses: Rayleigh, Mie, Brillouin, Raman
   • Define bending losses: micro and macro bending
   • Explain mechanism with ray / power loss concept
   • Draw suitable diagram showing scattering and bent fiber radiation
6. 7 Marks
   bClassify and explain various splicing techniques in optical fiber.
   How to answer

   • Define splicing and need for fiber joining
   • Classify into fusion, mechanical, V-groove, elastic tube
   • Explain procedure, advantages, losses for each
   • Compare loss and application
7. 7 Marks
   aHow signal degraded an optical fiber due to dispersion? Elaborate any case history.
   How to answer

   • Define dispersion: intermodal, intramodal, chromatic, PMD
   • Explain pulse broadening and ISI leading to signal degradation
   • Describe effect on bandwidth and BER
   • Elaborate example / case history
8. 7 Marks
   bWhat is the resonant frequency of LASER? Derive the expression of wavelength spacing between two modes.
   How to answer

   • Define resonant / longitudinal modes of LASER cavity
   • Apply standing wave condition $2L = m\lambda$
   • Derive resonant frequency and $\Delta \lambda$ between modes
   • State dependence on cavity length and refractive index
9. 7 Marks
   aHow is modulation of LASER diode done? Explain with a suitable diagram.
   How to answer

   • Define direct and external modulation of laser diode
   • Explain bias current, threshold and P-I characteristics
   • Describe modulation circuit operation
   • Draw block / circuit diagram of modulated laser transmitter
10. 7 Marks
    bExplain the different optical fiber connectors used.
    How to answer

    • Define need for demountable fiber connectors
    • List types: ST, SC, FC, LC, SMA etc.
    • Explain structure, coupling and loss
    • Compare applications
11. 7 Marks
    aHow is eye pattern useful in analyzing the performance of optical transmission?
    How to answer

    • Define eye pattern / eye diagram formation by superposition
    • Explain eye opening, jitter, noise, ISI interpretation
    • Relate eye closure to BER and performance
    • State use in evaluating optical transmission
12. 7 Marks
    bDerive a relation for digital receiver performance.
    How to answer

    • Define digital receiver: photodetector, amplifier, equalizer, decision circuit
    • Consider Gaussian noise, signal currents for 0/1
    • Derive BER / Q-factor / SNR relation
    • State condition for minimum error probability
13. 7 Marks
    aExplain the operating principle of WDM. Write down some applications of WDM.
    How to answer

    • Define WDM principle: multiple wavelengths on single fiber
    • Explain multiplexer, demultiplexer, EDFA operation
    • Draw WDM system block diagram
    • List applications: capacity enhancement, FTTH, backbone networks
14. 7 Marks
    bDiscuss about the MEMS technology. How does it beneficial for WDM? Explain.
    How to answer

    • Define MEMS technology and micro-opto-electro-mechanical structures
    • Explain micromirrors, switches, tunable filters
    • Relate to WDM: add-drop, cross-connect, equalization
    • State benefits: compact, low loss, reconfigurability
15. 14 Marks
    Write a short note (any four)

    1. a)EDFA
    2. b)Optical Fiber Connector
    3. c)Raman Amplifier
    4. d)Single Mode Fibers
    5. e)SONET
    6. f)Fiber Joints
    How to answer

    • Select any four topics from a) to f)
    • Give definition / principle for each
    • Explain structure / working with key points
    • Mention applications / advantages