EC-801 Optical Fiber Communication - May 2024 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
   aDraw the characteristic of optical fiber attenuation as a function of wave length and explain the properties of the different wavelength windows.
   How to answer

   • Attenuation vs wavelength curve with loss peaks and low-loss windows
   • First, second, third window wavelengths and loss values
   • Causes: Rayleigh scattering, OH absorption, IR absorption
   • Properties/applications of each window
2. 7 Marks
   bDerive the expression of numerical aperture in terms of core and cladding refractive index.
   How to answer

   • Snell's law at core-cladding and entrance face
   • Critical angle and total internal reflection condition
   • Derivation steps to $NA = \sqrt{n_1^2-n_2^2}$
   • Acceptance angle relation
3. 14 Marks
   Calculate numerical aperture of the step index fiber having core refractive index $n_1 = 1.48$ and cladding refractive index $n_2 = 1.46$. What is the maximum entrance angle for the fiber if outer medium is fiber.
   How to answer

   • Formula $NA = \sqrt{n_1^2-n_2^2}$
   • Substitute $n_1=1.48$, $n_2=1.46$ to get NA value
   • Acceptance / maximum entrance angle $\theta_a = \sin^{-1}(NA/n_0)$
   • Numerical result and interpretation
4. 7 Marks
   aWhat are the different mechanism causing absorption in optical fiber? How can absorption be reduced.
   How to answer

   • Intrinsic absorption: UV and IR absorption
   • Extrinsic absorption: impurity and OH ions
   • Atomic defects and radiation effects
   • Reduction by purification, dehydration, material control
5. 7 Marks
   bExplain VAD method of fiber fabrication.
   How to answer

   • VAD principle: vapor phase axial deposition
   • Soot deposition and sintering process
   • Apparatus and burner arrangement
   • Advantages for preform fabrication
6. 7 Marks
   aExplain about bending losses and core cladding losses. How can they be reduced.
   How to answer

   • Macrobending vs microbending losses mechanism
   • Core-cladding interface irregularity and mode coupling
   • Dependence on bend radius, NA, wavelength
   • Reduction by proper design, coating, installation
7. 7 Marks
   bExplain the working of OTDR.
   How to answer

   • OTDR principle: Rayleigh backscatter and Fresnel reflection
   • Block diagram: laser source, coupler, detector, display
   • Trace interpretation for loss, splice, break location
   • Applications in fault monitoring
8. 7 Marks
   aWhat is quantum efficiency? Derive the expression of quantum efficiency in terms of recombination life time.
   How to answer

   • Definition of quantum efficiency as electron-photon ratio
   • Carrier recombination and lifetime concepts
   • Derivation relating efficiency to recombination lifetime
   • Effect on photodetector responsivity
9. 7 Marks
   bWith the help of suitable diagram explain the working of PIN diode.
   How to answer

   • PIN structure: p-intrinsic-n layers diagram
   • Photon absorption and e-h pair generation in intrinsic region
   • Reverse bias operation and photocurrent collection
   • Characteristics and applications
10. 14 Marks
    Explain in detail the link power budget design analysis for optical link establishment.
    How to answer

    • Transmitter and receiver power levels and sensitivities
    • Losses: fiber, splice, connector, margins
    • Power budget equation and calculation steps
    • Example link design and rise-time consideration
11. 7 Marks
    aExplain the architecture and working of SONET.
    How to answer

    • SONET layers, frame structure and rates
    • Network elements: STS, OC, add-drop multiplexer
    • Synchronous architecture and protection
    • Working with diagram
12. 7 Marks
    bExplain the working of EDFA.

    1. a)LED
    2. b)Splicing
    3. c)Passive optical networks
    How to answer

    • EDFA structure: erbium-doped fiber, pump laser, WDM coupler
    • Stimulated emission and 980/1480 nm pumping
    • Gain, amplification in C-band
    • Advantages for WDM systems
13. 14 Marks
    Write short notes on any two of the following:

    1. a)LED
    2. b)Splicing
    3. c)Passive optical networks
    How to answer

    • LED: principle, structure, characteristics
    • Splicing: fusion/mechanical types and losses
    • PON: architecture, OLT/ONU, splitter
    • Any two with diagrams and applications