EC-801 Optical Fiber Communication - Jun 2025 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 the expression of Numerical Aperture in optical fiber.
   How to answer

   • Define acceptance angle and total internal reflection at core-cladding interface
   • Apply Snell's law at air-core and core-cladding interfaces
   • Derive $NA = \sin\theta_a = \sqrt{n_1^2-n_2^2}$
   • Give approximate form $NA \approx n_1\sqrt{2\Delta}$
2. 7 Marks
   bA silica fiber having core refractive index-1.5 and cladding refractive index is 1.47. Determine the critical angle at core cladding interface, numerical aperture and acceptance angle in air for the fiber.
   How to answer

   • Formula for critical angle $\phi_c = \sin^{-1}(n_2/n_1)$
   • Calculate $NA = \sqrt{n_1^2-n_2^2}$
   • Calculate acceptance angle $\theta_a = \sin^{-1}(NA)$
   • Substitute $n_1=1.5, n_2=1.47$ and give values in degrees
3. 7 Marks
   aExplain about the modes in planar guide.
   How to answer

   • Structure of planar dielectric waveguide
   • TE, TM and TEM mode concept and transverse resonance condition
   • Guided, leaky and radiation modes
   • Mode number and cutoff condition
4. 7 Marks
   bExplain about the ray transmission in step index fiber.
   How to answer

   • Step index profile and total internal reflection
   • Meridional and skew rays
   • Acceptance cone and numerical aperture
   • Ray path and condition for guidance
5. 7 Marks
   aDiscuss absorption losses in optical fibers. Also compare intrinsic and extrinsic absorption mechanism.
   How to answer

   • Definition of absorption loss in dB/km
   • Intrinsic absorption: UV and IR absorption
   • Extrinsic absorption: impurity and OH ion
   • Comparison table of causes and minimization
6. 7 Marks
   bA multimode step index fiber with a core refractive index of 1.5, a relative index difference of 3% and an operating wavelength of 0.82 $\mu m$. Estimate the critical radius of curvature at which large bending losses occur in the fiber.
   How to answer

   • Formula for critical bend radius $R_c$ in terms of $n_1, \Delta, \lambda$
   • Calculate $NA$ from $n_1$ and $\Delta$
   • Substitute $n_1=1.5, \Delta=0.03, \lambda=0.82\,\mu m$
   • Obtain $R_c$ value where macrobending loss rises sharply
7. 7 Marks
   aExplain modified chemical vapour deposition technique of fiber fabrication.
   How to answer

   • Principle of MCVD: chemical vapour deposition inside silica tube
   • Steps: deposition, collapse, drawing
   • Reactions and temperature zones
   • Advantages for low-loss fiber
8. 7 Marks
   bExplain the measurement done in fiber using OTDR.
   How to answer

   • Principle of OTDR: Rayleigh backscatter and Fresnel reflection
   • Block diagram and trace interpretation
   • Measurements: attenuation, splice loss, fault location
   • Advantages and limitations
9. 7 Marks
   aExplain the basic principle working of lasers.
   How to answer

   • Population inversion and stimulated emission
   • Components: active medium, pump, optical resonator
   • Threshold condition and coherence
   • Types relevant to optical communication
10. 7 Marks
    bExplain the principle working of PIN diode.
    How to answer

    • PIN structure: p-intrinsic-n layers
    • Photodetection principle under reverse bias
    • Drift current and responsivity
    • Advantages over PN diode
11. 14 Marks
    Design an optical link using link power budget.
    How to answer

    • Power budget equation: $P_s - P_r$ vs losses and margins
    • Steps: source power, detector sensitivity, allowances
    • Calculation of maximum link length
    • Example design flow
12. 7 Marks
    aExplain the working of SONET and SDH.
    How to answer

    • SONET/SDH hierarchy and frame structure
    • Multiplexing and synchronous operation
    • Network elements and topologies
    • Bit rates STM-n / OC-n
13. 7 Marks
    bExplain the principle working of EDFA.
    How to answer

    • Erbium-doped fiber and pump wavelengths 980/1480 nm
    • Three-level laser system and stimulated emission
    • Gain, ASE and wavelength window 1550 nm
    • Structure with WDM coupler and isolators
14. 14 Marks
    Write short notes on any two of the following.

    1. a)LED
    2. b)Raman Amplifier
    3. c)Splicing techniques
    How to answer

    • Any two topics with definition and working
    • Key features, advantages and applications
    • Relevant diagram or structure
    • Use in optical fiber communication