EC-801 Optical Fibre Communication - May 2026 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
   aWhat is OTDR? Explain its working principle and applications in measurement of fiber losses and faults.
   How to answer

   • Define OTDR - Optical Time Domain Reflectometer based on Rayleigh backscattering.
   • Block diagram and working: pulsed laser source, coupler, fiber under test, detector and signal processing.
   • Trace interpretation for attenuation, splice/connector loss, break/fault location and length measurement.
   • Applications in installation, maintenance and fault monitoring.
2. 7 Marks
   bWhat are the different types of optical fibers? Explain their structure and wave guiding fundamentals with neat diagrams.
   How to answer

   • List types: step-index multimode, graded-index multimode, single-mode fibers.
   • Structure: core, cladding, coating with refractive index profiles and diagrams.
   • Wave guiding fundamentals: total internal reflection, acceptance cone, modes.
   • Comparison of propagation characteristics.
3. 7 Marks
   aWhat is a Link Power Budget? Derive the optical power budget equation by considering source power, fiber attenuation, connector losses, splice losses, and receiver sensitivity. Explain the concept of system margin.
   How to answer

   • Define link power budget and its purpose in ensuring receiver sensitivity.
   • List power terms: source power, fiber attenuation, connector/splice losses, system margin.
   • Derive $P_s - P_r = \alpha L + N_c l_c + N_s l_s + M$ in dB/dBm.
   • Explain system margin for aging, temperature and future expansion.
4. 7 Marks
   bExplain the operational principle of a Raman Amplifier. Contrast its working mechanism, bandwidth capabilities, and noise characteristics with those of an EDFA.
   How to answer

   • Principle of stimulated Raman scattering with pump-signal power transfer in fiber itself.
   • Construction: high-power pump lasers, WDM couplers, distributed/lumped configuration.
   • Contrast gain medium, pumping need, bandwidth tunability vs fixed EDFA band.
   • Compare noise figure and gain characteristics.
5. 7 Marks
   aWhat is an Avalanche Photodiode (APD)? Explain its internal gain mechanism (impact ionization). Compare the performance, advantages, and disadvantages of PIN photodiodes versus APDs.
   How to answer

   • Define APD and high reverse bias operation.
   • Explain impact ionization and avalanche multiplication gain.
   • Compare PIN vs APD: sensitivity, gain, speed, noise, bias voltage, cost.
   • List advantages/disadvantages and applications.
6. 7 Marks
   bState and explain the basic optical laws: Snell's Law, Total Internal Reflection (TIR), and Critical Angle. Derive the mathematical expression for the Numerical Aperture (NA) and Acceptance Angle of a step-index fiber.
   How to answer

   • State Snell's law $n_1 \sin\theta_1 = n_2 \sin\theta_2$, TIR condition and critical angle $\theta_c = \sin^{-1}(n_2/n_1)$.
   • Derive NA using acceptance cone geometry at fiber entrance.
   • Obtain $NA = \sqrt{n_1^2-n_2^2}$ and acceptance angle $\theta_a = \sin^{-1}(NA)$.
   • Relate to step-index fiber light gathering ability.
7. 7 Marks
   aA step index fiber has core refractive index $n_1=1.48$ and cladding index $n=1.46$. Calculate the numerical aperture, critical angle, and acceptance angle in air.
   How to answer

   • Formula $NA = \sqrt{n_1^2-n_2^2}$, $\theta_c = \sin^{-1}(n_2/n_1)$.
   • Substitute $n_1=1.48$, $n_2=1.46$ to get NA.
   • Calculate critical angle and acceptance angle $\theta_a = \sin^{-1}(NA)$ in air.
   • Present numerical values with units/degrees.
8. 7 Marks
   bDescribe the principle, construction and working of LASER diode. Explain various laser structures used in optical sources.
   How to answer

   • Principle: stimulated emission, population inversion in forward-biased p-n junction.
   • Construction: active region, optical cavity with cleaved facets, confinement layers.
   • Working: threshold current, lasing action and output characteristics.
   • Types: homojunction, heterojunction, DFB, DBR, VCSEL structures.
9. 7 Marks
   aCalculate 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 $t_{sys} = \sqrt{t_{tx}^2 + t_{inter}^2 + t_{intra}^2 + t_{rx}^2}$.
   • Use NRZ bandwidth relation $B_{max} \approx 0.35/t_{sys}$ or $0.5/t_{sys}$.
   • Substitute 4 ns, 5 ns, 1 ns, 2 ns values.
   • Obtain maximum bit rate for 50 km link without repeaters.
10. 7 Marks
    bWhat is the basic principle of photodetection? Explain construction and working of PIN photodiode.
    How to answer

    • Principle of photodetection: photon absorption and electron-hole pair generation.
    • PIN structure: p-i-n layers, wide intrinsic absorption region.
    • Working under reverse bias, photocurrent generation and responsivity.
    • Features for optical receivers.
11. 7 Marks
    aExplain the architecture and working of SONET.
    How to answer

    • Define SONET as synchronous optical network standard.
    • Architecture: layers, STS/OC hierarchy, ring and point-to-point topologies.
    • Frame structure, multiplexing and add-drop multiplexers.
    • Protection and synchronization working.
12. 7 Marks
    bWhat is quantum efficiency? Derive the expression of quantum efficiency in terms of recombination life time.
    How to answer

    • Define quantum efficiency as electron-hole pairs per incident photon.
    • Relate absorption, recombination and carrier lifetime.
    • Derive $\eta = (1-R)(1-e^{-\alpha d})/(1+...)$ or in terms of recombination lifetime.
    • Interpret dependence on material and wavelength.
13. 7 Marks
    aDescribe different types of optical fiber connectors. Explain their characteristics and insertion loss.
    How to answer

    • List connector types: ST, SC, FC, LC, SMA etc.
    • Explain structure, alignment and coupling mechanism.
    • Characteristics: insertion loss, return loss, repeatability, durability.
    • Causes of insertion loss and its measurement.
14. 7 Marks
    bA 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

    • Formula for mode separation $\Delta \nu = c/2nL$ and longitudinal modes.
    • Substitute $L=4$ cm, $n=1.78$, $\lambda=0.55$.
    • Calculate frequency separation and estimate number of modes from linewidth.
    • Present results with units.
15. 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

    • For each chosen topic give definition and principle.
    • Explain construction/working with key features.
    • Mention applications, advantages/limitations.
    • Add neat sketch where relevant.