2024
Q1. State the requirements that are to be met in selecting materials for optical fibers.
Ans: Materials should have low loss, high transparency, good mechanical strength, thermal
stability, and compatible refractive indices for core and cladding to guide light efficiently.
Q2. What do you mean by pulse broadening in optical fiber?
Ans: Pulse broadening means the spreading of light pulses over time due to dispersion in the
fiber, which causes signal overlap and reduces data transmission quality.
Q3. Give the names of materials used for fabrication of optical fibers.
Ans: Common materials are silica (SiO₂), germania (GeO₂), doped silica, fluoride glass, and
plastic materials depending on the application and wavelength used.
Q4. Define the relative refractive index difference for an optical fiber and show how it may
be related to the numerical aperture.
Ans: It is the percentage difference between core and cladding refractive index. It affects
numerical aperture:
Δ = (n₁² - n₂²)/2n₁² ≈ NA²/(2n₁²)
Q5. What is quantum efficiency?
Ans: Quantum efficiency is the ratio of the number of electrons generated to the number of
incident photons. It shows how efficiently a photodetector converts light into current.
Q6. What is source to fiber power launching?
Ans: It refers to how optical power from a light source like a laser or LED is efficiently coupled
or launched into the fiber for transmission.
Q7. What is non-linear scattering? Write the names of two non-linear scatterings.
Ans: Non-linear scattering occurs at high optical power and causes signal loss. Two types are:
Raman scattering and Brillouin scattering.
Q8. What do you mean by modulation of LED?
Ans: Modulation of LED means changing its light output (intensity) according to the input
signal, used to transmit data through the fiber.
Q9. Define spectral width, group velocity, and group delay.
Ans:
Spectral width: Range of wavelengths emitted.
Group velocity: Speed of pulse envelope.
Group delay: Time taken by a pulse to travel through fiber.
Q10. What do you mean by modal noise in optical fiber and how it may be avoided?
Ans: Modal noise is caused by interference between modes in multimode fiber. It can be reduced
by using single-mode fiber or proper connectors.
2023
Q1. Define Step index and Graded index fiber.
Ans:
Step index fiber has a sudden change in refractive index between core and cladding.
Graded index fiber has a gradual change, reducing modal dispersion.
Q2. Name the important reasons of signal degradation.
Ans:
Main reasons are attenuation, dispersion (chromatic and modal), scattering, bending loss, and
connector misalignment, which reduce signal strength and quality.
Q3. Define the quantum efficiency and responsivity of a p-n diode?
Ans:
Quantum efficiency is the ratio of output electrons to input photons.
Responsivity is the output current per unit optical power (A/W).
Q4. What is bending loss?
Ans:
Bending loss occurs when light escapes the fiber due to sharp curves or bends, causing signal
power reduction and transmission loss.
Q5. Name three working principles of laser.
Ans:
1. Stimulated emission
2. Population inversion
3. Optical feedback using resonator cavity
Q6. A digital fiber link operating at 850 nm requires a maximum BER of 10⁻⁹. Find the
quantum limit in terms of the quantum efficiency.
Ans:
Quantum limit (min. photons/bit) ≈ 20/η,
where η = quantum efficiency. It's the minimum photons needed to maintain the required BER.
Q7. For a step index fiber, core refractive index (n₁) = 1.45 and Δ = 0.01. Calculate the
Numerical aperture and clad refractive index (n₂).
Ans:
n₂ = n₁√(1 - 2Δ) ≈ 1.435
NA = √(n₁² - n₂²) ≈ 0.269
Q8. State Snell’s law condition for TIR Phenomenon.
Ans:
Snell’s Law: n₁sinθ₁ = n₂sinθ₂.
For Total Internal Reflection (TIR), light must move from higher to lower index and incident
angle > critical angle.
Q9. What is the purpose of using optical switches in communication link?
Ans:
Optical switches are used for routing optical signals, managing traffic, reducing loss, and
enabling reconfiguration of optical networks without electrical conversion.
Q10. Write down the name of Non-linear effect of Optical fiber link.
Ans:
Common non-linear effects are:
1. Self-phase modulation
2. Four-wave mixing
3. Stimulated Raman scattering
2022
1. What is Dispersion?
Dispersion is the broadening of light pulses as they travel through an optical fiber due to
different speeds of light components, leading to signal distortion.
2. What is splicing in fiber?
Splicing is the process of joining two optical fibers end-to-end to form a continuous path, using
fusion or mechanical methods for permanent or temporary connections.
3. Explain the term Responsivity of photodiode.
Responsivity is the ratio of photocurrent generated to the incident optical power on a photodiode,
measured in A/W, indicating the efficiency of optical to electrical conversion.
4. Explain Quantum limit?
Quantum limit refers to the minimum optical power needed to detect a signal reliably, limited by
quantum noise due to the discrete nature of photons.
5. What is the object of optical Amplifiers?
The objective of optical amplifiers is to boost the power of optical signals directly, without
converting them to electrical signals, to extend transmission distances.
6. Give any two applications of LASER in optical communication.
(i) Used as a light source in fiber-optic communication.
(ii) Employed in high-speed data transmission over long distances.
7. Why a Graded index fiber with a parabolic index profile is preferred?
It reduces modal dispersion by making light rays travel at equal speeds through different paths,
improving bandwidth and signal quality over multimode fibers.
8. Explain power penalties on behalf of optical sources?
Power penalties are extra optical power required to overcome losses due to source imperfections
like chirping, mode noise, or alignment errors in the system.
9. What do you mean by signal degradation on optical fiber?
Signal degradation means loss of signal quality due to attenuation, dispersion, or nonlinear
effects in optical fibers, reducing transmission performance and clarity.
10. Give one merit and demerit of LED structure.
Merit: LEDs are low-cost and reliable light sources.
Demerit: They have low coupling efficiency and bandwidth compared to lasers.
