(SEM V) THEORY EXAMINATION 2020-21 OPTICAL COMMUNICATION

SECTION A (Any 2 Questions Explained Briefly)
 

What is acceptance angle and why is it important?
Acceptance angle is the maximum angle at which light can enter an optical fiber and still be guided through it by total internal reflection. Knowing the acceptance angle is important because it determines how efficiently light from a source can be coupled into the fiber and directly affects signal transmission efficiency.
 

What is Rayleigh scattering?
Rayleigh scattering is a loss mechanism in optical fibers caused by microscopic variations in material density and refractive index. These small inhomogeneities scatter light in different directions, leading to attenuation of the optical signal, especially at shorter wavelengths.
 

SECTION B (Any 2 Questions Explained)
 

Draw and explain the block diagram of a fiber optic communication system.
A fiber optic communication system consists of a transmitter, optical fiber channel, and receiver. The transmitter converts electrical signals into optical signals using sources like LEDs or LASERs. The optical fiber carries the light signal over long distances with low loss. The receiver uses photodetectors such as PIN or APD to convert optical signals back into electrical signals for further processing.
 

Derive an expression for numerical aperture of a step index fiber.
Numerical aperture is a measure of the light-gathering capacity of an optical fiber. For a step index fiber, it is derived using Snell’s law and total internal reflection conditions. It is given by the square root of the difference between the squares of the refractive indices of the core and cladding. Numerical aperture cannot be made very large because a high value increases dispersion and reduces bandwidth.
 

SECTION C (Any 2 Questions Explained)
 

Differentiate between step index and graded index fiber and explain ray propagation.
In step index fiber, the refractive index of the core is constant and changes abruptly at the cladding, causing rays to follow zig-zag paths. In graded index fiber, the refractive index gradually decreases from the center to the cladding, causing rays to follow curved paths. This gradual change reduces intermodal dispersion and improves signal quality.
 

Explain material and waveguide dispersion.
Material dispersion occurs because different wavelengths of light travel at different speeds in the fiber material. Waveguide dispersion occurs due to the dependence of propagation characteristics on fiber geometry and refractive index profile. Both types of dispersion cause pulse broadening and limit the bandwidth of optical communication systems.
 

Most Questions in This PDF Are Related To
 

Most questions in the Optical Communication (KEC-058) paper are related to optical fiber fundamentals, numerical aperture, acceptance angle, losses and dispersion in fibers, ray theory, step and graded index fibers, optical sources (LED and LASER), photodetectors (PIN and APD), optical receivers, noise analysis, and coherent optical communication systems. The paper mainly focuses on signal transmission, losses, dispersion mechanisms, and receiver performance.