(SEM. VII) THEORY EXAMINATION 2018-19 OPTICAL COMMUNICATION

SECTION A

(Attempt all questions in brief – 2 × 10 = 20 marks)

(a) Goos–Hänchen Effect

The Goos–Hänchen effect is a phenomenon in which a light beam undergoes a small lateral shift along the interface when it is totally internally reflected. This shift occurs due to phase change at the boundary and is significant in optical waveguides and fiber optics.

(b) Principle Used in Optical Fibers as Light Guides

Optical fibers work on the principle of total internal reflection. When light travels from a medium of higher refractive index (core) to a lower refractive index (cladding) at an angle greater than the critical angle, it is totally reflected and guided along the fiber.

(c) Step Index and Graded Index Fibers

In step index fiber, the refractive index of the core is constant and changes abruptly at the cladding. In graded index fiber, the refractive index of the core decreases gradually from the center to the outer edge, reducing signal distortion.

(d) Pulse Broadening

Pulse broadening refers to the spreading of an optical pulse as it travels through a fiber. It is caused by dispersion and limits the data transmission rate by causing overlapping of pulses.

(e) Advantages and Disadvantages of LED

LEDs are simple, low-cost, and highly reliable optical sources. However, they have low output power, wider spectral width, and limited bandwidth compared to laser diodes.

(f) Threshold Current Density of LASER

Threshold current density is the minimum current density required for a laser diode to begin stimulated emission. Below this level, the device operates as an LED.

(g) Noise in Optical Receiver

Optical receivers suffer from thermal noise, shot noise, and dark current noise. These noises degrade signal quality and increase the bit error rate.

(h) Avalanche Effect

Avalanche effect occurs when charge carriers gain enough energy to create additional carriers through impact ionization. It is used in avalanche photodiodes to achieve high gain.

(i) Error Detection and Correction Methods

Error detection and correction in optical links are achieved using parity checks, Hamming codes, and forward error correction techniques to improve transmission reliability.

(j) Power Penalties

Power penalty is the extra optical power required to maintain the same system performance due to dispersion, noise, or nonlinear effects in the fiber.

SECTION B

(Attempt any ONE – 10 marks)

(a) Optical Fiber Communication System and Skew Rays

An optical fiber communication system consists of a transmitter, optical fiber, and receiver. Skew rays follow a helical path through the fiber without crossing the fiber axis, unlike meridional rays which pass through the axis. Skew rays increase path length and cause modal dispersion compared to meridional rays.

(b) Phase Velocity and Group Velocity

Phase velocity is the speed at which the phase of a wave propagates, while group velocity is the speed at which energy or information travels. In optical fibers, group velocity is more important because it determines signal transmission speed. The relationship between them depends on refractive index and wavelength.

SECTION C

(Attempt any ONE – 10 marks)

(a) Multimode and Single-Mode Step Index Fibers

A multimode step index fiber allows multiple light paths, resulting in higher dispersion and lower bandwidth. A single-mode step index fiber allows only one propagation mode, minimizing dispersion and providing high bandwidth and long-distance communication. Single-mode fibers are preferred for modern optical networks.

(b) Modal Birefringence and Coherence Length

Modal birefringence occurs due to slight asymmetry in the fiber, causing different propagation constants for orthogonal modes. Coherence length is the distance over which light maintains a fixed phase relationship. It depends on spectral width and wavelength of the source and is important in fiber interferometry.

(c) Working Principle of LED and Quantum Efficiency

An LED works on the principle of spontaneous emission when electrons recombine with holes. Quantum efficiency is the ratio of emitted photons to injected electrons. It is maximized by improving material quality, reducing losses, and optimizing recombination efficiency.

(d) Semiconductor LASER Characteristics

A semiconductor laser operates on stimulated emission and produces highly directional, monochromatic light. Its characteristics include threshold current, output power, efficiency, and spectral width. Laser diodes are widely used in high-speed optical communication.