(SEM -VII) THEORY EXAMINATION 2019-20 Optical Communication

OPTICAL COMMUNICATION (REC-075)

B.Tech – Semester VII

SECTION A

(Attempt all questions)

(a) Numerical Aperture of a step index fiber

Numerical Aperture (NA) of a step index optical fiber is a measure of the light-gathering capacity of the fiber. It is defined as the sine of the maximum angle at which light can enter the fiber core and still be guided through the fiber by total internal reflection. Mathematically, it depends on the refractive indices of the core and cladding. A higher numerical aperture indicates that the fiber can accept light over a wider range of angles, which makes coupling light into the fiber easier.

(b) Intramodal dispersion

Intramodal dispersion, also known as chromatic dispersion, occurs due to the spreading of optical pulses because different wavelengths of light travel at different velocities within the same mode of propagation. Even when a single mode fiber is used, the spectral width of the source causes pulse broadening. Intramodal dispersion limits the bandwidth of optical communication systems and becomes significant at high data rates and long transmission distances.

(c) Need of cladding

Cladding is provided around the fiber core to ensure total internal reflection of light signals. It has a lower refractive index than the core, which allows light to remain confined within the core during propagation. Cladding also protects the core from surface imperfections, mechanical damage, and environmental effects. Without cladding, light losses would increase significantly, reducing transmission efficiency.

(d) Advantages and disadvantages of single mode and multimode fibers

Single mode fiber allows only one mode of light propagation, resulting in very low dispersion and high bandwidth. It is suitable for long-distance communication. However, it requires precise alignment and is more expensive. Multimode fiber supports multiple propagation paths, making it easier to couple light, but it suffers from modal dispersion, which limits bandwidth and transmission distance.

(e) Mode Field Diameter (MFD)

Mode Field Diameter is defined as the effective diameter of the optical power distribution in a single mode fiber. It represents the region where most of the optical power is concentrated. MFD is larger than the core diameter and plays a crucial role in determining splice loss, bending loss, and coupling efficiency between fibers.

(f) Skew rays and meridional rays

Meridional rays are the rays that pass through the fiber axis and are confined within a single plane. They follow a zig-zag path and undergo total internal reflection at the core-cladding interface. Skew rays, on the other hand, never cross the fiber axis and follow a helical path around it. Both types of rays contribute to signal propagation in multimode fibers.

(g) Rayleigh scattering and Mie scattering

Rayleigh scattering occurs due to microscopic variations in material density and refractive index that are much smaller than the wavelength of light. It is the dominant cause of attenuation in optical fibers. Mie scattering occurs due to larger imperfections such as core-cladding irregularities and diameter fluctuations. Both scattering mechanisms result in signal loss.

SECTION B

(Attempt any three)

(a) Fiber optic communication system

A fiber optic communication system consists of three main components: transmitter, optical fiber channel, and receiver. The transmitter converts electrical signals into optical signals using sources such as LEDs or laser diodes. These optical signals travel through the optical fiber, which acts as a transmission medium with very low attenuation and high bandwidth. At the receiver end, photodetectors such as PIN or avalanche photodiodes convert optical signals back into electrical form. Each component plays a vital role in ensuring reliable and high-speed data transmission.

(b) Point-to-point optical communication system

In a point-to-point optical communication system, a single transmitter communicates directly with a single receiver through an optical fiber link. The electrical input signal is converted into an optical signal, transmitted through the fiber, and then detected at the receiving end. This type of system is widely used in long-haul communication, data centers, and backbone networks due to its simplicity, security, and high performance.

(c) Optical sources used in fiber communication

Optical sources are devices that generate light signals for transmission. The most commonly used sources are Light Emitting Diodes (LEDs) and Laser Diodes. LEDs are inexpensive, have a wide spectral width, and are suitable for short-distance communication. Laser diodes provide high output power, narrow spectral width, and high modulation speed, making them ideal for long-distance and high-speed optical communication systems.

SECTION C

(Attempt any one)

5(a) Working principle of LED and quantum efficiency

An LED operates on the principle of spontaneous emission. When a forward bias is applied, electrons from the n-region recombine with holes in the p-region, releasing energy in the form of photons. The wavelength of emitted light depends on the bandgap energy of the semiconductor material. Quantum efficiency of an LED is defined as the ratio of the number of photons emitted to the number of electrons injected. To obtain maximum output power, parameters such as recombination efficiency, light extraction efficiency, and material quality must be optimized.

5(b) Materials used for optical sources and advantages of double heterostructure

Optical sources are fabricated using compound semiconductors such as GaAs, InP, and InGaAsP. Double heterostructure LEDs provide improved carrier confinement and reduced optical losses. This results in higher efficiency, better temperature stability, and increased output power. Surface-emitting LEDs emit light perpendicular to the junction, while edge-emitting LEDs emit light parallel to the junction and offer better coupling efficiency.