(SEM VII) THEORY EXAMINATION 2017-18 OPTICAL COMMUNICATION

SECTION A

(Short questions – clear explanation)

1. What is acceptance angle? Discuss its importance.

The acceptance angle of an optical fiber is the maximum angle at which light can enter the fiber and still be guided through total internal reflection. Light entering the fiber within this angle is successfully transmitted through the core. Acceptance angle is important because it determines how efficiently light from a source can be coupled into the fiber. A larger acceptance angle makes alignment easier and improves power coupling efficiency.

2. Compare the spectrum of a laser source and an LED source.

A laser source emits light with a very narrow spectral width, meaning it produces nearly monochromatic light. This property makes lasers suitable for high-speed and long-distance optical communication. An LED source, on the other hand, has a much broader spectral width, producing light over a wider range of wavelengths. LEDs are cheaper and simpler but suffer from higher dispersion, making them suitable mainly for short-distance communication.

SECTION B

(Medium questions – conceptual explanation)

3. What are the advantages of optical fiber communication systems?

Optical fiber communication offers several advantages over conventional copper systems. It provides very high bandwidth, allowing transmission of large amounts of data at high speed. Optical fibers have low signal attenuation, making them suitable for long-distance communication. They are immune to electromagnetic interference, lightweight, compact, and offer high security since signals are difficult to tap. These advantages make optical fiber the backbone of modern communication networks.

4. What are the different multichannel transmission techniques used in optical fiber communication?

Multichannel transmission techniques allow multiple signals to be transmitted simultaneously through a single optical fiber. One common technique is Wavelength Division Multiplexing (WDM), where different signals are transmitted using different wavelengths of light. Another technique is Time Division Multiplexing (TDM), where signals share the same wavelength but are transmitted in different time slots. These techniques increase the capacity and efficiency of optical fiber systems.

SECTION C

(Long questions – detailed explanation)

5. Explain absorption losses in optical fiber.

Absorption losses occur when part of the optical signal energy is absorbed by the fiber material and converted into heat. These losses mainly arise due to impurities such as hydroxyl ions and transition metal ions present in the glass. Intrinsic absorption is caused by the fiber material itself at specific wavelengths, while extrinsic absorption is due to impurities introduced during manufacturing. Absorption losses reduce signal strength and limit transmission distance.

6. Explain the working of a semiconductor laser and state the threshold condition for lasing action.

A semiconductor laser works by converting electrical energy into coherent light through stimulated emission. When a forward bias is applied, electrons and holes recombine in the active region and emit photons. These photons stimulate further emissions, producing a strong optical signal. The threshold condition for lasing action occurs when the optical gain equals the total losses in the cavity. Beyond this threshold current, continuous laser output is achieved.