What is an optical detector? State its three essential characteristics. Name the factor which decides how good a detector is.

Optical detector is a device at the receiver end, which converts light into electrical signal so that the transmitted information may be decoded.

Characteristics of optical detector:
- Size is compatible with that of the optical fibre.

- High sensitivity at the desired optical wave-length. 

- High response time for fast-speed data transmission/reception. 

The efficiency of generating electron-hole pairs in a photodiode decides the quality of detector.