Optical receivers convert incident optical power Pin into electric current through a photodiode. The relation Ip= RPinassumes that such a conversion is noise free. However, this is not the case even for a perfect receiver. Two fundamental noise mechanisms, shot noise and thermal noise, lead to fluctuations in the current even when the incident optical signal has a constant power. The relation Ip= RPin still holds if we interpret Ip as the average current. However, electrical noise induced by current fluctuations affects the receiver performance. The objective of this tutorial is to review the noise mechanisms and then discuss the signal-to-noise ratio (SNR) in optical receivers. The p-i-n and APD receivers are considered in separate subsections, as the SNR is also affected by the avalanche gain mechanism in APDs.
The design of an optical receiver depends on the modulation format used by the transmitter. Since most lightwave systems employ the binary intensity modulation, we focus on digital optical receivers. The figure below shows a block diagram of such a receiver. Its components can be arranged into three groups - the front end, the linear channel, and the decision circuit.
This tutorial focuses on reverse-biased p-n junctions that are commonly used for making optical receivers. Metal-semiconductor-metal (MSM) photodetectors are also discussed briefly.
The role of an optical receiver is to convert the optical signal back into electrical form and recover the data transmitted through the lightwave system. Its main component is a photodetector that converts light into electricity through the photoelectric effect.The requirements for a photodetector are similar to those of an optical source. It should have high sensitivity, fast response, low noise, low cost, and high reliability. Its size should be compatible with the fiber-core size. These requirements are best met by photodetectors made of semiconductor materials.
We have discussed the properties of optical sources. Although an optical source is a major component of optical transmitters, it is not the only component. Other components include a modulator for converting electrical data into optical form (if direct modulation is not used) and an electrical driving circuit for supplying current to the optical source. This tutorial covers the design of optical transmitters with emphasis on the packaging issues. 1. Source-Fiber Coupling The design object for any transmitter is to couple as much light as possible into the optical fiber. In practice, the coupling efficiency depends on the type of...
