Analysis and Design of Transimpedance Amplifiers for Optical Receivers

Chapter 3Photodetectors

The first element of an optical receiver is the photodetector. The characteristics of this device have a significant impact on the receiver's performance. To achieve a good receiver sensitivity, the photodetector must have a large response to the received optical signal, have a bandwidth that is sufficient for the incoming signal, and generate as little noise as possible.

We start with the three most common photodetectors: the p–i–n photodetector, the avalanche photodetector (APD), and the optically preamplified p–i–n detector, discussing their responsivity, bandwidth, and noise characteristics. Then, we turn our attention to photodetectors that are suitable for integration in a circuit technology, in particular, detectors compatible with CMOS technology (silicon-photonics detectors). Finally, we explore detectors for phase-modulated optical signals, such as QPSK and DQPSK, including the coherent detector with phase and polarization diversity.

3.1 p–i–n Photodetector

The p–i–n photodetector (or p–i–n photodiode) shown schematically in Fig. 3.1(a) and (b) is one of the simplest detectors. It consists of a p–n junction with a layer of intrinsic (undoped or lightly doped) semiconductor material sandwiched in between the p- and the n-doped material. The junction is reverse biased with to create a strong electric field in the intrinsic layer. The light enters ...