High‐Power Distributed Photodetectors for RF Photonic Applications

Abstract

Photodetector linearity is of paramount importance in external intensity modulated direct detection (IMDD) analogue fibre-optic links. This realization has created a demand for high bandwidth, high saturation current photodetectors. Additionally, high current photodetectors have the potential to simplify receiver designs by eliminating the need for impedance matched electronic amplifiers. Applications that can benefit from high-performance photodetectors include antenna remoting, fibre-radio and terahertz signal generation by photomixing.The system level benefits of high current photodetectors may be seen by plotting the link RF gain (G RF ), noise figure (NF), and spurious free dynamic range (SFDR) versus photocurrent for a hypothetical external IMDD link. The curves in Figure 3.1 clearly demonstrate the benefits of high current photodetectors. As the photocurrent increases so does the link performance until laser RIN dominates the noise floor. Link performance may be further improved by utilizing balanced detection, in which case laser RIN is suppressed, the shot-noise limited regime is extended, and with increasing photocurrent, G RF , NF and SFDR improve dramatically.Space charge effects and thermal runaway are the primary factors limiting photodetector high power operation [1]. Large photocurrent density in the absorption regions can screen the applied electric field leading to a lowering of the photogenerated carrier velocity and, ultimately, the complete collapse of the applied electric field [2]. Consequently, RF photoresponse decreases and nonlinearities generated by the photodetector start to degrade the system dynamic range. Microwave Photonics: Devices and Applications Edited by Stavros Iezekiel