Design of a Coherent Receiver Based on InAs Electron Avalanche Photodiode for Free-Space Optical Communications

In this paper, a coherent receiver based on InAs electron avalanche photodiode (e-APD) is designed for free-space optical (FSO) communications. According to the numerical simulations, it is found that the coherent receiver based on InAs e-APD can operate nearly in its shot-noise limit condition with a lower local oscillator (LO) power than traditional coherent receivers and get a better sensitivity improvement due to the gain provided by the coherent gain from the LO power and the internal gain of the e-APD. Hence, the multiple diversity receiver systems can be easier implemented and the number of the independent beams can be increased. In addition, this system is not sensitive to the change of the thermal noise of the coherent receiver and the dark current of the InAs e-APD that makes it more suitable for space environment. Furthermore, as the requirement of the internal gain for the InAs e-APD in this type of coherent receiver is not very high, the depletion region of InAs e-APD can be reduced. Hence, the coherent receiver based on InAs e-APD may become a potential choice in much higher speed FSO communication applications.