Abstract

This paper first proposes and verifies two simplified mode diversity receivers (MDRs) for free-space optical (FSO) communication systems under moderate-to-strong turbulence. The MDRs are designed and implemented for coherent detection of the FSO communication system based on few-mode-fiber (FMF) coupling. The first MDR replaces the coherent receiver with a Kramers–Kronig (KK) receiver, which reduces the number of photodetectors (PDs) and high-speed analog-to-digital converters by 3/4. The second MDR realizes few-mode KK detection by utilizing a few-mode local oscillator and only one PD. This method simplifies the system of a MDR to the extreme. Experimental results show that the average received power of a FSO communication system based on six-mode fiber has approximately 6 dB gain compared with the one based on single-mode fiber (SMF) under moderate-to-strong turbulence. The two proposed MDRs have approximate performance, improving by 5 dB to 6 dB compared to SMF-based heterodyne detection under moderate-to-strong turbulence. In addition, the simplified schemes reduce the volume and the cost of the optical receiver, and it has some practical significance for FSO communication.

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