Demonstration of a variable data-rate free-space optical communication architecture using efficient coherent techniques

Abstract

The next generation free-space optical (FSO) communications infrastructure will need to support a wide range of links from space-based terminals in low Earth orbit, geosynchronous Earth orbit, and deep space to the ground. Efficiently enabling such a diverse mission set requires an optical communications system architecture capable of providing excellent sensitivity (i.e., few photons-per-bit) while allowing reductions in data rate for increased system margin. Specifically, coherent optical transmission systems have excellent sensitivity and can trade data rate for system margin by adjusting the modulation format, the forward error correction (FEC) code rate, or by repeating blocks of channel symbols. These techniques can be implemented on a common set of hardware at a fixed system baud rate. Experimental results show that changing modulation formats between quaternary phase-shifted keying and binary phase-shifted keying enables a 3-dB scaling in data rate and a 3.5-dB scaling in system margin. Experimental results of QPSK transmission show a 5.6-dB scaling of data rate and an 8.9-dB scaling in system margin by varying the FEC code rate from rate-9/10 to rate-1/4. Experimental results also show a 45.6-dB scaling in data rate over a 41.7-dB range of input powers by block-repeating and combining a pseudorandom binary sequence up to 36,017 times.