Development of Real-Time 3-Gbit/s BPSK Optical Coherent Receiver Using Field Programmable Gate Array for Free-Space Laser Communications

Abstract

Coherent optical receivers have the following features: shot-noise-limited receiver sensitivity with sufficient local oscillator (LO) power; high frequency resolution at the intermediate frequency or baseband stage in highly dense frequency division multiplexed channels; improved receiver sensitivity compared with that of intensity modulation/direct detection (IMDD) systems; and active realization of multilevel signal transmission using not only intensity but also phase, frequency, and polarization states. With the advent of coherent binary phase shift keying (BPSK) receivers in orbit, it is absolutely imperative to establish interoperability between the different optical communications systems. A bread board model (BBM) of an optical receiver that can demodulate both IMDD and coherent optical signals for free-space laser communications has been developed. In this receiver, the carrier phase is recovered after homodyne detection by means of digital signal processing (DSP), which is implemented using a field programmable gate array (FPGA) for both estimating the carrier phase and recovering the payload data. The first BBM of the optical coherent receiver has the ability to demodulate 3-Gbit/s coherent BPSK and 6-Gbit/s quadrature phase-shift keying (QPSK) signals. In this paper, preliminary experimental results of the real-time coherent BPSK communication performance at the OC-48c data rate in the laboratory are presented.