Highly-sensitive coherent optical detection of M-ary frequency-shift keying signal.

We demonstrate M-ary frequency-shift keying (FSK) optical modulation and digital coherent detection, aiming at applications to space communications, where high receiver sensitivity is the most crucial consideration. The proposed FSK transmitter and receiver are based on the coherent orthogonal frequency-division multiplexing (OFDM) technique and feature simple configuration and low computational complexity. By offline bit-error rate measurements using a 256-FSK signal without the forward error-correction code, we obtain the receiver sensitivity as high as 3.5 photons per bit at the bit-error rate of 10(-3). The experimental result is in good agreement with simulations.