4-ary FSK coherent optical communication system

Experimental verification of the sensitivity advantage of M-ary orthogonal signalling over binary signalling is reported. A 110 Mbits 4-ary FSK system using conventional Fabry–Perot lasers with a 7% IF linewidth to data-rate ratio has achieved a sensitivity of 37 detected photons per bit at a 10−9 bit error rate. This exceeds the performance possible using ideal binary orthogonal FSK with zero linewidth lasers.