A high sensitivity receiver for multigigabit-per-second optical CPFSK transmission systems

A high sensitivity receiver using delay demodulation techniques for multigigabit-per-second continuous phase frequency shift keying (CPFSK) transmission systems was investigated experimentally and theoretically. A receiver sensitivity of 67 photons/b or -46.7 dBm at a 10^-9 BER was experimentally achieved at 2.488 Gb/s using 1.55-μm distributed feedback laser diodes (LDs) for the transmitter and local oscillator. This receiver enables transmission over 308 km through 1.3-μm zero-dispersion single-mode fiber. Experimental results are closely matched by computer simulation results which take into account the effects of eye degradation and noise filtering. These effects are mainly caused by the bandwidth limitations of the intermediate frequency bandpass filter and baseband low-pass filter. The simulation also takes into account the effects of nonflat LD frequency response, intrinsic finite frequency response of the delay demodulator, optical fiber chromatic dispersion, and laser phase noise