Optimum system parameters for multigigabit CPFSK optical heterodyne detection systems

To obtain a multigigabit continuous-phase frequency-shift-keying (CPFSK) system with a high receiver sensitivity, we theoretically and experimentally investigated the optimum modulation index parameter and IF center frequency, considering the modulation and demodulation baseband widths and the IF bandwidth. In a 6-Gb/s CPFSK experiment, we achieved a receiver sensitivity of -41.6 dBm, or 89 photons/bit. To use the 4- to 13-GHz IF band efficiently, we set the IF center frequency f _IF to 8.7 GHz, or 1.45 times the bit rate. To compensate for the insufficient 5-GHz FM-modulation bandwidth in the transmitter, we set the modulation index parameter β to 1.38, that is, a modulation index m of 0.8