Performance of novel low intermediate frequency coherent optical DPSK receiver using baseband detection

The authors have studied the performance of a heterodyne DPSK receiver operating with an intermediate frequency of only half the bit rate. The system degradation owing to laser phase noise and additive receiver noise has been analysed and is compared to experimentally obtained results. The theoretical shot noise limit of the system is 24.7 photoelectron/bit for a BER of 10^-9 which is only 0.9 dB from the shot noise limit of a conventional heterodyne DPSK system. The degradation of the system owing to laser phase noise is smaller than for a conventional heterodyne DPSK system because an IF linewidth of 0.8% of the bit rate introduces a 1 dB system penalty at 10^-9 BER compared to 0.5% for heterodyne DPSK systems. To achieve a system performance with less than 1 dB penalty from the optimum the IF may vary within a range of up to 7.5% of the bit rate. An experimental verification of the system has been carried out with a 1 Gbit/s system