The effect of nonideal optical hybrids on the performance of { 2×2 } and { 3×3 } CPFSK phase diversity homodyne receivers

The key component of CPFSK phase diversity liomodyne receivers is an optical hybrid. It provides a means of recovering the modulated optical signal without phase locking. However, all multiport receivers are sensitive to an optical hybrid imbalance. In this paper, an analysis of the effects of nonideal optical hybrids on the performance of {2 × 2} and {3×3} continuous phase frequency-shift keying (CPFSK) phase diversity homodyne receivers is done for the shot noise limited case. Asymptotic methods are employed, to give estimates of the bit-error-rate (BER) dependence for each type of the receiver. The results show that, for zero laser linewidths, the effect of a nonideal optical hybrid on the {3 × 3} CPFSK phase diversity receiver sensitivity is much less than that on the the {2 × 2} one and that, for {2 × 2} phase diversity receivers, CPFSK modulation is a little more sensitive to a nonideal optical hybrid than differential phase-shift keying (DPSK).