IMPACT OF FINITE FREQUENCY DEVIATION ON THE PERFORMANCE OF DUAL-FILTER HETERODYNE FSK LIGHTWAVE SYSTEMS

A detailed theoretical analysis is given for the impact of finite freq uency deviation on the sensitivity of dual-filter heterodyne frequency -shift keying (FSK) lightwave systems. Our analysis provides closed-fo rm signal-to-noise ratio (SNR) results for estimating the bit-error ra tio (BER) performance of the system. These closed-form results provide an insight into the impact of finite frequency deviation 2DELTAf(d), laser linewidth DELTAnu, bit rate R(b) and IF filter bandwidths on the system performance. Simulation results indicate that the accuracy of the approximate theory presented in this paper is within 1 dB for line widths up to 22% when BER = 10(-9). It is shown that there is a well-d efined relationship between the choice of frequency deviation and the tolerable amount of laser phase noise. When there is no phase noise, a frequency deviation of 2DELTAf(d) = 0.72 R(b) is sufficient for 1 dB sensitivity penalty with respect to the infinite frequency deviation c ase, whereas for a linewidth of DELTA(nu) = 0.50 R(b), the required fr equency deviation increases to 2DELTAf(d) = 3.42 R(b) for the same sen sitivity penalty. The sensitivity degradation can be very severe for a fixed linewidth as the frequency deviation gets smaller: for a linewi dth of 20%, the sensitivity penalty is only 0.54 dB when the frequency deviation is infinite, whereas it is 3.48 dB when the frequency devia tion is 2DELTAf(d) = R(b).