Performance of optically preamplified direct-detection receivers in the presence of laser chirp. Part 1: Ideal travelling-wave amplifier

Recently, optically preamplified receivers have attracted interest in high bit rate optical communication systems since they potentially offer improved sensitivity in comparison with conventional receivers. This paper outlines the performance degradation of these advanced receivers caused by the interaction between laser chirp and fibre chromatic dispersion. An ideal travelling-wave preamplifier structure is considered which allows us to estimate a best-case lower bound on chirp-induced penalty. By this means the intrinsic limitations are identified. We note that higher penalties will be associated with practical amplifiers due further impairments associated with residual facet reflectivity; in the interest of clarity these will be addressed in a separate paper. A SPICE 2 equivalent circuit model of a single-frequency laser transmitter is used to obtain wavelength chirp profiles. A comprehensive analysis is developed to estimate system penalties from the distorted received waveforms taking into account the various different noise components associated with the detection process. Illustrative results for 4 Gbit/s transmission are presented and compared with those for conventional pin and APD receivers. The results show that, even for ideal travelling-wave devices, these advanced receivers can be less tolerant to laser chirp than are conventional receivers, especially when signal-spontaneous beat noise dominates. We also find that inserting an optical bandpass filter between the amplifier and the detector (to reduce the spontaneous emission noise), will lead both to a higher extinction ratio penalty and a greater chirpinduced penalty.