Performance of optically preamplified direct detection receivers in presence of laser chirp. II. Near travelling-wave and Fabry-Perot amplifier cases

For pt.I see ibid., vol.136, no.5, p.249-55 (1989). The paper is concerned with performance degradation caused by laser chirp for optical receivers incorporating semiconductor laser preamplifiers with nonzero facet reflectivity. Transmission at 4 Gbit/s through a single-mode fibre (SMF) is specifically simulated, taking into account the interaction between laser wavelength chirp and both fibre chromatic dispersion and the optical amplifier transmission characteristics. The results indicate that the penalty, arising from the transiently varying mismatch between the instantaneous laser wavelength and the peak gain-wavelength of the amplifier, has strong dependence on both amplifier gain G₀ and facet reflectivity R, and may lead to severe performance degradation even in the absence of fibre chromatic dispersion. Biasing the laser source well above threshold-to reduce chirp-does not guarantee a reduction in the dynamic mismatch penalty, and it leads to a higher extinction ratio penalty which is again found to be an increasing function of both G₀ and R . To reduce the mismatch penalty the amplifier peak gain-wavelength should be tuned to be slightly shorter than the laser CW wavelength. Using this approach a 0.7 Å detuning can offer more than a 20 dB reduction in the mismatch penalty for Fabry-Perot amplifiers (FPAs) operating at high optical gain