Sensitivity degradation with laser wavelength chirp for direct-detection optical receivers

Long-span high-speed direct-detection optical fibre systems can be impaired by interactions between fibre chromatic dispersion and nonlinear data-dependent laser modulation phenomena such as frequency chirp and patterning. Strategies are outlined enabling system performance degradation in the presence of such impairments to be estimated, extending previous work to specific consideration of APD receivers. A SPICE2 equivalent-circuit model of a single-frequency laser transmitter is used to obtain optical power and wavelength chirp waveforms for pseudorandom data sequences. These are then used with two models for chirp-dispersion interaction to obtain distorted optical power waveforms at the receiver input. A comprehensive performance analysis for pin and APD receivers in the presence of laser chirp is presented, taking into account the various different noise components associated with the detection process. The results show that APD receivers are more sensitive to chirp than are pin receivers, and that this effect is more pronounced for high APD gains and large ionisation rate ratios. It is noted, however, that an overall improvement in receiver sensitivity may still be achieved with APD receivers