Penalty due to the residual laser radiation in ASK optical receivers

Intensity modulation direct detection systems are affected by the presence of residual laser radiation inherent in high rate systems. The effect of this is described by the extinction ratio which is used to calculate the penalty in the required input power in comparison to the same power when the extinction ratio is zero. For some special cases, analytical expressions for the optimum avalanche gain in the presence finite extinction are derived and representative numerical results are given.