Impact of Laser Diode Performance on Data Rate Capability of PPM Optical Communication

A model is developed for an APD receiver which is shown to have dominant noise contributions from signal-dependent excess noise and thermal noise at high data rate (HDR). For the HDR APD receiver with pulse position modulation (PPM), the data rate (R
_D
) can be written R
_D
-αρ
_a
where α is the modulation and coding efficiency and ρ
_a
is the average photon rate. It is shown that for an average-power-limited source the data rate capability is optimized by maximizing the coding efficiency α, but for a peak-power-limited source both α and ρ
_a
influence the data rate capability. Thus the optimum choice of PPM word size (M) is dependent on the source peak-to-average power ratio. Several commercially available single transverse-mode laser diodes were found to have peak-to-average power ratios of about 8-16. Finally, a method of coding for MY was devised to allow PPM to modulate the laser diode at M-8-16 while simultaneously allowing a Reed-Solomon (N,K) error correcting code to operate with high coding gain.