Impact of extinction ratio on return-to-zero coding gain in optical noise limited receivers

We experimentally confirm recent theoretical predictions, suggesting that return-to-zero (RZ) signals generated by means of a pulse carver or separate pulse source (Type I) perform better in the presence of finite data modulator extinction than RZ signals riding on a pedestal of constant optical power (Type II). As a consequence, the usually observed receiver sensitivity gain of RZ over nonreturn-to-zero is reduced (or even turned negative) for Type II RZ signals with poor data extinction. For Type I RZ signals, the RZ coding gain is largely unaffected by data modulator extinction.