Bit error probability reduction in direct detection optical receivers using RZ coding

We analyze the bit error probability reduction for direct detection ON-OFF keying optical receivers using return-to-zero (RZ) coding instead of the nonreturn-to-zero (NRZ) format. For the same average optical power, RZ is shown to outperform NRZ, even when employing the same receiver bandwidth. Results are given for receivers whose noise variance is i) dominated by a signal-independent term (e.g., simple pin diode receivers), ii) dominated by a signal-dependent term (e.g., optically preamplified receivers), and iii) made up of two equally important contributions [e,g,, avalanche photodiode (APD) receivers]. Based on semianalytic simulations including intersymbol interference, we show that the achievable RZ sensitivity gain is typically less for dominating signal-independent noise than for dominating signal-dependent noise, where it amounts to about 3 dB. We also quantitatively discuss the influence of the optical pulse shape on the achievable RZ coding gain, and show that finite extinction ratios can significantly reduce that gain, especially when the RZ signals are produced by direct-modulation methods.