Optically preamplified direct detection in the presence of modulator chirp

We analyze the impact of modulation induced chirp on optically preamplified direct-detection communication Systems transmitting over dispersion-free channels, as found in optical free-space communication links. We show that commonly used quantitative measures of chirp (the small-signa chirp parameter and the effective chirp parameter) can fail to describe changing chirp characteristics under large-signal modulation in a satisfactory manner. We present an extended chirp model that lends itself both to measurement and simulation in a straightforward way. Regarding receiver performance, we show that return-to-zero (RZ) coded data signals can experience receiver sensitivity degradations of several dB due to spectral broadening of the chirped signal. By optimizing optical and electrical receiver bandwidths the penalty can be kept considerably lower than for receivers optimized for chirp-free signals. For non return-to-zero (NRZ) coding, chirp can even improve receiver sensitivity by means of a pulse compression effect in the presence of narrow-band optical filters, leading to reduced inter-symbol interference (ISI). In our simulations and for the measurements, we exploit the variable chirp characteristics of a dual-drive electro-optic Mach-Zehnder modulator.