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We investigate the influence of chirp on free-space laser communication systems employing optically preamplified direct-detection receivers. We analyze the changes of receiver sensitivities in the presence of chirp and optimize both the optical and electrical receiver bandwidth with regard to sensitivity. For return-to-zero (RZ) coding with 33% duty cycle the receiver sensitivity decreases due to spectral broadening of the received signal by typically 3 dB when the chirp parameter amounts to alphaequals5. For optimized bandwidths, the penalty can be kept as low as 1 dB. In contrast, in the case of non-return-to-zero coding, chirp can reduce inter-symbol interference (ISI), thus improving receiver performance due to a pulse compression effect. The gain in sensitivity can be as large as to 1.8 dB when the magnitude of the chirp is alphaequals4. A larger chirp decreases the receiver sensitivity. In the case of NRZ, the optimum receiver bandwidths are quite insensitive with regard to chirp.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only. |
