XPM EFFECT IN COHERENT FDM SYSTEMS USING FSK AND HETERODYNE-DETECTIONSCHEME

Cross-phase modulation (XPM) induced by residual intensity modulation in coherent optical frequency-shift-keying (FSK) frequency division mu ltiplexing (FDM) transmission systems that use dispersion-shifted fibe rs is evaluated theoretically and experimentally in terms of spectral profile deformation. The bit-error rate is measured in a 2.5-Gbit/s 4- channel 40-km dispersion-shifted fiber transmission experiment. and we confirm experimentally and theoretically that the power penalty in th e presence of residual intensity modulation of over 4 mWp-p exceeds 1 dB. Experimental results show that the penalty due to XPM is large eve n when the power of the newly generated lights caused by four-wave mix ing is 20-dB less than that of signals. This confirms that residual in tensity modulation must be reduced in continuous-phase (CP)-FSK-FDM sy stems even though they are designed to avoid generating four-wave mixi ng.