Abstract
The reduction in the beat induced crosstalk caused by four-wave mixing (FWM) as a function of the wavelength difference between the signal light and FWM light is experimentally elucidated for wavelength division multiplexing (WDM) transmission employing unequal channel spacing. Experimental results show that the wavelength difference must be 2.7 times the bit rate in order to reduce the beat induced crosstalk due to FWM even if direct intensity-modulation whose power spectrum is extended by laser chirping is employed.