Influence of amplitude modulation induced by LD direct modulation on FM signal transmission

Transmission characteristics of sinusoidally frequency/phase modulated signals near the zero-dispersion wavelength are investigated. It is shown that waveform distortion is suppressed by transmission at a little longer wavelength than zero-dispersion wavelength for an LD direct modulated signal. This is because the amplitude modulation component induced by LD direct modulation is canceled by waveform distortion resulting from FM-AM conversion. This cancellation occurs most effectively when the phase difference between AM and FM of LD direct modulation is pi /2, which is expected for high-speed modulation. Since waveform distortion is less in the case of a wide frequency range, waveform distortion for an NRZ signal is also considered to be suppressed by transmission at a little longer wavelength than zero-dispersion wavelength.<>