A method and apparatus is provided that yields improved performance of
both single channel and WDM long-distance optical transmission systems by
synchronously modulating of the transmitted signal's amplitude. An
amplitude modulator receives an optical signal onto which data has been
modulated at a predetermined frequency. The modulator re-modulates the
amplitude of the optical signal in a continuous fashion with a waveform
that is periodic, whose fundamental frequency is equal to the same
predetermined frequency at which the data is modulated onto the optical
signal. The resulting signal (which is neither a pure NRZ or RZ signal)
is more tolerant to the distortions usually found in lightwave
transmission systems, thus giving superior transmission performance.