Abstract: An optical signal transmitter (10) is constituted by a coherent light generator (12) for generating a carrier wavelength and a modulator apparatus (16, 32) coupled with the output of the light generator. The modulator apparatus has a series arrangement of an optical intensity modulator (16) and an optical phase modulator (32) coupled between the input and an output of the modulator means. The intensity modulator (16) has an input (26) for a non return to zero (NRZ) digital signal of a predetermined data rate as defined by a clock generator. The phase modulator (32) has an input coupled with the clock generator which provides pulses to the phase modulator at the data rate, or at an even multiple of the data rate, but phase shifted relative to the data signal such that one end of each digital period is red shifted and the other end is blue shifted to facilitate detection.