A method and apparatus for variable duty cycle generation of optical RZ data signals uses a single, NRZ-driven phase modulator followed by a optical delay-line interferometer. If desired, a differential encoder precodes an NRZ data signal before being applied to the data input of a phase modulator arranged to modulate the light emerging from a continuously operating laser. The output of the modulator is then applied to an optical delay interferometer, such that the optical signal is split into first and second optical signals, each of which is applied to a respective one of the two interferometer arms. The interferometer is arranged such that the signal in one of the arms is controllably delayed with respect to the other (a) by a fine delay (on the other of the optical wavelength) to produce destructive interference in the absence of phase modulation by said phase modulator, and (b) by a coarse delay (on the order of a bit period), to produce RZ pulses having the desired duty cycle. The optical signals in each of the arms are then recombined to form the interferometer output, which is the desired optical RZ data signal.