If a digital signal is transmitted over a dispersive optical waveguide (3) with an optical transmitter (2) whose output signal is frequency-modulated (here: FSK-modulated) by the digital signal, the two wave trains which occur in FSK modulation arrive at the end of the fiber-optic link at different times (propagation time difference .DELTA..tau.). The electric output signal V of an optical-to-electrical transducer (5) at the receive side is intensity-modulated and passes through three ranges of values. To recover the digital signal therefrom, use is made of a novel circuit (6) whose output assumes a first state when a sample value is in the first range of values, a second state when a sample value is in the second range, and the same state as N bits earlier when a sample value is in the third range.