optical filter 30 e g a fabry perot filter tuned on one of the wavelengths u03bb1 and u03bb2 a photodetector d detects the signal transmitted at the output of the filter 30 it is followed by a capacitor 40 and an amplifier 41 supplying an output signal s1 or s2 as a function of whether the filter is tuned on of the two wavelengths the output signal will correspond to the switching signal or to its complement these signals are shown in part b the first s1 corresponds to the case where the filter 30 is turned on u03bb1 and the second s2 to the case where the filter is tuned on u03bb2 the applicants have produced such a system with on emission a gainasp on inp laser of the dfb type with a brs buried ridge structure such a structure can be obtained by mocvd the laser used has two identical electrodes of 200 micrometers each the electrode on the rear face of the substrate was connected to a 47 ohm resistor this laser had a continuous tuning range on 1 nm with a minimum band width of 15 mhz it was subject to a mode hop according to the value of the injected currents for example with i1 fixed at 50 ma and i 39 1 varying from 53 to 55 ma the oscillation mode hops by u03bb1 1520 8 nm to u03bb2 1521 8 nm u03b4 u03bb u03bb1 u03bb2 1 nm fig2 shows the optical spectrum of such a laser having this mode hop when a sinusoidal modulation current of 7 ma peak to peak at 700 mhz is applied to the current i 39 1 the.