Abstract: A first (1) and a second (6) narrowband laser device (1, 6) are each connected to an optical coupler means (3, 8) and also to each an input (13, 14) of a directional coupler (15). The coupler means (3, 8) have their respective outputs (5, 10) connected to a photo detector (18). The laser devices (1, 6) are each connected to their respective control circuit (17, 20). The control circuit (20) of the second laser device (6) is in turn connected to the output (19) of the photo diode (18) and to an exterior control signal (V2) with a frequency f.sub.0. Controlled by its control circuit (17) the first laser device (1) transmits a light signal (S1) with a fixed frequency f.sub.1, and the second laser device (6) transmits a light signal (S3) with a frequency f.sub.2. The photo detector (18) transmits a signal (V1) with a frequency f.sub.2 -f.sub.1 and the second control circuit (20) controls the second laser device (6) so that f.sub.2 -f.sub.1 =f.sub.0. An information carrying signal (V.sub.