Abstract: A high level from a reference high level register and a low level from a reference low level register are selectively applied by a change-over switch to a D-A converter in time intervals A+ and B+ of four intervals A+, B+, A- and B- and in the intervals A- and B-, respectively, whereby a rectangular output signal is produced an used to control the current to be injected into a semiconductor laser to control the optical frequency of its output light. The light is introduced, as clockwise and counterclockwise light beams, into a ring-shaped optical path, wherein both light beams propagate therethrough a plurality of times and interfere with each other a plurality of times. Such light beams are taken out of the ring-shaped optical path and their intensities are detected by first and second photodetectors.