Both the opposite running modes of the ring resonator cw and ccw (clockwise and counter clockwise) are superimposed with the help of a beam splitter and the deflection mirror for generating the interference of both modes. This arrangement creates two seperate beams with their individual interference patterns, out of phase by 180°, which are detected by the Si PIN photodetectors. The detectors are connected to the preamplifier (
474 308), which produces a TTL signal from both the signals by means of a comparator circuit, which are subsequently counted by the connected frequency counter (
474 311).