Abstract: In a light transmission device operable in response to a sequence of input digital signals to produce an output light beam, a modulated light beam is produced from a laser device as a result of frequency shift keying carried out under a designed modulation index and is transmitted as the output light beam on one hand and is sent to a Fabry-Perot interferometer on the other hand to produce an optical output beam which has an output level deviated in dependence upon a variation of a modulation index relative to the designed modulation index. A feedback signal is fed back to a drive circuit which is supplied with the input signals and which is operable to control the laser device. The feedback signal serves to adjust the modulation index of the laser device to the designed modulation index. A controller may be included to control a central frequency of the output light beam by producing a bias signal which is added to the feedback signal.

Abstract: Each subscriber is provided with an optical heterodyne detection circuit. An oscillation light of an optical local oscillator is divided to a local oscillation light for the heterodyne detection and a transmitting light for data communication. A channel for those lights is selected from vacant communication channels. The transmitting light may be obtained from a transmission light oscillator which is controlled by a control unit common to the optical local oscillator.

Abstract: An optical wavelength-division switching system comprises means for reproducing an electric signal for each channel from a wavelength division multiplexed optical signal, and means for converting the electric signal to a modulated optical signal. The means for reproducing the electric signal can be composed of an optical heterodyne or homodyne detection system, while the means for converting the electric signal can be composed of a distributed feedback laser diode having an optical frequency modulating characteristic so that the number of channels is much increased with the decrease of crosstalk.

Abstract: A laser (21) is frequency modulated by an electrical modulating signal of a bit rate into a modulated signal of a first and a second modulated frequency. A combination of an optical mixer (27) and an optical detector (31) carries out optical heterodyne or homodyne detection on the modulated optical signal to produce a detected signal having a first and a second frequency component. A component separator (32) is for selecting only one of the components from the detected signal. A detector (33) detects the selected component to produce a demodulated electrical signal representative of the modulating signal. The laser is preferably a semiconductor laser. More preferably, the semiconductor laser is frequency modulated after the modulating signal is converted to a current of an mBnB code sequence.

Abstract: In an FSK laser transmitting apparatus, a light source can change the output light frequency. A driver performs FSK of the light source with a plurality of values. A beam splitter splits the output light from the light source into two split beams. A delay circuit delays one of the two split beams with respect to the other. A beam coupler combines delayed and nondelayed beams as the two split beams from the beam splitter. A controller detects the beat frequency of combined light from the beam coupler and supplies a control signal to the driver so as to set the beat frequency to be a predetermined value.