Abstract: A synchronization control unit for synchronizing serial-to-parallel (S/P) converters operating in parallel, or parallel-to-serial (P/S) converters operating in parallel. More specifically, first and serial-to-parallel (S/P) converters produce respectively corresponding first and second clock signals and operate in parallel. The first and second clock signals are in synchronization, and a synchronization failure occurs when the first and second clock signals fall out of synchronization. When a synchronization failure occurs, the synchronization control unit resets one of the first and second clock signals so that synchronization between the first and second clock signals is restored within a definite time period. Moreover, the synchronization control unit can perform the same operation with parallel-to-serial (P/S) converters, instead of with S/P converters.

Abstract: An optical parallel receiving module receives optical signals transmitted in parallel in a plurality of channels by using optical fibers. The optical parallel receiving module includes photo-electric conversion elements for converting received signals which are electric signals as received signals, limiter amplifiers for comparing the received signals and a threshold level, and data decision circuits for recognizing data at a timing in every period of received data using a reference clock.

Abstract: A demodulator comprising an OR/NOR circuit, an exclusive OR/NOR circuit, and a delay circuit is disclosed as that capable of being suitably formed into a monolithic IC. Also, a polarization diversity receiver for coherent optical communication using the above demodulator as its constituent is disclosed as that having a smaller number of variable gain amplifiers and accordingly having the controlling circuit made simpler in structure.

Abstract: A receiver for coherent optical communication in a phase diversity system. An optical hybrid circuit receives a signal beam and receives a local-oscillator beam which has a predetermined phase shift. The optical hybrid circuit branches each beam into pairs, each pair having one signal beam and one local-oscillator beam. These beam pairs are output to first and second photodetectors which each apply a photoelectric conversion, thus producing first and second output signals. The output signals are output to an electrical 90.degree. hybrid circuit which shifts the first signal by 90.degree. and adds it to the second signal to produce a first output signal, and shifts the second signal by 90.degree. and adds it to the first signal to produce a second output signal. The first and second output signals are applied to first and second demodulators, respectively, which delay the signals by a delay corresponding to their frequency.

Abstract: A direct modulation PSK system and method, in a coherent optical fiber transmission system, in which an injection current supplied to a laser diode is adapted to be directly varied is disclosed. In this system or method, a modulating current pulse is superposed on a bias current for the laser diode so that a specified phase condition may be satisfied. Thereby, a differential coding circuit and an external modulator being indispensable for a DPSK system become unnecessary and, in addition, the system becomes less susceptible to the influence of wavelength dispersion than the DPSK system and able to reproduce the carrier.

Abstract: An optical signal receiver comprising an optical mixing/polarization splitting unit for producing IF optical signals having two orthogonally polarized components, an optical signal receiving unit for producing four corresponding IF electric signals, and a combining circuit for combining two IF electric signals having one polarization with the other two IF electric signals having the other polarization, suppressing fluctuation of the polarization and the intensity noise of the related local optical signal. The intensity noise suppression is also effective for IF electric signals having a higher frequency range.

Abstract: A polarized wave diversity optical receiver for coherent optical communication including: an optical local oscillating circuit (7) for oscillating local oscillating light; a mixing circuit (8) for mixing signal light and the local oscillating light to obtain two polarized components; a detecting circuit (9) for detecting the polarized component to output intermediate frequency signals (e.sub.S and e.sub.P); and a frequency control circuit (11) for controlling, in accordance with the intermediate frequency signals (e.sub.S and e.sub.P), the oscillating frequency of the optical local oscillating circuit (7). To ensure that the intermediate frequency does not disappear, the frequency control circuit (11) outputs a combined signal of a sum and a difference of the intermediate frequency signals (e.sub.S and e.sub.P).