This application is a National Stage Entry of PCT/JP2019/045840 filed on Nov. 22, 2019, which claims priority from Japanese Patent Application 2018-223133 filed on Nov. 29, 2018, the contents of all of which are incorporated herein by reference, in their entirety.

TECHNICAL FIELD

The present invention relates to an optical receiver and an optical space communication system, and particularly relates to an optical receiver and an optical space communication system that receive an optical phase modulation signal and are equipped with a satellite.

BACKGROUND ART

In recent years, needs for an increase in capacity of satellite communication have been increasing due to an increase in observation data of an earth observation satellite, and a broadband satellite communication service being found in a high throughput satellite and the like. As a technique for responding to the needs, an expectation on an optical space communication technique (inter-satellite optical communication technique, ground-satellite optical communication technique) for satellite mounting has been increasing.

In order to achieve the optical space communication for satellite mounting, an acquisition and tracking device for a propagating light beam, an optical amplifier, and an optical transceiver are required. Particularly, in order to establish communication at a distance of tens of thousands of kilometers, a high-sensitivity optical transceiver needs to be used. As seen in NPLs 1 and 2, a differential binary phase modulation/demodulation (DPSK: differential phase shift keying) method has been used as a modulation/demodulation method of such an optical transceiver. By using the differential binary phase modulation/demodulation method, higher sensitivity can be achieved than an intensity modulation/direct detection (IM/DD) method that has been used for satellite mounting.

Further, PTL 1 discloses an optical reception circuit of a differential phase modulation/demodulation method. The optical reception circuit in PTL 1 includes a one-bit delay interferometer, a differential amplifier in which two photodiodes are connected to two respective inputs, and two emitter followers or source follower circuits connected to two respective outputs of the differential amplifier. Further, in the optical reception circuit disclosed in PTL 1, one pair of optical signals having a differential phase being modulated is input to the one-bit delay interferometer, and the one-bit delay interferometer includes a one-bit delay element in one of a pair of waveguides, and outputs, to each of two photodiodes, one pair of two optical signals according to a phase difference between bits adjacent to each other. A difference between two input signals is output and demodulated in the differential amplifier, and a demodulation signal of a normal phase and a reversed phase being demodulated is output.

CITATION LIST

Patent Literature

[PTL 1] Japanese Patent No. 5339088

Non Patent Literature

[NPL 1] D. O. Caplan, et al, “Multi-rate DPSK Optical Transceivers for Free-Space Applications”, SPIE Photonics West LASE, 89710K, 2014[NPL 2] H. G. Rao, et al, “Electronics Design of a Multi-Rate DPSK Modem for Free-Space Optical Communications”, SPIE Photonics West LASE, 89710Y, 2014

SUMMARY OF INVENTION

Technical Problem

However, in the configuration of the one-bit delay interferometer described above, when an environmental temperature fluctuates, a delay amount between one pair of optical signals output from the one-bit delay interferometer fluctuates, and thus there is a risk that a peak of a differential signal thereof may decrease. Temperature control is performed on the one-bit delay interferometer in a space environment, but a heat transmission characteristic from a heater to the one-bit delay interferometer in a vacuum environment becomes poorer than that in an environment in an atmosphere, and thus it takes a long time until a temperature converges. Thus, there is a risk that a peak of a differential signal of one pair of optical signals output from the one-bit delay interferometer may decrease in a space environment, and demodulation may become unstable. In this way, it is difficult to apply a differential phase modulation/demodulation method to a space environment for satellite mounting in the space environment.

An object of the present invention is to provide, in an optical receiver of a differential phase modulation/demodulation method and an optical space communication system, a technique being able to suppress a peak fluctuation of a differential signal of adjacent bits, and apply the differential phase modulation/demodulation method to a space environment for satellite mounting.

Solution to Problem

An optical receiver according to the present invention is an optical receiver that receives an optical phase modulation signal, and includes: a differential light detection unit including an optical delay interferometer that outputs a first optical signal and a second optical signal interfering with each other by controlling, to one bit, a delay between two optical signals acquired by splitting the optical phase modulation signal, a first optical detector that performs differential light detection on the first optical signal and the second optical signal, and outputs a first main signal, and a peak signal detection unit that outputs a peak signal indicating intensity of differential light of the first optical signal and the second optical signal; and a digital signal processing unit including a level fluctuation frequency suppression unit that suppresses a level fluctuation frequency component of the peak signal, an optical delay interference control unit that generates, based on a peak signal having the level fluctuation frequency component being suppressed, an optical delay interference control signal on which overdrive amplification is performed, and applies the generated optical delay interference control signal to the optical delay interferometer, and a data recovery unit that recovers output data, based on the first main signal from the optical detector.

An optical space communication system according to the present invention includes: an optical reception device including the optical receiver described above; and an optical transmission device that transmits the optical phase modulation signal, wherein the optical transmission device includes an optical transmitter that outputs the optical phase modulation signal, and an acquisition and tracking device that performs acquisition and tracking of the optical phase modulation signal, and the optical transmitter includes a digital signal processing unit including a transmission control unit that outputs a laser drive current value causing an optical wavelength that cancels out a Doppler frequency generated on a satellite orbit, a signal generation unit that generates an input signal, based on input data, and a clock generation unit that generates a clock signal, and an optical transmission unit including a laser drive current control unit that performs control of a laser drive current, based on the laser drive current value, a transmission laser unit that outputs laser light by the laser drive current, and an optical modulation unit that performs modulation on the laser light, based on the input signal and the clock signal, and outputs the optical phase modulation signal.

Advantageous Effects of Invention

The present invention is able to, in an optical receiver of a differential phase modulation/demodulation method and an optical space communication system, suppress a peak fluctuation of a differential signal of adjacent bits, and apply the differential phase modulation/demodulation method to a space environment for satellite mounting.

