In recent years, movements of construction of a wireless communication network using a flying object such as a satellite and a balloon have been active. The wireless communication network using a flying object has an advantage that, in comparison with a communication network constructed on a surface of the ground, a base station is unlikely to encounter a disaster, for example, by constructing the base station on a flying object, and thereby disaster-resistant property is improved. Further, the wireless communication network using a flying object also has an advantage that a cost for construction such as installation of wiring facilities can be reduced. It is predicted that users of such a wireless communication network using a flying object increase, and with an increase of the users, it is supposed that frequency bands of radio waves used in the wireless communication network are tight. Therefore, as wireless communication used in the wireless communication network using a flying object, attention is paid to free space optics (FSO) using an optical frequency band expected to become markedly broadband, instead of microwaves that are currently mainstream.
In order to achieve a large-capacity free space optics system, a speeding-up technique of a bit rate of a transmission signal light and a wavelength division multiplexing technique are required. Further, in a free space optics system, since a signal light is propagated for a long distance between a flying object in midair and the ground, attenuation of the signal light during the propagation is large, and therefore a high-sensitive reception device is required. For the reception device, a technique common to an optical fiber communication technique, specifically, an optical transmission/reception technique using a single mode fiber (SMF) is applied. The reason why the single mode fiber is used in a free space optics system is that an optical transmission/reception technique such as a low-noise and high-gain direct optical amplification technique, a high-sensitive digital coherent reception technique, a high bit-rate transmission/reception technique, and a dense wavelength division multiplexing (DWDM) technique can be used.
In a free space optics system using a single mode fiber, it is necessary for a signal light (laser light) propagated in a free space to enter (be coupled with) a core having a small core diameter in the single mode fiber. Therefore, in a free space optics system between a flying object in midair such as an artificial satellite, and a ground station, in order to condense sufficient optical power, a reception device needs to include a telescope having a large aperture diameter. Herein, an aperture of the telescope is equal to or more than several times a spatial coherence radius of a signal light (laser light) propagated in the atmosphere, and therefore the signal light is likely to be affected by turbulence of the atmosphere such as wind. In other words, when passing through a portion where a local fluctuation of a refractive index of the atmosphere caused by a turbulence phenomenon and a thermal phenomenon occurs, a signal light is refracted and the signal light is deflected due to the refraction. Therefore, a disturbance of a beam spot of a signal light condensed by a telescope increases. In this manner, an intensity of a signal light that enters a reception device largely varies due to a disturbance of a beam spot, and therefore, in a free space optics system, a problem that stable communication is difficult occurs. In particular, when a large attenuation (fade) of a signal light due to a large intensity variation occurs, an error or lack of reception data is caused. Therefore, in a free space optics system, a problem that an overhead of forward error correction (FEC) increases and a problem that retransmission processing becomes necessary occur. Such problems cause a decrease in an effective throughput of a free space optics system.
As described above, in a free space optics system, due to a disturbance of a wave-front to which a signal light is subjected during atmospheric propagation, a problem that communication becomes unstable may occur. A large number of techniques for solving such the problem have been proposed heretofore.
A reception device disclosed in PTL 1 (Japanese Translation of PCT International Application Publication No. 2013-535871) includes a light condensing unit, a wavelength demultiplexer, a plurality of optical detectors, and a signal processing unit. The reception device is configured to collect light on a plurality of individual fiber end faces from the wavelength demultiplexer and converge a signal light to a single output fiber to be input to the optical detector by using a fiber bundle or the like that is gradually thinner. By such a configuration, in the reception device, stable fiber coupling is achieved even against a disturbance of a beam spot. Further, PTL 1 also discloses a configuration in which a core diameter is decreased to a diameter equal to that of a single mode fiber in a tapered shape, and thereby a coupled signal light is converged on a core of the single mode fiber and connection to an optical component adapted to a next-stage single mode fiber is made possible.
A communication device that constructs a free space optics system disclosed in PTL 2 (Japanese Registered Patent Publication No. 4701454) includes a light condensing unit, a fiber bundle, a plurality of optical receivers, a plurality of optical transmitters, and a transmission/reception control unit. The communication device is configured to transmit a signal light from the same position as a light condensing position of a signal light received from an opposite device. By this configuration, regardless of deflection of a light condensing system due to an atmospheric fluctuation and a peripheral temperature change, in the opposite device side, a signal light from the communication device is condensed to a signal light emission point.
NPL 1 (“Next-generation Extremely Large Optical Infrared Telescope Planning Instructions”) discloses an adaptive optics (AO) technique that measures and corrects distortion of a wave-front of a signal light. In the technique, a wave-front of a reception light distorted due to an atmospheric fluctuation is observed by a wave-front sensor and a wave-front correction element using a micro electro mechanical system (MEMS) or the like is controlled by using the observation result.