COMMUNICATION METHOD, OPTICAL RECEIVING APPARATUS, OPTICAL TRANSMITTING APPARATUS, AND COMMUNICATION SYSTEM

Abstract

This communication method is for a communication system comprising an optical transmission device and an optical reception device, wherein: a plurality of sensors detect a distortion of a wave front of a reference light signal, which is used for wave front observation and which arrives at a first time and a second time preceding a compensation start time at which wave front distortion compensation is performed; on the basis of the wave front distortion detected by each of the plurality of sensors, a spatial phase distribution for each of a plurality of atmospheric layers between the optical transmission device and the optical reception device is estimated at the first time and the second time; the spatial phase distribution for each of the plurality of atmospheric layers at the compensation start time is predicted by using the estimation results, estimated at the first time and the second time, for the spatial phase distribution for each of the plurality of atmospheric layers; operations by a wave front control device that compensates for distortion of a wave front of a light signal are controlled on the basis of the prediction results; and the distortion of the wave front of the light signal transmitted from the optical transmission device is received after being subjected to compensation by the wave front control device.　

Description

[Technical field]

[0001] The present invention relates to a communication method, an optical receiving device, an optical transmitting device, and a communication system. [Background technology]

[0002] In some cases, optical wireless communication is performed between a transmitter and a receiver using an optical signal propagating through the atmosphere. In this case, the wavefront of the optical signal is distorted due to the influence of the atmosphere. This causes spatial intensity patterns (speckles) in the optical signal arriving at the receiver. The intensity patterns vary over time according to atmospheric fluctuations. These intensity patterns are a major obstacle to realizing stable optical wireless communication.

[0003] Adaptive optics, which compensates for distortion of the wavefront of an optical signal by closed-loop control with the aim of suppressing the effects of atmospheric fluctuations, is being studied (see Non-Patent Document 1). In this adaptive optics, a receiver observes the effects of atmospheric fluctuations on an optical signal transmitted from a transmitter (opposing station). That is, the receiver observes the distortion of the wavefront (spatial phase distribution) of the arriving optical signal. Based on the observation results, the receiver derives a compensation pattern for the wavefront distortion. A wavefront control device provided in the receiver forms a compensation pattern for the wavefront distortion on the wavefront affected by the fluctuations. This improves the quality of optical wireless communication. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Yongxiong Ren, Guodong Xie, Hao Huang, Nisar Ahmed, Yan Yan, Long Li, Changjing Bao, Martin PJ Lavery, Moshe Tur, Mark A. Neifeld, Robert W. Boyd, Jeffrey H. Shapiro, and Alan E. Willner, “Adaptive-optics-based simultaneous pre- and post-turbulence compensation of multiple orbital-angular-momentum beams in a bidirectional free-space optical link”, Optica 1, 376-382 (2014) Summary of the Invention [Problem to be solved by the invention]

[0005] When wavefront distortion compensation is performed using the adaptive optics described in Non-Patent Document 1, there is a problem in that due to a control delay between the time when the wavefront is observed and the time when a wavefront distortion compensation pattern is formed by the wavefront control device, it is not possible to follow fluctuations in atmospheric turbulence, and the performance of the wavefront distortion compensation is degraded.

[0006] In view of the above circumstances, an object of the present invention is to provide a technique capable of improving the accuracy of compensating for distortion of the wavefront of an optical signal propagated through the atmosphere. [Means for solving the problem]

[0007] One aspect of the present invention is a communication method in a communication system including an optical transmitting device and an optical receiving device, comprising: detecting, by a plurality of sensors, a wavefront distortion of a reference optical signal used for wavefront observation arriving at a first time and a second time prior to a compensation start time for compensating for wavefront distortion; estimating a spatial phase distribution of each of a plurality of atmospheric layers between the optical transmitting device and the optical receiving device at the first time and the second time based on the wavefront distortion detected by each of the plurality of sensors; predicting a spatial phase distribution of each of the plurality of atmospheric layers at the compensation start time using the estimation results of the spatial phase distribution of each of the plurality of atmospheric layers estimated at the first time and the second time; controlling an operation of a wavefront control device that compensates for the wavefront distortion of an optical signal based on the prediction result; and receiving the wavefront distortion of the optical signal transmitted from the optical transmitting device after being compensated for by the wavefront control device.

[0008] One aspect of the present invention is an optical receiving device in a communication system including an optical transmitting device and an optical receiving device, the optical receiving device comprising: a plurality of sensors that detect distortion of a wavefront of a reference optical signal used for wavefront observation arriving at a first time and a second time prior to a compensation start time for compensating for wavefront distortion; a phase distribution prediction unit that estimates a spatial phase distribution of each of a plurality of atmospheric layers between the optical transmitting device and the optical receiving device at the first time and the second time based on the distortion of the wavefront detected by each of the plurality of sensors, and predicts the spatial phase distribution of each of the plurality of atmospheric layers at the compensation start time using the estimation results of the spatial phase distribution of each of the plurality of atmospheric layers estimated at the first time and the second time; a wavefront control device that compensates for the distortion of the wavefront of the optical signal transmitted from the optical transmitting device; a control unit that controls the operation of the wavefront control device based on the prediction result; and an optical receiving unit that receives an optical signal in which the wavefront distortion of the optical signal has been compensated for by the wavefront control device.

[0009] One aspect of the present invention is an optical transmission device in a communication system including an optical transmission device and an optical receiving device, the optical transmission device comprising: a plurality of sensors that detect wavefront distortion of a reference optical signal used for wavefront observation arriving at a first time and a second time prior to a compensation start time for compensating for wavefront distortion; a phase distribution prediction unit that estimates a spatial phase distribution of each of a plurality of atmospheric layers between the optical transmission device and the optical receiving device at the first time and the second time based on the wavefront distortion detected by each of the plurality of sensors, and predicts the spatial phase distribution of each of the plurality of atmospheric layers at the compensation start time using the estimation results of the spatial phase distribution of each of the plurality of atmospheric layers estimated at the first time and the second time; a wavefront control device that compensates for the wavefront distortion of an optical signal according to data to be transmitted; a control unit that controls operation by the wavefront control device based on the prediction result; and an optical transmission unit that transmits an optical signal with compensated wavefront distortion to the optical receiving device via the wavefront control device.

[0010] One aspect of the present invention is a communication system including an optical transmitting device and an optical receiving device, wherein the optical transmitting device includes an optical transmitting unit that transmits an optical signal corresponding to data to be transmitted and a reference optical signal used for wavefront observation to the optical receiving device, and the optical receiving device includes a plurality of sensors that detect distortion of the wavefront of the reference optical signal arriving at a first time and a second time prior to a compensation start time for compensating for wavefront distortion, a phase distribution prediction unit that estimates a spatial phase distribution of each of a plurality of atmospheric layers between the optical transmitting device and the optical receiving device at the first time and the second time based on the distortion of the wavefront detected by each of the plurality of sensors, and predicts the spatial phase distribution of each of the plurality of atmospheric layers at the compensation start time using the estimation results of the spatial phase distribution of each of the plurality of atmospheric layers estimated at the first time and the second time, a wavefront control device that compensates for the distortion of the wavefront of the optical signal, a control unit that controls the operation of the wavefront control device based on the prediction result, and an optical receiving unit that receives an optical signal in which the distortion of the wavefront of the optical signal has been compensated for by the wavefront control device.

[0011] One aspect of the present invention is a communication system including an optical transmitting device and an optical receiving device, wherein the optical transmitting device includes a plurality of sensors that detect a distortion of a wavefront of a reference optical signal used for wavefront observation arriving at a first time and a second time prior to a compensation start time for compensating for wavefront distortion, and the optical transmitting device estimates a spatial phase distribution of each of a plurality of atmospheric layers between the optical transmitting device and the optical receiving device at the first time and the second time based on the distortion of the wavefront detected by each of the plurality of sensors, and performs the compensation start time compensation using the estimation results of the spatial phase distribution of each of the plurality of atmospheric layers estimated at the first time and the second time. The communication system includes a phase distribution prediction unit that predicts the spatial phase distribution of each of the multiple atmospheric layers at a start time, a wavefront control device that compensates for wavefront distortion of an optical signal according to data to be transmitted, a control unit that controls operation of the wavefront control device based on the predicted result, and an optical transmitting unit that transmits an optical signal with the wavefront distortion compensated for to the optical receiving device via the wavefront control device, wherein the optical receiving device includes an optical transmitting unit that transmits the reference optical signal to the optical transmitting device, and an optical receiving unit that receives the optical signal with the wavefront distortion of the optical signal compensated for by the wavefront control device. Effect of the Invention

[0012] According to the present invention, it is possible to improve the accuracy of compensating for distortion of the wavefront of an optical signal propagated through the atmosphere. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram for explaining an overview of the present invention. [Diagram 2] FIG. 1 is a diagram for explaining an overview of the present invention. [Diagram 3] 1 is a diagram illustrating an example of the configuration of a communication system according to a first embodiment. [Figure 4] FIG. 4 is a sequence diagram showing a processing flow of the communication system in the first embodiment. [Figure 5A] FIG. 13 is a diagram showing a phase distribution to which the phase change given by the first layer of atmosphere has been added for simulation purposes. [Figure 5B]FIG. 13 is a diagram showing the estimated phase distribution due to the first layer of the atmosphere. [Figure 6A] FIG. 13 shows a phase distribution with different phase changes given by the second layer of atmosphere for simulation. [Figure 6B] FIG. 13 shows a phase distribution with different phase changes given by the second layer of atmosphere for simulation. [Figure 6C] FIG. 13 shows a phase distribution with different phase changes given by the second layer of atmosphere for simulation. [Figure 6D] FIG. 13 is a diagram showing the estimated phase distribution due to the second layer of the atmosphere. [Figure 6E] FIG. 13 is a diagram showing the estimated phase distribution due to the second layer of the atmosphere. [Figure 6F] FIG. 13 is a diagram showing the estimated phase distribution due to the second layer of the atmosphere. [Figure 7] FIG. 11 is a diagram illustrating an example of the configuration of a communication system according to a second embodiment. [Figure 8] FIG. 4 is a sequence diagram showing a processing flow of the communication system in the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. (overview) First, an overview of the present invention will be described with reference to Figures 1 and 2. As shown in Figure 1, the atmosphere between an optical transmitter and an optical receiver is expressed as a superposition of multiple atmospheric layers (atmospheric layer 1 and atmospheric layer 2 in Figure 1). Here, the atmospheric phase distribution in atmospheric layer 1 is denoted as φ_atm1, and the atmospheric phase distribution in atmospheric layer 2 is denoted as φ_atm2. Here, the atmospheric phase distribution represents the wavefront distortion (spatial phase distribution) suffered by light passing through the atmosphere. If the wind speed and wind direction of each atmospheric layer can be estimated, the optical receiver can predict the influence of atmospheric fluctuations after a control delay and form an appropriate wavefront distortion compensation pattern.

[0015] However, the wavefront (spatial phase distribution) observed by the sensor in the optical receiving device is the result of superimposing the phase distributions of multiple atmospheric layers. Therefore, it is necessary to estimate the wind speed and direction for each atmospheric layer separately from the wavefront (spatial phase distribution) observed by the sensor. The sensor in the optical receiving device is, for example, a wavefront sensor.

[0016] In the present invention, as shown in Fig. 2 as an example, a reference optical signal having a wavelength different from that of an optical signal for data transmission (hereinafter simply referred to as "optical signal") is transmitted from an optical transmitting device, and the reference optical signal is received by a plurality of sensors (e.g., three sensors) in an optical receiving device. The reference optical signal is a signal used for observing a wavefront, has a larger beam diameter than an optical signal, and is received by each sensor. Each sensor detects distortion of the wavefront of the reference optical signal by observing the wavefront (spatial phase distribution) of the reference optical signal.

[0017] The phase distribution prediction unit included in the optical receiving device estimates the phase distribution of the first layer (atmospheric layer 1) and the phase distribution of the second layer (atmospheric layer 2) based on the wavefront distortion of the reference optical signal detected by each sensor. The phase distribution prediction unit executes this process at times tn-1 (first time) and tn (second time), which are times prior to the wavefront distortion compensation time (compensation start time) tn', and estimates the atmospheric wind speed and wind direction of each atmospheric layer by correlation calculation of the phase distribution estimates at times tn-1 and tn. The wavefront distortion compensation time tn' is the time at which compensation for wavefront distortion is performed.

[0018] Then, the phase distribution prediction unit predicts the atmospheric phase distribution of each atmospheric layer at the wavefront distortion compensation time tn' based on the estimated results of the wind speed and wind direction, and causes the wavefront control device to form a wavefront distortion compensation pattern that compensates for the phase distribution. In this way, since the wavefront control device is controlled at a time prior to the wavefront distortion compensation time tn', it is possible to suppress control delay at the wavefront distortion compensation time tn' and to follow the fluctuation of the atmospheric turbulence, so that it is possible to improve the accuracy of compensating for the wavefront distortion of an optical signal propagated through the atmosphere. A specific configuration for implementing the above processing will now be described.

[0019] (First embodiment) FIG. 3 is a diagram showing an example of the configuration of a communication system 100 in the first embodiment. The communication system 100 is an optical communication system that performs wireless communication (optical wireless communication) using optical signals. The communication system 100 includes one or more optical transmitting devices 10 and one or more optical receiving devices 20. In the communication system 100, an optical signal propagates through the atmosphere between the optical transmitting device 10 and the optical receiving device 20. In the first embodiment, the optical receiving device 20 compensates for distortion occurring in the wavefront of the optical signal propagated through the atmosphere by adaptive optics. In the first embodiment, it is assumed that there are two layers of the atmosphere between the optical transmitting device 10 and the optical receiving device 20.

[0020] The optical transmitter 10 includes a signal generator 11, a reference light source 12, a beam splitter 13, and an optical transmitter 14.

[0021] The signal generator 11 generates an optical signal according to data to be transmitted.

[0022] The reference light source 12 outputs a reference optical signal of a predetermined wavelength used for wavefront observation. The predetermined wavelength is a wavelength different from the wavelength of the optical signal generated by the signal generating unit 11.

[0023] The beam splitter 13 outputs the reference optical signal output by the reference light source 12 to the optical transmitter 14 on the same optical axis as the optical signal generated by the signal generator 11 .

[0024] The optical transmitter 14 transmits the reference optical signal output from the beam splitter 13 and the optical signal. The optical transmitter 14 transmits the reference optical signal with a beam diameter larger than that of the optical signal. The beam diameter larger than that of the optical signal is preferably a size that can be received by at least a plurality of sensors provided in the optical receiver 20.

[0025] The optical receiving device 20 includes a plurality of sensors 21-1 to 21-3, a beam splitter 22, a wavefront control device 23, an optical receiving section 24, a phase distribution predicting section 25, a control section 26, and a reference light source 27.

[0026] The sensors 21-1 to 21-3 are wave-front sensors. The sensors 21-1 to 21-3 are arranged at different positions, and detect distortion of the wavefront of the reference optical signal by observing the wavefront (spatial phase distribution) of the incoming reference optical signal. The sensors 21-1 to 21-3 output a signal representing the distortion of the wavefront of the reference optical signal to the phase distribution prediction unit 25.

[0027] The wavefront control device 23 compensates for distortion of the wavefront of the optical signal according to data to be transmitted. The wavefront control device 23 is, for example, a spatial light phase modulator or a deformable mirror. In the following description, the wavefront control device 23 will be described as a spatial light phase modulator as an example. The wavefront control device 23 modulates the phase of the wavefront of the input optical signal (including the reference optical signal). The wavefront control device 23 forms a wavefront distortion compensation pattern for compensating for the wavefront distortion of the input optical signal according to the control of the control unit 26, and compensates for the wavefront distortion by modulating the phase of the wavefront of the optical signal.

[0028] The beam splitter 22 splits the optical signal (including the reference optical signal) whose wavefront has been phase-modulated by the wavefront control device 23, into an optical receiving unit 24 and a sensor 21-2.

[0029] The optical receiving unit 24 converts the optical signal into an electrical signal. The optical receiving unit 24 executes a predetermined signal processing (e.g., demodulation processing) on ​​the converted electrical signal. The optical receiving unit 24 obtains data transmitted from the optical transmitting device 10 by using the optical signal from the electrical signal by performing the predetermined signal processing.

[0030] The phase distribution prediction unit 25 predicts the atmospheric phase distribution of each atmospheric layer (atmospheric layer 1 and atmospheric layer 2) at the wavefront distortion compensation time tn' based on signals representing the wavefront distortion of the reference optical signals obtained from the sensors 21-1 to 21-3. The phase distribution prediction unit 25 outputs the prediction results of the atmospheric phase distribution of each atmospheric layer (atmospheric layer 1 and atmospheric layer 2) at the wavefront distortion compensation time tn' to the control unit 26.

[0031] The control unit 26 derives a wavefront distortion pattern for compensating for the phase distribution included in the prediction result based on the prediction result output from the phase distribution prediction unit 25. The control unit 26 controls the operation of phase modulation by the wavefront control device 23 according to the derived wavefront distortion pattern.

[0032] Fig. 4 is a sequence diagram showing the flow of processing in the communication system 100 in the first embodiment. It is assumed that the wave-front control device 23 has not yet been subjected to phase adjustment by the control unit 26 at the start of the processing in Fig. 4.

[0033] The signal generating unit 11 of the optical transmitting device 10 generates an optical signal (step S101). The signal generating unit 11 outputs the generated optical signal to the beam splitter 13. The reference light source 12 generates a reference optical signal having a wavelength different from the wavelength of the optical signal generated by the signal generating unit 11 (step S102). The reference light source 12 outputs the generated reference optical signal to the beam splitter 13.

[0034] The optical signal generated by the signal generating unit 11 and the reference optical signal generated by the reference light source 12 are input to the optical transmitting unit 14 via the beam splitter 13. The optical transmitting unit 14 transmits the optical signal and the reference optical signal to the optical receiving device 20 (step S103). The optical transmitting device 10 continuously transmits the optical signal and the reference optical signal to the optical receiving device 20. The reference optical signal transmitted from the optical transmitting device 10 is received by each of the sensors 21-1 to 21-3 of the optical receiving device 20. The sensor 21-2 receives the reference optical signal via the wavefront control device 23 and the beam splitter 22.

[0035] Each of the sensors 21-1 to 21-3 observes the wavefront (spatial phase distribution) of the reference optical signal at time tn-1 (step S104). As a result, each of the sensors 21-1 to 21-3 detects the distortion of the wavefront of the reference optical signal at time tn-1. Here, the phase distributions φ1(x,y), φ2(x,y), and φ3(x,y) observed by each of the sensors 21-1 to 21-3 are expressed as the following formula (1). Here, the direction from the optical transmitter 10 to the optical receiver 20 is the z-axis, and the plane perpendicular to the z-axis is the xy plane. In this case, (x,y) in the above φ1(x,y) is a description to supplementarily indicate that φ1 is not a single phase value, but represents a phase distribution in the xy plane. As for (x,y) in φ2(x,y) and φ3(x,y), φ1 in the above description may be read as φ2 or φ3. The phase distribution φ1(x,y) represents the phase distribution observed by the sensor 21-1, the phase distribution φ2(x,y) represents the phase distribution observed by the sensor 21-2, and the phase distribution φ3(x,y) represents the phase distribution observed by the sensor 21-3.

[0036]

number

[0037] In formula (1), φ_atm1(x,y) represents the phase distribution of atmospheric layer 1 (first layer) passing through on the path from the optical transmitter 10 to the optical receiver 20, φ_atm2,1(x,y) represents the phase distribution of atmospheric layer 2 (second layer) passing through on the path from the optical transmitter 10 to the sensor 21-1 of the optical receiver 20, φ_atm2,2(x,y) represents the phase distribution of atmospheric layer 2 (second layer) passing through on the path from the optical transmitter 10 to the sensor 21-2 of the optical receiver 20, and φ_atm2,3(x,y) represents the phase distribution of atmospheric layer 2 (second layer) passing through on the path from the optical transmitter 10 to the sensor 21-3 of the optical receiver 20. Furthermore, in formula (1), ni(x,y) (i is an integer of 1 or more) represents the wavefront observation error at each of the sensors 21-1 to 21-3.

[0038] Each of the sensors 21-1 to 21-3 outputs a signal representing the distortion of the wavefront of the reference optical signal to the phase distribution prediction unit 25. The phase distribution prediction unit 25 estimates the atmospheric phase distribution of each atmospheric layer at time tn-1 based on the signal representing the distortion of the wavefront of the reference optical signal obtained from each of the sensors 21-1 to 21-3 (step S105). First, the phase distribution prediction unit 25 estimates the phase distribution φ_atm1'(x,y) of the first layer at time tn-1 based on the following equation (2).

[0039]

number

[0040] The phase distribution φ_atm1´(x,y) in the first layer is common to each transmit / receive path as shown in Figure 2, so it can be estimated by averaging φ1(x,y), φ2(x,y), and φ3(x,y).

[0041] Next, the phase distribution prediction unit 25 estimates the phase distributions φ_atm2,1′(x,y), φ_atm2,2′(x,y), and φ_atm2,3′(x,y) of the second layer at the time tn-1 based on the following equation (3).

[0042]

number

[0043] As shown in equation (3), the second layer phase distributions φ_atm2,1'(x,y), φ_atm2,2'(x,y), and φ_atm2,3'(x,y) can be estimated by subtracting the estimated values ​​of the first layer phase distribution from the phase distributions observed by each sensor 21-1 to 21-3.

[0044] Thereafter, each of the sensors 21-1 to 21-3 observes the wavefront (spatial phase distribution) of the reference optical signal at time tn (step S106). As a result, each of the sensors 21-1 to 21-3 detects the distortion of the wavefront of the reference optical signal at time tn. Each of the sensors 21-1 to 21-3 outputs a signal representing the distortion of the wavefront of the reference optical signal to the phase distribution prediction unit 25.

[0045] The phase distribution prediction unit 25 estimates the atmospheric phase distribution of each atmospheric layer at time tn based on the signal representing the distortion of the wavefront of the reference optical signal obtained from each of the sensors 21-1 to 21-3 (step S107). The method of deriving the atmospheric phase distribution of each atmospheric layer at time tn is the same as that of step S105. As a result, the phase distribution prediction unit 25 can obtain an estimate of the atmospheric phase distribution of each atmospheric layer at time tn-1 (hereinafter referred to as the "time tn-1 estimate") and an estimate of the atmospheric phase distribution of each atmospheric layer at time tn (hereinafter referred to as the "time tn estimate").

[0046] The phase distribution prediction unit 25 estimates the wind speed and wind direction of the atmosphere of each atmospheric layer by calculating the correlation between the time tn-1 estimated value and the time tn estimated value (step S108). Specifically, the phase distribution prediction unit 25 estimates the wind speed and wind direction of the atmosphere of each atmospheric layer by calculating the cross-correlation between the two-dimensional phase distribution estimated values ​​on the xy plane at two times (time tn-1 and time tn). As a result, the phase distribution prediction unit 25 estimates the direction and speed at which the atmosphere is moving between time tn-1 and time tn for each atmospheric layer using the time tn-1 estimated value and the time tn estimated value. Then, the phase distribution prediction unit 25 predicts the phase distribution of the atmosphere of each atmospheric layer at the wavefront distortion compensation time tn' based on the estimated wind speed and wind direction of the atmosphere of each atmospheric layer (step S109).

[0047] The phase distribution prediction unit 25 outputs the prediction result to the control unit 26. Based on the prediction result, the control unit 26 derives a wavefront distortion pattern for compensating for the phase distribution included in the prediction result. The control unit 26 performs wavefront distortion compensation control by controlling the operation of phase modulation by the wavefront control device 23 according to the derived wavefront distortion pattern (step S110). This makes it possible to compensate for the wavefront distortion of the optical signal input to the wavefront control device 23. The optical receiving unit 24 receives the optical signal with the wavefront distortion compensated for (step S111).

[0048] Next, the results of estimating the spatial distribution of the phase change in each atmospheric layer using the above method are shown in Figures 5 and 6. Figure 5(A) shows a phase distribution to which a phase change given by the first atmosphere layer is added for simulation purposes, and Figure 5(B) shows an estimated result of the phase distribution due to the first atmosphere layer (e.g., phase distribution φ_atm1'(x,y)). Figures 6(A), 6(B), and 6(C) show a phase distribution to which a different phase change given by the second atmosphere layer is added for simulation purposes, and Figures 6(D), 6(E), and 6(F) show an estimated result of the phase distribution due to the second atmosphere layer.

[0049] More specifically, Figure 6(A) shows a phase distribution for simulation purposes to which different phase changes are imparted by the atmosphere in atmospheric layer 2 (second layer) through which the optical signal passes on the path from the optical transmitting device 10 to the sensor 21-1 of the optical receiving device 20, and Figure 6(D) shows an estimated result of the phase distribution in Figure 6(A) (e.g., the phase distribution φ_atm2,1'(x,y)).

[0050] Figure 6(B) shows a phase distribution for simulation purposes that is given different phase changes in the atmosphere in atmospheric layer 2 (second layer) through which the optical signal passes on the path from the optical transmitting device 10 to the sensor 21-2 of the optical receiving device 20, and Figure 6(E) shows an estimated result of the phase distribution in Figure 6(B) (e.g., the phase distribution φ_atm2,2'(x,y)).

[0051] Figure 6(C) shows a phase distribution for simulation purposes that is given different phase changes in the atmosphere in atmospheric layer 2 (second layer) through which the optical signal passes on the path from the optical transmitting device 10 to the sensor 21-3 of the optical receiving device 20, and Figure 6(F) shows an estimated result of the phase distribution in Figure 6(C) (e.g., the phase distribution φ_atm2,3'(x,y)).

[0052] As shown in FIG. 5(B) and FIG. 6(D) to FIG. 6(F), it is understood that the phase distribution is estimated with high accuracy.

[0053] According to the communication system 100 configured as above, it is possible to improve the accuracy of compensating for the wavefront distortion of the optical signal propagated through the atmosphere. Specifically, the optical transmitter 10 transmits a reference optical signal to the optical receiver 20, and the optical receiver 20 estimates the phase distribution of each atmospheric layer between the optical transmitter 10 and the optical receiver 20 at a time before the wavefront distortion compensation time tn' based on the reference optical signal. The optical receiver 20 estimates the wind direction and wind speed of the atmosphere in each atmospheric layer based on the estimated value of the phase distribution of each atmospheric layer at each estimated time (for example, time tn-1, tn), and predicts the phase distribution of the atmosphere at the wavefront distortion compensation time tn' from the estimation result. Then, the optical receiver 20 controls the wavefront control device 23 in advance so as to compensate for the predicted phase distribution. This makes it unnecessary to control the wavefront control device 23 after the wavefront distortion compensation time tn'. In this way, the control delay can be suppressed, and therefore the fluctuation of the atmospheric turbulence can be followed. This makes it possible to improve the accuracy of compensating for distortion of the wavefront of an optical signal propagated through the atmosphere.

[0054] (Modification of the first embodiment) In the above embodiment, the operation has been described when the wavefront control device 23 is a spatial light phase modulator. Below, the operation will be described when the wavefront control device 23 is a deformable mirror. When the wavefront control device 23 is a deformable mirror, the wavefront control device 23 forms a wavefront distortion compensation pattern for compensating for the wavefront distortion of the input optical signal according to the control of the control unit 26, and compensates for the wavefront distortion of the optical signal.

[0055] Second embodiment The second embodiment differs from the first embodiment in that the optical transmitter compensates for distortion occurring in the wavefront of the transmitted optical signal before the optical signal is transmitted. The second embodiment will be described focusing on the differences from the first embodiment.

[0056] FIG. 7 is a diagram showing a configuration example of a communication system 100a in the second embodiment. The communication system 100a includes one or more optical transmitting devices 10a and one or more optical receiving devices 20a. In the communication system 100a, an optical signal transmitted from the optical transmitting device 10a toward the optical receiving device 20a propagates through the atmosphere between the optical transmitting device 10a and the optical receiving device 20a. In the communication system 100a, a reference optical signal transmitted from the optical receiving device 20a toward the optical transmitting device 10a propagates through the atmosphere between the optical transmitting device 10a and the optical receiving device 20a. In the second embodiment, the optical transmitting device 10a compensates for distortion occurring in the wavefront of the optical signal propagated through the atmosphere by adaptive optics. In the second embodiment, it is assumed that there are two layers of the atmosphere between the optical transmitting device 10a and the optical receiving device 20a.

[0057] The optical transmitting device 10a includes a signal generating unit 11, an optical transmitting unit 14, a plurality of sensors 15-1 to 15-3, a phase distribution predicting unit 16, a control unit 17, and a wavefront control device 18. The optical receiving device 20a includes an optical receiving unit 24, a reference light source 27, and an optical transmitting unit 28.

[0058] The sensors 15-1 to 15-3 are wavefront sensors. The sensors 15-1 to 15-3 are arranged at different positions, and detect the distortion of the wavefront of the reference optical signal by observing the wavefront (spatial phase distribution) of the incoming reference optical signal. The sensors 15-1 to 15-3 output a signal representing the distortion of the wavefront of the reference optical signal to the phase distribution prediction unit 16.

[0059] The phase distribution prediction unit 16 predicts the atmospheric phase distribution of each atmospheric layer (atmospheric layer 1 and atmospheric layer 2) at the wavefront distortion compensation time tn' based on signals representing the wavefront distortion of the reference optical signals obtained from the sensors 15-1 to 15-3. The phase distribution prediction unit 16 outputs the prediction results of the atmospheric phase distribution of each atmospheric layer (atmospheric layer 1 and atmospheric layer 2) at the wavefront distortion compensation time tn' to the control unit 17.

[0060] The control unit 17 derives a wavefront distortion pattern for compensating for the phase distribution included in the prediction result based on the prediction result output from the phase distribution prediction unit 16. The control unit 17 controls the operation of phase modulation by the wavefront control device 18 according to the derived wavefront distortion pattern.

[0061] The wavefront control device 18 compensates for distortion of the wavefront of the optical signal according to the data to be transmitted. The wavefront control device 18 is, for example, a spatial light phase modulator or a deformable mirror. In the following description, the wavefront control device 18 will be described as a spatial light phase modulator as an example. The wavefront control device 18 modulates the phase of the wavefront of the optical signal transmitted by the optical transmitter 14. The wavefront control device 18 forms a wavefront distortion compensation pattern for compensating for the wavefront distortion of the input optical signal according to the control of the controller 17, and compensates for the wavefront distortion by modulating the phase of the wavefront of the optical signal.

[0062] The reference light source 27 outputs a reference optical signal of a predetermined wavelength used for wavefront observation. The predetermined wavelength is a wavelength different from the wavelength of the optical signal transmitted by the optical transmitter 10a.

[0063] The optical transmitter 28 transmits the reference optical signal output by the reference light source 27 to the optical transmitter 10a. For convenience of explanation, in Fig. 7, the reference optical signal transmitted from the optical transmitter 28 to the sensor 15-2 and the optical signal transmitted from the wavefront control device 18 to the optical receiver 24 are shown as propagating on different optical axes, but in reality, the reference optical signal and the optical signal are propagated on the same optical axis. Note that the optical transmitter 28 transmits the reference optical signal with a beam diameter large enough to be received by at least the multiple sensors 15 included in the optical transmitter 10a.

[0064] Fig. 8 is a sequence diagram showing the flow of processing in the communication system 100a in the second embodiment. It is assumed that the wave-front control device 18 has not yet been subjected to phase adjustment by the control unit 17 at the start of the processing in Fig. 8.

[0065] The reference light source 27 of the optical receiving device 20a generates a reference optical signal (step S201). The reference light source 27 outputs the generated reference optical signal to the optical transmitting unit 28. The optical transmitting unit 28 transmits the reference optical signal to the optical transmitting device 10a (step S202). The optical receiving device 20a continues to transmit the reference optical signal to the optical transmitting device 10a. The reference optical signal transmitted from the optical receiving device 20a is received by each of the sensors 15-1 to 15-3 of the optical transmitting device 10a.

[0066] Each of the sensors 15-1 to 15-3 observes the wavefront (spatial phase distribution) of the reference optical signal at time tn-1 (step S203). As a result, each of the sensors 15-1 to 15-3 detects the distortion of the wavefront of the reference optical signal at time tn-1. Here, the phase distributions φ1(x,y), φ2(x,y), and φ3(x,y) observed by each of the sensors 15-1 to 15-3 are expressed as in the above formula (1). In this case, the phase distribution φ1(x,y) represents the phase distribution observed by the sensor 15-1, the phase distribution φ2(x,y) represents the phase distribution observed by the sensor 15-2, and the phase distribution φ3(x,y) represents the phase distribution observed by the sensor 15-3.

[0067] In the second embodiment, in equation (1), φ_atm1(x,y) represents the phase distribution of atmospheric layer 1 (first layer) passed through on the path from the optical receiving device 20a to the optical transmitting device 10a, φ_atm2,1(x,y) represents the phase distribution of atmospheric layer 2 (second layer) passed through on the path from the optical receiving device 20a to the sensor 15-1 of the optical transmitting device 10a, φ_atm2,2(x,y) represents the phase distribution of atmospheric layer 2 (second layer) passed through on the path from the optical receiving device 20a to the sensor 15-2 of the optical transmitting device 10a, and φ_atm2,3(x,y) represents the phase distribution of atmospheric layer 2 (second layer) passed through on the path from the optical receiving device 20a to the sensor 15-3 of the optical transmitting device 10a. Furthermore, in the second embodiment, in formula (1), ni(x, y) (i is an integer equal to or greater than 1) represents the wavefront observation error in each of sensors 15-1 to 15-3.

[0068] Each of the sensors 15-1 to 15-3 outputs a signal representing the distortion of the wavefront of the reference optical signal to the phase distribution prediction unit 16. The phase distribution prediction unit 16 estimates the atmospheric phase distribution of each atmospheric layer at time tn-1 based on the signal representing the distortion of the wavefront of the reference optical signal obtained from each of the sensors 15-1 to 15-3 (step S204). First, the method of estimating the atmospheric phase distribution of each atmospheric layer by the phase distribution prediction unit 16 is the same as that in the first embodiment, and therefore a description thereof will be omitted.

[0069] Then, each of the sensors 15-1 to 15-3 observes the wavefront (spatial phase distribution) of the reference optical signal at time tn (step S205). As a result, each of the sensors 15-1 to 15-3 detects the distortion of the wavefront of the reference optical signal at time tn. Each of the sensors 15-1 to 15-3 outputs a signal representing the distortion of the wavefront of the reference optical signal to the phase distribution prediction unit 16.

[0070] The phase distribution prediction unit 16 estimates the atmospheric phase distribution of each atmospheric layer at time tn based on the signal representing the distortion of the wavefront of the reference optical signal obtained from each of the sensors 15-1 to 15-3 (step S206). The method of deriving the atmospheric phase distribution of each atmospheric layer at time tn is the same as that of step S204. In this way, the phase distribution prediction unit 16 can obtain a time tn-1 estimated value and a time tn estimated value.

[0071] The phase distribution prediction unit 16 estimates the atmospheric wind speed and wind direction of each atmospheric layer by calculating the correlation between the time tn-1 estimated value and the time tn estimated value (step S207). For example, the phase distribution prediction unit 16 estimates the direction and speed at which the atmosphere is moving for each layer using the time tn-1 estimated value and the time tn estimated value. Then, the phase distribution prediction unit 16 predicts the atmospheric phase distribution of each atmospheric layer at the wavefront distortion compensation time tn' based on the estimated atmospheric wind speed and wind direction of each atmospheric layer (step S208).

[0072] The phase distribution prediction unit 16 outputs the prediction result to the control unit 17. The control unit 17 derives a wavefront distortion pattern for compensating for the phase distribution included in the prediction result based on the prediction result. The control unit 17 performs wavefront distortion compensation control by controlling the operation of phase modulation by the wavefront control device 18 according to the derived wavefront distortion pattern (step S209). This makes it possible to compensate for the wavefront distortion of the optical signal input to the wavefront control device 18.

[0073] The signal generating unit 11 generates an optical signal (step S210). The signal generating unit 11 outputs the generated optical signal to the optical transmitting unit 14. The optical transmitting unit 14 transmits the optical signal to the optical receiving device 20a via the wavefront control device 18 (step S211). The optical signal transmitted from the optical transmitting unit 14 reaches the optical receiving unit 24 of the optical receiving device 20a after wavefront distortion has been compensated for in advance by the wavefront control device 18. The optical receiving unit 24 receives the optical signal with the wavefront distortion compensated for (step S212).

[0074] According to the communication system 100a configured as above, the optical receiving device 20a transmits a reference optical signal to the optical transmitting device 10a, and the optical transmitting device 10a compensates for distortion occurring in the wavefront of the transmitted optical signal based on the reference optical signal before transmitting the optical signal. This makes it unnecessary to control the wavefront control device 18 after the wavefront distortion compensation time tn'. In this way, it is possible to suppress control delay and therefore to follow the fluctuation of atmospheric turbulence. This makes it possible to improve the accuracy of compensating for distortion of the wavefront of an optical signal propagated through the atmosphere.

[0075] (Modification of the second embodiment) In the above embodiment, the operation has been described when the wavefront control device 18 is a spatial light phase modulator. Below, the operation will be described when the wavefront control device 18 is a deformable mirror. When the wavefront control device 18 is a deformable mirror, the wavefront control device 18 forms a wavefront distortion compensation pattern for compensating for the wavefront distortion of the input optical signal according to the control of the control unit 17, and compensates for the wavefront distortion of the optical signal.

[0076] (Modifications common to the first and second embodiments) In the first embodiment, the number of sensors 21 included in the optical receiving device 20 is three, and in the second embodiment, the number of sensors 15 included in the optical transmitting device 10a is three. The number of sensors 21 and sensors 15 may be four or more. In this case, the beam diameter of the reference optical signal is controlled to be large enough to be received by four or more sensors.

[0077] A part or all of the processes performed by the optical transmitters 10, 10 and the optical receivers 20, 20a in the above-mentioned embodiments may be realized by a computer. In this case, a program for realizing this function may be recorded in a computer-readable recording medium, and the program recorded in the recording medium may be read into a computer system and executed to realize the functions. Note that the "computer system" here includes hardware such as an OS and peripheral devices. Also, the "computer-readable recording medium" refers to portable media such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, and a storage device such as a hard disk built into a computer system.

[0078] Furthermore, the term "computer-readable recording medium" may include a medium that dynamically holds a program for a short period of time, such as a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line, and a medium that holds a program for a certain period of time, such as a volatile memory inside a computer system that serves as a server or client in such a case. The above program may be one for implementing part of the above-mentioned functions, or may be one that can realize the above-mentioned functions in combination with a program already recorded in the computer system, or may be one that is realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0079] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and designs that do not deviate from the gist of the present invention are also included. [Industrial Applicability]

[0080] The present invention is applicable to optical communication systems that perform wireless communication using optical signals. [Explanation of symbols]

[0081] 10, 10a...optical transmitting device, 20, 20a...optical receiving device, 11...signal generating unit, 12...reference light source, 13...beam splitter, 14...optical transmitting unit, 15-1 to 15-3...sensors, 16...phase distribution predicting unit, 17...control unit, 18...wavefront control device, 21-1 to 21-3...sensors, 22...beam splitter, 23...wavefront control device, 24...optical receiving unit, 25...phase distribution predicting unit, 26...control unit, 27...reference light source, 28...optical transmitting unit

Claims

1. A communication method in a communication system including an optical transmitting device and an optical receiving device, comprising: detecting, by a plurality of sensors, a distortion of a wavefront of a reference optical signal used for wavefront observation, the reference optical signal arriving at a first time and a second time before a compensation start time for compensating for the wavefront distortion; estimating a spatial phase distribution of each of a plurality of atmospheric layers between the optical transmitting device and the optical receiving device at the first time and the second time based on the distortion of the wavefront detected by each of the plurality of sensors; predicting a spatial phase distribution of each of the plurality of atmospheric layers at the compensation start time using estimation results of the spatial phase distribution of each of the plurality of atmospheric layers estimated at the first time and the second time; Controlling an operation by a wavefront control device to compensate for the distortion of the wavefront of the optical signal based on the predicted result; receiving the optical signal transmitted from the optical transmitting device after compensating for distortion of the wavefront of the optical signal by the wavefront control device; Communication methods.

2. transmitting the reference light signal with a beam diameter larger than the optical signal so that the reference light signal can be received by the plurality of sensors; The communication method according to claim 1 .

3. The plurality of atmospheric layers are composed of two layers, estimating a spatial phase distribution of the first layer of atmosphere at the first time and the second time by averaging spatial phase distributions representing distortion of the wavefront detected by each of the plurality of sensors; estimating a spatial phase distribution of a second layer of the atmosphere at the first time and the second time by subtracting an estimated value of the spatial phase distribution of the first layer of the atmosphere from a spatial phase distribution representing the distortion of the wavefront detected by each of the plurality of sensors; estimating atmospheric wind speeds and wind directions in each atmospheric layer based on the estimated values ​​of the spatial phase distribution of the first layer of the atmosphere and the estimated values ​​of the spatial phase distribution of the second layer of the atmosphere at the first time and the second time, respectively; predicting a spatial phase distribution of each of the plurality of atmospheric layers at the compensation start time based on an atmospheric wind speed and a wind direction of each atmospheric layer at the first time and an atmospheric wind speed and a wind direction of each atmospheric layer at a second time; The communication method according to claim 1 or 2.

4. estimating atmospheric wind speed and direction for each atmospheric layer by performing a correlation calculation between the estimation results of the spatial phase distribution of each atmospheric layer at the first time and the second time; The communication method according to claim 3.

5. An optical receiving apparatus in a communication system including an optical transmitting apparatus and an optical receiving apparatus, a plurality of sensors for detecting distortion of a wavefront of a reference light signal used for wavefront observation arriving at a first time and a second time before a compensation start time for compensating for the wavefront distortion; a phase distribution prediction unit that estimates a spatial phase distribution of each of a plurality of atmospheric layers between the optical transmitting device and the optical receiving device at the first time and the second time based on the distortion of the wavefront detected by each of the plurality of sensors, and predicts a spatial phase distribution of each of the plurality of atmospheric layers at the compensation start time using the estimation results of the spatial phase distribution of each of the plurality of atmospheric layers estimated at the first time and the second time; a wavefront control device that compensates for distortion of a wavefront of an optical signal transmitted from the optical transmitter; A control unit that controls an operation by the wavefront control device based on a result of the prediction; an optical receiving unit that receives an optical signal in which distortion of the wavefront of the optical signal has been compensated for by the wavefront control device; An optical receiving device comprising:

6. An optical transmitting apparatus in a communication system including an optical receiving apparatus, a plurality of sensors for detecting distortion of a wavefront of a reference light signal used for wavefront observation arriving at a first time and a second time before a compensation start time for compensating for the wavefront distortion; a phase distribution prediction unit that estimates a spatial phase distribution of each of a plurality of atmospheric layers between the optical transmitting device and the optical receiving device at the first time and the second time based on the distortion of the wavefront detected by each of the plurality of sensors, and predicts a spatial phase distribution of each of the plurality of atmospheric layers at the compensation start time using the estimation results of the spatial phase distribution of each of the plurality of atmospheric layers estimated at the first time and the second time; a wavefront control device that compensates for distortion of a wavefront of an optical signal according to data to be transmitted; A control unit that controls an operation by the wavefront control device based on a result of the prediction; an optical transmitting unit that transmits an optical signal in which the distortion of a wavefront has been compensated to the optical receiving device via the wavefront control device; An optical transmitting device comprising:

7. A communication system including an optical transmitting device and an optical receiving device, The optical transmitter comprises: an optical transmitter that transmits to the optical receiving device an optical signal corresponding to data to be transmitted and a reference optical signal used for wavefront observation; Equipped with The optical receiving device comprises: a plurality of sensors that detect distortion of a wavefront of the reference optical signal arriving at a first time and a second time before a compensation start time for compensating for the wavefront distortion; a phase distribution prediction unit that estimates a spatial phase distribution of each of a plurality of atmospheric layers between the optical transmitting device and the optical receiving device at the first time and the second time based on the distortion of the wavefront detected by each of the plurality of sensors, and predicts a spatial phase distribution of each of the plurality of atmospheric layers at the compensation start time using the estimation results of the spatial phase distribution of each of the plurality of atmospheric layers estimated at the first time and the second time; a wavefront control device for compensating for distortion of a wavefront of the optical signal; A control unit that controls an operation by the wavefront control device based on a result of the prediction; an optical receiving unit that receives an optical signal in which distortion of the wavefront of the optical signal has been compensated for by the wavefront control device; A communication system comprising:

8. A communication system including an optical transmitting device and an optical receiving device, The optical transmitter comprises: a plurality of sensors for detecting distortion of a wavefront of a reference light signal used for wavefront observation arriving at a first time and a second time before a compensation start time for compensating for the wavefront distortion; a phase distribution prediction unit that estimates a spatial phase distribution of each of a plurality of atmospheric layers between the optical transmitting device and the optical receiving device at the first time and the second time based on the distortion of the wavefront detected by each of the plurality of sensors, and predicts a spatial phase distribution of each of the plurality of atmospheric layers at the compensation start time using the estimation results of the spatial phase distribution of each of the plurality of atmospheric layers estimated at the first time and the second time; a wavefront control device that compensates for distortion of a wavefront of an optical signal according to data to be transmitted; A control unit that controls an operation by the wavefront control device based on a result of the prediction; an optical transmitting unit that transmits an optical signal in which the distortion of a wavefront has been compensated to the optical receiving device via the wavefront control device; Equipped with The optical receiving device comprises: an optical transmitter that transmits the reference optical signal to the optical transmitter; an optical receiving unit that receives an optical signal in which distortion of the wavefront of the optical signal has been compensated for by the wavefront control device; A communication system comprising:

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