Optical compensator of free-space optical communication system and optical communication device including optical compensator

Abstract

To provide an optical compensator for free-space optical communication that does not perform mechanical control.SOLUTION: An optical compensator includes detection means for detecting a first phase distribution of a reception beam received from a counterpart optical communication device, the first phase distribution being in a first plane perpendicular to the propagation direction of the reception beam, adjustment means for adjusting a transmission beam to be transmitted to the counterpart optical communication device, and control means for determining a second phase distribution based on the first phase distribution and controlling the adjustment means so that the phase distribution of the transmission beam in a second plane perpendicular to the propagation direction of the transmission beam becomes the second phase distribution.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to Free-Space Optical Communication (FSO) technology. [Background technology]

[0002] FSO communication is communication that transmits optical beams into space, and can be used in situations where it is not possible or difficult to install optical fiber, or where it is desirable to reduce the cost increase associated with laying optical fiber.

[0003] Non-Patent Document 1 discloses an optical communication device for FSO communication between a satellite and a ground station. The optical communication device has an optical compensator that controls the direction of a transmitted optical beam by compensating for changes in the relative position with the optical communication device of the communication partner. According to Non-Patent Document 1, the optical compensator detects the arrival direction of the optical beam from the communication partner using a quadrant photodetector (QPD), and controls the attitude of mirrors and lenses so that the optical beam is transmitted in the detected arrival direction. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] E.Ciaramella,et.al., IEEE Journal on selected areas in communications,2009,27.9:1639-1645. Summary of the Invention [Problem to be solved by the invention]

[0005] In the configuration of Non-Patent Document 1, the optical compensator includes an actuator for mechanically controlling lenses and mirrors. The actuator includes a gimbal, a motor, and the like. For example, when an optical communication device including the optical compensator described in Non-Patent Document 1 is mounted on a satellite, the actuator needs to be configured to withstand the acceleration at the time of launching the satellite, and the robustness required of the actuator is high. The mass of the actuator increases to ensure the necessary robustness, but the increased mass is disadvantageous for launching a satellite.

[0006] The present disclosure provides an optical compensator for FSO communication that can solve the above problems. [Means for solving the problem]

[0007] According to one aspect of the present disclosure, an optical compensator of an optical communication device for free space optical communications includes a detection means for detecting a first phase distribution of a received beam received from a counter optical communication device, the first phase distribution being in a first plane perpendicular to a propagation direction of the received beam, an adjustment means for adjusting a transmitted beam to be transmitted to the counter optical communication device, and a control means for determining a second phase distribution based on the first phase distribution and controlling the adjustment means so that the phase distribution of the transmitted beam in a second plane perpendicular to the propagation direction of the transmitted beam becomes the second phase distribution. Effect of the Invention

[0008] According to the present disclosure, an optical compensator for FSO communication that can solve the above problems is provided. [Brief description of the drawings]

[0009] [Figure 1] 1 is a block diagram of an optical transceiver including an optical compensator, according to some embodiments. [Diagram 2] 1 is an illustration of a spatial light phase sensor and a spatial light phase modulator, according to some embodiments. [Diagram 3] 1 is a block diagram of an optical transceiver including an optical compensator, according to some embodiments. [Figure 4]1 is a block diagram of an optical transceiver including an optical compensator, according to some embodiments. [Diagram 5] 1 is a block diagram of an optical transceiver including an optical compensator, according to some embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more features among the multiple features described in the embodiments may be arbitrarily combined. In addition, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0011] First Embodiment 1 is a configuration diagram of an optical communication device according to this embodiment. The optical communication device includes an optical transmitter 3 that outputs an optical beam modulated with data to be transmitted to a communication partner optical communication device, and an optical receiver 4 that demodulates the optical beam received from the communication partner optical communication device. In the following description, the optical beam transmitted to the communication partner optical communication device is referred to as a "transmitted beam", and the optical beam received from the communication partner optical communication device is referred to as a "received beam". Moreover, the optical communication device to be described is simply referred to as an "optical communication device", and the communication partner optical communication device is referred to as a "counterpart optical communication device". The configuration of the "counterpart optical communication device" may be the same as that of FIG. 1.

[0012] The optical communication device also includes an optical antenna 2 for transmitting a transmission beam into space and receiving a reception beam from space. The optical antenna 2 is used for the purposes of expanding the aperture of the transmission beam from the optical transmitter 3 and concentrating the reception beam, and is also called a beam extender (BEX: Beam EXtender).

[0013] Furthermore, the optical communication device has an optical compensator 1 that compensates for changes in the relative position with respect to the opposing optical communication device and controls the transmission beam to be directed toward the opposing optical communication device. The configuration of the optical compensator 1 will be described below. The optical circulator 11 outputs the reception beam from the optical antenna 2 to the optical splitter 13. The optical splitter 13 is a branching section that branches the reception beam into at least two, and outputs one branched reception beam to the optical receiver 4 and outputs the other branched reception beam to the spatial optical phase sensor 14.

[0014] The spatial optical phase sensor 14 detects the phase of the received beam at each position (region) in a plane perpendicular to the incident direction (propagation direction) of the received beam. FIG. 2(A) is an explanatory diagram of the configuration of the spatial optical phase sensor 14. The X-axis and Y-axis in FIG. 2(A) are two axes perpendicular to each other, and the incident direction of the received beam is the Z-direction perpendicular to the plane including both the X-axis and the Y-axis (hereinafter, the XY plane). The spatial optical phase sensor 14 has a plurality of regions arranged two-dimensionally in the XY plane. Each square in FIG. 2(A) represents one region, which will be also referred to as a "cell" below. The spatial optical phase sensor 14 detects a phase difference Δθ of the received beam with respect to a first reference phase for each cell. The first reference phase can be arbitrarily determined by the spatial optical phase sensor 14. The phase difference Δθ is, for example, a value within the range of -π<Δθ≦+π. A positive phase difference Δθ indicates that the phase detected in the cell is ahead of the first reference phase, and a negative phase difference Δθ indicates that the phase detected in the cell is behind the first reference phase.

[0015] In addition, the cells are specified by coordinates in the XY plane, with the number of cells as a unit. For example, the coordinates of the shaded cell in Fig. 2(A) are (x1, y1), the coordinates of the cell to the left of the shaded cell are (x1-1, y1), and the coordinates of the cell above the shaded cell are (x1, y1+1). The phase difference Δθ detected in each cell is output to the control unit 15.

[0016] The spatial light phase modulator 12, like the spatial light phase sensor 14, has a plurality of cells arranged two-dimensionally in a plane (XY plane) perpendicular to the incident direction (propagation direction) of the transmission beam. FIG. 2B is an explanatory diagram of the configuration of the spatial light phase modulator 12, and each square represents one cell. Each cell of the spatial light phase modulator 12 is also specified by coordinates in the XY plane. For example, the coordinates of the shaded cell in FIG. 2B are (x2, y2), the coordinates of the cell to the left of the shaded cell are (x2-1, y2), and the coordinates of the cell one level above the shaded cell are (x2, y2+1). In addition, each cell of the spatial light phase modulator 12 and each cell of the spatial light phase sensor 14 have a 1:1 correspondence. The corresponding cells can be determined based on the correspondence between the area where the spatial light phase sensor 14 is irradiated with the reception beam and the area where the spatial light phase modulator 12 is irradiated with the transmission beam. For example, the cell of the spatial light phase modulator 12 corresponding to the cell of the spatial light phase sensor 14 illuminated by the center of the received beam is the cell of the spatial light phase modulator 12 through which the center of the transmitted beam passes. In the following, the cell of the spatial light phase sensor 14 at the coordinates (x1, y1) in Fig. 2(A) corresponds to the cell of the spatial light phase modulator 12 at the coordinates (x2, y2) in Fig. 2(B). In the following description, in order to distinguish between the cell of the spatial light phase sensor 14 and the cell of the spatial light phase modulator 12, the cell of the spatial light phase sensor 14 is referred to as the "first cell" and the cell of the spatial light phase modulator 12 is referred to as the "second cell."

[0017] The spatial light phase modulator 12 gives a phase change to the transmission beam passing through each second cell. Note that the phase in the XY plane of the transmission beam incident on the spatial light phase modulator 12 from the optical transmitter 3 is assumed to be uniform. The amount of phase change given to the transmission beam by each second cell is controlled by the control unit 15. The control unit 15 determines the amount of phase change in the second cell so that the phase difference of the phase of the transmission beam after passing through the second cell with respect to the second reference phase has the same absolute value as the phase difference detected in the first cell corresponding to the second cell, but the sign is inverted. Note that the second reference phase can be arbitrarily determined by the control unit 15. For example, it is assumed that the phase difference Δθ detected in the first cell at the coordinates (x1, y1) in FIG. 2(A) is +θ1. In this case, the control unit 15 sets the amount of phase change given by the second cell so that the phase of the transmission beam after passing through the second cell of the spatial light phase modulator 12 at the coordinates (x2, y2) in FIG. 2(B) is −θ1 with respect to the second reference phase.

[0018] Since the phase of the transmission beam incident on each second cell of the spatial optical phase modulator 12 from the optical transmitter 3 is uniform, the spatial phase distribution of the transmission beam after passing through the spatial optical phase modulator 12 (hereinafter, second phase distribution) is the inverse of the spatial phase distribution of the reception beam incident on the spatial optical phase sensor 14 (hereinafter, first phase distribution). Therefore, by transmitting a transmission beam having a second phase distribution obtained by inverting the first phase distribution of the reception beam, the transmission beam is transmitted toward the opposing optical communication device, similar to beam control in wireless communication. The optical circulator 11 outputs the transmission beam that has passed through the spatial optical phase modulator 12 to the optical antenna 2.

[0019] In this embodiment, the phases of the transmission beams incident on each second cell of the spatial light phase modulator 12 from the optical transmitter 3 are aligned, but this embodiment can be applied even when the phases of the transmission beams incident on each second cell of the spatial light phase modulator 12 from the optical transmitter 3 are not aligned. In this case, the phases of the transmission beams incident on each second cell of the spatial light phase modulator 12 are measured by a measurement unit (not shown) and set in the control unit 15. The control unit 15 sets the amount of phase change provided by each second cell of the spatial light phase modulator 12, taking into account the phases of the transmission beams incident on each second cell of the spatial light phase modulator 12, so that the second phase distribution is the inverse of the first phase distribution.

[0020] In this embodiment, the second phase distribution is the inversion of the first phase distribution, but the second phase distribution is not limited to the inversion of the first phase distribution. For example, when the second phase distribution is the inversion of the first phase distribution, the composite phase distribution obtained by combining (adding) the first phase distribution and the second phase distribution is a uniform distribution, that is, a distribution in which the phase change amount is 0. Although this state is ideal, it is sufficient that the maximum phase change amount of the composite phase distribution obtained by combining the first phase distribution and the second phase distribution is equal to or less than a first threshold value obtained in advance.

[0021] It is assumed that the optical communication device receives the optical beam transmitted by the opposing optical communication device at time t0 at time t1, and the transmitted beam, to which a phase change has been applied based on the detection result of the spatial optical phase sensor 14 at time t1, reaches the opposing optical communication device at time t2. In this case, the transmitted beam is transmitted toward the location where the opposing optical communication device was at time t0, not toward the location where the opposing optical communication device was at time t2. Therefore, in cases where the relative positional relationship between the optical communication device and the opposing optical communication device changes over time, such as in communications between an artificial satellite and a ground station, and the delay between the optical communication device and the opposing optical communication device cannot be ignored, the transmitted beam will not be transmitted toward the opposing optical communication device.

[0022] However, even if the relative positional relationship between the optical communication device and the opposing optical communication device changes over time, such as in communication between an artificial satellite and a ground station, how it changes is usually known. For example, when the optical communication device is installed in a ground station and the opposing optical communication device is mounted on an artificial satellite, the change amount (vector) of the position of the artificial satellite from time t0 to time t2 is determined from the orbital information of the artificial satellite. In addition, the period from time t0 to time t2 can be determined from the delay amount between the optical communication device and the opposing optical communication device. Therefore, the control unit 15 can determine the phase change amount to be set in each second cell of the spatial light phase modulator 12 by correcting the phase change amount to be given to each second cell obtained based on the detection result of the spatial light phase sensor 14 at time t0 by the phase change amount corresponding to the change amount (vector) of the position of the artificial satellite from time t0 to time t2. In this way, even when the relative positional relationship between the optical communication device and the opposing optical communication device changes over time and the delay between the optical communication device and the opposing optical communication device cannot be ignored, the transmission beam can be directed to the opposing optical communication device.

[0023] Furthermore, even if an optical beam with a uniform spatial phase distribution is transmitted into the atmosphere, the spatial phase distribution of the optical beam will not be uniform due to atmospheric turbulence. However, in the configuration of this embodiment, the detection result of the spatial optical phase sensor 14 includes the phase change due to atmospheric turbulence, so that the phase change due to atmospheric turbulence can also be compensated for, making the spatial phase distribution of the transmitted beam in the opposite optical communication uniform, that is, reducing the unevenness of the wavefront.

[0024] The optical communication device according to the present embodiment can also be applied to a case where the relative positional relationship with the opposing optical communication device does not change over time. Even when the relative positional relationship with the opposing optical communication device does not change over time, the phase change due to atmospheric fluctuation can be compensated for as described above.

[0025] The above configuration makes it possible to realize an optical compensator for FSO communications that does not require actuators for driving mirrors and lenses, and that compensates for temporal changes in the relative positional relationship with the opposing optical communications device through electrical control, or that compensates for phase changes due to atmospheric turbulence.

[0026] Second Embodiment Next, the second embodiment will be described, focusing on the differences from the first embodiment. Fig. 3 is a configuration diagram of an optical communication device according to this embodiment. An optical splitter 13 splits the received beam from the optical antenna 2 into at least two beams, and outputs them to a spatial optical phase sensor 14 and a spatial optical phase modulator 12. The received beam that has passed through the spatial optical phase modulator 12 is output to an optical receiver 4 by an optical circulator 11. The processing of the transmitted beam is the same as in the first embodiment.

[0027] In the first embodiment, the reception beam received and demodulated by the optical receiver 4 did not pass through the spatial light phase modulator 12. In this embodiment, the optical receiver 4 receives the reception beam that has passed through the spatial light phase modulator 12. The amount of phase change provided by each second cell of the spatial light phase modulator 12 is the inverse of the phase distribution of the reception beam detected by the spatial light phase sensor 14, so the spatial phase distribution of the reception beam approaches uniformity due to the phase change provided by the spatial light phase modulator 12. In other words, it is possible to reduce the unevenness of the wavefront of the reception beam.

[0028] <Third embodiment> Next, the third embodiment will be described, focusing on the differences from the second embodiment. Fig. 4 is a configuration diagram of an optical communication device according to this embodiment. In this embodiment, a "transmission type" spatial optical phase sensor 14 is used. Therefore, the optical splitter 13 in Fig. 3 can be omitted.

[0029] <Fourth embodiment> Next, the fourth embodiment will be described with a focus on the differences from the first to third embodiments. FIG. 5 is a configuration diagram of an optical communication device according to this embodiment. In this embodiment, a wavefront sensor 17 is used instead of the spatial light phase sensor 14 of the first to third embodiments, and a spatial light modulator 16 is used instead of the spatial light phase modulator 12. The wavefront sensor 17 detects the intensity (amplitude) of the received beam in each first cell in addition to the phase difference of the received beam in each first cell with respect to the first reference phase. The spatial light modulator 16 controls the intensity (amplitude) as well as the phase of the transmitted beam passing through each second cell. The control unit 15 sets the amount of phase change in each second cell of the spatial light modulator 16 so that the phase is controlled in the same manner as in the first to third embodiments.

[0030] Regarding intensity, the control unit 15 sets an attenuation amount for each second cell of the spatial light modulator 16 so that the intensity of the transmission beam after passing through the corresponding second cell increases as the detection level in the first cell increases. As an example, when the intensity of the reception beam detected in the first cell by the wavefront sensor 17 is X times the first reference intensity, the control unit 15 sets an attenuation amount to be applied in the corresponding second cell so that the intensity of the transmission beam after passing through the corresponding second cell becomes X times the second reference intensity. Note that the first reference intensity is a value arbitrarily determined by the wavefront sensor 17, and the second reference intensity is a value arbitrarily determined by the control unit 15.

[0031] In other words, the control unit 15 determines the spatial intensity distribution (first intensity distribution) of the received beam based on the detection result of the intensity of the received beam by the wavefront sensor 17. The control unit 15 determines the second intensity distribution based on the first intensity distribution, and sets the attenuation amount in each second cell of the spatial light modulator 16 so that the spatial intensity distribution of the transmitted beam that has passed through the spatial light modulator 16 becomes the determined second intensity distribution. For example, the control unit 15 determines the same distribution as the first intensity distribution as the second intensity distribution. With this configuration, the second complex amplitude distribution that takes into account both the second intensity distribution and the second phase distribution of the transmitted beam becomes a complex conjugate distribution of the first complex amplitude distribution that takes into account both the first intensity distribution and the first phase distribution of the received beam.

[0032] As described above, by making the second complex amplitude distribution the complex conjugate distribution of the first complex amplitude distribution, it is possible to accurately direct the transmission beam toward the opposing optical communication device.

[0033] In this embodiment, the second intensity distribution is the same as the first intensity distribution, but it is sufficient that the maximum value of the difference between the second intensity distribution and the first intensity distribution is equal to or smaller than a second threshold value obtained in advance.

[0034] With the above configuration, it is possible to provide an optical compensator for FSO communications that can resolve the issues mentioned above, thereby contributing to Goal 9 of the United Nations Sustainable Development Goals (SDGs) to "build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0035] The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention. [Explanation of symbols]

[0036] 14: spatial light phase sensor, 12: spatial light phase modulator, 15: control unit

Claims

1. 1. An optical compensator for an optical communication device for free space optical communications, comprising: a detector for detecting a first phase distribution of a received beam from an opposing optical communication device, the first phase distribution being in a first plane perpendicular to a propagation direction of the received beam; an adjustment means for adjusting a transmission beam to be transmitted to the opposing optical communication device; a control means for determining a second phase distribution based on the first phase distribution and for controlling the adjustment means so that a phase distribution of the transmission beam in a second plane orthogonal to a propagation direction of the transmission beam becomes the second phase distribution; The optical compensator comprises:

2. The optical compensator according to claim 1 , wherein the control means determines the second phase distribution so that a maximum amount of phase change in a composite phase distribution obtained by combining the first phase distribution and the second phase distribution is equal to or less than a first threshold value.

3. 2. The optical compensator according to claim 1, wherein the control means determines a phase distribution obtained by inverting a phase indicated by the first phase distribution as the second phase distribution.

4. 2. The optical compensator according to claim 1, wherein the control means determines a third phase distribution obtained by inverting the first phase distribution, and determines the second phase distribution by correcting the third phase distribution based on a time change in the relative positional relationship between the opposing optical communication device and the optical communication device and a period from when the opposing optical communication device transmits the receiving beam to when the opposing optical communication device receives the transmitting beam.

5. 5. The optical compensator of claim 4, wherein the control means determines a first position of the counter optical communication device when the counter optical communication device transmits the receiving beam and a second position of the counter optical communication device when the counter optical communication device receives the transmitting beam based on a change over time in the relative positional relationship between the counter optical communication device and the optical communication device and a period from when the counter optical communication device transmits the receiving beam to when the counter optical communication device receives the transmitting beam, and determines the second phase distribution by correcting the third phase distribution based on the first position and the second position.

6. The optical compensator of claim 1 , wherein the adjusting means is a spatial optical phase modulator.

7. the detection means has a plurality of first cells arranged on the first plane; the adjustment means includes a plurality of second cells arranged on the second plane, each of the plurality of second cells corresponding to one of the plurality of first cells; Each of the plurality of first cells detects a first phase difference of the reception beam incident on the first cell with respect to a first reference phase; 2. The optical compensator of claim 1, wherein the control means controls the adjustment means so that the absolute value of a second phase difference of the transmission beam passing through a second cell relative to a second reference phase is equal to the absolute value of the first phase difference detected in a first cell corresponding to the second cell, and the sign is inverted.

8. The detection means further detects a first intensity distribution of the receive beam in the first plane; 2. The optical compensator of claim 1, wherein the control means further determines a second intensity distribution based on the first intensity distribution and controls the adjustment means so that the intensity distribution of the transmitted beam in the second plane becomes the second intensity distribution.

9. The optical compensator according to claim 8 , wherein the control means determines the second intensity distribution so that a maximum value of a difference between the first intensity distribution and the second intensity distribution is equal to or smaller than a second threshold value.

10. An optical compensator according to any one of claims 1 to 9; an optical transmitter that generates the transmission beam and outputs it to the adjustment means; an optical receiver for demodulating the received beam; an optical antenna that transmits the transmission beam adjusted by the adjustment means into space and receives the reception beam from the opposing optical communication device; An optical communication device comprising:

11. 11. The optical communication device according to claim 10, wherein the optical receiver receives the reception beam from the optical antenna via the adjustment means.

12. 11. The optical communication device according to claim 10, wherein the optical receiver receives the reception beam from the optical antenna without passing through the adjustment means.

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