Methods for improving the coupling efficiency between telescopes and optical fibers

Abstract

To improve the coupling efficiency between a telescope and an optical fiber or optical communication sensor at a ground station receiving light from a satellite. [Solution] Light emitted from the optical transmitter 41 passes through the telescope 11, is reflected by the optical reflector 43, enters the tracking camera 21 via the first optical system 23, and is received by the tracking camera 21. The drive control unit 31 maintains the state in which the first optical system 23 is controlled so that the light received by the tracking camera 21 is centered on the lens, and controls the second optical system 27 so that the light reflected by the optical reflector 43 enters the center of the sensor unit 25, and while maintaining the second optical system 27, causes the telescope 11 to perform an operation that simulates orbital tracking. The drive control unit 31 obtains drive control information to control one or more of the first optical system 23, the second optical system 27, and the attitude of the telescope 11 using the attitude of the telescope 11 during the orbital tracking process and the light receiving position in the sensor unit.

Description

Technical Field

[0001] This invention relates to a method for improving the coupling efficiency between a telescope and an optical fiber, etc.

Background Art

[0002] In recent years, communication services by satellite constellations have become popular. On the other hand, due to the limited RF frequency resources, attention has been focused on optical communication technologies between satellites and the ground for future increases in communication capacity. Furthermore, in order to realize the globalization of information-theoretically secure key sharing, the mounting of quantum key distribution devices on satellites is expected.

[0003] The technology of mounting a quantum key distribution (QKD) device on a satellite is already known. For example, Japanese Unexamined Patent Application Publication No. 2022-535712 describes a system and method for quantum key distribution via a hybrid quantum channel. Japanese Patent Application Publication No. 2024-505095 describes a QKD exchange system and protocol.

[0004] In quantum key distribution, the average number of photons per pulse of the emitted light is determined, and it is a very weak optical signal. However, considering the downlink to the ground, a system using a wavelength band that is safe for human eyes is expected. However, the wavelength of 1.5 μm in the eye-safe wavelength band is longer than visible light, and in principle, it is more difficult to collect light than visible light. On the other hand, in order to improve quantum key distribution and normal optical communication performance, it is necessary to reduce communication path loss as much as possible. In the terrestrial system, an optical system using a telescope is required. That is, the problem is how to improve the coupling efficiency to an optical fiber (for example, a single-mode fiber) and an optical communication sensor used in a telescope system and high-speed optical communication.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] Therefore, methods are needed to improve the coupling efficiency between a telescope and an optical fiber or optical communication sensor. In particular, in a quantum key distribution system, methods are needed to improve the coupling efficiency between a telescope and an optical fiber in a ground station having a satellite and a telescope that receives light from the satellite. [Means for solving the problem]

[0007] This method is based on the findings from past implementations, which show that by performing a simulated operation that tracks the satellite's orbit, measuring the alignment deviation of the ground station's optical system, predicting the actual deviation during satellite tracking, and then preparing an optical system with an offset adjustment for that deviation, the coupling efficiency between the telescope and the optical fiber or optical communication sensor can be improved.

[0008] The first invention relates to a method for improving the coupling efficiency between a telescope 11 and an optical fiber 13 or optical communication sensor in an optical circuit section 17 for an optical ground station 15 that transmits an optical signal received by the telescope 11 to an optical fiber 13 or an optical communication sensor.

[0009] The optical circuit section 17 includes a tracking camera 21, a first optical system 23, a sensor section 25, a second optical system 27, an optical coupling section 29, and a drive control section 31. The tracking camera 21 is an element for receiving at least a portion of the light output from the telescope 11. The first optical system 23 is an element for guiding the light output from the telescope 11 to the tracking camera 21. The sensor unit 25 is an element for receiving at least a portion of the light output from the telescope 11. The second optical system 27 is an element for guiding the light output from the telescope 11 to the sensor unit 25. The optical coupling section 29 is an element for optically coupling the telescope 11 with the optical fiber 13 or the optical communication sensor. The drive control unit 31 is an element for controlling the attitude of the first optical system 23, the second optical system 27, and the telescope 11.

[0010] The above method includes the following steps. An optical transmitter 41 is installed to output light to the optical coupling unit 29. An optical reflector 43 is installed to reflect the light emitted from the optical transmitter 41, through the optical circuit section 17, and after passing through the telescope 11. Light is emitted from the light transmitter 41. Light emitted from the optical transmitter 41 passes through the optical circuit section 17, then through the telescope 11, is reflected by the optical reflector 43, and enters the tracking camera 21 via the first optical system 23, where it is received by the tracking camera 21. The drive control unit 31 controls the first optical system 23 so that the light received by the tracking camera 21 is centered on the lens of the tracking camera 21 (first control step). The drive control unit 31 maintains the control state of the first optical system 23 in the first control step and controls the second optical system 27 so that the light reflected by the optical reflection unit 43 is incident on the center of the sensor unit 25 (second control step). The drive control unit 31 maintains the control state of the second optical system 27 in the second control process and causes the telescope 11 to perform an operation that simulates orbital tracking (orbital tracking process). In the trajectory tracking process, the drive control unit 31 stores the light reception position at the sensor unit observed by the sensor unit 25 (light position storage process). The drive control unit 31 stores the relationship between the attitude of the telescope 11 during the orbit tracking process and the light-receiving position in the sensor unit (telescope attitude and light-receiving position storage process). Using the attitude of the telescope 11 during the orbit tracking process and the light-receiving position in the sensor unit, drive control information is obtained to control one or more of the attitudes of the first optical system 23, the second optical system 27, and the telescope 11 (drive control information acquisition process). [Effects of the Invention]

[0011] According to this invention, a method for improving the coupling efficiency between a telescope and an optical fiber or an optical communication sensor can be provided. According to this invention, particularly in a quantum key distribution system, in a ground station having a telescope for receiving light from a satellite, a method for improving the coupling efficiency between the telescope and an optical fiber or an optical communication sensor can be provided.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a conceptual diagram for explaining an example of the relationship between a satellite, a ground station, and a receiver. [Figure 2] FIG. 2 is a block diagram for explaining an optical circuit section of an optical ground station. [Figure 3] FIG. 3 is a block diagram for explaining a configuration example of an alignment adjustment system. [Figure 4] FIG. 4 is a conceptual diagram for explaining an example of a system in an embodiment. [Figure 5] FIG. 5 is a block diagram showing an alignment adjustment system used in an embodiment.

Embodiments of the Invention

[0013] This invention relates to a method for improving the coupling efficiency between a telescope and an optical fiber or an optical communication sensor. One example of the use of this invention is quantum key distribution (QKD) using a satellite, and since the coupling efficiency between a telescope and an optical fiber (particularly a single-mode fiber) can be effectively improved, hereinafter, taking quantum key distribution using a satellite as an example, a method for improving the coupling efficiency between a telescope and an optical fiber or an optical communication sensor will be described.

[0014] FIG. 1 is a conceptual diagram for explaining an example of the relationship among a satellite, a terrestrial station, and a receiver. In the example shown in FIG. 1, an optical terrestrial station 15 receives an optical signal from a satellite 3, and the received optical signal is appropriately analyzed by the optical terrestrial station 15 to communicate with a receiver 7. The satellite 3 is an element that moves while communicating with the optical terrestrial station 15 according to a predetermined orbit. Satellite communication itself is already known. Therefore, known satellite communication can be appropriately used. The example of the satellite 3 is a satellite for quantum key distribution adapted to generate a quantum key distribution (QKD) free-space signal for a telescope 11. In this example, the satellite 3 is compatible with the telescope 11, and the telescope 11 can receive the QKD free-space signal output from the satellite 3.

[0015] The optical terrestrial station 15 includes a telescope 11 for receiving an optical signal from the satellite, an optical circuit unit 17 for receiving light from the telescope 11, and an optical fiber 13 (or an optical coupling unit 29) for connecting the light passing through the optical circuit unit 17 to the receiver 7. The optical terrestrial station 15 may have an optical communication sensor instead of the optical fiber 13. The optical communication sensor may be a small optical communication sensor used for free-space coupling. Instead of the optical fiber 13 in FIG. 1, the optical communication sensor may be optically coupled to an optical coupler 29. In the example of FIG. 1, the receiver 7 exists within a QKD remote receiving station 9. The optical terrestrial station 15 may be fixed on the ground or movable. Examples of the optical terrestrial station 15 are a QKD fixed terrestrial station and a QKD portable terrestrial station. The optical terrestrial station 15, for example, transmits a QKD free-space signal to the optical fiber 13 via the optical circuit unit 17, and transmits it via the optical fiber 13 to the QKD remote receiving station 9 installed inside the optical terrestrial station 15 or at a location different from the optical terrestrial station 15. A preferred example of the optical fiber 13 is a single-mode fiber. In the following example, the mode in which the telescope 11 is optically coupled to the optical fiber 13 via the optical circuit unit 17 will be used for explanation. However, using the same principle, the mode in which the telescope 11 is optically coupled to the optical communication sensor via the optical circuit unit 17 can also be explained.

[0016] Figure 2 is a block diagram illustrating the optical circuit section of the optical ground station. As shown in Figure 2, the optical circuit section 17 includes a tracking camera 21, a first optical system 23, a sensor section 25, a second optical system 27, an optical coupling section 29, and a drive control section 31. The tracking camera 21 is an element for receiving at least a portion of the light output from the telescope 11. The tracking camera 21 may be controlled by a drive control unit 31, which will be described later, and the drive control unit 31 may be able to control the position and orientation of the telescope 11 based on the output of the tracking camera 21. The tracking camera 21 can display images taken by the telescope 11. The tracking camera 21 only needs to be able to detect light over a certain area, and may be a general-purpose camera, a high-resolution camera for precise tracking, or a CCD camera, etc.

[0017] The first optical system 23 is an element for guiding the light output from the telescope 11 to the tracking camera 21. In the example in Figure 2, the first optical system 23 includes a pupil 33 that receives the light output from the telescope 11, a polarizing plate 35 for adjusting the polarization plane of the light emitted from the pupil 33, a beam splitter (BS) 37 for separating the light emitted from the polarizing plate 35, and a first movable mirror 39 to which a portion of the light separated by the BS 37 is input. The attitude (direction and position, or both, hereinafter the same) of the first movable mirror 39 can be adjusted by the drive control unit 31. The drive control unit 31 may also be able to adjust the aperture diaphragm of the pupil 33. The drive control unit 31 may also be able to adjust the polarization plane of the polarizing plate 35. When the drive control unit 31 adjusts the position and direction of the first movable mirror 39, the position at which the light emitted from the telescope 11 enters the tracking camera 21 can be adjusted. The beam splitter (BS) functions as a light separation unit. The mirror is used to adjust the direction of light propagation, and therefore functions as a light propagation direction adjustment unit. In the first optical system 23, the pupil 33 and polarizer 35 are optional.

[0018] The sensor unit 25 is an element for receiving at least a portion of the light output from the telescope 11. The sensor unit 25 can be any device (light sensor) that can receive light over a certain area and detect the received light. Examples of the sensor unit 25 include a quadrature detection (QD) sensor and a position sensor.

[0019] The second optical system 27 is an element for guiding the light output from the telescope 11 to the sensor unit 25. The second optical system 27 can also be described as an element for guiding the light that has passed through the first optical system 23 (light output from the telescope 11) to the sensor unit 25. In the example in Figure 2, the second optical system 27 includes a first wavelength filter 51 into which the light that has passed through the first optical system 23 is incident, a second movable mirror 53 into which the light that has passed through the first wavelength filter 51 is incident, a second wavelength filter 55 into which the light that has passed through the second movable mirror is incident, and a second beam splitter (BS) 57 into which the light that has passed through the second wavelength filter 55 is incident. A portion of the light separated by the BS 57 is then emitted to the sensor unit 25. In Figure 2, 59 is an optional element, a lens. The lens 59 can adjust the reflected light. The second movable mirror 53 is designed so that its orientation can be adjusted by the drive control unit 31. When the drive control unit 31 adjusts the orientation of the second movable mirror 31, the position at which the light emitted from the telescope 11 enters the sensor unit 25 can be adjusted.

[0020] The optical coupler 29 is an element for optically coupling the telescope 11 and the optical fiber 13. In the example shown in Figure 2, the optical coupler 29 functions as an element for coupling the light emitted from the telescope 11, which has passed through the first optical system 23 and the second optical system 27, with the optical fiber 13. Examples of optical couplers 29 include photocouplers and fiber couplers.

[0021] The drive control unit 31 is an element for controlling the attitudes of the first optical system 23, the second optical system 27, and the telescope 11. The drive control unit 31 may be implemented by a computer. In addition to controlling the attitudes of the first optical system 23, the second optical system 27, and the telescope 11, the drive control unit 31 may also control various drive systems. Although not specifically shown, it is preferable that the drive control unit 31 is connected to the various elements wirelessly or by wire to enable the exchange of information with the elements to be controlled.

[0022] A computer has an input unit, an output unit, a control unit, an arithmetic unit, and a memory unit, and each element is connected by a bus or the like to enable the exchange of information. Computers usually handle digital information. For example, a computer may store programs or various kinds of information in its memory unit. When predetermined information is input from the input unit, the control unit reads the program stored in the memory unit. The control unit then reads the information stored in the memory unit as appropriate and transmits it to the arithmetic unit. The control unit also transmits the input information to the arithmetic unit as appropriate. The arithmetic unit performs calculations using the received information and stores it in the memory unit. The control unit reads the calculation results stored in the memory unit and outputs them from the output unit. In this way, various processes and steps are executed. Each unit and each means is responsible for executing these various processes. A computer may have a processor, and the processor may implement various functions and steps. A computer may be standalone. A computer may have some of its functions distributed between a server and terminals. In that case, it is preferable that the server and terminals can exchange information via a network such as the internet or an intranet. A computer may include a processor and memory connected to the processor. The memory may store instructions, and when executed by the processor, these instructions may cause the computer to perform various processes and function as various components. The computer may build a learning model by providing various training data and perform various calculations through machine learning. In this case, the computer may perform various analyses and interpretations using the learning model created by AI (artificial intelligence) machine learning and deep learning.

[0023] Next, we will describe a method for improving the coupling efficiency between the telescope and the optical fiber. This method includes the following steps. Alignment adjustment system construction process An optical transmitter 41 and an optical reflector 43 are installed at the optical ground station 15. If either or both of the optical transmitter 41 and the optical reflector 43 are already installed at the optical ground station 15, the already installed equipment may be used. An optical receiver 45 may also be attached to the optical ground station 15. In this way, an alignment adjustment system can be constructed. Figure 3 is a block diagram illustrating an example configuration of the alignment adjustment system. Of the elements in Figure 3, those included in Figure 2 are described by reference and their explanations are omitted.

[0024] The optical transmitter 41 is an element for outputting light to the optical coupling unit 29 included in the optical circuit unit 17 of the optical ground station 15. The optical transmitter 41 can function as any light source.

[0025] The optical reflector 43 is an element that reflects light emitted from the light transmitter 41, through the optical circuit section 17, and after passing through the telescope 11. Examples of the optical reflector 43 include mirrors, reflectors, and corner cubes.

[0026] The alignment adjustment system constructed in this manner is used to obtain offset information for improving the coupling efficiency between the telescope and the optical fiber. This specification also describes a method for obtaining offset information for improving the coupling efficiency between the telescope and the optical fiber using the alignment adjustment system.

[0027] Light emission process First, light is emitted from the optical transmitter 41. The light emitted from the optical transmitter 41 is assumed to be light emitted from satellite 3 and reaching telescope 11, and may, for example, be light with wavelengths in the infrared region, or it may be weak light. The light output by the optical transmitter 41 may be continuous light or pulsed light.

[0028] First control process Light emitted from the optical transmitter 41 passes through the optical circuit section 17, then through the telescope 11, is reflected by the optical reflector 43, and enters the tracking camera 21 via the first optical system 23, where it is received by the tracking camera 21. Specifically, light emitted from the optical transmitter 41 is propagated through the optical coupling section 29 to the optical circuit section 17. In the example in Figure 3, the light that has passed through the optical coupling section 29 passes through the lens 59 and then through the second optical system 27. The light that has passed through the second optical system 27 passes through the first optical system 23 and is emitted to the telescope 11. The light emitted to the telescope 11 passes through the telescope 11 and is reflected by the optical reflector 43. The light reflected by the optical reflector 43 enters the telescope 11 (as if it were light from a satellite). Light passing through the telescope 11 enters the tracking camera 21 via the first optical system 23 and is received by the tracking camera 21. Specifically, light passing through the telescope 11 enters the first BS 37 via the pupil 33 and polarizer 35. A portion of the light entering the first BS has its direction of travel adjusted by the first movable mirror 37 and proceeds to the tracking camera 21. The light entering the tracking camera 21 is received (photographed) by the tracking camera 21. Information regarding the light received by the tracking camera and information regarding the position of the received light in the lens of the tracking camera, or both, are transmitted to the drive control unit 31.

[0029] The drive control unit 31 controls the first optical system 23 so that the light received by the tracking camera 21 is centered on the lens of the tracking camera 21 (first control step). When the drive control unit 31 receives the above control information from the tracking camera 21, it calculates a control command to control the first optical system 23 (for example, the attitude of the first movable mirror 39) based on the program's commands. The drive control unit 31 stores the calculated control command in the memory unit as appropriate and outputs a command to control the first optical system 23 to the first optical system 23. The first optical system 23 (for example, the first movable mirror 39) has an actuator that is driven based on the command. Therefore, the first optical system 23 can be controlled based on the command from the drive control unit 31. The alignment of the first optical system 23 is achieved by the first control step.

[0030] Second control process The drive control unit 31 maintains the control state of the first optical system 23 in the first control step and controls the second optical system 27 so that the light reflected by the optical reflector 43 is incident on the center of the sensor unit 25 (second control step). The first control step aligns the first optical system 23. The first optical system 23 is maintained in that state. Then, the light output from the aligned first optical system 23 passes through the second optical system 27 and is incident on the sensor unit 25. In the example in Figure 3, the light output from the first optical system 23 passes through the first wavelength filter 51 to remove unwanted wavelength components and reaches the second movable mirror 53. The light whose direction of travel has been adjusted by the second movable mirror 53 passes through the second wavelength filter and is incident on the second BS57. A portion of the light incident on the second BS57 has its direction of travel adjusted so that it is directed towards the sensor unit 25. The sensor unit 25 receives the light. Light incident on the sensor unit 25 is received (captured) by the sensor unit 25. Information regarding the light received by the sensor unit 25 and information regarding the position of the received light on the sensor surface of the sensor unit 25, or both, are transmitted to the drive control unit 31.

[0031] The drive control unit 31 controls the second optical system 27 so that the light received by the sensor unit 25 is centered on the lens of the sensor unit 25. For example, when the drive control unit 31 receives the above control information from the sensor unit 25, it calculates a control command to control the second optical system 27 (for example, the posture of the second movable mirror 53) based on the program's commands. The drive control unit 31 stores the calculated control command in the memory unit as appropriate and outputs a command to control the second optical system 27 to the second optical system 27. The second optical system 27 (for example, the second movable mirror 53) has an actuator that is driven based on the command. Therefore, the second optical system 27 can be controlled based on the command from the drive control unit 31. The alignment of the second optical system 27 is achieved by the second control step.

[0032] Trajectory tracking process The drive control unit 31 maintains the control state of the second optical system 27 in the second control process and causes the telescope 11 to perform an action that simulates orbital tracking (orbital tracking process). An action that simulates orbital tracking means an action that simulates the action the telescope 11 would take when tracking the orbit of the satellite 3, rather than actually tracking the satellite 3. The memory unit of the drive control unit 31 stores information on the change in the attitude of the telescope 11 when the telescope 11 received light from the satellite in the past and tracked the satellite's orbit, or control information for changing the telescope 11 to that attitude. Such stored information can be obtained by storing information on the change in the attitude of the telescope 11 when the telescope 11 actually tracked the satellite 3, or by storing commands for driving the telescope 11. The telescope 11 has a telescope drive unit 61 for driving the telescope 11. Therefore, the attitude (position and orientation) of the telescope 11 can be controlled by driving the drive unit 61 based on the commands output by the drive control unit 31. The drive control unit 31 reads the control information described above from the memory unit and drives the drive unit 61 to cause the telescope 11 to perform an operation that simulates orbital tracking. In other words, even if the telescope 11 does not actually trace the change in its relative position with the satellite 3, the drive control unit 31 can cause the telescope 11 to perform an operation as if it were tracking the orbit of the satellite 3. Note that there may be multiple types of satellites 3, and information corresponding to each satellite may be stored so that an operation simulating orbital tracking corresponding to each satellite 3 can be performed. In the orbital tracking process, although the telescope 11 is not actually tracking the satellite 3, the light reflected by the optical reflector 43 and incident on the telescope 11 functions as if it were light from the satellite 3.

[0033] Optical position memory process The drive control unit 31 stores the light reception position at the sensor unit observed by the sensor unit 25 during the orbit tracking process (light position storage process). As explained earlier, during the orbit tracking process, the telescope 11 performs an operation that simulates orbit tracking while maintaining the control state of the second optical system 27 in the second control process (the state in which the first and second optical systems 23 and 27 are aligned). During the orbit tracking process, the light transmitter 41 continues to emit light. As a result, the position of the incident light at the sensor unit observed by the sensor unit 25 changes in accordance with the attitude of the telescope 11. During the orbit tracking process, the drive control unit 31 stores the light reception position at the sensor unit, which changes over time in relation to the change in the attitude of the telescope 11, in the storage unit. The drive control unit 31 may also store the light reception position at each timing of the orbit tracking process in the storage unit.

[0034] Telescope attitude and light-receiving position memory process The drive control unit 31 stores the relationship between the attitude of the telescope 11 during the orbit tracking process and the light-receiving position at the sensor unit. For example, the drive control unit 31 associates the attitude of the telescope 11 with the light-receiving position at the sensor unit at multiple timings during the orbit tracking process and stores this information in the memory unit.

[0035] To obtain multiple data points, the orbit tracking process, the optical position memory process, and the telescope attitude and light reception position memory process may be repeated.

[0036] Drive control information acquisition process Using the attitude of the telescope 11 during the orbit tracking process and the light-receiving position in the sensor unit, drive control information is obtained to control one or more of the attitudes of the first optical system 23, the second optical system 27, and the telescope 11 (drive control information acquisition process). The drive control unit 31 reads information from the memory unit regarding the relationship between the attitude of the telescope 11 and the light-receiving position of the sensor unit, and analyzes drive control information such that, in a given attitude of the telescope 11, the light-receiving position of the sensor unit is at the center of the sensor unit. The drive control unit 31 may also store in advance drive control information for controlling one or more of the first optical system 23, the second optical system 27, and the attitude of the telescope 11, corresponding to the light-receiving positions of various sensor units, and read the corresponding drive control information from the memory unit for the attitude of the telescope 11. In this way, offset information (information for correcting the discrepancy between the actual attitude of the telescope 11 and the light-receiving position of the sensor unit) can be obtained to correct the drive control information of the first optical system 23, the second optical system 27, and the telescope 11 when the telescope 11 is actually tracking the satellite. Therefore, in this system, when the telescope 11 actually tracks the satellite, the sensor unit can observe the signal appropriately by controlling one or more of the first optical system 23, the second optical system 27, and the attitude of the telescope 11, while taking offset information into consideration.

[0037] Trajectory tracking process The drive control information acquisition step may include a step of controlling the second optical system 27 to pre-apply an offset when a correlation is found between the attitude of the telescope 11 in the orbit tracking step and the light-receiving position at the sensor unit. The drive control unit 31 examines whether there is a correlation between the attitude of the telescope 11 during the orbit tracking process and the light-receiving position in the sensor unit. The drive control unit 31 may also have a correlation analysis unit. For example, if the orientation of the telescope 11 is facing a specific direction, and the light-receiving position in the sensor unit is located in that specific direction, then it can be said that there is a correlation between the attitude of the telescope 11 and the light-receiving position in the sensor unit. Therefore, when tracking the satellite 3 and the orientation of the telescope 11 becomes that specific direction, control information (offset information) should be obtained such that the light received by the sensor unit is in the opposite direction to that specific direction. Then, when actually tracking the satellite 3 and the orientation of the telescope 11 becomes that specific direction, the drive control unit 31 controls the drive system with that offset applied, so that the sensor unit 25 can appropriately receive the optical signal from the satellite 3.

[0038] The trajectory tracking process may be performed using a pre-trained model obtained through machine learning.

[0039] How to build a pre-trained model In this case, a trained model is constructed using training data to estimate the offset. To construct the trained model, for example, a dataset is prepared in which the attitude of the telescope 11 and the light-receiving position at the sensor unit 25 are used as features, and the offset (correction information for adjusting the drive system) is used as a label. These datasets are used as training data with answers (labeled data) to train the AI ​​model. The AI ​​learns from these datasets and is trained to predict the correct label for new data. In this way, a trained model can be constructed in which the attitude of the telescope 11 and the light-receiving position at the sensor unit 25 are used as features. Furthermore, the accuracy of the trained model can be improved by inputting sample features into the trained model and feeding back the resulting label, the offset (performing model learning). This invention also provides a trained model for estimating the offset. Moreover, the computer described above may be a computer that further has a trained model for estimating the offset.

[0040] Method for estimating offsets using a pre-trained model When the optical ground station 15 actually tracks satellite 3, it reads out the attitude of the telescope 11 corresponding to the position of satellite 3 and the light-receiving position at the sensor unit 25 as feature quantities. The computer inputs the features into a trained model. As described above, the trained model is trained using multiple datasets so that when features are input, it can output offset information, which is a label. Therefore, when features are input into the trained model, offset information can be obtained. The offset information obtained in this way is stored in the memory unit as appropriate. Then, when the optical ground station 15 actually tracks satellite 3, the drive control unit 31 controls the attitude of the telescope 11, taking into account the obtained offset information.

[0041] Quantum key distribution (QKD) method Next, the quantum key distribution (QKD) method will be described. This method uses the previously described method and includes steps to improve the coupling efficiency between the telescope and the optical fiber as appropriate. Then, in this method, satellite 3 outputs the QKD free-space signal. Next, the telescope 11 of the optical ground station 15 receives the QKD free-space signal. The optical ground station 15 transmits the QKD free-space signal to the optical fiber 13 via the optical circuit section 17, and also transmits it to the QKD remote receiving station 7 via the optical fiber 13. At this time, the relative relationship between satellite 3 and optical ground station 15 changes. From the perspective of optical ground station 15, satellite 3 is observed to be displacing along a predetermined orbit. Optical ground station 15 changes the attitude of telescope 11 in response to the displacement of satellite 3. At this time, since the offset is stored in the memory unit, the drive control unit 31 does not control the attitude of telescope 11 to correspond to the actually observed position of satellite 3, but rather changes the attitude of telescope 11 after taking the read-out offset into consideration. In this way, the quantum key signal can be appropriately received and transmitted to the QKD remote receiving station 7. [Examples]

[0042] Figure 4 is a conceptual diagram illustrating an example of the system in the embodiment. Figure 5 is a block diagram showing the alignment adjustment system used in the embodiment. The following describes a highly efficient optical coupling method for optical communication telescopes using the above-mentioned ground stations. 1. First, light is emitted from the optical transmitter. The light is reflected by the corner cube after passing through the optical circuit. The light reflected by the corner cube enters the tracking camera. The first movable mirror is adjusted so that the light entering the tracking camera aligns with the center of the camera's lens. In the example in Figure A1, a portion of the light reflected by the corner cube, separated by the first beam splitter (BS), enters the tracking camera via the first movable mirror. By adjusting the position and orientation of the first movable mirror, the position of the light entering the tracking camera can be adjusted.

[0043] 2.1 Adjust the second movable mirror so that light enters the center of the QD sensor (or position sensor). In the example in Figure A1, the remaining light separated by the first beam splitter (BS) from the light reflected by the corner cube enters the second beam splitter (BS) via the second movable mirror. A portion of the light that enters the second beam splitter (BS) enters the QD sensor (or position sensor). By adjusting the position and orientation of the second movable mirror, the position of the light entering the QD sensor (or position sensor) can be adjusted. In this way, primary alignment of the optical system can be performed.

[0044] 3.2 The telescope performs an operation that simulates tracking the planned orbit for observation. During step 4.3, the position of the light (light receiving position) at the QD sensor (or position sensor) is recorded. 5. Record the relationship between the telescope's status (position and orientation) and the light-receiving position of the QD sensor (or position sensor). Repeat steps 6.4-5 (or 3-5) to confirm the correlation between the telescope's attitude (position and orientation) and the light-receiving position of the QD sensor (or position sensor). If a correlation is observed in 7.6, an offset is pre-applied to the movement of the second movable mirror so that it focuses light towards the center of the position sensor relative to the attitude (position) of the telescope during actual observation, thereby forming an optical link with the satellite. In fact, when quantum key distribution was performed based on this protocol, the optical coupling performance between the telescope and the single-mode fiber was significantly improved.

[0045] In Japan, temperatures fluctuate from below freezing to nearly 40 degrees Celsius. Therefore, temperature differences can cause optical axis misalignment due to expansion and contraction of the optical system. Furthermore, while telescopes have Cassegrain, Nasmyth, and Coudet focuses, optical axis misalignment becomes increasingly apparent as the optical path lengthens and the number of mirrors increases. Additionally, when tracking low-Earth orbit satellites, telescopes adjust their elevation azimuth, and this process also results in optical axis misalignment due to mechanical inaccuracies. According to this invention, these misalignments can be measured in advance, facilitating optical axis alignment during actual optical communication. Therefore, this invention is an extremely useful technology for optical communication between satellites and the ground. [Industrial applicability]

[0046] This invention can be preferably used in the field of satellite communications. In particular, this invention can be used in the field of quantum key distribution technology in which a quantum key distribution device is mounted on a satellite. [Explanation of Symbols]

[0047] 3 satellites 7 Receiver 9 QKD Remote Receiving Stations 11 Telescope 13 Optical Fiber 15 Optical ground station 17 Optical circuit section 21 Tracking Camera 23. First optical system 25 Sensor section 27. Second Optical System 29 Optical coupling part 31 Drive control unit 33 Hitomi 35 Polarizing plates 37 Beam Splitter 39. First movable mirror 41 Optical Transmitter 43 Optical reflector 51 First wavelength filter 53. Second movable mirror 55 Second wavelength filter 57. Second beam splitter 59 lenses 61 Telescope drive mechanism

Claims

1. A method for improving the coupling efficiency between a telescope and an optical fiber in an optical circuit section (17) for an optical ground station (15) that transmits an optical signal received by a telescope (11) to an optical fiber (13) or an optical communication sensor, The optical circuit section (17) is A tracking camera (21) that receives at least a portion of the light output from the telescope (11), A first optical system (23) for guiding the light output from the telescope (11) to the tracking camera (21), A sensor unit (25) that receives at least a portion of the light output from the telescope (11), A second optical system (27) for guiding the light output from the telescope (11) to the sensor unit (25), An optical coupling section (29) for optically coupling with the optical fiber (13), The system includes the first optical system (23), the second optical system (27), and a drive control unit (31) for controlling the attitude of the telescope (11), The steps include installing an optical transmitter (41) for outputting light to the optical coupling unit (29), The steps include installing an optical reflector (43) for reflecting light that has passed through the telescope (11), The process of emitting light from the light transmitter (41), The process involves the light emitted from the optical transmitter (41) passing through the telescope (11), being reflected by the optical reflector (43), and being received by the tracking camera (21) via the first optical system (23). The drive control unit (31) performs a first control step of controlling the first optical system (23) so that the light received by the tracking camera (21) is centered on the lens of the tracking camera (21), The drive control unit (31) performs a second control step in which, while maintaining the control state of the first optical system (23) in the first control step, the drive control unit (31) controls the second optical system (27) so that the light reflected by the optical reflector (43) is incident on the center of the sensor (25), The drive control unit (31) performs an orbit tracking step in which it causes the telescope (11) to perform an operation that simulates orbit tracking while maintaining the control state of the second optical system (27) in the second control step, The drive control unit (31) performs a light position storage step in which it stores the light reception position observed by the sensor unit (25) during the trajectory tracking step, The drive control unit (31) performs a telescope attitude and light-receiving position storage step, which is a step of storing the relationship between the attitude of the telescope (11) in the orbit tracking step and the light-receiving position in the sensor unit, A drive control information acquisition step is a step for obtaining drive control information for controlling one or more of the first optical system (23), the second optical system (27), and the attitude of the telescope (11) using the attitude of the telescope (11) in the orbit tracking step and the light receiving position in the sensor unit, Methods that include...

2. The method according to claim 1, The optical ground station (15) is configured so that the telescope (11) can receive the QKD free-space signal from a quantum key distribution satellite (3) adapted to generate the QKD free-space signal. A method for transmitting the QKD free-space signal via the optical circuit section (17) to the optical fiber (13), and then transmitting it via the optical fiber (13) to a QKD remote receiving station (9) located at a different location from the optical ground station (15).

3. The method according to claim 2, The optical fiber (13) is a single-mode fiber in this method.

4. The method according to claim 2, The drive control information acquisition step is a method that includes a step of controlling the second optical system (27) to pre-apply an offset when a correlation is found between the attitude of the telescope (11) in the orbit tracking step and the light receiving position in the sensor unit.

5. A quantum key distribution (QKD) method using any of the methods of claims 2 to 4, The process involves the satellite (3) outputting the QKD free-space signal, The process involves the telescope (11) of the optical ground station (15) receiving the QKD free space signal, A method comprising the optical ground station (15) transmitting the QKD free-space signal to the optical fiber (13) via the optical circuit section (17), and transmitting it to the QKD remote receiving station (9) via the optical fiber (13).

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