Optical space communication transceiver terminal and optical space communication system

Abstract

An optical space communication transceiver terminal (1A) comprises: an optical transceiver (2, 2A) which converts a transmission optical signal modulated at a symbol rate synchronized with a transmission clock signal and outputs it, and generates reception data based on the received optical signal; an intensity modulation unit (4) which generates a transmission optical signal modulated by intensity; a transmission clock source (3) which supplies a common transmission clock signal to the optical transceiver (2, 2A) and the intensity modulation unit (4); an optical fiber coupling unit (5) which outputs the intensity modulated transmission optical signal as collimated light and couples the input optical signal with the optical fiber; an optical fiber amplifier (9) which amplifies the optical signal coupled with the optical fiber; a tracking reflector (6) which outputs the optical signal to the optical fiber coupling unit (5) and outputs the transmission optical signal to space; a beam splitter (7) which distributes a part of the optical signal; and an angle sensor (8) which outputs a clock signal extracted from the optical signal distributed by the beam splitter (7) to the optical transceiver (2, 2A).

Description

Technical Field

[0001] The present disclosure relates to an optical space communication transceiver terminal and an optical space communication system. Background Art

[0002] Since optical space communication has the characteristics that the miniaturization of transceivers and low power consumption can be expected, research is being conducted on feeder links used in next-generation satellite communication networks (for example, refer to non-patent document 1). For transceivers in optical space communication, optical fiber amplifiers are used to improve receiving sensitivity. An optical fiber amplifier is an amplifier that amplifies an optical signal coupled to an optical fiber. The optical signal amplified by the optical fiber amplifier is output to an optical transceiver. The optical transceiver extracts a clock signal corresponding to the symbol rate from the optical signal amplified by the optical fiber amplifier, samples the received optical signal synchronously with the extracted clock signal, and thereby performs demodulation processing on the received optical signal.

[0003] Prior art literature

[0004] Non-patent literature

[0005] Non-patent literature 1: H.Hauschildt et al., "HydRON: High thRoughput OpticalNetwork," 2019 IEEE International Conference on Space Optical Systems and Applications (ICSOS), Portland, OR, USA, 2019. Summary of the invention

[0006] Problem that the invention aims to solve

[0007] When using an optical fiber amplifier, it is necessary to couple the optical signal propagating in the atmosphere to the optical fiber by converging it. However, the optical signal propagating in the atmosphere is affected by atmospheric fluctuations, causing wavefront distortion, and thus, loss occurs when coupling with the optical fiber. As a result, the SNR (Signal Noise Ratio) of the received optical signal amplified by the optical fiber amplifier decreases. In addition, the loss of the optical signal generated when coupling with the optical fiber varies with atmospheric fluctuations, so the SNR of the received optical signal also varies with time.

[0008] When the SNR of the received optical signal changes with time and becomes lower than the SNR required for extracting the clock signal, the optical transceiver cannot extract the clock signal from the received optical signal, and communication is impossible. In this case, the conventional optical space communication system has the following problem: communication cannot be restored until the clock signal is extracted from the received optical signal amplified by the optical fiber amplifier after the SNR of the received optical signal is restored. In addition, the communication capacity of the optical space communication decreases in accordance with the time taken to restore the communication.

[0009] The present invention is used to solve the above-mentioned problem, and its purpose is to obtain an optical space communication transceiver terminal and an optical space communication system, which can extract the clock signal without waiting for the recovery of the SNR of the received optical signal when communication is impossible due to the decrease in the SNR of the received optical signal caused by the loss generated when the optical signal is coupled with the optical fiber.

[0010] Means used to solve problems

[0011] The optical space communication transceiver terminal disclosed in the present invention performs bidirectional communication with an optical space communication transceiver terminal of the same structure and paired with the optical space communication transceiver terminal, and comprises: an optical transceiver, which converts transmission data into a transmission optical signal modulated at a symbol rate synchronized with a transmission clock signal and outputs it, demodulates and decodes a signal obtained by sampling the received optical signal at a period synchronized with the reception clock signal, thereby generating reception data; an intensity modulation unit, which superimposes the transmission clock signal on the transmission optical signal output from the optical transceiver to generate an intensity modulated transmission optical signal, and outputs the generated transmission optical signal; a transmission clock source, which supplies a common transmission clock signal to the optical transceiver and the intensity modulation unit; and an optical fiber coupling unit, which outputs the intensity modulated transmission optical signal as collimated light, and outputs the input optical signal The optical signal is coupled to the optical fiber; an optical fiber amplifier, which amplifies the optical signal coupled to the optical fiber through the optical fiber coupling part, and outputs the amplified optical signal as a received optical signal to the optical transceiver; a tracking reflector, which outputs the optical signal propagated in the space to the optical fiber coupling part side, outputs the transmission optical signal set as collimated light by the optical fiber coupling part to the space, and when a propagation angle error is input, outputs an optical signal after the propagation angle is corrected based on the input propagation angle error; a beam splitter, which distributes a part of the optical signal propagated between the tracking reflector and the optical fiber coupling part; and an angle sensor, which outputs the propagation angle error detected based on the optical signal distributed by the beam splitter to the tracking reflector, and outputs the clock signal extracted from the optical signal distributed by the beam splitter to the optical transceiver as a received clock signal.

[0012] Effects of the Invention

[0013] According to the present disclosure, the angle sensor extracts the clock signal from the optical signal before coupling with the optical fiber. Thus, when the optical space communication transceiver terminal of the present disclosure cannot communicate due to the decrease in the SNR of the received optical signal caused by the loss generated when coupling the optical signal with the optical fiber, it is possible to extract the clock signal without waiting for the recovery of the SNR of the received optical signal. Thus, compared with the case where the clock signal is extracted while waiting for the recovery of the SNR of the received optical signal, the time until the communication is restored can be shortened. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a block diagram showing the configuration of the free-space optical communication system according to Embodiment 1.

[0015] Figure 2 This is a block diagram showing the structure of the optical space communication transceiver terminal according to the first embodiment.

[0016] Figure 3 It is a block diagram showing a configuration example of an intensity modulation unit.

[0017] Figure 4 This is a block diagram showing the structure of Example (1) of the angle sensor.

[0018] Figure 5 This is a block diagram showing the structure of Example (2) of the angle sensor.

[0019] Figure 6 This is a block diagram showing the structure of an optical space communication transceiver terminal according to the second embodiment.

[0020] Figure 7 It is a block diagram showing the structure of the ARQ control unit.

[0021] Figure 8 It is a flowchart showing the operation of the ARQ control unit.

[0022] Fig. 9 It is a block diagram showing a hardware configuration for realizing the functions of an ARQ control unit.

[0023] Fig.10 : is a block diagram showing the hardware configuration for executing software that realizes the functions of the ARQ control unit.

[0024] Fig.11 This is a block diagram showing the structure of an optical transceiver included in the free-space optical communication transceiver terminal according to the third embodiment.

[0025] Fig.12 This is a flowchart showing the operations of the transmission unit and the reception unit included in the optical transceiver in the third embodiment.

[0026] Fig.13 This is a block diagram showing a hardware configuration for realizing the functions of a transmission unit and a reception unit included in the optical transceiver in the third embodiment.

[0027] Fig.14 This is a block diagram showing a hardware configuration for executing software that realizes the functions of the transmission unit and the reception unit included in the optical transceiver in the third embodiment. DETAILED DESCRIPTION

[0028] Implementation method 1.

[0029] Figure 1 1 is a block diagram showing the structure of the optical space communication system 1 according to the first embodiment. Figure 1 In the optical space communication system 1, a system for communicating using light propagating in space is provided with a pair of optical space communication transceiver terminals 1A. Figure 1 As shown, the free-space optical communication transceiver terminal 1A performs bidirectional communication with a paired free-space optical communication transceiver terminal 1A of the same configuration.

[0030] Figure 2 1 is a block diagram showing the structure of an optical space communication transceiver terminal 1A. The optical space communication transceiver terminal 1A includes an optical transceiver 2, a transmission clock source 3, an intensity modulation unit 4, an optical fiber coupling unit 5, a tracking reflector 6, a beam splitter 7, an angle sensor 8, and an optical fiber amplifier 9. The optical transceiver 2, the intensity modulation unit 4, the optical fiber coupling unit 5, and the optical fiber amplifier 9 are connected to each other through optical fibers. In addition, the optical transceiver 2, the transmission clock source 3, the tracking reflector 6, and the angle sensor 8 are connected to each other through signal lines.

[0031] The optical transceiver 2 converts the transmission data (1) into a transmission optical signal (3) modulated at a symbol rate synchronized with the transmission clock signal (5) and outputs the signal. In addition, the optical transceiver 2 samples the reception optical signal (4) at a period synchronized with the reception clock signal (8), demodulates and decodes the sampled signal, and thereby generates reception data (2).

[0032] The transmission clock source 3 supplies a common transmission clock signal (5) to the optical transceiver 2 and the intensity modulation unit 4. When the transmission clock signal (5) having a frequency of 100 MHz is outputted by the transmission clock source 3, the optical transceiver 2 generates a transmission optical signal (3) modulated at a symbol rate of 2 GHz by multiplying the frequency of the transmission clock signal (5) by, for example, 20 times.

[0033] The intensity modulator 4 superimposes the transmission clock signal (5) on the transmission optical signal (3) output from the optical transceiver 2 to generate the transmission optical signal (3) subjected to intensity modulation, and outputs the transmission optical signal (6) after intensity modulation to the optical fiber coupling unit 5. For example, the intensity modulator 4 is an intensity modulator that superimposes the transmission clock signal (5) as a modulation signal on the transmission optical signal (3). In the intensity modulation process of the transmission optical signal, the modulation amplitude of the intensity modulator 4 is small, and the intensity of the transmission optical signal (3) does not vary significantly. Thus, the influence of the variation of the intensity of the transmission optical signal (3) after intensity modulation on the modulation and demodulation process of the optical signal performed by the optical transceiver 2 can be reduced.

[0034] The intensity modulation unit 4 has, for example, Figure 3 The structure shown. Figure 3 4 is a block diagram showing a configuration example of the intensity modulator 4. The intensity modulator 4 includes an electro-optical converter 41 and an optical multiplexer 42. The electro-optical converter 41 is Figure 3 The electrical-to-optical converter 41 converts the transmission clock signal (5) supplied from the transmission clock source 3 from an electrical signal into an optical signal.

[0035] The optical combiner 42 combines the transmission optical signal (3) output from the optical transceiver 2 with the transmission clock signal converted into an optical signal by the electro-optical converter 41. Thus, the transmission optical signal (3) is intensity modulated by superimposing the transmission clock signal (5). In addition, the transmission clock signal (5) converted into an optical signal is an optical signal of a wavelength different from that of the transmission optical signal (3). That is, the transmission optical signal (6) intensity modulated by the optical combiner 42 is a signal obtained by wavelength multiplexing signals of different wavelengths. The transmission optical signal (6) is output from the optical combiner 42 to the optical fiber coupling unit 5.

[0036] The optical fiber coupling unit 5 outputs the transmission optical signal (6) intensity-modulated by the intensity modulation unit 4 as collimated light (parallel light), and couples the optical signal input from the tracking reflector 6 side with the optical fiber. For example, the optical fiber coupling unit 5 includes a condenser lens for condensing the optical signal from the tracking reflector 6, and an optical system holder for holding the condenser lens and the optical fiber. The optical fiber held by the optical system holder is connected to the input side of the optical fiber amplifier 9. The optical signal from the tracking reflector 6 is condensed by the condenser lens and then incident on the optical fiber.

[0037] When an optical signal transmitted from a paired optical space communication transceiver terminal 1A having the same structure and propagating in space is incident on the tracking reflector 6, the tracking reflector 6 outputs the incident optical signal to the optical fiber coupling unit 5. For example, the tracking reflector 6 includes at least a reflector unit that reflects the incident light to the optical fiber coupling unit 5 and a reflector driving unit that changes the angle of the reflector unit.

[0038] When the propagation angle error (7) of the optical signal is input from the angle sensor 8, the tracking mirror 6 outputs an optical signal after the propagation angle is corrected based on the input propagation angle error (7). The propagation angle error (7) is the difference in propagation angle between the light beams that are consistent with the optical axis. For example, the mirror drive unit changes the angle of the mirror unit in a manner that eliminates the propagation angle error (7), thereby correcting the propagation angle of the optical signal incident on the tracking mirror 6. The optical signal after the propagation angle is corrected is output to the optical fiber coupling unit 5 side via the beam splitter 7.

[0039] When the transmission light signal that is collimated by the optical fiber coupling unit 5 is incident on the tracking mirror 6 via the beam splitter 7, the tracking mirror 6 outputs the incident collimated light to space. As a result, the transmission light signal is transmitted to the paired optical space communication transceiver terminal 1A. In addition, the tracking mirror 6 corrects the propagation angle of the transmission light signal (6) that is collimated by the optical fiber coupling unit 5 based on the propagation angle error (7) detected by the angle sensor 8, and outputs the light signal with the corrected propagation angle to space. As a result, the propagation direction of the light signal transmitted to the paired optical space communication transceiver terminal 1A is compensated.

[0040] The beam splitter 7 distributes a part of the optical signal propagating between the optical fiber coupling unit 5 and the tracking reflector 6 to the angle sensor 8. For example, the beam splitter 7 distributes a part of the transmission optical signal output from the optical fiber coupling unit 5 to the tracking reflector 6 to the angle sensor 8, and distributes a part of the transmission optical signal output from the tracking reflector 6 to the optical fiber coupling unit 5 to the angle sensor 8.

[0041] The angle sensor 8 detects the propagation angle error (7) of the optical signal distributed by the beam splitter 7, and outputs the detected propagation angle error (7) to the tracking mirror 6. In addition, the angle sensor 8 extracts a clock signal from the optical signal distributed by the beam splitter 7, and outputs the extracted clock signal as a reception clock signal (8) to the optical transceiver 2. The reception clock signal (8) is a sine wave signal of a clock frequency, or a signal composed of a repetition of a 0 (low) value and a 1 (high) value, and only its frequency is used as a clock.

[0042] The angle sensor 8 has, for example, Figure 4 The structure shown. Figure 4 1 is a block diagram showing the structure of example (1) of the angle sensor 8. Figure 4 In FIG. 8 , the angle sensor 8 includes a lens 81 , a four-division light receiving element 82 (hereinafter referred to as a four-division PD 82 ), and a band pass filter 83 (hereinafter referred to as a BPF 83 ).

[0043] The lens 81 converges the optical signal divided by the beam splitter 7 to the center of the light receiving surface of the 4-divided PD 82. The 4-divided PD 82 is a divided light receiving element having a light receiving surface divided into four light receiving areas. The BPF 83 is a band pass filter that sets the frequency band of the transmission clock signal (5) in the paired optical space communication transceiver terminal 1A as a pass band.

[0044] The center of the light receiving surface of the 4-split PD82 is a position adjacent to all the 4 light receiving areas. The 4-split PD82, for example, includes a calculation unit that calculates a propagation angle error (7) of the light signal focused on the light receiving surface based on the intensity of the light received in the 4 light receiving areas. When the light focusing position of the light signal is offset relative to the center of the light receiving surface, a difference in the intensity of the light received between the light receiving areas occurs. The calculation unit of the 4-split PD82 calculates the offset of the light focusing position relative to the center of the light receiving surface based on the difference in the intensity of the light received in the 4 light receiving areas, respectively, and the total intensity of the light received in the 4 light receiving areas. Furthermore, the calculation unit detects the propagation angle error (7) of the light signal focused on the light receiving surface based on the offset of the light focusing position relative to the center of the light receiving surface.

[0045] In the optical space communication system 1, the optical signal transmitted from the optical space communication transceiver terminal 1A on the transmitting side to the optical space communication transceiver terminal 1A on the receiving side is an optical signal whose intensity varies in synchronization with the transmission clock signal (5). That is, the variation of the total intensity of the light received in the four light receiving areas in the light receiving surface of the four-divided PD 82 corresponds to the variation of the intensity of the optical signal transmitted from the optical space communication transceiver terminal 1A on the transmitting side.

[0046] The BPF 83 allows the transmission clock signal (5) of the transmitting optical space communication transceiver terminal 1A to pass through the frequency band of intensity fluctuation output from the 4-division PD 82, thereby extracting the clock signal. The clock signal extracted by the BPF 83 is output to the optical transceiver 2 as the reception clock signal (8).

[0047] Figure 5 : is a block diagram showing the structure of Example (2) of the angle sensor 8. Figure 5 In FIG. 8 , the angle sensor 8 includes a beam splitter 84, a lens 85, a four-division light receiving element 86 (hereinafter referred to as four-division PD 86), a photoelectric converter 87, and a bandpass filter 88 (hereinafter referred to as BPF 88). The photoelectric converter 87 is Figure 5 The beam splitter 84 distributes the optical signal distributed by the beam splitter 7 to the four-division PD 86 and the photoelectric converter 87. The lens 85 converges the optical signal distributed by the beam splitter 7 to the center of the light receiving surface of the four-division PD 86.

[0048] The 4-divided PD 86 is a light receiving element having a light receiving surface divided into four light receiving areas, similar to the 4-divided PD 82. The center of the light receiving surface is a position adjacent to all the four light receiving areas. The 4-divided PD 86 includes a calculation unit that calculates the propagation angle error (7) of the light signal focused on the light receiving surface based on the intensity of the light received in the four light receiving areas.

[0049] When the focusing position of the optical signal is offset from the center of the light receiving surface, a difference in the intensity of the light received between the light receiving areas occurs. The calculation unit of the 4-division PD 86 calculates the offset of the focusing position from the center of the light receiving surface based on the difference in the intensity of the light received in the four light receiving areas and the total intensity of the light received in the four light receiving areas. Furthermore, the calculation unit detects the propagation angle error (7) of the optical signal focused on the light receiving surface based on the offset of the focusing position from the center of the light receiving surface.

[0050] The photoelectric converter 87 converts the optical signal divided by the beam splitter 7 into an electric signal. The BPF 88 is a bandpass filter that sets the frequency band of the transmission clock signal (5) in the paired optical space communication transceiver terminal 1A as a passband.

[0051] In the optical space communication transceiver terminal 1A, for example, a single-mode optical fiber is used. Single-mode optical fiber is generally an optical fiber with low influence of mode dispersion, and its core diameter is restricted. Figure 4 and Figure 5 The light receiving surfaces of the 4-split PD 82, 4-split PD 86, and the photoelectric converter 87 shown are not restricted and have a light receiving area larger than the core diameter of the optical fiber. Therefore, even if the wavefront of the input light is distorted, the angle sensor 8 can suppress the decrease in the intensity of the detected light signal compared to the intensity of the light signal when coupled with the optical fiber. That is, the angle sensor 8 can detect the intensity change based on the light signal before coupling with the optical fiber. Therefore, even if the wavefront of the light is distorted due to the influence of atmospheric fluctuations, the clock signal can be stably extracted from the received light signal and can be continuously supplied to the optical transceiver 2.

[0052] The clock signal extracted from the received optical signal by the angle sensor 8 is a clock signal synchronized with the symbol rate of the transmitted optical signal from the paired optical space communication transceiver terminal 1A of the same structure. Therefore, the optical transceiver 2 can achieve sampling synchronized with the received optical signal by sampling the received optical signal synchronously with the clock signal extracted by the angle sensor 8. When the SNR of the received optical signal (4) is affected by atmospheric fluctuations and drops to a state in which communication is impossible and then recovers to a state in which communication is possible, the optical transceiver 2 can use the clock signal extracted by the angle sensor 8 to directly demodulate and decode the signal. In this way, the time until communication is restored can be shortened, and a decrease in communication capacity can be prevented.

[0053] The optical signal transmitted from the optical space communication transceiver terminal 1A on the transmitting side to the optical space communication transceiver terminal 1A on the receiving side is an intensity modulated optical signal whose intensity varies synchronously with the transmission clock signal (5). That is, the intensity variation of the electrical signal converted from the optical signal by the photoelectric converter 87 corresponds to the intensity variation of the transmission optical signal transmitted from the optical space communication transceiver terminal 1A on the transmitting side.

[0054] The BPF 88 allows the frequency band of the transmission clock signal (5) in the transmitting optical space communication transceiver terminal 1A to pass through the frequency band in which the intensity of the electrical signal output from the photoelectric converter 87 varies, thereby extracting the clock signal. The clock signal extracted by the BPF 88 is output to the optical transceiver 2 as the reception clock signal (8).

[0055] The optical fiber amplifier 9 is connected to the optical fiber held by the optical fiber coupling unit 5. The optical fiber amplifier 9 is an optical element that amplifies the intensity of the optical signal coupled to the optical fiber through the optical fiber coupling unit 5 to improve the light receiving sensitivity. The intensity of the light beam of the received light incident on the optical fiber is amplified according to the excitation light, and the received light signal (4) with the amplified intensity is emitted to the optical transceiver 2.

[0056] The specific operation of the free-space optical communication transceiver terminal 1A is as follows.

[0057] The optical transceiver 2 modulates transmission data (1) input from the outside at a symbol rate synchronized with a transmission clock signal (5) supplied from a transmission clock source 3, and outputs the modulated transmission optical signal (3) to the intensity modulation unit 4.

[0058] The intensity modulator 4 superimposes the transmission clock signal (5) supplied from the transmission clock source 3 on the transmission optical signal (3) to generate the transmission optical signal (3) subjected to intensity modulation, and outputs the transmission optical signal (6) subjected to intensity modulation to the optical fiber coupling unit 5. In addition, due to the intensity variation of the optical signal generated when the optical transceiver 2 modulates the optical signal, crosstalk to the reception clock signal (8) may be generated. Therefore, by setting the frequency of the transmission clock signal (5) superimposed on the transmission optical signal (3) to a frequency of, for example, 1 / 10 of the symbol rate or less, Figure 4 and Figure 5 The BPF 83 or 88 shown is capable of suppressing crosstalk to the reception clock signal (8).

[0059] The optical fiber coupling unit 5 converts the transmission light signal (6) intensity-modulated by the intensity modulation unit 4 into collimated light, and outputs the collimated light of the transmission light signal (6) to the beam splitter 7. The beam splitter 7 outputs the collimated light of the transmission light signal (6) output from the optical fiber coupling unit 5 to the tracking mirror 6, and distributes a part of the transmission light signal (6) to the angle sensor 8. The angle sensor 8 detects the propagation angle error (7) of the light signal distributed by the beam splitter 7, and outputs the detected propagation angle error (7) to the tracking mirror 6.

[0060] The tracking reflector 6 corrects the propagation angle of the transmission optical signal (6) which is collimated by the optical fiber coupling unit 5 based on the propagation angle error (7) detected by the angle sensor 8. The optical signal with the corrected propagation angle is output to space. Thus, the propagation direction of the optical signal transmitted to the paired optical space communication transceiver terminal 1A is compensated.

[0061] When the optical signal transmitted from the paired optical space communication transceiver terminal 1A and propagated in space is incident on the tracking reflector 6, the tracking reflector 6 outputs the incident optical signal to the optical fiber coupling unit 5 via the beam splitter 7. The beam splitter 7 outputs the optical signal output from the tracking reflector 6 to the optical fiber coupling unit 5, and distributes a part of the optical signal to the angle sensor 8.

[0062] The light output from the optical fiber coupling unit 5 to the tracking reflector 6 and the light output from the tracking reflector 6 to the optical fiber coupling unit 5 are arranged on the same optical axis. The optical fiber coupling unit 5 is provided with a wavelength filter for separating the input light according to each wavelength. The optical fiber coupling unit 5 uses the wavelength filter to separate the light in the above two directions and couples it with the optical fiber. Thus, even when optical signals with different wavelengths are transmitted between the paired optical space communication transceiver terminals 1A, the received and received light can be separated in the optical fiber coupling unit 5. In addition, an optical circulator for separating the light in the two directions can also be provided in the optical fiber coupling unit 5.

[0063] The light output from the fiber coupling unit 5 to the tracking reflector 6 and the light output from the tracking reflector 6 to the fiber coupling unit 5 are parallelized. The angle sensor 8 detects the propagation angle error (7) of the light output from the tracking reflector 6 to the fiber coupling unit 5 in the two directions. The tracking reflector 6 can also correct the propagation angles of both the light output from the fiber coupling unit 5 to the tracking reflector 6 and the light output from the tracking reflector 6 to the fiber coupling unit 5 using the propagation angle error (7) detected based on the light output from the tracking reflector 6 to the fiber coupling unit 5.

[0064] The optical fiber coupling unit 5 couples the optical signal output from the tracking reflector 6 to the optical fiber. The optical fiber amplifier 9 amplifies the intensity of the received optical signal (4) coupled to the optical fiber through the optical fiber coupling unit 5, and outputs the amplified received optical signal (4) to the optical transceiver 2 through the optical fiber. The angle sensor 8 extracts the received clock signal (8) from a part of the optical signal distributed by the beam splitter 7, and outputs the extracted received clock signal (8) to the optical transceiver 2.

[0065] The optical transceiver 2 samples the received optical signal (4) propagating in the optical fiber at a period synchronized with the received clock signal (8), and demodulates and decodes the sampled signal, thereby generating received data (2) and outputting it to the outside.

[0066] In addition, Figure 3 When the optical combiner 42 shown in the figure uses the transmission clock signal (5) having a wavelength different from that of the transmission optical signal (3) as a modulation signal for wavelength multiplexing, the optical transceiver 2 performs coherent detection on the received optical signal (4) (the transmission optical signal (6) transmitted from the paired optical space communication transceiver terminal 1A). As a result, the optical transceiver 2 can demodulate and decode the transmission optical signal having a wavelength different from that of the transmission clock signal based on the received optical signal (4) (the transmission optical signal (6) after intensity modulation).

[0067] In addition, Figure 4 and Figure 5 , a structure in which the 4-divided PD 82 or the 4-divided PD 86 detects the propagation angle error of the optical signal is shown, but the light receiving element provided in the angle sensor 8 may be a light receiving element having a light receiving surface divided into a plurality of regions, and is not limited to being divided into 4. For example, a light receiving element that can detect the offset between the center position of the light receiving surface and the focusing position of the optical signal may be a light receiving element whose light receiving surface is divided into 2 regions or a light receiving element whose light receiving surface is divided into 5 or more regions.

[0068] As described above, in the optical space communication transceiver terminal 1A of embodiment 1, the angle sensor 8 extracts the clock signal from the optical signal before coupling with the optical fiber. As a result, when the optical space communication transceiver terminal 1A cannot communicate due to a decrease in the SNR of the received optical signal caused by the loss generated when coupling the optical signal with the optical fiber, it is possible to extract the clock signal without waiting for the recovery of the SNR of the received optical signal. As a result, compared with the case of extracting the clock signal while waiting for the recovery of the SNR of the received optical signal, the time until the communication is restored can be shortened.

[0069] In the optical space communication transceiver terminal 1A of embodiment 1, the angle sensor 8 includes a 4-divided PD 82 or 86 for receiving an optical signal on a light receiving surface divided into four areas, a lens 81 or 85 for focusing the optical signal at the center position of the light receiving surface of the 4-divided PD 82 or 86, and a BPF 83 or 88 for setting the frequency band of the transmission clock signal (5) from the paired optical space communication transceiver terminal 1A of the same structure as a passband. The angle sensor 8 detects the propagation angle error of the optical signal based on the offset between the focusing position of the optical signal and the center position of the 4-divided PD 82 or 86. The clock signal extracted by the BPF 83 or 88 is output to the optical transceiver 2 as a received clock signal. For example, the 4-divided PD 82 or 86 has a light receiving area larger than the core diameter of the optical fiber, so the angle sensor 8 can detect the intensity change based on the optical signal before coupling with the optical fiber. Therefore, even if the optical signal is affected by atmospheric fluctuations, the angle sensor 8 can stably extract the clock signal from the received optical signal and can continuously supply it to the optical transceiver 2.

[0070] In the optical space communication system 1 of the first embodiment, a pair of optical space communication transceiver terminals 1A of the same structure are provided, and bidirectional communication is performed between the paired optical space communication transceiver terminals 1A of the same structure. Thus, when communication is impossible due to a decrease in the SNR of a received optical signal caused by a loss generated when coupling an optical signal to an optical fiber, a clock signal can be extracted without waiting for the recovery of the SNR of the received optical signal. Thus, compared with the case where the clock signal is extracted while waiting for the recovery of the SNR of the received optical signal, the time until communication is restored can be shortened.

[0071] Implementation method 2.

[0072] Figure 6 1 is a block diagram showing the structure of an optical free-space communication transceiver terminal 1B according to Embodiment 2. Figure 6 In the Figure 2 The same components are denoted by the same reference numerals and their description is omitted. Figure 1 In the shown free-space optical communication system 1, bidirectional communication is performed with a paired free-space optical communication transceiver terminal 1B of the same configuration.

[0073] like Figure 6 As shown, the optical space communication transceiver terminal 1B includes an optical transceiver 2, a transmission clock source 3, an intensity modulation unit 4, an optical fiber coupling unit 5, a tracking reflector 6, a beam splitter 7, an angle sensor 8A, an optical fiber amplifier 9, an ARQ (Automatic RepeatreQuest) control unit 10, a low-speed receiver 11 and a low-speed transmitter 12.

[0074] In the optical space communication transceiver terminal 1B, the optical transceiver 2, the intensity modulation unit 4, the optical fiber coupling unit 5 and the optical fiber amplifier 9 are connected to each other through optical fibers. In addition, the optical transceiver 2, the transmission clock source 3, the tracking reflector 6, the angle sensor 8A, the ARQ control unit 10, the low-speed receiver 11 and the low-speed transmitter 12 are connected to each other through signal lines.

[0075] The angle sensor 8A inputs the optical signal distributed by the beam splitter 7, and generates an intensity modulation signal (8a) of the input optical signal. The intensity modulation signal (8a) is an electrical signal indicating the intensity variation of the optical signal. For example, the intensity modulation signal (8a) is a digital signal composed of an arrangement of 0 (high) values ​​and 1 (low) values, and is a signal including a frame retransmission request, etc. The intensity modulation signal (8a) includes a sinusoidal wave component such as a receiving clock signal (8), and the low-speed receiver 11 can extract the above sinusoidal wave component from the intensity modulation signal (8a). In addition, the angle sensor 8A has, for example, the same as the angle sensor 8. Figure 4 and Figure 5 The 4-division PD 82 or 86 detects the propagation angle error of the incident light and detects the intensity variation of the incident light. In addition, the BPF 83 or 88 is a bandpass filter that sets the frequency band of the signal output from the low-speed transmitter 12 of the paired optical space communication transceiver terminal 1B as a passband.

[0076] The signal that has passed through the BPF 83 or 88 among the signals indicating the intensity variation detected by the 4-division PD 82 or 86 is output as an intensity modulation signal (8a). The intensity modulation signal (8a) output from the angle sensor 8A is output to the low-speed receiver 11. The low-speed receiver 11 demodulates and decodes the intensity modulation signal (8a) to reproduce a clock signal synchronized with the symbol rate. The clock signal reproduced by the low-speed receiver 11 is output to the optical transceiver 2.

[0077] The ARQ control unit 10 outputs a transmission frame (1a) input from the outside as transmission data (1) to the optical transceiver 2. In addition, the ARQ control unit 10 outputs a transmission frame corresponding to a sequence number indicated by a retransmission request signal (9) output from the low-speed receiver 11 as transmission data to the optical transceiver 2. In addition, the ARQ control unit 10 stores the reception data (2) output from the optical transceiver 2 as a reception frame (2a), and when the sequence number of the reception frame (2a) is lost, the ARQ control unit 10 outputs a retransmission request signal (10) of the reception frame (2a) corresponding to the lost sequence number to the low-speed transmitter 12.

[0078] The low-speed receiver 11 demodulates and decodes the intensity modulation signal (8a) output from the angle sensor 8A to reproduce a clock signal synchronized with the symbol rate, and outputs the reproduced clock signal as a reception clock signal (8) to the optical transceiver 2. In addition, when the low-speed receiver 11 reproduces a retransmission request signal (9) by demodulating and decoding the intensity modulation signal (8a), the low-speed receiver 11 outputs the reproduced retransmission request signal (9) to the ARQ control unit 10. The retransmission request signal (9) is a signal indicating a retransmission request for a lost communication frame, and includes a sequence number corresponding to the corresponding frame. In addition, the sequence number is stored in the header of the communication frame, etc.

[0079] The low-speed transmitter 12 converts the retransmission request signal (10) output from the ARQ control unit 10 into a signal modulated at a symbol rate synchronized with the transmission clock signal (5), and outputs the converted signal to the intensity modulation unit 4. The symbol rate is set to be the same as or substantially the same as the frequency of the transmission clock signal (5). Even in the absence of the retransmission request signal (10) from the ARQ control unit 10, the low-speed transmitter 12 continuously outputs the random signal modulated at the symbol rate synchronized with the transmission clock signal (5) to the intensity modulation unit 4. In the intensity modulation process of the intensity modulation unit 4, the modulation amplitude of the intensity modulation unit 4 is small and the intensity of the transmission optical signal (3) does not vary greatly. As a result, the influence of the intensity variation of the transmission optical signal (3) after intensity modulation on the modulation and demodulation process of the optical signal performed by the optical transceiver 2 can be reduced. By using a modulation speed that is lower than the modulation speed of the optical transceiver (2), even if the intensity variation is reduced, demodulation can be performed by the low-speed receiver.

[0080] Figure 7 is a block diagram showing the structure of the ARQ control unit 10. Figure 7 In the embodiment, the ARQ control unit 10 includes a transmission buffer 101, a transmission data output unit 102, a reception buffer 103, a reception frame output unit 104, a sequence loss determination unit 105, and a retransmission request output unit 106. The transmission buffer 101 is an internal buffer that stores transmission frames (1a) input from the outside in the order of sequence numbers. The transmission data output unit 102 outputs the transmission frames (1a) read out from the transmission buffer 101 in the order of sequence numbers to the optical transceiver 2 as transmission data (1).

[0081] The reception buffer 103 is an internal buffer that stores the reception data (2) output from the optical transceiver 2 as reception frames (2a) in sequence number order. The reception frame output unit 104 outputs the reception frames (2a) stored in the reception buffer 103 to the outside in sequence number order.

[0082] The sequence loss determination unit 105 is a determination unit that determines whether the sequence number of the received frame (2a) stored in the reception buffer 103 has been lost. When the sequence loss determination unit 105 determines that the sequence number has been lost, the retransmission request output unit 106 outputs a retransmission request signal (10) of the received frame (2a) corresponding to the lost sequence number to the low-speed transmitter 12.

[0083] The specific operation of the free-space optical communication transceiver terminal 1B is as follows.

[0084] The ARQ control unit 10 outputs the externally input transmission frame (1a) as transmission data (1) to the optical transceiver 2. The optical transceiver 2 modulates the transmission data (1) input from the ARQ control unit 10 at a symbol rate synchronized with a transmission clock signal (5) supplied from a transmission clock source 3, and outputs the modulated transmission optical signal (3) to the intensity modulation unit 4.

[0085] The low-speed transmitter 12 outputs a signal modulated at a symbol rate synchronized with the transmission clock signal (5) to the intensity modulation unit 4. When there is a retransmission request signal (10) from the ARQ control unit 10, the low-speed transmitter 12 outputs a signal modulated at a symbol rate synchronized with the transmission clock signal (5) to the intensity modulation unit 4.

[0086] The intensity modulating unit 4 generates an intensity modulated transmission optical signal (6) by superimposing the signal output from the low-speed transmitter 12 on the transmission optical signal (3), and outputs the intensity modulated transmission optical signal (6) to the optical fiber coupling unit 5. In addition, due to the intensity variation of the optical signal generated when the optical transceiver 2 modulates the optical signal, crosstalk to the intensity modulated signal (8a) may be generated. Therefore, by setting the frequency of the intensity modulated signal (8a) superimposed on the transmission optical signal (3) to a frequency of, for example, 1 / 10 of the symbol rate or less, Figure 4 and Figure 5 The BPF 83 or 88 shown is capable of suppressing crosstalk to the intensity modulated signal (8a).

[0087] The optical fiber coupling unit 5 converts the transmission light signal (6) intensity-modulated by the intensity modulation unit 4 into collimated light, and outputs the collimated light of the transmission light signal (6) to the beam splitter 7. The beam splitter 7 outputs the collimated light of the transmission light signal (6) output from the optical fiber coupling unit 5 to the tracking mirror 6, and distributes a part of the transmission light signal (6) to the angle sensor 8A. The angle sensor 8A detects the propagation angle error (7) of the light signal distributed by the beam splitter 7, and outputs the detected propagation angle error (7) to the tracking mirror 6.

[0088] The tracking reflector 6 corrects the propagation angle of the transmission optical signal (6) which is collimated by the optical fiber coupling unit 5 based on the propagation angle error (7) detected by the angle sensor 8A. The optical signal with the corrected propagation angle is output to space. Thus, the propagation direction of the optical signal transmitted to the paired optical space communication transceiver terminal 1B is compensated.

[0089] When the optical signal transmitted from the paired optical space communication transceiver terminal 1B and propagated in space is incident on the tracking reflector 6, the tracking reflector 6 outputs the incident optical signal to the optical fiber coupling unit 5 via the beam splitter 7. The beam splitter 7 outputs the optical signal output from the tracking reflector 6 to the optical fiber coupling unit 5, and distributes a part of the optical signal to the angle sensor 8A.

[0090] The light output from the optical fiber coupling unit 5 to the tracking reflector 6 and the light output from the tracking reflector 6 to the optical fiber coupling unit 5 are arranged on the same optical axis. The optical fiber coupling unit 5 is provided with a wavelength filter for separating the input light according to each wavelength. The optical fiber coupling unit 5 separates the light in the above two directions using the wavelength filter and couples it with the optical fiber. Thus, even when optical signals with different wavelengths are transmitted between the paired optical space communication transceiver terminals 1B, the received and received light can be separated in the optical fiber coupling unit 5. In addition, an optical circulator for separating the light in the two directions can also be provided in the optical fiber coupling unit 5.

[0091] The light output from the fiber coupling unit 5 to the tracking mirror 6 and the light output from the tracking mirror 6 to the fiber coupling unit 5 are parallelized. The angle sensor 8A detects the propagation angle error (7) of the light output from the tracking mirror 6 to the fiber coupling unit 5 in the two directions. The tracking mirror 6 can also correct the propagation angles of both the light output from the fiber coupling unit 5 to the tracking mirror 6 and the light output from the tracking mirror 6 to the fiber coupling unit 5 using the propagation angle error (7) detected based on the light output from the tracking mirror 6 to the fiber coupling unit 5.

[0092] The optical fiber coupling unit 5 couples the optical signal output from the tracking reflector 6 to the optical fiber. The optical fiber amplifier 9 amplifies the intensity of the received optical signal (4) coupled to the optical fiber through the optical fiber coupling unit 5, and outputs the received optical signal (4) with the amplified intensity to the optical transceiver 2 through the optical fiber. The angle sensor 8A generates an intensity modulated signal (8a) from a part of the optical signal distributed by the beam splitter 7, and outputs the intensity modulated signal (8a) to the low-speed receiver 11.

[0093] The low-speed receiver 11 demodulates and decodes the intensity modulation signal (8a) output from the angle sensor 8A, and reproduces the reception clock signal (8) synchronized with the symbol rate. The reception clock signal (8) reproduced by the low-speed receiver 11 is output to the optical transceiver 2. The optical transceiver 2 samples the reception optical signal (4) propagated in the optical fiber at a period synchronized with the reception clock signal (8). The optical transceiver 2 demodulates and decodes the sampled signal to generate reception data (2), and outputs the generated reception data (2) to the ARQ control unit 10.

[0094] The ARQ control unit 10 stores the received data (2) output from the optical transceiver 2 as a received frame (2a), and in the case where a sequence number is lost in the stored received frame (2a), a retransmission request signal (10) of the received frame (2a) corresponding to the lost sequence number is output to the low-speed transmitter 12.

[0095] In addition, Figure 3 When the optical combiner 42 shown in the figure uses a signal having a wavelength different from the transmission optical signal (3) as a modulated signal for wavelength multiplexing, the optical transceiver 2 performs coherent detection on the received optical signal (4) (the transmission optical signal (6) transmitted from the paired optical space communication transceiver terminal 1B). Thus, the optical transceiver 2 can demodulate and decode the transmission optical signal having a wavelength different from the transmission clock signal based on the received optical signal (4) (the transmission optical signal (6) after intensity modulation).

[0096] Figure 8 : is a flowchart showing the operation of the ARQ control unit 10.

[0097] The transmission data output unit 102 checks whether a retransmission request signal (9) is received from the low-speed receiver 11 (step ST1). If the retransmission request signal (9) is not received (step ST1; No), the process proceeds to step ST4. In addition, if the transmission data output unit 102 receives a retransmission request signal for a communication frame corresponding to a specific sequence number (step ST1; Yes), it checks whether a transmission frame (1a) is stored in the transmission buffer 101 (step ST2). If the transmission frame (1a) is not stored in the transmission buffer 101 (step ST2; No), the process proceeds to step ST4.

[0098] When the transmission buffer 101 stores the transmission frame (1a) (step ST2; yes), the transmission data output unit 102 outputs the transmission frame (1a) stored in the transmission buffer 101 as the transmission data (1) to the optical transceiver 2 (step ST3). The transmission buffer 101 stores the transmission frames (1a) input from the outside in the order of sequence numbers (step ST4). If there is a frame that has not been output among the transmission frames stored in the transmission buffer 101, the transmission data output unit 102 outputs the frame to the optical transceiver 2 (step ST5).

[0099] The reception buffer 103 stores the reception data (2) output from the optical transceiver 2 as reception frames (2a) in the order of sequence numbers (step ST6). Next, the sequence loss determination unit 105 determines whether there is a sequence number loss in the reception frames (2a) stored in the reception buffer 103 (step ST7). Here, if there is a sequence number loss (step ST7; yes), the retransmission request output unit 106 outputs a retransmission request signal (10) of the communication frame of the lost sequence number to the low-speed transmitter 12 (step ST8).

[0100] If there is no sequence number loss (step ST7; No), the received frame output unit 104 reads out the received frames that have not been output from the received frames stored in the receiving buffer 103, and outputs the read received frame (2a) to the outside (step ST9). After that, the process returns to step ST1, and a series of processes starting from step ST1 are repeated.

[0101] The loss of sequence numbers occurs when the frame corresponding to the lost sequence number cannot be demodulated and decoded by the optical transceiver 2. For example, the wavefront of light propagating in space is affected by atmospheric fluctuations and distorted. When the optical signal with distorted wavefront is coupled to the optical fiber, the intensity decreases (loss). As a result, the SNR of the received optical signal (4) is lower than the SNR required for demodulation, and thus the received frame cannot be demodulated and decoded.

[0102] The ARQ control unit 10 compensates for the loss of sequence numbers of the received frames stored in the reception buffer 103 and then outputs the frames in the order of the sequence numbers. In addition, the frames stored in the transmission buffer 101 and the reception buffer 103 are discarded after a predetermined time has passed.

[0103] Fig. 9 2 is a block diagram showing a hardware configuration for realizing the functions of the ARQ control unit 10 . Fig.101 is a block diagram showing a hardware configuration of software that executes the functions of the ARQ control unit 10. The functions of the transmission buffer 101, transmission data output unit 102, reception buffer 103, reception frame output unit 104, sequence loss determination unit 105, and retransmission request output unit 106 included in the ARQ control unit 10 are implemented by processing circuits. Figure 8 The processing circuit of the processes from step ST1 to step ST9 shown in the figure may be dedicated hardware or a CPU (Central Processing Unit) that executes a program stored in a memory.

[0104] In the above processing circuit is Fig. 9 In the case of the dedicated hardware processing circuit 200 shown, the processing circuit 200 corresponds to, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination thereof. The functions of the transmission buffer 101, the transmission data output unit 102, the reception buffer 103, the reception frame output unit 104, the sequence loss determination unit 105, and the retransmission request output unit 106 included in the ARQ control unit 10 may be implemented by different processing circuits, or may be implemented by one processing circuit.

[0105] In the above processing circuit is Fig.10 In the case of the processor 201 shown in FIG. 1 , the functions of the transmission buffer 101, the transmission data output unit 102, the reception buffer 103, the reception frame output unit 104, the sequence loss determination unit 105, and the retransmission request output unit 106 included in the ARQ control unit 10 are implemented by software, firmware, or a combination of software and firmware. The software or firmware is described in the form of a program and stored in the memory 202.

[0106] The processor 201 implements the functions of the transmission buffer 101, the transmission data output unit 102, the reception buffer 103, the reception frame output unit 104, the sequence loss determination unit 105, and the retransmission request output unit 106 of the ARQ control unit 10 by reading and executing the program stored in the memory 202. For example, the ARQ control unit 10 includes the memory 202, which is used to store the results of the execution when the processor 201 executes the program. Figure 8The computer 200 may be a computer-readable storage medium storing a program for causing a computer to function as the transmission buffer 101, the transmission data output unit 102, the reception buffer 103, the reception frame output unit 104, the sequence loss determination unit 105, and the retransmission request output unit 106.

[0107] The memory 202 corresponds to, for example, RAM (Random Access Memory), ROM (Read Only Memory), flash memory, EPROM (Erasable Programmable ROM), EEPROM (Electrically-EPROM), and other non-volatile or volatile semiconductor memories, magnetic disks, floppy disks, optical disks, high-density disks, mini disks, DVDs, and the like.

[0108] It is also possible that part of the functions of the transmission buffer 101, the transmission data output unit 102, the reception buffer 103, the reception frame output unit 104, the sequence loss determination unit 105, and the retransmission request output unit 106 included in the ARQ control unit 10 is realized by dedicated hardware, and the remaining part is realized by software or firmware. For example, the transmission buffer 101 and the reception buffer 103 are realized by the processing circuit 200 which is dedicated hardware, and the transmission data output unit 102, the reception frame output unit 104, the sequence loss determination unit 105, and the retransmission request output unit 106 are realized by the processor 201 reading and executing the program stored in the memory 202. In this way, the processing circuit can realize the above functions by hardware, software, firmware, or a combination thereof.

[0109] As described above, in the optical space communication transceiver terminal 1B of the second embodiment, the angle sensor 8A extracts the intensity modulation signal from the optical signal before coupling with the optical fiber. The low-speed receiver 11 demodulates and decodes the intensity modulation signal output from the angle sensor 8A, reproduces the clock signal synchronized with the symbol rate, and outputs the reproduced clock signal as the reception clock signal (8) to the optical transceiver 2. Even when communication is impossible due to a decrease in the SNR of the received optical signal caused by the loss generated when the optical signal is coupled to the optical fiber, the angle sensor 8A can stably extract the intensity modulation signal. Therefore, compared with the optical transceiver 2, the low-speed receiver 11 can stably demodulate and decode the retransmission request signal, and can also stably supply the reception clock signal to the optical transceiver 2. As a result, the optical space communication transceiver terminal 1B can shorten the time until communication is restored compared to the case of extracting the clock signal while waiting for the recovery of the SNR of the received optical signal, and can prevent a decrease in communication capacity. Furthermore, the low-speed receiver 11 can stably demodulate and decode the retransmission request signal of the communication frame and output it to the ARQ control unit 10. This makes it possible to respond to the retransmission request without delay, thereby suppressing transmission delay.

[0110] In the optical space communication transceiver terminal 1B of the second embodiment, the ARQ control unit 10 includes a transmission buffer 101, a transmission data output unit 102, a reception buffer 103, a reception frame output unit 104, a sequence loss determination unit 105, and a retransmission request output unit 106. By having these components, even if the sequence number of the frame stored in the reception buffer 103 is lost, the ARQ control unit 10 can make a retransmission request for the corresponding frame without delay.

[0111] In the optical space communication system 1 of the second embodiment, a pair of optical space communication transceiver terminals 1B of the same structure are provided, and bidirectional communication is performed between the paired optical space communication transceiver terminals 1B of the same structure. Thus, when communication is impossible due to a decrease in the SNR of the received optical signal caused by a loss generated when coupling the optical signal to the optical fiber, a clock signal can be extracted without waiting for the recovery of the SNR of the received optical signal. Thus, compared with the case of extracting the clock signal while waiting for the recovery of the SNR of the received optical signal, the time until the communication is restored can be shortened.

[0112] Implementation method 3.

[0113] Fig.11 1 is a block diagram showing the structure of an optical transceiver 2A provided in an optical free-space communication transceiver terminal 1A or 1B according to Embodiment 3. The optical transceiver 2A is a replacement for Figure 1 The optical space communication transceiver terminal 1A shown or Figure 6 The optical transceiver 2 in the optical space communication transceiver terminal 1B shown in FIG. Fig.11As shown, the optical transceiver 2A includes a transmission unit 21 , an optical modulator 22 , a Doppler shift estimator 23 , a reception unit 24 , and a coherent detector 25 .

[0114] The transmission unit 21 outputs a signal obtained by modulating the transmission data (1) at a symbol rate synchronized with the transmission clock signal (5). The transmission unit 21 includes a modulation signal conversion unit 211. The transmission unit 21 is, for example, a DSP (Digital Signal Processor). The modulation signal conversion unit 211 converts the input transmission data (1) into a signal modulated at a symbol rate synchronized with the transmission clock signal (5).

[0115] The optical modulator 22 converts the modulated signal output from the transmitting unit 21 into a transmission optical signal (3) and outputs it to the intensity modulating unit 4. The Doppler shift estimator 23 estimates the Doppler shift amount of the receiving clock signal (8) input from the angle sensor 8 or the low-speed receiver 11. The Doppler shift amount is a value obtained by dividing the frequency difference generated by the Doppler shift by the frequency before the frequency shift. For example, the Doppler shift estimator 23 detects the frequency difference between the receiving clock signal (8) and an internal clock signal having the same nominal frequency as the receiving clock signal (8), and divides it by the above-mentioned nominal frequency, thereby calculating the Doppler shift amount. In addition, the receiving clock signal (8) is also input to the receiving unit 24.

[0116] The coherent detector 25 performs coherent detection on the received optical signal (4) output from the optical fiber amplifier 9. The receiving unit 24 decodes the received data (2) using the received optical signal (4) after coherent detection, the estimated value of the Doppler shift amount in the received clock signal (8), and the received clock signal (8). The receiving unit 24 is, for example, a DSP.

[0117] like Fig.11 As shown, the receiving unit 24 includes a sampling unit 241, an adaptive equalization unit 242, an SNR estimation unit 243, a frequency difference estimation unit 244, a frequency difference compensation unit 245, a phase difference estimation unit 246, a phase difference compensation unit 247, and a decoding unit 248. The sampling unit 241 samples the received optical signal (4) after coherent detection at a symbol rate synchronized with the received clock signal (8). For example, the sampling unit 241 samples the signal after coherent detection at a symbol rate synchronized with the received clock signal (8) or a rational multiple of the symbol rate.

[0118] The adaptive equalizer 242 adaptively equalizes the signal sampled by the sampling unit 241. The adaptive equalizer 242 performs equalization in such a way that the average error between the output signal point and the ideal signal point within a certain period of time becomes the minimum. For example, when the modulation method is QPSK (Quadra Phase-Shift Keying), adaptive equalization can be achieved by a constant envelope algorithm.

[0119] The SNR estimation unit 243 estimates the SNR of the adaptively equalized signal. For example, the SNR estimation unit 243 estimates the amount of error by averaging the error between the equalized signal and an ideal signal point over a certain period of time, thereby estimating the SNR.

[0120] The frequency difference estimation unit 244 estimates the frequency difference of the adaptively equalized signal using the signal adaptively equalized by the adaptive equalization unit 242, the estimated value of the SNR, and the estimated value of the Doppler frequency shift. For example, in the frequency difference estimation process, multiple samples are used to suppress the influence of noise, and successive calculations using multiple samples are performed to average the estimated values. During the period when the optical signal is affected by atmospheric fluctuations and the SNR decreases, when the frequency difference based on the Doppler frequency shift changes, the initial value of the successive calculation in the frequency difference estimation is offset from the true value, and it takes time to converge. In this case, the frequency difference cannot be compensated, and even if the SNR of the received optical signal is restored, it takes time to restore the communication.

[0121] Therefore, the optical transceiver 2A estimates the Doppler shift amount based on the receiving clock signal (8) which is not easily affected by the distortion of the wavefront caused by the influence of atmospheric waves and is stably supplied, thereby, the receiving unit 24 can stably calculate the change of the frequency difference based on the Doppler shift. However, in addition to the Doppler shift, the wavelength of the light source itself also changes due to the influence of temperature, so the frequency difference is estimated based on the signal that has been adaptively equalized for the receiving clock signal (8). When the SNR of the received optical signal is restored, the frequency difference calculated using the estimated value of the Doppler shift amount is used as the initial value, and the frequency difference is estimated based on the signal that has been adaptively equalized for the receiving clock signal (8), thereby accelerating the convergence of the estimated value and shortening the time until communication is restored.

[0122] The frequency difference estimation unit 244 estimates the frequency difference of the adaptively equalized signal when the estimated value of the SNR estimated by the SNR estimation unit 243 is greater than the threshold. When the estimated value of the SNR becomes greater than the threshold again after falling below the threshold, the frequency difference estimation unit 244 estimates the frequency difference of the adaptively equalized signal using the estimated value of the frequency difference immediately before the estimated value of the SNR falls below the threshold and the estimated value of the frequency difference calculated based on the transition of the estimated value of the Doppler shift amount after the estimated value of the SNR falls below the threshold as initial values.

[0123] For example, the initial value of the estimated value of the frequency difference is obtained by f0+fc(d1-d0) in the following case: after the estimated value of the frequency difference estimated from the signal subjected to adaptive equalization is f0 (Hz) and the estimated value of the SNR is lower than the threshold value when the estimated value of the Doppler frequency shift is d0, the estimated value of the SNR becomes higher than the threshold value when the estimated value of the Doppler frequency shift is d1. Here, fc is the frequency of the transmitted optical signal.

[0124] The frequency difference compensation unit 245 compensates the frequency difference of the adaptively equalized signal based on the frequency difference estimated by the frequency difference estimation unit 244. The phase difference estimation unit 246 estimates the phase difference of the signal whose frequency difference is compensated by the frequency difference compensation unit 245. The phase difference compensation unit 247 compensates the phase difference of the adaptively equalized signal based on the phase difference estimated by the phase difference estimation unit 246. For example, in the case where the modulation method is QPSK, for example, the phase difference can be estimated and compensated according to the method described in reference 1 by using the Viterbi algorithm. In addition, the frequency difference estimation unit 244 can estimate the frequency difference of the adaptively equalized signal according to the method described in reference 2, and can also remove the phase modulation component by calculating the fourth power of the QPSK signal. The decoding unit 248 decodes the signal for which the phase difference has been compensated.

[0125] (Reference 1)

[0126] A.Viterbi, "Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission," in IEEE Transactions on Information Theory, vol.29, no.4, pp.543-551, July 1983.

[0127] (Reference 2)

[0128] A. Leven, et al., "Frequency Estimation in Intradyne Reception," IEEEPhotonics Technology Letters, vol.19, no.6, pp.366-368 (2007).

[0129] Fig.12 It is a flowchart showing the operations of the transmission unit 21 and the reception unit 24 .

[0130] The modulated signal converter 211 converts the transmission data (1) input from the outside or the ARQ control unit 10 into a signal modulated at a symbol rate synchronized with the transmission clock signal (5) (step ST1a). The modulated signal output from the transmitter 21 is converted into a transmission optical signal (3) by the optical modulator 22 and then output to the intensity modulator 4.

[0131] The coherent detector 25 performs coherent detection on the received optical signal (4) received by the paired optical space communication transceiver terminal 1A or 1B. The sampling unit 241 samples the received optical signal (4) after coherent detection at a symbol rate synchronized with the reception clock signal (8). The adaptive equalizer 242 performs adaptive equalization on the signal sampled by the sampling unit 241 (step ST2a).

[0132] The SNR estimation unit 243 estimates the SNR of the signal adaptively equalized by the adaptive equalization unit 242 (step ST3a). Next, the frequency difference estimation unit 244 estimates the frequency difference in the adaptively equalized signal using the signal adaptively equalized by the adaptive equalization unit 242, the estimated value of the SNR, and the estimated value of the Doppler shift amount (step ST4a). The frequency difference compensation unit 245 compensates for the frequency difference of the adaptively equalized signal based on the frequency difference estimated by the frequency difference estimation unit 244 (step ST5a).

[0133] Next, the phase difference estimation unit 246 estimates the phase difference of the signal whose frequency difference has been compensated by the frequency difference compensation unit 245 (step ST6a). The phase difference compensation unit 247 compensates the phase difference of the signal subjected to adaptive equalization based on the phase difference estimated by the phase difference estimation unit 246 (step ST7a). Finally, the decoding unit 248 decodes the signal whose phase difference has been compensated, and outputs the decoded data as received data (2) to the outside or the ARQ control unit 10 (step ST8a).

[0134] Fig.13 2 is a block diagram showing a hardware configuration for realizing the functions of the transmission unit 21 and the reception unit 24 . Fig.14 2 is a block diagram showing a hardware configuration for executing software that realizes the functions of the transmission unit 21 and the reception unit 24. Fig.13 and Fig.14 In the embodiment, the optical modulator 300 is Fig.11 The optical modulator 22 is shown. The optical modulator 300 is connected to a processing circuit 305, a processor 306, and a memory 307 via a DA converter (DAC) 301. The DAC 301 converts the signal input to the optical modulator 300 from a digital signal to an analog signal. The Doppler shift estimator 302 is Fig.11 The Doppler shift estimator 23 is shown. The coherent detector 303 is Fig.11 The coherent detector 25 is shown. The coherent detector 303 is connected to a processing circuit 305, a processor 306, and a memory 307 via an AD converter (ADC) 304. The ADC 304 converts a signal output from the coherent detector 303 from an analog signal to a digital signal.

[0135] The functions of the transmitting unit 21 and the receiving unit 24 of the optical transceiver 2A are realized by the processing circuit. Fig.12 The processing circuit of the processes of step ST1a to step ST8a shown in the figure may be dedicated hardware or a CPU that executes a program stored in a memory.

[0136] In the above processing circuit is Fig.13 In the case of the dedicated hardware processing circuit 305 shown, the processing circuit 305 corresponds to, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC, an FPGA, or a combination thereof. The functions of the transmitting unit 21 and the receiving unit 24 included in the optical transceiver 2A may be implemented by different processing circuits, or these functions may be implemented uniformly by one processing circuit.

[0137] In the above processing circuit is Fig.14 In the case of the processor 306 shown, the functions of the transmitter 21 and the receiver 24 of the optical transceiver 2A are implemented by software, firmware, or a combination of software and firmware. The software or firmware is described in the form of a program and stored in the memory 307.

[0138] The processor 306 realizes the functions of the transmitter 21 and the receiver 24 of the optical transceiver 2A by reading and executing the program stored in the memory 307. For example, the optical transceiver 2A includes the memory 307, which is used to store the program executed by the processor 306. Fig.12 The memory 307 may be a computer-readable storage medium storing a program for causing the computer to function as the sending unit 21 and the receiving unit 24.

[0139] The memory 307 corresponds to, for example, a nonvolatile or volatile semiconductor memory such as RAM, ROM, flash memory, EPROM, EEPROM, a magnetic disk, a floppy disk, an optical disk, a high-density disk, a mini disk, a DVD, or the like.

[0140] Alternatively, a part of the functions of the transmitting unit 21 and the receiving unit 24 included in the optical transceiver 2A may be implemented by dedicated hardware, and the remaining part may be implemented by software or firmware. For example, the transmitting unit 21 implements the function by a processing circuit 305 which is dedicated hardware, and the receiving unit 24 implements the function by a processor 306 reading and executing a program stored in a memory 307. In this way, the processing circuit can implement the above functions by hardware, software, firmware, or a combination thereof.

[0141] As described above, in the optical space communication transceiver terminal 1A or 1B of implementation mode 3, the receiving unit 24 samples the received optical signal after coherent detection at a symbol rate synchronized with the receiving clock signal, performs adaptive equalization on the sampled signal, estimates the SNR in the adaptively equalized signal, estimates the frequency difference in the adaptively equalized signal using the adaptively equalized signal, the estimated value of the SNR, and the estimated value of the Doppler frequency shift, compensates for the frequency difference of the adaptively equalized signal based on the estimated frequency difference, estimates the phase difference of the signal compensated for the frequency difference, compensates for the phase difference of the adaptively equalized signal based on the estimated phase difference, and outputs the data obtained by decoding the phase difference compensated signal as received data.

[0142] By providing the optical space communication transceiver terminal 1A or 1B with the optical transceiver 2A, the effects shown in embodiments 1 or 2 are obtained. In addition, as described above, since the time for estimating the frequency difference of the received clock signal (8) is shortened, the time until communication is restored when the SNR of the received optical signal is restored is shortened.

[0143] In the optical space communication transceiver terminal 1A or 1B of the third embodiment, the frequency difference estimation unit 244 estimates the frequency difference in the signal subjected to adaptive equalization when the SNR is above the threshold value, and when the SNR becomes above the threshold value again after falling below the threshold value, estimates the frequency difference in the signal subjected to adaptive equalization using the estimated value of the frequency difference just before the SNR falls below the threshold value and the estimated value of the frequency difference calculated based on the transition of the estimated value of the Doppler frequency shift amount after the SNR falls below the threshold value as initial values. Thus, since the time for estimating the frequency difference of the reception clock signal (8) is shortened, the time until the communication is restored when the SNR of the received optical signal is restored is shortened.

[0144] In addition, in the combination of each embodiment, or in the modification of arbitrary constituent elements of each embodiment, or in each embodiment, arbitrary constituent elements may be omitted.

[0145] Industrial Applicability

[0146] The optical space communication transceiver terminal disclosed in the present invention can be used in a feeder link in a satellite communication network, for example.

[0147] Description of Reference Numerals

[0148] 1 optical space communication system, 1A, 1B optical space communication transceiver terminal, 2, 2A optical transceiver, 3 transmission clock source, 4 intensity modulation unit, 5 optical fiber coupling unit, 6 tracking reflector, 7 beam splitter, 8, 8A angle sensor, 9 optical fiber amplifier, 10 ARQ control unit, 11 low-speed receiver, 12 low-speed transmitter, 21 transmission unit, 22, 300 optical modulator, 23, 302 Doppler frequency shift estimator, 24 receiving unit, 25, 303 coherent detector, 41 electro-optical converter, 42 optical combiner, 81, 85 lens, 83, 88 band pass filter (BPF ), 84 beam splitter, 87 photoelectric converter, 101 sending buffer, 102 sending data output unit, 103 receiving buffer, 104 receiving frame output unit, 105 sequence loss determination unit, 106 retransmission request output unit, 200, 305 processing circuit, 201, 306 processor, 202, 307 memory, 211 modulation signal conversion unit, 241 sampling unit, 242 adaptive equalization unit, 243 SNR estimation unit, 244 frequency difference estimation unit, 245 frequency difference compensation unit, 246 phase difference estimation unit, 247 phase difference compensation unit, 248 decoding unit.

Claims

1. An optical space communication transceiver terminal, which performs two-way communication with an optical space communication transceiver terminal of the same structure and paired with it, characterized in that: The optical space communication transceiver terminal has: An optical transceiver that converts the transmission data into a transmission optical signal modulated at a symbol rate synchronized with a transmission clock signal and outputs the signal, and demodulates and decodes a signal obtained by sampling the reception optical signal at a period synchronized with a reception clock signal, thereby generating reception data; an intensity modulation unit that superimposes a transmission clock signal on a transmission optical signal output from the optical transceiver to generate an intensity-modulated transmission optical signal, and outputs the generated transmission optical signal; a transmission clock source, which supplies a common transmission clock signal to the optical transceiver and the intensity modulation unit; An optical fiber coupling unit that outputs the intensity-modulated transmission optical signal as collimated light and couples the input optical signal to the optical fiber; an optical fiber amplifier, which amplifies the optical signal coupled to the optical fiber through the optical fiber coupling portion, and outputs the amplified optical signal as a received optical signal to the optical transceiver; a tracking reflector that outputs the optical signal propagating in the space to the optical fiber coupling part side, outputs the transmission optical signal set as collimated light by the optical fiber coupling part to the space, and when a propagation angle error is input, outputs an optical signal after the propagation angle is corrected based on the input propagation angle error; a beam splitter that distributes a portion of the optical signal propagating between the tracking mirror and the fiber coupling portion; and An angle sensor outputs a propagation angle error detected based on the optical signal distributed by the beam splitter to the tracking mirror, and outputs a clock signal extracted from the optical signal distributed by the beam splitter to the optical transceiver as a reception clock signal.

2. The optical space communication transceiver terminal according to claim 1, characterized in that: The angle sensor comprises: A split-type light-receiving element that receives a light signal in a light-receiving surface that is split into a plurality of parts; a lens that converges the optical signal to the center position of the light receiving surface of the split-type light receiving element; and A bandpass filter that sets the frequency band of the transmission clock signal from the paired optical space communication transceiver terminals of the same structure as the passband, The angle sensor detects a propagation angle error of the optical signal based on a deviation between a focusing position of the optical signal and a center position of the split-type light receiving element. The angle sensor outputs the clock signal extracted by the bandpass filter to the optical transceiver as a reception clock signal.

3. An optical space communication transceiver terminal, which performs two-way communication with an optical space communication transceiver terminal of the same structure and paired with it, characterized in that: The optical space communication transceiver terminal has: An optical transceiver that converts the transmission data into a transmission optical signal modulated at a symbol rate synchronized with a transmission clock signal and outputs the signal, and demodulates and decodes a signal obtained by sampling the reception optical signal at a period synchronized with a reception clock signal, thereby generating reception data; a low-speed transmitter that converts a retransmission request signal of a communication frame into a signal modulated at a symbol rate synchronized with a transmission clock signal and outputs the signal; a transmission clock source, which supplies a common transmission clock signal to the optical transceiver and the low-speed transmitter; an intensity modulator that superimposes the signal output from the low-speed transmitter on the transmission light signal output from the optical transceiver to generate an intensity-modulated transmission light signal, and outputs the generated transmission light signal; An optical fiber coupling unit that outputs the intensity-modulated transmission optical signal as collimated light and couples the input optical signal to the optical fiber; an optical fiber amplifier, which amplifies the optical signal coupled to the optical fiber through the optical fiber coupling portion, and outputs the amplified optical signal as a received optical signal to the optical transceiver; a tracking reflector that outputs the optical signal propagated in the space to the optical fiber coupling part side, outputs the transmission optical signal set as collimated light by the optical fiber coupling part to the space, and when a propagation angle error is input, outputs an optical signal after the propagation angle is corrected based on the input propagation angle error; a beam splitter that distributes a portion of the optical signal propagating between the tracking reflector and the fiber coupling portion; an angle sensor that outputs an electrical signal representing intensity variation of the optical signal split by the beam splitter; a low-speed receiver that reproduces a clock signal synchronized with a symbol rate by demodulating and decoding the electrical signal output from the angle sensor, outputs the reproduced clock signal as a reception clock signal to the optical transceiver, and outputs the reproduced retransmission request signal when the retransmission request signal is reproduced by demodulating and decoding the electrical signal; as well as An ARQ control unit outputs a transmission frame input from the outside as transmission data to the optical transceiver, outputs a transmission frame corresponding to a sequence number indicated by the retransmission request signal output from the low-speed receiver as transmission data to the optical transceiver, stores reception data output from the optical transceiver as reception frames, and in the case where a sequence number of a reception frame is lost, outputs the retransmission request signal of the reception frame corresponding to the lost sequence number to the low-speed transmitter.

4. The optical space communication transceiver terminal according to claim 3, characterized in that: The ARQ control unit comprises: a transmission buffer storing input transmission frames in the order of sequence numbers; a transmission data output unit, which outputs the transmission frames read out from the transmission buffer in the order of sequence numbers as transmission data to the optical transceiver; a receiving buffer storing the received data output from the optical transceiver as a receiving frame in the order of sequence numbers; a reception frame output unit, which outputs the reception frames stored in the reception buffer in the order of sequence numbers; a determination unit configured to determine whether a sequence number of a received frame stored in the receiving buffer is lost; as well as A retransmission request output unit outputs a retransmission request signal of a received frame corresponding to the lost sequence number to the low-speed transmitter when the determination unit determines that the sequence number has been lost.

5. The optical space communication transceiver terminal according to any one of claims 1 to 4, characterized in that: The optical transceiver comprises: a transmitting unit that converts the transmission data into a signal modulated at a symbol rate synchronized with a transmission clock signal and outputs the signal; An optical modulator that converts a modulated signal output from the transmitting unit into a transmission optical signal and outputs the signal to the intensity modulating unit; a Doppler frequency shift estimator that estimates the amount of Doppler frequency shift of a received clock signal; A coherent detector for performing coherent detection on a received optical signal; as well as a receiving unit that decodes received data using the received optical signal after coherent detection, the estimated value of the Doppler frequency shift amount of the received clock signal, and the received clock signal, The receiving unit comprises: a sampling unit for sampling the received optical signal after coherent detection at a symbol rate synchronized with the received clock signal; An adaptive equalization unit, which performs adaptive equalization on the sampled signal; An SNR estimation unit estimates a signal-to-noise ratio of a signal subjected to adaptive equalization; a frequency difference estimation unit for estimating a frequency difference of the adaptively equalized signal using the signal adaptively equalized by the adaptive equalization unit, the estimated value of the signal-to-noise ratio, and the estimated value of the Doppler shift amount; a frequency difference compensating unit for compensating for a frequency difference of a signal subjected to adaptive equalization based on the frequency difference estimated by the frequency difference estimating unit; a phase difference estimation unit that estimates a phase difference of a signal compensated for a frequency difference; a phase difference compensating unit that compensates for a phase difference of a signal subjected to adaptive equalization based on the phase difference estimated by the phase difference estimating unit; as well as The decoding unit decodes the signal with the phase difference compensated, and outputs the decoded data as received data.

6. The optical space communication transceiver terminal according to claim 5, characterized in that: The frequency difference estimation unit estimates the frequency difference of the adaptively equalized signal when the signal-to-noise ratio is greater than a threshold value, and estimates the frequency difference in the adaptively equalized signal using, as initial values, an estimated value of the frequency difference immediately before the signal-to-noise ratio falls below the threshold and an estimated value of the frequency difference calculated based on a shift in an estimated value of the Doppler frequency shift amount after the signal-to-noise ratio falls below the threshold when the signal-to-noise ratio becomes greater than the threshold again.

7. An optical space communication system, characterized in that: The space optical communication system comprises the space optical communication transceiver terminal according to claim 1 or 3, Bidirectional communication is performed between paired optical space communication transceiver terminals of the same structure.

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