Optical receiver and optical transmitter

Abstract

To increase the amount of information transmitted in an optical communication system that uses PPM.SOLUTION: An optical receiver that receives signal light in which pulse position modulated symbols are repeated includes first light receiving means through Mth light receiving means (M is an integer greater than or equal to 2), and the mth light receiving means (m is an integer from 1 to M) is configured to detect one symbol out of M symbols and not to detect the same symbol as the other (M-1) light receiving means.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to optical communication technology. [Background Art]

[0002] In deep-space communications such as communication between artificial satellites and the ground, and communication between the Moon and the ground, it is necessary to use a modulation method with high energy efficiency and a reception technology with high sensitivity, in consideration of power limitations at space stations such as artificial satellites and large losses caused by ultra-long-distance transmission. Non-Patent Document 1 discloses that energy-efficient pulse position modulation (PPM) is used for communication between an artificial satellite and the ground. PPM is a method that associates the position (number) of a time slot with data. For example, when using four time slots from the 1st to the 4th, the first time slot is associated with bit "00", the second time slot is associated with bit "10", the third time slot is associated with bit "01", and the fourth time slot is associated with bit "11". When transmitting bit "01" in such an association, the optical transmitter transmits light (pulsed light) within the third time slot, and stops transmitting light in other time slots. To increase the amount of information transmission in PPM, it is necessary to shorten the duration of each time slot.

[0003] In addition, Non-Patent Document 1 discloses using a single photon detector (SPD) to receive PPM signals with high sensitivity. [Prior Art Documents] [Non-Patent Documents]

[0004] [Non-Patent Document 1] David J.Geisler,"Modem Module Development for NASA's Orion Spacecraft:Achieving FSO Communications Over Lunar Distances",Th3J.1.pdf,OFC 2020,OSA 2020 [Overview of the project] [Problems that the invention aims to solve]

[0005] Some SPDs (Single Photon Diodes) utilize superconductivity, but in this case, the single photon detector needs to be cooled to extremely low temperatures. Since cooling to such low temperatures requires large equipment, superconducting single photon detectors are unsuitable for use in space agencies. On the other hand, SPDs can be constructed by operating avalanche photodiodes (APDs) in Geiger mode. However, in the case of SPDs using APDs, the photon detection interval must be greater than the guard time. In other words, an SPD using APDs cannot detect a subsequent photon even if it arrives within the guard time after detecting a previous photon. Therefore, the amount of information transmitted is limited by the guard time of the SPD.

[0006] This disclosure provides a technology for increasing the amount of information transmitted in optical communication systems using PPM. [Means for solving the problem]

[0007] According to one aspect of this disclosure, an optical receiver that receives a signal light in which pulsed position-modulated symbols are repeated is: It includes a single-photon detector, and once it detects light, it cannot detect light for a minimum period of time. The system comprises a first to a Mth light-receiving means (where M is an integer of 2 or more), and the mth light-receiving means (where m is an integer from 1 to M) is configured to detect one symbol for every M symbols and not to detect the same symbol as the other (M-1) light-receiving means, and the period from the end timing of the first symbol detected by the mth light-receiving means to the start timing of the second symbol detected by the mth light-receiving means after the first symbol is the period during which the mth light-receiving means The aforementionedIt is longer than the minimum period. [Effects of the Invention]

[0008] According to this disclosure, the amount of information transmitted in optical communication systems using PPM can be increased. [Brief explanation of the drawing]

[0009] [Figure 1] A diagram showing the configuration of signal light transmitted by an optical transmitter according to several embodiments. [Figure 2] Configuration diagrams of an optical transmitter according to several embodiments. [Figure 3] Configuration diagrams of optical receivers according to several embodiments. [Figure 4] Configuration diagrams of an optical transmitter according to several embodiments. [Figure 5] Configuration diagrams of optical receivers according to several embodiments. [Figure 6] Configuration diagrams of optical receivers according to several embodiments. [Modes for carrying out the invention]

[0010] The embodiments will be described in detail below with reference to the attached drawings. Note that the following embodiments do not limit the invention as defined in the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more of the features described in the embodiments may be arbitrarily combined. Furthermore, identical or similar configurations will be given the same reference numeral, and redundant descriptions will be omitted.

[0011] <First Embodiment> Figure 1 shows the structure of the signal light transmitted by the optical transmitter. The signal light consists of a repetition of multiple symbols. As shown in Figure 1, the duration of one symbol (symbol duration) is T. One symbol contains N time slots (N is an integer greater than or equal to 2). Each of the N time slots corresponds to one of the log2N bits of data, as described above. For example, when N=4, the four time slots correspond one-to-one with the four 2-bit data "00", "01", "10", and "11". Therefore, the optical transmitter can carry log2N bits of data in one symbol.

[0012] Figure 2 is a diagram of the configuration of the optical transmitter according to this embodiment. Light source 10 transmits light of wavelength λ1 to optical modulator 11. Light source 20 transmits light of wavelength λ2, which is different from wavelength λ1, to optical modulator 21. Light source 30 transmits light of wavelength λ3, which is different from wavelengths λ1 and λ2, to optical modulator 31. The distribution unit 41 divides the input data into log2N bits and outputs it to optical modulators 11, 21 and 31 in a round-robin format, for example. Optical modulators 11, 21 and 31 output light (pulsed light) of the wavelength input from the corresponding light source in time slots corresponding to the input log2N bits of data. Hereinafter, the pulse position modulated (PPM) light output by optical modulators 11, 21 and 31 will also be referred to as PPM light.

[0013] The timing at which the three optical modulators 11, 21, and 31 output PPM light is different from that of the others. In this embodiment, since three modulators are used, the symbols at which optical modulators 11, 21, and 31 output PPM light occur are once every three symbols. For example, the symbols at which optical modulator 11 outputs PPM light are the 1st symbol, 4th symbol, 7th symbol, etc., which are symbols whose numbers, when divided by the number of optical modulators (3), leave a remainder of 1. The symbols at which optical modulator 21 outputs PPM light are the 2nd symbol, 5th symbol, 8th symbol, etc., which are symbols whose numbers, when divided by the number of optical modulators (3), leave a remainder of 2. The symbols at which optical modulator 31 outputs PPM light are the 3rd symbol, 6th symbol, 9th symbol, etc., which are symbols whose numbers, when divided by the number of optical modulators (3), leave a remainder of 0.

[0014] The wavelength multiplexer 40 outputs PPM light (wavelength multiplexed PPM light) obtained by wavelength multiplexing the PPM light of wavelength λ1 from the optical modulator 11, the PPM light of wavelength λ2 from the optical modulator 21, and the PPM light of wavelength λ3 from the optical modulator 31. The wavelength multiplexed PPM light is transmitted to the optical receiver.

[0015] In this embodiment, three wavelengths are used, and therefore, the number of optical modulators is set to three. More generally, if the number of wavelengths used is M (where M is an integer greater than or equal to 2), then M optical modulators are provided, from the first optical modulator to the Mth optical modulator. The Mth optical modulator transmits PPM light with a symbol whose remainder when divided by M is 0, and the mth optical modulator (where m is an integer from 1 to M-1) transmits PPM light with a symbol whose remainder when divided by M is m.

[0016] FIG. 3 is a configuration diagram of the optical receiver according to the present embodiment. The wavelength separator 50 wavelength-separates the received wavelength-multiplexed PPM light into PPM light of wavelength λ1, PPM light of wavelength λ2, and PPM light of wavelength λ3. The SPD 51 detects the PPM light of wavelength λ1 and outputs a detection timing. The SPD 52 detects the PPM light of wavelength λ2 and outputs a detection timing. The SPD 53 detects the PPM light of wavelength λ3 and outputs a detection timing. The detection timings of the PPM light output by SPD 51 to SPD 53 are output to a demodulation unit (not shown). The demodulation unit determines data transmitted by an optical transmitter based on the detection timings of the PPM light output by SPD 51 to SPD 53.

[0017] As shown in FIG. 3, the minimum interval at which each of SPD 51 to 53 detects pulsed light is a period from the end timing of a certain symbol to the start timing of a symbol three symbols after the certain symbol, that is, 2T corresponding to two symbol periods. If the number of wavelengths used is M, the minimum interval at which each SPD detects a photon is (M-1)T. Therefore, by setting the number of wavelengths used or the symbol period T such that this minimum interval is larger than the guard time of each SPD, each SPD can detect PPM light.

[0018] With the above configuration, it is possible to transmit a larger amount of information than the amount of information determined by the guard time of each individual SPD.

[0019] Although wavelength division multiplexing is used in the above embodiment, polarization division multiplexing can be used instead of or in addition to wavelength division multiplexing. For example, by using right-handed polarization and left-handed polarization orthogonal to right-handed polarization, the minimum interval at which each SPD detects a photon can be set to T. Note that polarization division multiplexing is not limited to multiplexing of circularly polarized waves, and if linearly polarized waves can be used, the configuration may be such that polarization division multiplexing is performed with two orthogonal linearly polarized waves. When both wavelength division multiplexing and polarization division multiplexing are used, M is the number of combinations of the number of wavelengths and the number of polarizations used.

[0020] Note that, as shown in FIG. 2, the light source and the optical modulator are used as a set. This set may also be referred to as one optical modulation unit.

[0021] <Second Embodiment> Next, the second embodiment will be described focusing on differences from the first embodiment. FIG. 4 is a configuration diagram of an optical transmitter according to the present embodiment. A light source 10 transmits light of wavelength λ1 to an optical modulator 11. The optical modulator 11 modulates the light of wavelength λ1 with input data and outputs PPM light. Specifically, the optical modulator 11 divides the input data into every log₂N bits, and outputs light in a time slot corresponding to the log₂N bits of data for each symbol, thereby outputting PPM light.

[0022] FIG. 5 is a configuration diagram of an optical receiver according to the present embodiment. An optical switch 60 separates the received PPM light into three portions equal to the number of SPDs provided in the optical receiver for each symbol. Therefore, as shown in FIG. 5, one symbol is input to SPD 51 to SPD 53 every three symbols. Accordingly, similar to the first embodiment, the minimum interval at which each of SPD 51 to 53 detects pulsed light is 2T, which corresponds to two symbol periods. If the number of SPDs to be used is M, the minimum interval at which each SPD detects pulsed light is (M-1)T. Therefore, by setting the number of SPDs to be used or the symbol period T such that this minimum interval is larger than the guard time of each SPD, each SPD can detect PPM light.

[0023] <Third Embodiment> Next, the third embodiment will be described focusing on differences from the second embodiment. The configuration of the optical transmitter of the present embodiment is the same as that of the second embodiment. FIG. 6 is a configuration diagram of an optical receiver according to the present embodiment. In the present embodiment, an optical coupler 70 is used instead of the optical switch 60 of the second embodiment. Therefore, all symbols are input to SPD 51 to SPD 53.

[0024] However, in this embodiment, the timing control circuit 71 controls the period during which SPD51 to SPD53 are active. Specifically, the timing control circuit 71 controls each SPD such that when an SPD is active for the duration of one symbol, it becomes inactive for the next two consecutive symbols. The timing control circuit 71 ensures that the periods during which SPD51 to SPD53 are active differ from each other. The shaded symbols in Figure 6 indicate the symbols detected by the corresponding SPD when that SPD is activated. Therefore, similar to the second embodiment, the minimum interval during which each of SPD51 to 53 detects pulse light is 2T, which corresponds to a two-symbol period.

[0025] If the number of SPDs used is M, the timing control circuit 71 activates each SPD only once at a different timing for each M symbol. As a result, the minimum interval at which each SPD detects pulse light is (M-1)T. Therefore, by setting the number of SPDs used or the symbol period T so that this minimum interval is greater than the guard time of each SPD, each SPD can detect PPM light.

[0026] <Summary> As described in each embodiment above, the optical receiver has M light-receiving units, for example, SPDs. The SPDs are, for example, avalanche photodiodes operated in Geiger mode. The optical receiver receives signal light from the optical transmitter, which consists of a repetition of pulse-position modulated symbols. Each SPD is configured to detect the symbols of the signal light only once for every M symbols. Each SPD is configured not to detect the same symbols as the other (M-1) SPDs. This configuration allows adjustment of the minimum interval at which each SPD detects light. For example, the period from the end timing of the first symbol detected by a certain SPD to the start timing of the second symbol detected by that SPD after the first symbol can be made longer than the minimum period during which the SPD can continuously detect light. Therefore, the amount of information transmitted in the optical communication system can be increased by M times compared to using a single light-receiving unit.

[0027] In the first and second embodiments, the optical receiver is provided with a separation unit that separates the received signal light and outputs the first signal light to the Mth signal light. The first signal light to the Mth signal light are each output to one of the M SPDs. In the first embodiment, the separation unit corresponds to a wavelength separator 50. As described in the first embodiment, when polarization multiplexing is used, the separator becomes a polarization separator. Furthermore, when wavelength multiplexing and polarization multiplexing are used in combination, the separation unit includes a wavelength separator 50 and a polarization separator. In the second embodiment, the separation unit corresponds to an optical switch 60.

[0028] On the other hand, in the third embodiment, the optical receiver is provided with a branching unit that branches the received signal light from the first signal light to the Mth signal light. The first to Mth signal lights are each output to one of the M SPDs. The branching unit is, for example, an optical coupler 70. Furthermore, in the third embodiment, the optical receiver is provided with a state setting unit that sets the operating state of the M SPDs to either an active state in which light is detected or an inactive state in which light is not detected. The state setting unit corresponds to the timing control circuit 71 in the third embodiment. The state setting unit sets the operating state of each SPD such that each SPD detects one symbol for every M symbols and does not detect the same symbol as the other (M-1) SPDs.

[0029] Furthermore, according to this embodiment, an optical transmitter is provided that transmits signal light in which pulse position modulated symbols are repeated. The optical transmitter is provided with a plurality of modulation units that generate pulse position modulated PPM light symbols based on the data to be transmitted, and a multiplexing unit that multiplexes the symbols generated by the plurality of modulation units. The multiplexing unit performs at least one of wavelength multiplexing and polarization multiplexing. The timing at which each modulation unit generates symbols is made different from that of the others.

[0030] The above configuration makes it possible to increase the amount of information transmitted in optical communication systems using PPM. Therefore, it becomes possible to contribute to Goal 9 of the United Nations-led Sustainable Development Goals (SDGs), "Build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0031] The invention is not limited to the embodiments described above, and various modifications and changes are possible within the scope of the gist of the invention. [Explanation of symbols]

[0032] 51-53: SPD

Claims

1. An optical receiver that receives signal light in which pulse position modulated symbols are repeated, It includes a single-photon detector and comprises first to M light-receiving means (where M is an integer of 2 or more) which, once light is detected, are unable to detect light for a minimum period of time. The mth light-receiving means (where m is an integer from 1 to M) is configured to detect one symbol for every M symbols, and not to detect the same symbol as the other (M-1) light-receiving means. An optical receiver in which the period from the end timing of the first symbol detected by the m light receiving means to the start timing of the second symbol detected by the m light receiving means after the first symbol is greater than the minimum period of the m light receiving means.

2. The aforementioned optical receiver The system further comprises separation means that outputs a first signal light to a Mth signal light by separating the signal light using the aforementioned symbols as units, The optical receiver according to claim 1, wherein the m-th light receiving means detects the m-th signal light.

3. Each of the repeating symbols has a wavelength or polarization that is one of M different wavelengths or polarizations. The optical receiver according to claim 2, wherein the separation means outputs the M signal light from the first signal light by wavelength separation or polarization separation of the signal light.

4. The wavelength and polarization of each of the repeating symbols are one of M different combinations of wavelength and polarization. The optical receiver according to claim 2, wherein the separation means outputs the M signal light from the first signal light by wavelength separation and polarization separation of the signal light.

5. The optical receiver according to claim 2, wherein the separation means includes an optical switch.

6. The aforementioned optical receiver A branching means that outputs the Mth signal light from the first signal light by branching the aforementioned signal light, A state setting means that sets the operating state of the M light receiving means from the first light receiving means to either an active state that detects light or an inactive state that does not detect light, Equipped with, The m-th signal light is input to the m-th light receiving means, The optical receiver according to claim 1, wherein the state setting means sets the operating state of the m light receiving means such that the m light receiving means detects one symbol for every M symbols and does not detect the same symbol as the other (M-1) light receiving means.

7. The optical receiver according to any one of claims 1 to 6, wherein the single-photon detector is an avalanche photodiode operated in Geiger mode.

8. An optical transmitter that transmits a signal light in which pulsed position-modulated symbols are repeated to an optical receiver including a single-photon detector and having multiple light-receiving means that, upon detecting light, are unable to detect light for a minimum period of time, Multiple modulation means for generating pulse-position modulated symbols based on the data to be transmitted, A multiplexing means for multiplexing the symbols generated by the plurality of modulation means, Equipped with, The aforementioned multiplexing is at least one of wavelength multiplexing and polarization multiplexing. The timing at which the multiple modulation means generate the symbols differs for each of them. An optical transmitter in which the period from the end timing of the first symbol generated by the first modulation means among the plurality of modulation means to the start timing of the second symbol generated by the first modulation means after the first symbol is greater than the minimum period of the light receiving means among the plurality of light receiving means that detects the symbol generated by the first modulation means.

9. The optical transmitter according to claim 8, wherein the wavelengths of light used by each of the plurality of modulation means to generate the symbols are all different.

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