Optical transmitter, optical receiver, and optical communication system

Abstract

To provide an optical transmitter, an optical receiver, and an optical communication system that increase the amount of information transmission in the optical communication system using pulse position modulation.SOLUTION: An optical transmitter includes one or more modulation means for generating modulated light that is pulse position-modulated based on data to be transmitted, and repeats changing the frequency of the modulated light linearly from a first frequency to a second frequency. An optical receiver includes demultiplexing means for demultiplexing the modulated light into a plurality of de-multiplexed modulated lights according to the frequency of the modulated light, and a plurality of light receiving means for receiving the plurality of respective de-multiplexed modulated lights.SELECTED DRAWING: Figure 4

Description

[Technical field]

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

[0002] In deep space communications such as communications between a satellite and the ground or between the moon and the ground, it is necessary to use a modulation method with high energy efficiency and a receiving technology with high sensitivity, taking into consideration the power limitations in space stations such as satellites and the large losses due to ultra-long distance transmission. Non-Patent Document 1 discloses the use of energy-efficient pulse position modulation (PPM) for communications between a satellite and the ground. PPM is a method of associating data with a time slot position (number) for transmitting an optical pulse signal. For example, when using four time slots from the first to the fourth, the first time slot is associated with bits "00", the second time slot is associated with bits "10", the third time slot is associated with bits "01", and the fourth time slot is associated with bits "11". When transmitting bit "01" in such an association, the optical transmitter transmits an optical pulse signal in the third time slot and stops transmitting optical pulse signals in the other time slots. In order to increase the amount of information transmitted in PPM, it is necessary to shorten the period of each time slot.

[0003] Furthermore, Non-Patent Document 1 discloses the use of a single photon detector (SPD) in order to receive a PPM signal with high sensitivity. [Prior art documents] [Non-patent literature]

[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 Summary of the Invention [Problem to be solved by the invention]

[0005] Some single photon detectors use superconductivity, but in this case, the detector must be cooled to extremely low temperatures. Since cooling to extremely low temperatures requires large equipment, single photon detectors using superconductivity are not suitable for use in space stations. On the other hand, a single photon detector can be constructed by operating an avalanche photodiode (APD) in Geiger mode. However, in the case of a single photon detector using an APD, the detection interval between photons must be longer than the guard time. In other words, after detecting a photon, a single photon detector using an APD cannot detect the next photon, even if the next photon arrives within the guard time.

[0006] For example, as shown in FIG. 1, each frame is composed of eight time slots TS#1 to TS#8. The period of each TS is T. Since one frame has eight time slots, three bits of data can be transmitted in one frame. The optical transmitter transmits an optical pulse signal only in the time slots corresponding to the data to be transmitted among TS#1 to TS#8 in each frame. The optical pulse signal is transmitted near the center timing of each time slot. Depending on the data to be transmitted, an optical pulse signal is transmitted in TS#8 of the Nth frame, and an optical pulse signal is transmitted in TS#1 of the following (N+1)th frame. To detect such an optical signal, the period T must be greater than the guard time of the single photon detector. Therefore, the amount of information transmission is limited by the guard time of the single photon detector.

[0007] The present invention provides a technique for increasing the amount of information transmitted in optical communication systems that use pulse position modulation. [Means for solving the problem]

[0008] According to one embodiment of the present disclosure, an optical transmitter includes one or more modulation means for generating modulated light that is pulse-position modulated based on data to be transmitted, and the one or more modulation means repeatedly change the frequency of the modulated light linearly from a first frequency to a second frequency. The period during which the frequency of the modulated light is linearly changed from the first frequency to the second frequency is M times (M is an integer equal to or greater than 2) the period of a time slot in the pulse position modulation. It is characterized by: Effect of the Invention

[0009] The present invention makes it possible to increase the amount of information transmitted in optical communication systems that use pulse position modulation. [Brief description of the drawings]

[0010] [Figure 1] FIG. [Diagram 2] FIG. [Diagram 3] FIG. 2 is a configuration diagram of an optical transmitter according to an embodiment. [Figure 4] FIG. 4 is a diagram illustrating the operation of the optical transmitter according to the embodiment. [Diagram 5] FIG. 2 is a configuration diagram of an optical receiver according to an embodiment. [Figure 6] FIG. 2 is a configuration diagram of an optical transmitter according to an embodiment. [Figure 7] FIG. 4 is a diagram illustrating the operation of the optical transmitter according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more features among the multiple features described in the embodiments may be arbitrarily combined. In addition, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0012] Before describing the embodiment, a comparative example will be described first. Fig. 2 shows the configuration of an optical receiver in the comparative example. In the following description, it is assumed that each frame is composed of eight time slots TS#1 to TS#8 as shown in Fig. 1. In Fig. 2, the optical receiver has four single photon detectors (SPDs) 51. In the following description, the four SPDs 51 are referred to as SPD#1 to SPD#4 when they need to be distinguished from one another.

[0013] The optical switch 50 outputs TS#1 and TS#5 to SPD#1, TS#2 and TS#6 to SPD#2, TS#3 and TS#7 to SPD#3, and TS#4 and TS#8 to SPD#4. Each SPD51 detects whether or not an optical pulse signal is being transmitted in the input time slot. Therefore, the minimum interval at which each SPD51 must detect an optical pulse signal is four times that when a single SPD51 is used. In other words, if the guard time of the SPD51 is constant, the time slot period T can be reduced to one-fourth by using four SPD51. This means that the amount of information transmission can be four times that when a single SPD51 is used. However, it is difficult to increase the switching speed of the optical switch 50, and the amount of information transmission is limited by the switching speed of the optical switch 50.

[0014] First Embodiment Fig. 3 is a configuration diagram of the optical transmitter 1 according to this embodiment. For example, a light source 10, which is a laser diode, generates a carrier light and outputs it to an optical modulator 11. The optical modulator 11 pulse-position modulates the carrier light based on input data and outputs modulated light. The frame structure of the modulated light in this example is as shown in Fig. 1. Based on the frame structure of Fig. 1, a synchronization unit 15 outputs a signal to the light source 10 for changing the frequency (wavelength) of the carrier light output by the light source 10. Therefore, the light source 10 outputs a carrier light whose frequency changes over time.

[0015] Fig. 4 is an explanatory diagram of a change in the frequency of the carrier light output by the light source 10. As shown in Fig. 4, in this example, the light source 10 linearly increases the frequency from the start timing of time slot #1 to the end timing of time slot #4. After that, the light source 10 makes the frequency of the carrier light at the start timing of time slot #5 the same as that at the start timing of time slot #1, and linearly increases the frequency from the start timing of time slot #5 to the end timing of time slot #8. Note that the frequency of the carrier light at the end timing of time slot #8 is made the same as the frequency of the carrier light at the end timing of time slot #4.

[0016] Therefore, the optical pulse signal transmitted through TS#1 and the optical pulse signal transmitted through TS#5 have the same frequency. Similarly, the optical pulse signal transmitted through TS#2 and the optical pulse signal transmitted through TS#6 have the same frequency, the optical pulse signal transmitted through TS#3 and the optical pulse signal transmitted through TS#7 have the same frequency, and the optical pulse signal transmitted through TS#4 and the optical pulse signal transmitted through TS#8 have the same frequency. In the following description, the frequency of the optical pulse signals transmitted through TS#k and TS#(k+4) (k is an integer from 1 to 4) is represented as f#k.

[0017] Fig. 5 shows the configuration of the optical receiver 2 according to this embodiment. The optical demultiplexer 20 separates the input modulated light according to frequency. In this example, the optical demultiplexer 20 outputs modulated light of frequency f#k to the SPD#k. As explained in the modified example of Fig. 2, the amount of information transmission can be four times as much as when a single SPD 51 is used. Furthermore, in this embodiment, instead of the optical switch 50 that operates dynamically, the optical demultiplexer 20 that operates statically is used, so the amount of information transmission is not limited by the switching speed of the optical switch 50 as in the comparative example.

[0018] As described above, in this embodiment, the frequency of the carrier light is repeatedly changed linearly from the first frequency to the second frequency, and the optical modulator 11 performs pulse position modulation on the carrier light whose frequency changes in this manner to output modulated light. With this configuration, even if the time slot period is shortened, the modulated light can be frequency-separated by the optical demultiplexer 20 in the optical receiver 2, and therefore the amount of information transmission can be increased.

[0019] Second Embodiment Next, the second embodiment will be described with a focus on differences from the first embodiment. In the first embodiment, as shown in FIG. 4, the light source 10 must significantly change the frequency of the output carrier light at the end timing of TS#4 (start timing of TS#5) and the end timing of TS#8 (start timing of TS#1). In FIG. 4, the frequency of the carrier light is depicted as changing instantaneously at the end timing of TS#4 and the end timing of TS#8 for the sake of simplicity, but in reality, the frequency of the carrier light changes with a certain slope. In other words, a certain period X is required for the frequency at the end timing of TS#4 to change to the frequency at the start timing of TS#5. Therefore, in the configuration of the first embodiment, the amount of information transmission is limited by this period X. For this reason, multiple light sources are used in this embodiment.

[0020] 6 is a configuration diagram of the optical transmitter 1 according to this embodiment. The optical transmitter 1 of this embodiment includes a light source 12, an optical modulator 13, and an optical multiplexer 14 in addition to the light source 10, the optical modulator 11, and the synchronization unit 15. The light source 12, like the light source 11, generates a carrier light and outputs it to the optical modulator 13. The optical modulator 13, like the optical modulator 11, pulse position modulates the carrier light based on input data and outputs modulated light. The optical multiplexer 14 outputs modulated light obtained by multiplexing the modulated light from the optical modulator 11 and the modulated light from the optical modulator 13. The synchronization unit 15 controls the frequency of the carrier light output by the light source 12 in addition to the light source 10.

[0021] 7(A) shows the change in frequency of the optical carrier output by the light source 10, and FIG. 7(B) shows the change in frequency of the optical carrier output by the light source 12. The light source 10 linearly increases the frequency from the start of time slot #1 to the end of time slot #4. Thereafter, the light source 10 does not output the optical carrier until the start of the next time slot #1. Similarly, the light source 12 linearly increases the frequency from the start of time slot #5 to the end of time slot #8. Thereafter, the light source 10 does not output the optical carrier until the start of the next time slot #5.

[0022] With the above configuration, it is possible to prevent the amount of information transmission from being limited by the period X during which the frequency of the carrier light output by the light source is significantly changed. The configuration of the optical receiver 2 in this embodiment is the same as that in the first embodiment.

[0023] In this embodiment, the number of light sources is two, but the number P of light sources can be any number equal to or greater than two. The P light sources are configured such that when one light source is outputting a carrier light, the remaining (P-1) light sources stop outputting the carrier light. This control is performed by the synchronization unit 15. Specifically, assuming that the period during which the frequency of the carrier light is changed linearly is Q, each of the P light sources is configured to stop outputting the carrier light for a period of Q×(P-1) following the linear change in the frequency of the carrier light. The period Q is 4T in the example of FIG. 7.

[0024] In this embodiment, one optical modulator is provided corresponding to one light source, but since there is only one light source that outputs a carrier light at a given moment, the number of optical modulators can be reduced to one by inputting carrier lights from multiple light sources to the same single optical modulator. In this case, the optical multiplexer 14 is omitted.

[0025] <Other> In each of the above embodiments, the frequency of the carrier light is changed linearly, but the intensity (amplitude) of the carrier light can also be changed according to the frequency of the carrier light. This is because the transmission loss differs according to the frequency. In other words, the optical transmitter 1 can be configured to change the intensity (amplitude) of the carrier light according to the frequency of the carrier light so that the intensity of the modulated light received by the optical receiver 2 is constant regardless of the frequency. In addition, in each of the above embodiments, the light source and the optical modulator are different members. However, it is also possible to configure the light source to directly output modulated light that has been pulse-position modulated by controlling the emission of the light source by a drive signal based on data. In other words, it is also possible to configure the light source in each of the above embodiments and the optical modulator in each of the above embodiments to be realized by one member. Note that this member may be called either a light source or an optical modulator.

[0026] In each of the above embodiments, the light source linearly increases the frequency of the carrier light in a period spanning four time slots (4T), but the frequency of the carrier light can also be linearly decreased. Also, in each of the above embodiments, the light source linearly changes the frequency of the carrier light in a period spanning four time slots (4T). This is because the number of SPDs 51 in the optical receiver 2 is four. Therefore, more generally, when the number of SPDs 51 in the optical receiver 2 is M, the light source linearly changes the frequency of the carrier light in a period spanning M time slots (M×T). Here, M is an integer equal to or greater than 2.

[0027] In the above-mentioned embodiments, the number M of the optical receivers 2 is set to 4, which is half the number of time slots in a frame. This is to make the number of tie slots input to each SPD 51 a predetermined number. For example, in the example shown in FIG. 2, only TS#1 and TS#5 are input to SPD#1, and TS#2 to #4 and TS#6 to #8 are not input to SPD#1. By setting the number M of the SPDs 51 to 1 / integer of the number of time slots in a frame or an integer multiple of the number of time slots in a frame, the number of tie slots input to each SPD 51 can be set to a predetermined number. This configuration is advantageous because it is easy to understand the correspondence between each SPD 51 and the data value detected by the SPD 51. However, the number M of the SPDs 51 can be set to a value other than 1 / integer of the number of time slots in a frame or an integer multiple of the number of time slots in a frame. For example, even if one frame has 8 time slots, the number M of the SPDs 51 can be set to 7. In this case, the time slot numbers input to SPD#51 will be a repetition of TS#1, TS#8, TS#7, TS#6, TS#5, TS#4, TS#3, TS#2, and TS#1.

[0028] Furthermore, there is provided an optical communication system including the optical transmitter and optical receiver described in each embodiment.

[0029] The above configuration can increase the amount of information transmitted in optical communication systems that use pulse position modulation, which can contribute to Goal 9 of the United Nations' Sustainable Development Goals (SDGs), which is to "build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0030] The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention. [Explanation of symbols]

[0031] 10, 12: light source, 11, 13: optical modulator

Claims

1. one or more modulation means for generating modulated light which is pulse-position modulated based on data to be transmitted; the one or more modulation means repeatedly change the frequency of the modulated light linearly from a first frequency to a second frequency; an optical transmitter characterized in that a period during which the frequency of the modulated light is linearly changed from the first frequency to the second frequency is M times (M is an integer equal to or greater than 2) a period of a time slot in the pulse position modulation.

2. the one or more modulation means is a modulation means; 2. The optical transmitter according to claim 1, wherein the one of the modulation means, when the frequency of the modulated light reaches the second frequency, subsequently changes the frequency of the modulated light linearly from the first frequency to the second frequency.

3. the one or more modulation means being a plurality of modulation means; 2. The optical transmitter according to claim 1, wherein, when one of the plurality of modulation means is outputting the modulated light, another of the plurality of modulation means other than the one modulation means stops outputting the modulated light.

4. The number of the plurality of modulation means is P (P is an integer equal to or greater than 2), a period during which the plurality of modulation means linearly change the frequency of the modulated light from the first frequency to the second frequency is Q; The optical transmitter according to claim 3, characterized in that the multiple modulation means are configured to stop outputting the modulated light for a subsequent period of Q×(P-1) when the frequency of the modulated light is linearly changed from the first frequency to the second frequency.

5. Each of the one or more modulation means includes a light source that generates a carrier light, and a modulator that pulse position modulates the carrier light from the light source based on the data to generate the modulated light, 5. An optical transmitter as claimed in claim 1, wherein the frequency of the modulated light is changed linearly from the first frequency to the second frequency by linearly changing the frequency of the carrier light from the first frequency to the second frequency.

6. the one or more modulation means includes a plurality of light sources each generating a carrier light, and a modulator that pulse position modulates the carrier light from the plurality of light sources based on the data to generate the modulated light, the frequency of the carrier light is linearly changed from the first frequency to the second frequency, so that the frequency of the modulated light is linearly changed from the first frequency to the second frequency; 2. The optical transmitter according to claim 1, wherein when one of the plurality of light sources is outputting the carrier light, a light source other than the one of the plurality of light sources is configured to stop outputting the carrier light.

7. The number of the plurality of light sources is P (P is an integer equal to or greater than 2), a period during which the plurality of light sources linearly change the frequency of the carrier light from the first frequency to the second frequency is Q; The optical transmitter of claim 6, wherein the plurality of light sources are configured to stop outputting the carrier light for a subsequent period of Q×(P−1) when the frequency of the carrier light is linearly changed from the first frequency to the second frequency.

8. 8. An optical transmitter according to claim 1, wherein a receiver for receiving the modulated light comprises: a splitting means for splitting the modulated light into M number of split modulated lights in accordance with a frequency of the modulated light; and M number of receiving means for receiving each of the M number of split modulated lights.

9. 9. The optical transmitter according to claim 1, wherein the one or more modulation means change an intensity of the modulated light in accordance with a frequency of the modulated light.

10. 1. An optical receiver for receiving pulse-position modulated light whose frequency changes linearly from a first frequency to a second frequency, a demultiplexing means for demultiplexing the modulated light into a plurality of demultiplexed modulated lights in accordance with a frequency of the modulated light; a plurality of light receiving means for receiving each of the plurality of demultiplexed modulated lights; Equipped with an optical receiver characterized in that the number of the plurality of light receiving means is M (M is an integer equal to or greater than 2), and a period during which the frequency of the modulated light changes linearly from the first frequency to the second frequency is M times a period of a time slot in the pulse position modulation.

11. 11. The optical receiver of claim 10, wherein said plurality of light receiving means are single photon detectors.

12. 12. The optical receiver of claim 11, wherein the single photon detector is an avalanche photodiode operated in Geiger mode.

13. An optical transmitter according to any one of claims 1 to 9; An optical receiver according to any one of claims 10 to 12; An optical communication system comprising:

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