Optical Communication Systems

Abstract

To improve the reliability of optical communication systems.SOLUTION: An optical transmitting device has first to Mth transmission processing means, where mth transmission processing means (m is an integer from 1 to M) has M 2-input 2-output directional couplers. When P is 2 or more, each of N (N=2M) output ports of kth transmission processing means (k is an integer from 1 to M-1) is connected to one of N input ports of (k+1)th transmission processing means so that light input to each of N input ports of the first transmission processing means is output from all of N output ports of the Mth transmission processing means. Transmission light output from each of the N output ports of the Mth transmission processing means is carried via one of N optical links. An optical receiving device has first to Mth reception processing means similar to the first to Mth transmission processing means. Each of N input ports of the first reception processing means is configured to receive the transmission light from one of the N optical links.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a technique for improving the reliability of an optical communication system. [Background technology]

[0002] Free-Space Optics (FSO) communication is a communication method that emits optical signals into free space rather than using a fixed medium such as an optical fiber. Non-Patent Document 1 discloses the application of FSO communication to a portion of an optical link that carries optical signals from an optical transmitter to an optical receiver. Specifically, an optical transmitter transmits an optical signal to a first optical fiber. A first lens is provided at the end of the first optical fiber opposite to the end connected to the optical transmitter. The optical signal propagating through the first optical fiber is emitted into space via the first lens. Meanwhile, an optical receiver is connected to a second optical fiber. A second lens is provided at the end of the second optical fiber opposite to the end connected to the optical receiver. The optical signal emitted into space by the first lens enters the second optical fiber via the second lens and is received by the optical receiver via the second optical fiber. In the following description, the section from the first lens to the second lens, in which the optical signal propagates through free space, is also referred to as the FSO section.

[0003] By providing an FSO section in part of an optical link, it may be possible to quickly restore communications, for example, in the event of a disaster. However, if some kind of obstacle enters the FSO section, the power of the optical signal entering the second optical fiber via the second lens decreases. The same applies when weather conditions that attenuate optical signals, such as fog or rain, occur in the FSO section. Optical links and optical receiving devices generally include optical amplifiers that amplify optical signals. When the power of the optical signal input to the optical amplifier is attenuated, noise increases at the output of the optical amplifier, degrading the signal-to-noise ratio (SNR) of the optical signal.

[0004] For this reason, a configuration in which an optical transmitter and an optical receiver are connected via multiple optical links can be considered. Specifically, the optical transmitter transmits the same optical signal to multiple optical links. The optical receiver selects one optical signal with an SNR greater than a predetermined value from the optical signals received from the multiple optical links, demodulates the selected optical signal, and outputs it to a downstream device. When the SNR of the selected optical signal deteriorates below the predetermined value, the optical receiver switches the optical signal used for demodulation to another optical signal with an SNR greater than the predetermined value. However, momentary interruptions occur when switching optical signals, reducing the reliability of the optical communication system. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Abdelmoula Bekkali,et.al.,"New Generation Free-Space Optical Communication Systems With Advanced Optical Beam Stabilizer",JOURNAL OF LIGHTWAVE TECHNOLOGY,VOL.40,NO.5,March 1, 2022,pp1509-1518 Summary of the Invention [Problem to be solved by the invention]

[0006] The present disclosure provides a technique for improving the reliability of optical communication systems. [Means for solving the problem]

[0007] According to one embodiment of the present disclosure, an optical communication system includes an optical transmission device, and a plurality of optical communication devices, each of which includes N=2 P (P is 2 The optical transmitter has first to M-th transmission processing means (M=N / 2), and the m-th transmission processing means (m is an integer from 1 to M) has two inputs and two outputs. SenderIt has M directional couplers . before The M number of the first transmission processing means Sender The light input to each of the N input ports of the directional coupler is transmitted to the M transmission processing means. Sender Each of the N output ports of the kth transmission processing means (k is an integer between 1 and M-1) is connected to one of the N input ports of the (k+1)th transmission processing means so that light is output from all of the N output ports of the directional coupler. The optical transmitting device is configured so that the transmitted light output from each of the N output ports of the Mth transmission processing means is carried via one of the N optical links. The optical receiving device has first to Mth reception processing means, and the mth reception processing means is a 2-input, 2-output optical link. Receiving side It has M directional couplers . before The M number of the first reception processing means Receiving side The light input to each of the N input ports of the directional coupler is input to the M reception processing means. Receiving side Each of the N output ports of the kth reception processing means is connected to one of the N input ports of the (k+1)th reception processing means so that the light is output from all of the N output ports of the directional coupler. Each of the N input ports of the first reception processing means is configured to receive transmitted light from one of the N optical links. [Effects of the Invention]

[0008] According to the present disclosure, the reliability of an optical communication system can be improved. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a configuration diagram of an optical communication system according to an embodiment. [Figure 2] FIG. 1 is a configuration diagram of an optical communication system according to an embodiment. [Figure 3] 1 is a configuration diagram of an optical transmitting device in an optical communication system according to an embodiment. [Figure 4] 1 is a configuration diagram of an optical receiving device in an optical communication system according to an embodiment. [Figure 5]FIG. 1 is a configuration diagram of an optical communication system according to an embodiment. [Figure 6] FIG. 1 is a configuration diagram of an optical communication system according to an embodiment. [Figure 7] 1 is a configuration diagram of an optical transmitting device in an optical communication system according to an embodiment. [Figure 8] 1 is a configuration diagram of an optical receiving device in an optical communication system according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more features among the multiple features described in the embodiments may be arbitrarily combined. Furthermore, the same reference numerals are used for the same or similar components, and redundant explanations will be omitted.

[0011] First Embodiment FIG. 1 is a configuration diagram of an optical communication system according to this embodiment. Note that in the following figures, components not necessary for explaining the embodiment are omitted. An optical transmitter and an optical receiver are connected by two optical links, optical link 91 and optical link 92. Although not shown, optical link 91 and optical link 92 each have at least one FSO section. Furthermore, optical link 91 and optical link 92 may include one or more optical amplifiers. Note that in this embodiment, optical link 91 and optical link 92 are adjusted so that the phase difference between two signal lights with the same phase simultaneously input from the optical transmitter to optical link 91 and optical link 92 is an integer multiple of 2π at the reception point of the optical receiver. This can be achieved, for example, by providing a device capable of adjusting the phase of light, such as an optical phase shifter, in at least one of optical links 91 and 92 and adjusting the phase of light using this device.

[0012] The optical transmitter has a two-input, two-output directional coupler 1. In the following description, the two input ports of the directional coupler 1 are referred to as input port #1 and input port #2, and the two output ports of the directional coupler 1 are referred to as output port #1 and output port #2. First, the characteristics of the directional coupler will be described below.

[0013] A first optical signal input to input port #1 is output from output port #1 and output port #2 with equal power. The phase of the first optical signal output from output port #2 is delayed by π / 2 relative to the phase of the first optical signal output from output port #1. Similarly, a second optical signal input to input port #2 is output from output port #1 and output port #2 with equal power. The phase of the second optical signal output from output port #1 is delayed by π / 2 relative to the phase of the second optical signal output from output port #2.

[0014] Furthermore, when a first optical signal is input to input port #1 and a second optical signal is input to input port #2, a signal obtained by multiplexing the first optical signal and the second optical signal is output from output port #1 and output port #2, respectively. However, a signal obtained by multiplexing the first optical signal with the second optical signal having a phase delay of π / 2 is output from output port #1, and a signal obtained by multiplexing the first optical signal with the second optical signal having a phase delay of π / 2 is output from output port #2.

[0015] In this embodiment, signal light #1 to be transmitted to an optical receiving device is input to input port #1 of directional coupler 1. Therefore, output port #1 and output port #2 of directional coupler 1 each output signal light #1. Signal light #1 output from output port #1 is transmitted to the optical receiving device via optical link 91. Signal light #1 output from output port #2 is transmitted to the optical receiving device via optical link 92. In the following description, as shown in FIG. 1 , signal light #1 transmitted to the optical receiving device via optical link 91 will also be referred to as transmitted light #1, and signal light #1 transmitted to the optical receiving device via optical link 92 will also be referred to as transmitted light #2. As described above, the phase of transmitted light #2 lags behind the phase of transmitted light #1 by π / 2.

[0016] The optical amplifier 21 of the optical receiving device amplifies the transmitted light #1 received via the optical link 91 and inputs it to the input port #1 of the directional coupler 3. The optical amplifier 22 of the optical receiving device amplifies the transmitted light #2 received via the optical link 92 and inputs it to the input port #2 of the directional coupler 3. The directional coupler 3 is similar to the directional coupler 1. Therefore, a signal obtained by multiplexing the transmitted light #1 with the transmitted light #2, with the phase of the transmitted light #1 delayed by π / 2, is output from the output port #2 of the directional coupler 3. As described above, the transmitted light #1 and the transmitted light #2 are both signal light #1, and the phase of the transmitted light #1 leads the phase of the transmitted light #2 by π / 2. Therefore, the output port #2 of the directional coupler 3 outputs a signal light obtained by multiplexing the two signal light #1 in phase, i.e., signal light #1. Note that the output port #1 of the directional coupler 3 outputs a signal obtained by multiplexing the transmitted light #2 with the phase delayed by π / 2 and with the transmitted light #1. As described above, both transmitted light #1 and transmitted light #2 are signal light #1, and the phase of transmitted light #1 is ahead of the phase of transmitted light #2 by π / 2. Therefore, what is output from output port #2 of directional coupler 3 is the combination of two signal lights #1 in opposite phase, i.e., no signal light is output.

[0017] For example, in the configuration of FIG. 1, assume that attenuation in the FSO section of optical link 91 increases, resulting in an increase in noise at the output of optical amplifier 21. In this case, the noise power is also split into two in directional coupler 3, with half of the noise power being output from output port #1 and the other half being output from output port #2. Therefore, the noise power contained in signal light #1 output from output port #2 of directional coupler 3 is half the noise power at the output of optical amplifier 21, thereby suppressing degradation of SNR. Furthermore, even if attenuation in the FSO section of optical link 91 becomes very large, communication can continue without interruption as long as the SNR of transmitted light #2 (the same as signal light #1) received via optical link 92 is good. The same applies when the SNR of optical link 92 deteriorates. As described above, configuring an optical communication system as shown in FIG. 1 can improve the reliability of the optical communication system.

[0018] Second Embodiment Next, the second embodiment will be described, focusing on the differences from the first embodiment. In the first embodiment, only one signal light #1 is transmitted using two optical links 91 and 92. In this embodiment, two signal lights #1 and #2 are transmitted using two optical links 91 and 92. Figure 2 shows the configuration of an optical communication system according to this embodiment.

[0019] As shown in FIG. 2 , in this embodiment, signal light #2 is input to input port #2 of directional coupler 1. Therefore, output ports #1 and #2 of directional coupler 1 output transmitted light #1 and transmitted light #2, respectively, which are obtained by multiplexing signal light #1 and signal light #2. As described above, transmitted light #1 is obtained by multiplexing signal light #1 with signal light #2, the phase of which is delayed by π / 2, and transmitted light #2 is obtained by multiplexing signal light #1 with signal light #2, the phase of which is delayed by π / 2. Therefore, for the same reason that signal light #1 is output from output port #2 of directional coupler 3 in the optical receiving device, signal light #2 is output from output port #1 of directional coupler 3.

[0020] For the same reasons as those described in the first embodiment, even if the signal light is attenuated in the FSO section of optical link 91 or optical link 92 and noise increases at the output of the optical amplifier at the subsequent stage, the influence is equally divided among the signal light by the directional coupler 3 of the optical receiving device. Therefore, even if attenuation increases in the FSO section of the optical link, communication can be continued without interruption. Furthermore, compared to the first embodiment, the same number of signal light as the number of optical links can be transmitted.

[0021] Third Embodiment Next, the third embodiment will be described, focusing on the differences from the second embodiment. In the second embodiment, two optical links 91 and 92 were used to transmit two signal lights #1 and #2. In this embodiment, four optical links 91 to 94 are used to transmit four signal lights #1 to #4. FIG. 3 shows the configuration of an optical transmitter in an optical communication system according to this embodiment. In this embodiment, the optical transmitter has four directional couplers 1-11, 1-12, 1-21, and 1-22. The four directional couplers 1-11, 1-12, 1-21, and 1-22 are arranged in a 2×2 matrix. The four directional couplers 1-11, 1-12, 1-21, and 1-22 and the optical waveguides connecting the output ports and input ports of the directional couplers may be formed on a silicon substrate.

[0022] Signal light #1 is input to input port #1 of directional coupler 1-11, and signal light #2 is input to input port #2 of directional coupler 1-11. Similarly, signal light #3 is input to input port #1 of directional coupler 1-12, and signal light #4 is input to input port #2 of directional coupler 1-12. Output port #1 of directional coupler 1-11 is connected to input port #1 of directional coupler 1-21, and output port #2 of directional coupler 1-11 is connected to input port #1 of directional coupler 1-22. Furthermore, output port #1 of directional coupler 1-12 is connected to input port #2 of directional coupler 1-21, and output port #2 of directional coupler 1-12 is connected to input port #2 of directional coupler 1-22. Note that the optical waveguides connecting the output and input ports of the two directional couplers can be configured so that they are the same length.

[0023] 3, output port #1 of directional coupler 1-21 is connected to optical link 91, output port #2 of directional coupler 1-21 is connected to optical link 92, output port #1 of directional coupler 1-22 is connected to optical link 93, and output port #2 of directional coupler 1-22 is connected to optical link 94. Each of optical links 91 to 94 has at least one FSO section. Furthermore, optical links 91 to 94 may have one or more optical amplifiers. In this embodiment, as in the above embodiment, optical links 91 to 94 are adjusted so that the phase difference between four light beams of the same phase that are simultaneously input to optical links 91 to 94 becomes an integer multiple of 2π at the reception point of the optical receiving device.

[0024] FIG. 4 shows the configuration of an optical receiving device in an optical communication system according to this embodiment. The optical receiving device includes optical amplifiers 21 to 24 that amplify transmitted light #1 to transmitted light #4 from optical links 91 to 94. Similarly to the optical transmitting device, the optical receiving device also includes four directional couplers 3-11, 3-12, 3-21, and 3-22. The configurations of the four directional couplers 3-11, 3-12, 3-21, and 3-22 are similar to the configurations of the four directional couplers 1-11, 1-12, 1-21, and 1-22 in the optical transmitting device. Transmitted light #1 is input to input port #1 of directional coupler 3-11, and transmitted light #2 is input to input port #2 of directional coupler 3-11. Transmitted light #3 is input to input port #1 of directional coupler 3-12, and transmitted light #4 is input to input port #2 of directional coupler 3-12.

[0025] For the same reasons as those described in the above embodiment, the signal output from output port #1 of directional coupler 3-11 is the signal input to input port #2 of directional coupler 1-21, and is designated by the same reference symbol 43 in the drawings and will hereinafter also be referred to as signal 43. The signal output from output port #2 of directional coupler 3-11 is the signal input to input port #1 of directional coupler 1-21, and is designated by the same reference symbol 41 in the drawings and will hereinafter also be referred to as signal 41. The signal output from output port #1 of directional coupler 3-12 is the signal input to input port #2 of directional coupler 1-22, and is designated by the same reference symbol 44 in the drawings and will hereinafter also be referred to as signal 44. The signal output from output port #2 of directional coupler 3-12 is the signal input to input port #1 of directional coupler 1-22, and is designated by the same reference symbol 42 in the drawings and will hereinafter also be referred to as signal 42.

[0026] Signals 43 and 44 are input to input port #1 and input port #2 of the directional coupler 3-21. Signals 43 and 44 are output from output port #1 and output port #2 of the directional coupler 1-12 by inputting signal light #3 and signal light #4 to input port #1 and input port #2 of the directional coupler 1-12. Therefore, for the same reasons as described in the above embodiment, output port #1 and output port #2 of the directional coupler 3-21 output signal light #4 and signal light #3, respectively. Signals 41 and 42 are input to input port #1 and input port #2 of the directional coupler 3-22. Signals 41 and 42 are output from output port #1 and output port #2 of the directional coupler 1-11 by inputting signal light #1 and signal light #2 to input port #1 and input port #2 of the directional coupler 1-11. Therefore, for the same reasons as explained in the above embodiment, the output port #1 and output port #2 of the directional coupler 3-22 output the signal light #2 and the signal light #1, respectively.

[0027] For the same reasons as those explained in the first and second embodiments, even if the signal light is attenuated in the FSO section of the optical link and noise increases at the output of the optical amplifier in the subsequent stage, the influence is divided equally among the signal lights by the directional couplers 3-11, 3-12, 3-21, and 3-22 of the optical receiving device. Therefore, even if the attenuation in the FSO section of the optical link increases, communication can be continued without interruption.

[0028] <Summary of the above embodiment> In the second embodiment, two signal lights are transmitted over two optical links, and in the third embodiment, four signal lights are transmitted over four optical links. More generally, N=2 P A configuration can be adopted in which up to N signal lights can be transmitted through (P is an integer of 1 or more) optical links. Note that the second embodiment is an example in which P=1, and the third embodiment is an example in which P=2. In order to transmit N signal lights through N optical links, the optical transmitting device has M 2 a transmission processing section including M directional couplers; 2 A receiving processing section including M directional couplers is provided, where M=N / 2.

[0029] The transmission processing unit has first to Mth transmission processing units. The mth transmission processing unit (m is an integer from 1 to M) has M directional couplers (2 inputs, 2 outputs). The (2M=N) input ports of the M directional couplers of the first transmission processing unit constitute the N input ports of the transmission processing unit and are used to input N signal light. The (2M=N) output ports of the M directional couplers of the Mth transmission processing unit constitute the N output ports of the transmission processing unit, and each output port is connected to one of the N optical links. When P is 2 or greater, each of the N output ports of the kth transmission processing unit (k is an integer from 1 to M-1) is connected to one of the N input ports of the (k+1)th transmission processing unit. The connection between the kth transmission processing unit and the (k+1)th transmission processing unit is made so that when light is input to any of the N input ports of the first transmission processing unit, the light is branched into N parts and output from all of the N output ports of the Mth transmission processing unit.

[0030] The receiving processor has the same configuration as the transmitting processor, and includes a first receiving processor through an Mth receiving processor. The mth receiving processor has M directional couplers (two inputs, two outputs). The (2M=N) input ports of the M directional couplers of the first receiving processor constitute the N input ports of the receiving processor, and each input port receives transmitted light transmitted from the optical transmitter via the optical link. The (2M=N) output ports of the M directional couplers of the Mth receiving processor constitute the N output ports of the receiving processor, and each output port outputs signal light. When P is 2 or greater, each of the N output ports of the kth receiving processor (k is an integer between 1 and M-1) is connected to one of the N input ports of the (k+1)th receiving processor. The connection between the kth receiving processor and the (k+1)th receiving processor is such that, regardless of which of the N input ports of the first receiving processor is used, the light is branched into N and output from all of the N output ports of the Mth receiving processor.

[0031] In the above configuration, the number of transmitted signal lights can be any number between 1 and N, and it is not necessary to always transmit N signal lights. In other words, the number of signal lights actually transmitted can be configured to increase in order according to demand. For example, the first embodiment is an example in which the number of transmitted signal lights is 1 when P=1 (i.e., N=2).

[0032] <Fourth embodiment> The first to third embodiments are based on the premise that each optical link is adjusted so that the phase difference between signal lights of the same phase simultaneously incident on each optical link is an integer multiple of 2π at the receiving point of the optical receiving device. However, the phase of each signal light may fluctuate due to various factors. Therefore, a configuration for dynamic phase compensation will be described below.

[0033] FIG. 5 illustrates a configuration in which a phase fluctuation compensation configuration is added to the configuration of the first embodiment shown in FIG. 1. The following mainly describes the differences from the configuration shown in FIG. 1. Transmitted light #1 and transmitted light #2 are input to input ports #1 and #2 of directional coupler 3 of the optical receiving device via optical phase shifter 51 and optical phase shifter 52. Optical phase shifter 51 and optical phase shifter 52 shift the phase of the transmitted light passing through. The amount of phase shift is controlled by controller 6. Output port #1 of directional coupler 3 of the optical receiving device is connected to controller 6. Controller 6 determines the amount of phase shift in optical phase shifter 51 and optical phase shifter 52 based on the level of light input from output port #1 of directional coupler 3, and sets the amount of phase shift in optical phase shifter 51 and optical phase shifter 52. As described above, since no light is input to input port #2 of directional coupler 1 of the optical transmitting device, ideally, the level of light output from output port #1 of directional coupler 3 is zero.

[0034] The control unit 6 adjusts the amounts of phase shift applied by the optical phase shifter 51 and the optical phase shifter 52 so that the level of the input light is minimized. When the amount of phase shift applied by the optical phase shifter 51 is increased (when the phase is delayed further), the control unit 6 adjusts the amounts of phase shift applied by the optical phase shifters 51 and 52 so that the amount of phase shift applied by the optical phase shifter 52 is decreased. Similarly, when the amount of phase shift applied by the optical phase shifter 51 is decreased, the control unit 6 adjusts the amounts of phase shift applied by the optical phase shifters 51 and 52 so that the amount of phase shift applied by the optical phase shifter 52 is increased.

[0035] As described above, by adjusting the amount of phase shift in the optical phase shifter 51 and the optical phase shifter 52 so that the level of the light output from the output port #1 of the directional coupler 3 is minimized, ideally to 0, even if the phase of each transmitted light fluctuates due to various factors, the phase fluctuation can be compensated for and a decrease in the level of the signal light #1 output from the directional coupler 3 can be prevented. Note that in this embodiment, two optical phase shifters 51 and two optical phase shifters 52 are provided corresponding to the optical link 91 and the optical link 92, respectively, but a configuration in which only one of the optical phase shifters 51 and 52 is provided may also be used.

[0036] Fifth Embodiment Fig. 6 shows an example in which a configuration for compensating for phase fluctuations is added to the configuration of the second embodiment shown in Fig. 2. The following description focuses on the differences from the configurations shown in Fig. 2 and Fig. 5. The fourth embodiment (Fig. 5) utilizes the fact that no light is input to input port #2 of directional coupler 1 of the optical transmitter. However, in this embodiment, signal light #2 is input to input port #2 of directional coupler 1 of the optical transmitter.

[0037] For this reason, a coupler 7 is provided in the optical transmitting device. The coupler 7 multiplexes the signal light #1 and the supervisory light and inputs the multiplexed signal light #1 to the input port #1 of the directional coupler 1. The supervisory light is, for example, continuous light with a frequency fs. Note that the frequency fs is a frequency outside the band of the signal light #1 and the signal light #2. In the optical receiving device, the supervisory light is output from the output port #2 of the directional coupler 3, just like the signal light #1. A filter 9 is provided to remove the supervisory light output from the output port #2 of the directional coupler 3 and to pass only the signal light #1.

[0038] Furthermore, a frequency separator 8 is connected to output port #1 of the directional coupler 3. The frequency separator 8 separates the band of the signal light #2 from a band including the frequency fs of the supervisory light and outputs the separated light. The port of the frequency separator 8 that outputs light in the band including the frequency fs of the supervisory light is connected to the control unit 6. When the phase state is ideal, the supervisory light is output only from output port #2 of the directional coupler 3, and is not output from output port #1 of the directional coupler 3. Therefore, as in the fourth embodiment, the control unit 6 adjusts the phase shift amounts of the optical phase shifters 51 and 52 so that the level of the supervisory light output from output port #1 of the directional coupler 3 is minimized.

[0039] With the above configuration, even if the phase of each transmitted light fluctuates due to various factors, the phase fluctuation can be compensated for, and a decrease in the level of the signal light output from the directional coupler 3 can be prevented. In this embodiment, the frequency separator 8 is connected to output port #1 of the directional coupler 3, the filter 9 is connected to output port #2 of the directional coupler 3, and the control unit 6 controls the optical phase shifter to minimize the level of the monitor light output from output port #1 of the directional coupler 3. However, a configuration in which the filter 9 is connected to output port #1 of the directional coupler 3, the frequency separator 8 is connected to output port #2 of the directional coupler 3, and the optical phase shifter is controlled to maximize the level of the monitor light output from output port #2 of the directional coupler 3 may also be used.

[0040] Sixth Embodiment Figures 7 and 8 show an arrangement in which a configuration for compensating for phase fluctuations is provided in the arrangement of the third embodiment shown in Figures 3 and 4. The following description will focus on the differences between the arrangement shown in Figures 3 and 4 and the arrangement shown in the fifth embodiment (Figure 6).

[0041] In this embodiment, three supervisory light beams #1 to #3 are used, which is one less than the number of optical links (four). The supervisory light beams #1 to #3 are, for example, continuous light beams with frequencies fs1 to fs3. The frequencies fs1 to fs3 are different from one another and are outside the bands of the signal light beams #1 to #4.

[0042] 7, on the transmitting side, a coupler 7 is provided in the optical waveguide connecting the output port #1 of the directional coupler 1-11 and the input port #1 of the directional coupler 1-21, and monitor light #1 is input thereto. A coupler 7 is also provided in the optical waveguide connecting the output port #2 of the directional coupler 1-12 and the input port #2 of the directional coupler 1-22, and monitor light #2 is input thereto. Furthermore, monitor light #3 is branched into two by a coupler 71, and these are input to the coupler 7 provided in the optical waveguide connecting the output port #2 of the directional coupler 1-11 and the input port #1 of the directional coupler 1-22, and the optical coupler 7 provided in the optical waveguide connecting the output port #1 of the directional coupler 1-12 and the input port #2 of the directional coupler 1-21.

[0043] Because the supervisory light #1 is multiplexed with the signal 41, if the phase state is ideal, on the receiving side, the supervisory light #1 is output from output port #2 of the directional coupler 3-11 and not from output port #1. For this reason, a frequency separator 8 is provided in the optical waveguide connecting the output port #2 of the directional coupler 3-11 and the input port #1 of the directional coupler 3-22, and the supervisory light #1 is input to the control unit 6. Similarly, because the supervisory light #2 is multiplexed with the signal 44, if the phase state is ideal, on the receiving side, the supervisory light #2 is output from output port #1 of the directional coupler 3-12 and not from output port #2. For this reason, a frequency separator 8 is provided in the optical waveguide connecting the output port #1 of the directional coupler 3-12 and the input port #2 of the directional coupler 3-12, and the supervisory light #2 is input to the control unit 6.

[0044] The control unit 6 adjusts the optical phase shifter 51 and the optical phase shifter 52 so that the level of the supervisory light #1 output from output port #2 of the directional coupler 3-11 is maximized, and adjusts the optical phase shifter 53 and the optical phase shifter 54 so that the level of the supervisory light #2 output from output port #1 of the directional coupler 3-12 is maximized. By adjusting the optical phase shifter 51 and the optical phase shifter 52 so that the level of the supervisory light #1 output from output port #2 of the directional coupler 3-11 is maximized, the phase difference between the transmitted light #1 and the transmitted light #2 is compensated. Furthermore, by adjusting the optical phase shifter 53 and the optical phase shifter 54 so that the level of the supervisory light #2 output from output port #1 of the directional coupler 3-12 is maximized, the phase difference between the transmitted light #3 and the transmitted light #4 is compensated.

[0045] Furthermore, the supervisory light #3 is multiplexed with the signals 42 and 43. Therefore, when the phase state is ideal, the supervisory light #3 is output from the output port #1 of the directional coupler 3-11 and the output port #2 of the directional coupler 3-12 on the receiving side. As described above, the phase difference between the transmitted light #1 and the transmitted light #2 is compensated for by the control of the optical phase shifter based on the supervisory light #1 and the supervisory light #2, and the phase difference between the transmitted light #3 and the transmitted light #4 is also compensated for. However, the phase difference between the first group of the transmitted light #1 and the transmitted light #2 and the second group of the transmitted light #3 and the transmitted light #4 is not compensated for. For this reason, as shown in FIG. 8, a frequency separator 8 is provided in each of the optical waveguides connecting the output port #1 of the directional coupler 3-11 to the input port #1 of the directional coupler 3-12 and the optical waveguide connecting the output port #2 of the directional coupler 3-12 to the output port #2 of the directional coupler 3-22, and the supervisory light #3 is extracted from each of them. The extracted two supervisory light beams #3 are multiplexed by coupler 7 and input to control unit 6. If the phase state between the first group of transmitted light beams #1 and #2 and the second group of transmitted light beams #3 and #4 is ideal, the phases of the two supervisory light beams #3 are in phase, and the level of supervisory light beam #3 output by coupler 7 is maximized. On the other hand, if the phase state between the first and second groups is not ideal, the phase difference between the two supervisory light beams #3 will cause the level of supervisory light beam #3 output by coupler 7 to decrease.

[0046] Therefore, the control unit 6 controls the optical phase shifters 51 to 54 so that the level of the input supervisory light #3 is maximized. At this time, the control unit 6 controls the optical phase shifters 51 and 52 so that the difference between the phase shift amounts provided by the optical phase shifters 51 and 52 does not change, in order to maintain the adjustment using supervisory light #1. Similarly, the control unit 6 controls the optical phase shifters 53 and 54 so that the difference between the phase shift amounts provided by the optical phase shifters 53 and 54 does not change, in order to maintain the adjustment using supervisory light #2. More specifically, when the adjustments of the optical phase shifters 51 and 52 based on the level of supervisory light #1 are completed, the phase shift amounts provided by the optical phase shifters 51 and 52 are the first shift amount and the second shift amount, and when the adjustments of the optical phase shifters 53 and 54 based on the level of supervisory light #2 are completed, the phase shift amounts provided by the optical phase shifters 53 and 54 are the third shift amount and the fourth shift amount. In this case, the control unit 6 adjusts the shift amounts in the optical phase shifters 51 to 54 so that the level of the light of frequency fs3 input to the control unit 6 is maximized, under the condition that the difference between the first and second shift amounts is not changed and the difference between the third and fourth shift amounts is not changed. By controlling the optical phase shifters 51 to 54 in this way, the phase difference between the first and second groups can be compensated for, and the phase can be brought closer to an ideal state.

[0047] In this embodiment, the optical phase shifters 51 and 52 are adjusted so that the monitor light #1 output from output port #2 of the directional coupler 3-11 is maximized, and the optical phase shifters 53 and 54 are adjusted so that the monitor light #2 output from output port #1 of the directional coupler 3-12 is maximized. However, a configuration in which the optical phase shifters 51 and 52 are adjusted so that the monitor light #1 output from output port #1 of the directional coupler 3-11 is minimized, and the optical phase shifters 53 and 54 are adjusted so that the monitor light #2 output from output port #2 of the directional coupler 3-12 is minimized, may also be used. In this case, the frequency separator 8 connected to output port #1 of the directional coupler 3-11 separates the signal 43, the signal with frequency fs1, and the signal with frequency fs3, and the frequency separator 8 connected to output port #2 of the directional coupler 3-12 separates the signal 43, the signal with frequency fs2, and the signal with frequency fs3. In this case, there is no need to provide the frequency separator 8 connected to the output port #2 of the directional coupler 3-11 and the output port #1 of the directional coupler 3-12.

[0048] Generally, N=2 P In a configuration in which up to N signal lights are transmitted over (P is an integer greater than or equal to 1) optical links, the phases of the N transmitted lights can be dynamically controlled by using (N-1) supervisory lights.

[0049] In the above embodiments, the optical communication system is configured such that each of the multiple optical links includes one or more FSO sections. However, at least one of the multiple optical links may include one or more FSO sections. Furthermore, the optical communication system may be configured such that none of the multiple optical links includes an FSO section. Furthermore, in the above embodiments, the optical receiving device includes an optical amplifier. However, the optical amplifier may be an external device to the optical receiving device. Furthermore, in the fourth to sixth embodiments, the optical phase shifter and the control unit are components of the optical receiving device. However, the optical phase shifter and the control unit may be external devices to the optical receiving device. Furthermore, the bands of the signal lights may completely overlap, partially overlap, or not overlap.

[0050] The above configuration can improve the reliability of optical communication systems, thereby contributing to Goal 9 of the United Nations' Sustainable Development Goals (SDGs), which is to "Build resilient infrastructure, promote sustainable industrialization, and foster innovation." [Explanation of symbols]

[0051] 1-11, 1-12, 1-21, 1-22, 3-11, 3-12, 3-21, 3-22: directional couplers, 91 to 94: optical links

Claims

1. An optical transmitting device, the optical transmitting device, and N=2 Ｐ an optical receiving device connected by optical links (P is an integer of 2 or more), The optical transmitter comprises: The system has first to Mth transmission processing means (M=N / 2), the mth transmission processing means (m is an integer from 1 to M) has M transmission-side directional couplers each having two inputs and two outputs; each of the N output ports of the kth transmission processing means (k is an integer from 1 to M−1) is connected to one of the N input ports of the (k+1)th transmission processing means so that light input to each of the N input ports of the M transmission-side directional couplers of the first transmission processing means is output from all of the N output ports of the M transmission-side directional couplers of the Mth transmission processing means; the optical transmitting device is configured so that transmitted light output from each of the N output ports of the M transmission processing means is carried via one of the N optical links; The optical receiving device The system includes first reception processing means to Mth reception processing means, the m-th reception processing means has M two-input, two-output reception-side directional couplers; each of the N output ports of the kth reception processing means is connected to one of the N input ports of the (k+1)th reception processing means so that light input to each of the N input ports of the M reception-side directional couplers of the first reception processing means is output from all of the N output ports of the M reception-side directional couplers of the Mth reception processing means; An optical communication system, wherein each of the N input ports of said first receive processing means is configured to receive transmitted light from one of said N optical links.

2. 2. The optical communication system according to claim 1, wherein the optical receiving device has an optical amplifier connected to each of the N optical links, and each of the N input ports of the first reception processing means is configured to receive transmitted light from one of the N optical links via the optical amplifier.

3. 2. The optical communication system of claim 1, further comprising an optical phase shifter for shifting the phase of transmitted light carried on each of said N optical links.

4. P=2, a first input port and a second input port of a first directional coupler of the two transmitting-side directional couplers of the first transmission processing means are used to input a first signal light and a second signal light; a first input port and a second input port of a second directional coupler of the two transmitting-side directional couplers of the first transmission processing means are used to input a third signal light and a fourth signal light, a signal from a first output port of the first directional coupler is input to a first input port of a third directional coupler of the two transmitting side directional couplers of the M transmission processing means, and a signal from a first output port of the second directional coupler is input to a second input port of the third directional coupler; a signal from the second output port of the first directional coupler is input to a first input port of a fourth directional coupler of the two transmitting side directional couplers of the M transmission processing means, and a signal from the second output port of the second directional coupler is input to a second input port of the fourth directional coupler; a first transmission light from a first output port of the third directional coupler is output to a first optical link; a second transmission light from a second output port of the third directional coupler is output to a second optical link; a third transmission light from the first output port of the fourth directional coupler is output to a third optical link; 4. The optical communication system according to claim 3, wherein a fourth transmission light from a second output port of the fourth directional coupler is output to a fourth optical link.

5. the first transmission light is input to a first input port of a fifth directional coupler of the two reception-side directional couplers of the first reception processing means, and the second transmission light is input to a second input port of the fifth directional coupler; the third transmission light is input to a first input port of a sixth directional coupler of the two reception-side directional couplers of the first reception processing means, and the fourth transmission light is input to a second input port of the sixth directional coupler; a signal from the first output port of the fifth directional coupler is input to a first input port of a seventh directional coupler of the two receiving-side directional couplers of the M receiving processing means, and a signal from the first output port of the sixth directional coupler is input to a second input port of the seventh directional coupler; 5. The optical communication system according to claim 4, wherein a signal from the second output port of the fifth directional coupler is input to a first input port of an eighth directional coupler of the two receiving side directional couplers of the M receiving processing means, and a signal from the second output port of the sixth directional coupler is input to a second input port of the eighth directional coupler.

6. a first supervisory light having a frequency different from the bands of the first to fourth signal lights is input to the first input port of the third directional coupler; a second optical supervisory channel having a frequency different from the band of the first optical signal, the fourth optical signal, and the first optical supervisory channel is input to the second input port of the fourth directional coupler; 6. The optical communication system according to claim 5, wherein third supervisory light having a frequency different from the bands of the first signal light, the fourth signal light, the first supervisory light, and the second supervisory light is input to the second input port of the third directional coupler and the first input port of the fourth directional coupler.

7. 7. The optical communication system according to claim 6, further comprising: control means for controlling the optical phase shifter based on the first monitor light output from the second output port of the fifth directional coupler, the second monitor light output from the first output port of the sixth directional coupler, the third monitor light output from the first output port of the fifth directional coupler, and the third monitor light output from the second output port of the sixth directional coupler.

8. the optical phase shifter comprises a first optical phase shifter that shifts the phase of the first transmission light, a second optical phase shifter that shifts the phase of the second transmission light, a third optical phase shifter that shifts the phase of the third transmission light, and a fourth optical phase shifter that shifts the phase of the fourth transmission light, The control means adjusting the phase shift amounts of the first transmission light and the second transmission light in the first optical phase shifter and the second optical phase shifter to a first shift amount and a second shift amount, respectively, so that the level of the first supervisory light is maximized, and adjusting the phase shift amounts of the third transmission light and the fourth transmission light in the third optical phase shifter and the fourth optical phase shifter to a third shift amount and a fourth shift amount so that the level of the second supervisory light is maximized; 8. The optical communication system according to claim 7, wherein, after adjusting the amounts of phase shift in the first to fourth optical phase shifters so that the level of the first monitor light and the level of the second monitor light are maximized, the amounts of phase shift in the transmitted light in the optical phase shifters are adjusted so that the level of combined light obtained by combining the third monitor light output from the first output port of the fifth directional coupler and the third monitor light output from the second output port of the sixth directional coupler is maximized, under the condition that a difference between the first shift amount and the second shift amount is not changed and a difference between the third shift amount and the fourth shift amount is not changed.

9. An optical communication system comprising an optical transmitting device, an optical receiving device connected to the optical transmitting device by two optical links, an optical phase shifter that shifts the phase of light carried by each of the two optical links, and control means, the optical transmitter has one transmitting-side directional coupler with two inputs and two outputs; the optical transmitting device is configured such that signal light is input to only one of two input ports of the transmitting-side directional coupler, and transmitted light output from each of two output ports of the transmitting-side directional coupler is carried via one of the two optical links; the optical receiving device has one receiving-side directional coupler with two inputs and two outputs; each of the two input ports of the receiving directional coupler is configured to receive transmitted light from one of the two optical links; an optical communication system in which the control means controls the optical phase shifter based on the level of light output from a second output port, which is one of the two output ports of the receiving side directional coupler and is different from a first output port from which the signal light is output.

10. An optical communication system as described in Claim 9, wherein the control means adjusts the amount of phase shift of the transmitted light in the optical phase shifter so that the level of light output from the second output port is minimized.

11. An optical communication system comprising an optical transmitting device, an optical receiving device connected to the optical transmitting device by two optical links, and an optical phase shifter that shifts the phase of light carried by each of the two optical links, the optical transmitter has one transmitting-side directional coupler with two inputs and two outputs; the optical transmitting device is configured such that a first input port of the two input ports of the transmitting-side directional coupler is used to input a first signal light, a second input port of the two input ports of the transmitting-side directional coupler is used to input a second signal light, and transmitted light output from each of the two output ports of the transmitting-side directional coupler is carried via one of the two optical links; the optical receiving device has one receiving-side directional coupler with two inputs and two outputs; An optical communication system, wherein each of the two input ports of the receiving directional coupler is configured to receive transmitted light from one of the two optical links.

12. A supervisory light having a frequency outside the band of the first signal light and the second signal light is input to the first input port of the transmitting side directional coupler, The optical communication system includes:

12. The optical communication system according to claim 11, further comprising a control means for controlling the optical phase shifter based on the level of the monitor light output from a monitor port of the two output ports of the receiving side directional coupler.

13. The monitoring port is one of the two output ports of the receiving-side directional coupler from which the second signal light is output, 13. The optical communication system according to claim 12, wherein said control means adjusts the amount of shift in the phase of said transmission light in said optical phase shifter so that the level of said supervisory light is minimized.

14. The monitoring port is one of the two output ports of the receiving-side directional coupler from which the first signal light is output, 13. The optical communication system according to claim 12, wherein said control means adjusts the amount of shift in the phase of said transmission light in said optical phase shifter so that the level of said supervisory light is maximized.

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