Optical transmitting device and optical receiving device

Abstract

To suppress the effects of reflected light without using dithering.SOLUTION: An optical transmitter connected to an optical receiver via an optical transmission path includes generating means for generating carrier light, control means for controlling the frequency of the carrier light in accordance with a time-varying frequency pattern, and generating means for generating signal light to be transmitted to the optical receiver by modulating the carrier light generated by the generating means on the basis of an electrical signal, and in the time-varying frequency pattern, the difference in frequencies separated by a predetermined period of time is greater than the bandwidth of the signal light.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present disclosure relates to a technique for suppressing the influence of reflected light in an optical communication system.

Background Art

[0002] There are a plurality of connection points for connecting optical fibers to each other in an optical transmission line. Note that optical fibers are connected to each other by an optical connector or fusion splicing. At this connection point, a part of the signal light transmitted from the optical transmission device to the optical reception device is reflected toward the optical transmission device. A part of the signal light reflected toward the optical transmission device can be further reflected toward the optical reception device at another connection point. Since the optical transmission device has an isolator or the like that blocks light propagating in the opposite direction to the transmitted signal light, there is no problem even if the reflected signal light reaches the optical transmission device. On the other hand, the optical reception device receives received light including signal light that has reached the optical reception device without being reflected in the optical transmission line (hereinafter referred to as direct light) and signal light that has reached the optical reception device after being reflected an even number of times in the optical transmission line (hereinafter referred to as reflected light). Since the propagation delays of the direct light and the reflected light are different, the reflected light becomes interference light of the direct light and affects the demodulation of the direct light.

[0003] Patent Document 1 and Non-Patent Document 1 disclose a configuration that performs dithering in order to suppress the influence of reflected light. Specifically, Non-Patent Document 1 discloses a configuration that generates dithering light using a phase modulator dedicated to dithering. Further, Patent Document 1 discloses a configuration that generates transmission light including signal light and dithering light by driving a light source with both a signal for carrying information and a signal for dithering.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Non-Patent Documents

[0005] [Non-Patent Document 1] Byung Gon Kim,et.al.,"Reflection-Tolerant RoF-Based Mobile Fronthaul Network for 5G Wireless Systems",JOURNAL OF TECHNOLOGY,VOL.37,NO.24,December 15, 2019 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, if the level of reflected light is high, dithering cannot suppress the effects of reflected light.

[0007] This disclosure provides a technology that suppresses the effects of reflected light without using dithering. [Means for solving the problem]

[0008] According to one aspect of the present disclosure, an optical transmitting device connected to an optical receiving device via an optical transmission path includes: generating means for generating carrier light; control means for controlling the frequency of the carrier light according to a time variation pattern of frequency; and generating means for generating signal light to be transmitted to the optical receiving device by modulating the carrier light generated by the generating means based on an electrical signal, wherein in the time variation pattern, the difference in frequencies separated by a predetermined period is greater than the bandwidth of the signal light. [Effects of the Invention]

[0009] According to this disclosure, the effects of reflected light can be suppressed without dithering. [Brief explanation of the drawing]

[0010] [Figure 1] Configuration diagrams of optical communication systems according to several embodiments. [Figure 2] Configuration diagrams of an optical transmission device according to several embodiments. [Figure 3]A diagram showing the time variation patterns of the carrier light frequency in several embodiments. [Figure 4] Diagrams illustrating the configuration of the light source according to several embodiments. [Figure 5] A diagram showing the received light received by an optical receiver according to several embodiments. [Figure 6] Configuration diagrams of optical receiving devices according to several embodiments. [Figure 7] Configuration diagrams of optical communication systems according to several embodiments. [Figure 8] A diagram showing the time evolution patterns of the frequencies of multiple carrier lights generated by multiple light sources according to several embodiments. [Figure 9] Figure 8 is a diagram illustrating the various methods of utilizing carrier light. [Figure 10] Configuration diagrams of an optical transmission device according to several embodiments. [Figure 11] Configuration diagrams of an optical transmission device according to several embodiments. [Modes for carrying out the invention]

[0011] 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 combined arbitrarily. Furthermore, identical or similar configurations will be given the same reference numeral, and redundant descriptions will be omitted.

[0012] <First Embodiment> FIG. 1 is a configuration diagram of an optical communication system according to the present embodiment. The optical transmission device 1 transmits signal light to the optical reception device 2 via the optical transmission line 3. The signal light is obtained by modulating the carrier light with an electrical signal that carries information. Note that another optical reception device 2 may be arranged in the station building where the optical transmission device 1 is arranged, and another optical transmission device 1 may be arranged in the station building where the optical reception device 2 is arranged. The another optical transmission device 1 transmits signal light to the another optical reception device 2 via another optical transmission line 3. The optical transmission device 1 and the another optical reception device 2 may be implemented as one optical communication device. Similarly, the another optical transmission device 1 and the optical reception device 2 may be implemented as one optical communication device.

[0013] The optical transmission line 3 has a plurality of connection points (reflection points) 31, 32, and 33. Note that in FIG. 1, the number of reflection points is three, but the number of reflection points may be any number of two or more. The signal light transmitted by the optical transmission device 1 to the optical transmission line 3 may be reflected at each of the reflection points 31, 32, and 33. The reflected light reflected at an odd number of reflection points reaches the optical transmission device 1, and the reflected light reflected at an even number of reflection points reaches the optical reception device 2. As described above, since the optical transmission device 1 has an isolator that blocks the reflected light reaching the optical transmission device 1, the reflected light to the optical transmission device 1 does not affect the optical communication system. On the other hand, the reflected light reaching the optical reception device 2 becomes the interference light of the signal light that reaches the optical reception device 2 without being reflected at the reflection point, that is, the direct light, and thus affects the demodulation of the signal light in the optical reception device 2.

[0014] In FIG. 1, D1 is the time required for the signal light to propagate between the reflection point 31 and the reflection point 32, and D2 in FIG. 1 is the time required for the signal light to propagate between the reflection point 32 and the reflection point 33. For example, the first reflected light that is reflected at the reflection point 32 and then at the reflection point 31 reaches the optical receiver 2 with a delay of 2×D1 compared to the direct light. Also, the second reflected light that is reflected at the reflection point 33 and then at the reflection point 32 reaches the optical receiver 2 with a delay of 2×D2 compared to the direct light. Furthermore, the third reflected light that is reflected at the reflection point 33 and then at the reflection point 31 reaches the optical receiver 2 with a delay of 2×(D1 + D2) compared to the direct light. Additionally, for example, there is also reflected light that reaches the optical receiver 2 after being reflected in the order of the reflection points 33, 31, 32, 31, etc. However, since the level of the reflected light reflected at four or more reflection points at the optical receiver 2 is sufficiently smaller than that of the direct light, it is not considered in this embodiment.

[0015] Furthermore, the levels of the first to third reflected lights reflected at two reflection points at the optical receiver 2 are also different. For example, in FIG. 1, let D1 < D2. This means that the optical path length between the reflection point 31 and the reflection point 32 is shorter than the optical path length between the reflection point 32 and the reflection point 33. As the optical path length increases, the level of the reflected light can become smaller. Assuming that the level of the reflected light becomes smaller as the optical path length increases, in FIG. 1, the level of the first reflected light is the highest. In such a case, the reflected light that most affects the demodulation of the direct light at the optical receiver 2 is the reflected light with the smallest propagation delay difference with respect to the direct light. Therefore, in this embodiment, the optical communication system is configured to suppress the influence of the reflected light with the smallest propagation delay with respect to the direct light. In the following description, the delay difference between this direct light and the reflected light with the smallest propagation delay is denoted as ΔT. In the case of FIG. 1, ΔT = 2×D1. Note that in the following description of this embodiment, "reflected light" means the reflected light with the smallest propagation delay.

[0016] FIG. 2 is a configuration diagram of the optical transmission device 1 according to the present embodiment. The light source unit 11 generates carrier light and transmits it to the modulation unit 12. The modulation unit 12 generates signal light by modulating the carrier light with an electrical signal, and outputs the generated signal light to the optical transmission line 3. The modulation method by the modulation unit 12 is arbitrary, and intensity modulation (amplitude modulation), phase modulation, quadrature amplitude modulation, etc. can be used. In the case of a directly modulated laser, the light source unit 11 and the modulation unit 12 are integrated.

[0017] The control unit 13 has information indicating a time change pattern of the frequency, and controls the frequency of the carrier light generated by the light source unit 11 by transmitting a control signal according to the time change pattern to the light source unit 11. The information indicating the time change pattern is stored in the control unit 13 in advance. FIG. 3(A) shows an example of the time change pattern of the frequency. According to FIG. 3(A), the frequency of the carrier light repeatedly decreases with time from frequency f2 to frequency f1. That is, the time change pattern of the frequency indicates that the frequency of the carrier light changes in a sawtooth shape with one cycle being from the frequency f2 to the frequency f1. In FIG. 3(A), the frequency f2 is higher than the frequency f but the frequency f2 may be lower than the frequency f1. That is, in one cycle, the configuration may be such that the frequency of the carrier light increases with time from the frequency f2 to the frequency f1.

[0018] The light source unit 11 may, for example, have one light source and generate carrier light with a changing frequency as shown in Figure 3(A). Alternatively, as shown in Figure 4(A), the light source unit 11 may have two light sources, #1 and #2, and generate carrier light with a changing frequency as shown in Figure 3(A). In the configuration of Figure 4(A), the control unit 13 controls the light source unit 11 so that while one of the light sources, #1 or #2, is generating carrier light, the other light source does not generate carrier light. The control unit 13 then controls the light source unit 11 so that light sources #1 and #2 alternately generate carrier light every period. The combiner combines the carrier light from light sources #1 and #2 and outputs it. Although two light sources are used in Figure 4(A), a configuration using three or more light sources is also possible. More generally, when using N light sources (where N is an integer greater than or equal to 2), the control unit 13 controls the light source unit 11 so that the N light sources sequentially generate carrier light for one period, repeating this process in units of N periods.

[0019] Figure 5 shows the received light received by the optical receiver 2. The received light includes direct light and reflected light. In Figure 5, the solid line represents direct light, and the dotted line represents reflected light with the smallest delay difference from the direct light. Figure 5 shows the case where the modulation method in the modulation unit 12 is amplitude modulation. The direct light has a carrier light 80, an upper sideband 81, and a lower sideband 82, and its bandwidth (the difference between the highest frequency of the upper sideband 81 and the lowest frequency of the lower sideband 82) is B. The reflected light has a carrier light 90, an upper sideband 91, and a lower sideband 92, and its bandwidth is the same as the direct light, B. As is clear from Figure 5, if the frequency difference FD between the carrier light 80 and the carrier light 90 is greater than the bandwidth B of the signal light, the reflected light does not interfere with the direct light. Although Figure 5 shows the case applied to double sideband, it can also be applied to single sideband.

[0020] Here, the reflected light originates from the signal light transmitted from the optical transmitter 1 a time ΔT before the direct light. As mentioned above, time ΔT is the propagation delay difference between the reflected light and the direct light. Therefore, by ensuring that the frequency difference FD at two time positions separated by a time ΔT from the carrier light output by the light source 11 is always greater than the bandwidth B, the reflected light will not interfere with the direct light. Accordingly, the time variation pattern of the frequency shown in Figure 3(A) is set such that the frequency difference FD at two time positions separated by a time ΔT is always greater than the bandwidth B.

[0021] Figure 6 shows an example of the configuration of an optical receiver according to this embodiment. Figure 6(A) shows an example of the configuration when direct detection is performed. The conversion unit 22 photoelectrically converts the received light, which includes direct light and reflected light from the optical transmission path 3, and outputs an electrical signal. The electrical signal includes beat components between the direct light, beat components between the reflected light, and beat components between the direct light and the reflected light. Note that only the beat component between the direct light is necessary for demodulation. Since the level of the reflected light is lower than the level of the direct light, the levels of the three beat components are such that the level of the beat component between the direct light is the highest and the level of the beat component between the reflected light is the lowest.

[0022] For example, if the frequency of the carrier light is not changed in the optical transmitter 1, the beat component between reflected light and the beat component between direct light and reflected light will both be in the same bandwidth as the beat component between direct light. Therefore, both the beat component between reflected light and the beat component between direct light and reflected light will affect demodulation. However, in this embodiment, since the reflected light is outside the bandwidth of the direct light, the beat component between direct light and reflected light will be outside the bandwidth of the beat component between direct light. In other words, the beat component between direct light and reflected light will not affect demodulation. On the other hand, the beat component between reflected light will be in the same bandwidth as the beat component between direct light, but since the beat component between reflected light is sufficiently small compared to the beat component between direct light, it will not pose a problem for demodulation.

[0023] Figure 6(B) shows another configuration example when direct detection is performed. In the example in Figure 6(B), the received light, including direct and reflected light from the optical transmission path 3, is input to the optical variable filter 21. The passband of the optical variable filter 21 is controlled by the control unit 23. The control unit 23 has the same time-varying pattern information stored in the control unit 13 stored in advance. Then, according to the frequency change of the direct light carrier light 80 indicated by the time-varying pattern, the control unit 23 controls the passband of the optical variable filter 21 so that the passband of the optical variable filter 21 includes the direct light but does not include the reflected light. Therefore, the optical variable filter 21 suppresses the reflected light and outputs the direct light to the conversion unit 22. As a result, the level of the beat component between reflected light can be suppressed more than in the configuration of Figure 6(A).

[0024] Furthermore, control unit 13 and control unit 23 need to operate in synchronously. For this reason, control unit 13 and control unit 23 may be configured to send and receive a synchronization signal. The synchronization signal may be sent and received, for example, via the optical transmission path 3. Alternatively, the synchronization signal may be sent and received via a transmission path other than the optical transmission path 3. Moreover, instead of sending and receiving a synchronization signal between control unit 13 and control unit 23, the control unit 13 and control unit 23 can be configured to operate in synchronously based on time obtained from a GPS system or the like.

[0025] Figure 6(C) shows an example configuration for coherent detection. In the example in Figure 6(C), the received light, including direct and reflected light from the optical transmission path 3, is input to the demodulation unit 24. The demodulation unit 24 performs coherent detection of the received light using local light from the light source 25. The control unit 23 has the same time-varying pattern information stored in the control unit 13 pre-stored. The control unit 23 then controls the light source 25 to change the frequency of the local light according to the frequency change of the direct light carrier 80 indicated by the time-varying pattern. In the case of homodyne detection, the frequency of the local light changes in the same way as the frequency of the direct light carrier 80. On the other hand, in the case of heterodyne detection, the frequency of the local light changes with a constant frequency difference from the frequency of the direct light carrier 80. The beat component of the local light and reflected light is outside the bandwidth of the beat component of the local light and direct light required for demodulation, so the beat component of the local light and reflected light does not affect demodulation.

[0026] In Figure 3(A), the frequency of the carrier light only decreased or increased within a single period. However, the time evolution pattern of the carrier light frequency is not limited to that shown in Figure 3(A). For example, as shown in Figure 3(B), the frequency of the carrier light may decrease and increase within a single period. According to Figure 3(B), the frequency of the carrier light decreases from frequency f2 towards frequency f1, increases to frequency f3 upon reaching frequency f1, then decreases towards frequency f2, increases to frequency f4 upon reaching frequency f2, and then decreases towards frequency f3.

[0027] Figure 4(B) shows an example configuration of the light source unit 11 for changing the frequency of carrier light, as shown in Figure 3(B). Light source #1 generates carrier light with frequencies f1 to f2, light source #2 generates carrier light with frequencies f2 to f3, and light source #3 generates carrier light with frequencies f3 to f4. The control unit 13 controls the light source unit 11 so that only light source #1 generates carrier light during the first 1 / 3 of a cycle, only light source #2 generates carrier light during the next 1 / 3, and only light source #3 generates carrier light during the last 1 / 3. This makes it possible to change the frequency of the carrier light as shown in Figure 3(B).

[0028] Furthermore, for example, in a time-division duplexing (TDD) system, where a transmission period in which signal light is transmitted from the optical transmitter 1 to the optical receiver 2 alternates with a non-transmission period in which signal light is not transmitted from the optical transmitter 1 to the optical receiver 2, a period in one cycle in which the light source 11 does not output carrier light can be provided, as shown in Figure 3(C). In Figure 3(C), the period in one cycle in which carrier light is output corresponds to the transmission period, and the period in one cycle in which carrier light is not output corresponds to the non-transmission period. In this case, the time variation pattern is set such that the frequency difference when two time positions separated by a period ΔT are both within the transmission period is always greater than FD. Note that in Figure 3(C), the light source 11 does not generate carrier light during the non-transmission period, but it is also possible to have a configuration in which carrier light that changes from frequency f2 to frequency f1 is generated during the non-transmission period.

[0029] <Second Embodiment> Next, the second embodiment will be described. Figure 7 shows the optical communication system according to this embodiment. The optical communication system of the first embodiment was a so-called P2P system in which an optical transmitter 1 and an optical receiver 2 communicate one-to-one. The optical communication system according to this embodiment is a so-called P2MP system, such as a passive optical network (PON), in which an optical transmitter 1 and optical receivers 2 communicate one-to-many. As shown in Figure 7, the optical transmitter 1 transmits signal light to multiple optical receivers 2 via, for example, an optical transmission path 3 for P2MP. In Figure 7, the number of optical receivers 2 is shown as 3, but the number of optical receivers 2 can be any number of 2 or more. The following description will focus on the differences from the first embodiment.

[0030] Figure 8 is an explanatory diagram of the carrier light generated by the light source unit 11 according to this embodiment. The optical transmitter 1 has at least the same number of light source units 11 as the number of optical receivers 2. In Figure 8, the number of light source units 11 in the optical transmitter 1 is set to 3, and each light source unit 11 is denoted as light source unit #1, #2, and #3. Also, P#1 to P#4 in Figure 8 each represent one period that changes the carrier light. As shown in Figure 8, light source unit #1, light source unit #2, and light source unit 3 generate carrier light that changes from frequency f2 to frequency f1, carrier light that changes from frequency f3 to frequency f2, and carrier light that changes from frequency f4 to frequency f3 in each period P.

[0031] Figure 9 shows which light source 11's generated carrier light is used as the signal light transmitted to which optical receiver 2 during each of the periods P#1 to P#4. According to Figure 9, during period P#1, the carrier light generated by light source #1 is used as the signal light to optical receiver #1; during period P#2, the carrier light generated by light source #2 is used as the signal light to optical receiver #1; and during period P#3, the carrier light generated by light source #3 is used as the signal light to optical receiver #1. From period P#4 onward, periods P#1 to P#3 are repeated. Therefore, the frequency of the carrier light used as the signal light to optical receiver #1 changes as shown in Figure 3(B). The same applies to optical receivers #2 and #3. Therefore, similar to the first embodiment, the influence of reflected light in optical receivers #1 to #3 can be suppressed.

[0032] Figure 10 shows an example configuration of the optical transmitter 1 according to this embodiment. Carrier light generated by light sources #1 to #3 is output to the optical switch 14. The control unit 13 controls the frequency of the carrier light generated by light sources #1 to #3 according to the time variation pattern of the frequency. The modulation unit 12 has modulators #1 to #3. Modulator #1 generates signal light to be transmitted to the optical receiver #1 by modulating the carrier light from the optical switch 14 based on electrical signal #1, modulator #2 generates signal light to be transmitted to the optical receiver #2 by modulating the carrier light from the optical switch 14 based on electrical signal #2, and modulator #3 generates signal light to be transmitted to the optical receiver #3 by modulating the carrier light from the optical switch 14 based on electrical signal #3.

[0033] The control unit 13 outputs the carrier light from the three light sources 11 input to the optical switch 14 to the appropriate modulator based on switching information as shown in Figure 9. In this case, optical receivers #1 to #3 in Figure 9 should be read as modulators #1 to #3. The multiplexer combines the signal light from modulators #1 to #3 and outputs it to the optical transmission path 3.

[0034] Figure 11 shows another configuration example of the optical transmitter 1 according to this embodiment. The carrier light generated by light source units #1 to #3 is output to modulators #1 to #3, respectively. Electrical signals #1 to #3, which are transmitted to optical receivers #1 to #3, are input to an electrical switch. Based on the switching information shown in Figure 9, the control unit 13 controls the system so that in period P#1, electrical signal #1 is output to modulator #1; in period P#2, electrical signal #1 is output to modulator #2; in period P#3, electrical signal #1 is output to modulator #3; and in period P#4, electrical signal #1 is output to modulator #1. In this case, optical receivers #1 to #3 in Figure 9 are read as electrical signals #1 to #3, and light source units #1 to #3 are read as modulators #1 to #3.

[0035] Furthermore, optical receivers #1 to #3 may be configured, for example, as shown in Figure 6(B) or Figure 6(C), so that signal light different from the received signal light does not affect demodulation.

[0036] <Third Embodiment> Next, the differences between the third embodiment and the first and second embodiments will be explained. In the first and second embodiments, the time variation pattern of the carrier light frequency generated by the light source 11 was set based on the propagation delay difference ΔT between the reflected light with the smallest propagation delay and the direct light, such that the frequency difference at time positions separated by a period ΔT becomes greater than the bandwidth B of the signal light. However, the level of reflected light in the optical receiver 2 also changes depending on the amount of reflection at each reflection point. Therefore, even reflected light with the second or third smallest propagation delay difference from the direct light may affect the demodulation of the direct light.

[0037] Therefore, in this embodiment, one or more reflected light beams that affect the demodulation of the direct light are determined according to a predetermined criterion, and the propagation delay difference between each of the determined one or more reflected light beams and the direct light beams is determined. Then, according to the maximum value ΔTmax of the propagation delay difference of each of the one or more reflected light beams, the time variation pattern of the frequency of the carrier light generated by the light source 11 is set such that the frequency difference at a time position separated by a period ΔTmax is greater than the bandwidth B of the signal light beam.

[0038] For example, the predetermined criterion can be a criterion based on the level of reflected light in the optical receiver 2. In this case, reflected light whose level in the optical receiver 2 is greater than the threshold is considered to be reflected light that affects the demodulation of the direct light. Alternatively, the predetermined criterion can be the number of reflections. For example, reflected light with two reflections can be considered to be reflected light that affects the demodulation of the direct light. For example, in the case of the optical transmission path 3 as shown in Figure 1, the third reflected light has the largest delay among the first to third reflected light, and in this case, the maximum value ΔTmax is 2 × (D1 + D2).

[0039] With the above configuration, the effects of reflected light can be suppressed without dithering. 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."

[0040] 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]

[0041] 11: Light source unit, 13: Control unit, 12: Modulation unit

Claims

1. An optical transmitting device connected to an optical receiving device via an optical transmission path, A means for generating carrier light, A control means for controlling the frequency of the carrier light according to the time variation pattern of the frequency, A generating means that generates signal light to be transmitted to the optical receiving device by modulating the carrier light generated by the generating means based on an electrical signal, Equipped with, An optical transmitting device in which, in the aforementioned time-varying pattern, the difference in frequencies separated by a predetermined period is greater than the bandwidth of the signal light.

2. The optical transmitting device according to claim 1, wherein the predetermined period is a period based on the propagation delay difference between the direct light, which is the signal light that reaches the optical receiving device without being reflected in the optical transmission path, and the first reflected light among one or more reflected lights, which are the signal light that reaches the optical receiving device after being reflected an even number of times in the optical transmission path.

3. The optical transmitting device according to claim 2, wherein the first reflected light is the reflected light among the one or more reflected lights that has the smallest propagation delay difference with the direct light.

4. The optical transmitting device according to claim 2, wherein the first reflected light is the reflected light with the largest propagation delay difference from the direct light among the reflected light whose level in the optical receiving device is greater than a threshold.

5. The optical transmitting device according to claim 2, wherein the first reflected light is the reflected light that has been reflected only twice in the optical transmission path and reached the optical receiving device, and which has the largest propagation delay difference from the direct light.

6. The optical transmitting device communicates with the optical receiving device using time-division duplexing. The optical transmitting device according to claim 1, wherein the control means controls the frequency of the carrier light such that, during the period in which the optical transmitting device transmits the signal light to the optical receiving device, the difference in the frequencies of the carrier light separated by a predetermined period is greater than the bandwidth of the signal light.

7. The optical transmitting device according to claim 1, wherein the optical transmitting device communicates with a plurality of optical receiving devices via the optical transmission path and has at least the same number of generating means as the optical receiving devices.

8. An optical transmitting device according to any one of claims 1 to 7 and an optical receiving device connected via the optical transmission path, A variable optical filter into which the received light from the optical transmission path is input, A control means for controlling the passband of the optically tunable filter according to the aforementioned time-varying pattern, A conversion means for converting the received light that has passed through the optically tunable filter into photoelectric energy, An optical receiving device equipped with the following features.

9. An optical transmitting device according to any one of claims 1 to 7 and an optical receiving device connected via the optical transmission path, A local light generation means that generates local light whose frequency changes according to the aforementioned time-varying pattern, A demodulation means for demodulating the received light from the optical transmission path based on the local light to generate the electrical signal, An optical receiving device equipped with the following features.

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