Optical transmitter and optical receiver

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 reducing the effects of reflected light in an optical communication system. [Background technology]

[0002] There are a plurality of connection points in the optical transmission path for connecting optical fibers to each other. The optical fibers are connected to each other by optical connectors or fusion splicing. At this connection point, a part of the signal light transmitted from the optical transmitting device to the optical receiving device is reflected toward the optical transmitting device. The part of the signal light reflected toward the optical transmitting device may be further reflected toward the optical receiving device at another connection point. Since the optical transmitting 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 transmitting device. On the other hand, the optical receiving device receives received light including the signal light that reaches the optical receiving device without being reflected in the optical transmission path (hereinafter referred to as direct light) and the signal light that reaches the optical receiving device after being reflected an even number of times in the optical transmission path (hereinafter referred to as reflected light). Since the propagation delays of the direct light and the reflected light are different, the reflected light becomes an 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 in which dithering is performed to suppress the influence of reflected light. Specifically, Non-Patent Document 1 discloses a configuration in which dithering light is generated using a phase modulator dedicated to dithering. Patent Document 1 also discloses a configuration in which a light source is driven by both a signal carrying information and a signal for dithering to generate transmission light including signal light and dithering light. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2010-232764 A [Non-patent literature]

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

[0006] However, when the level of reflected light is high, dithering cannot reduce the effect of reflected light.

[0007] The present disclosure provides a technique for suppressing the effects of reflected light without relying on 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 comprises a generating means for generating carrier light, a control means for controlling the frequency of the carrier light in accordance with a time-varying frequency pattern, and a 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 a difference in frequencies separated by a predetermined period in the time-varying frequency pattern is greater than the bandwidth of the signal light. Effect of the Invention

[0009] According to the present disclosure, the effects of reflected light can be suppressed without relying on dithering. [Brief description of the drawings]

[0010] [Figure 1] 1 is a block diagram of an optical communication system according to some embodiments. [Diagram 2] FIG. 2 is a block diagram of an optical transmitting device according to some embodiments. [Diagram 3]4A and 4B are diagrams illustrating time-varying patterns of the frequency of a carrier light according to some embodiments. [Figure 4] FIG. 2 is a block diagram of a light source unit according to some embodiments. [Diagram 5] 2 illustrates received light received by an optical receiving device according to some embodiments. [Figure 6] FIG. 2 is a block diagram of an optical receiving device according to some embodiments. [Figure 7] 1 is a block diagram of an optical communication system according to some embodiments. [Figure 8] 6A and 6B are diagrams showing time-varying patterns of frequencies of multiple carrier lights generated by multiple light source units according to some embodiments. [Figure 9] FIG. 9 is an explanatory diagram of a method of using a plurality of optical carriers shown in FIG. 8. [Figure 10] FIG. 2 is a block diagram of an optical transmitting device according to some embodiments. [Figure 11] FIG. 2 is a block diagram of an optical transmitting device according to some embodiments. 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] First Embodiment FIG. 1 is a configuration diagram of an optical communication system according to this embodiment. An optical transmitter 1 transmits a signal light to an optical receiver 2 via an optical transmission path 3. The signal light is obtained by modulating a carrier light with an electrical signal carrying information. Another optical receiver 2 may be disposed in the station where the optical transmitter 1 is disposed, and another optical transmitter 1 may be disposed in the station where the optical receiver 2 is disposed. The other optical transmitter 1 transmits a signal light to the other optical receiver 2 via another optical transmission path 3. The optical transmitter 1 and the other optical receiver 2 may be implemented as one optical communication device. Similarly, the other optical transmitter 1 and the optical receiver 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. Although the number of reflection points is three in FIG. 1, the number of reflection points may be any number of two or more. The signal light transmitted from 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, the optical transmission device 1 has an isolator that blocks the reflected light that reaches the optical transmission device 1, so that the reflected light to the optical transmission device 1 does not affect the optical communication system. On the other hand, the reflected light that reaches the optical reception device 2 becomes the signal light that reaches the optical reception device 2 without being reflected at the reflection points, that is, the interference light of the direct light, and therefore 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. Further, 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. Furthermore, for example, there is also reflected light that reaches the optical receiver 2 by reflecting in the order of the reflection points 33, 31, 32, 31. 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 becomes longer, the level of the reflected light can become smaller. Assuming that the level of the reflected light becomes smaller as the optical path length becomes longer, 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. In the following description of this embodiment, "reflected light" means the reflected light with the smallest propagation delay.

[0016] 2 is a configuration diagram of the optical transmission device 1 according to this 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. Any modulation method can be used by the modulation unit 12, and intensity modulation (amplitude modulation), phase modulation, quadrature amplitude modulation, etc. can be used. Note that in a directly modulated laser, the light source unit 11 and the modulation unit 12 are integrated into one configuration.

[0017] The control unit 13 has information indicating a time-varying pattern of the frequency, and transmits a control signal according to the time-varying pattern to the light source unit 11 to control the frequency of the carrier light generated by the light source unit 11. The information indicating the time-varying pattern is stored in the control unit 13 in advance. FIG. 3(A) shows an example of a time-varying pattern of the frequency. According to FIG. 3(A), the frequency of the carrier light repeatedly decreases over time from frequency f2 to frequency f1. That is, the time-varying pattern of the frequency shows that the frequency of the carrier light is changed in a sawtooth wave shape, with one period being the period from frequency f2 to frequency f1. In FIG. 3(A), the frequency f2 is higher than frequency f1, but the frequency f2 may be lower than frequency f1. That is, the frequency of the carrier light may be increased over time from frequency f2 to frequency f1 in one period.

[0018] The light source unit 11 may have, for example, one light source, and may generate carrier light whose frequency changes as shown in FIG. 3(A) by using this one light source. Alternatively, as shown in FIG. 4(A), the light source unit 11 may have two light sources, light source #1 and light source #2, and may generate carrier light whose frequency changes as shown in FIG. 3(A) by using these two light sources. In the configuration of FIG. 4(A), the control unit 13 controls the light source unit 11 so that while one of the light source #1 and the light source #2 generates carrier light, the other light source does not generate carrier light. Then, the control unit 13 controls the light source unit 11 so that the light source #1 and the light source #2 alternately generate carrier light every one period. The multiplexing unit multiplexes the carrier light from the light source #1 and the light source #2 and outputs it. Although two light sources are used in FIG. 4(A), a configuration using three or more light sources may be used. More generally, when N light sources (N is an integer equal to or greater than 2) are used, the control unit 13 controls the light source unit 11 so that the N light sources sequentially generate one cycle of carrier light, repeating this process in units of N cycles.

[0019] FIG. 5 shows the received light received by the optical receiver 2. The received light includes direct light and reflected light. The solid line in FIG. 5 shows the direct light, and the dotted line shows the reflected light with the smallest delay difference with the direct light. FIG. 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 B, which is the same as that of the direct light. As is clear from FIG. 5, if the frequency difference FD between the carrier light 80 and the carrier light 90 is larger than the bandwidth B of the signal light, the reflected light does not interfere with the direct light. Although FIG. 5 shows the case where it is applied to a double sideband, it can also be applied to a single sideband.

[0020] Here, the reflected light is from the signal light transmitted from the optical transmitter 1 a time ΔT earlier than the direct light. As described above, the time ΔT is the propagation delay difference between the reflected light and the direct light. Therefore, by making the frequency difference FD between two time positions separated by a period ΔT of the carrier light output from the light source unit 11 always larger than the bandwidth B, the reflected light does not interfere with the direct light. Therefore, the frequency time change pattern shown in FIG. 3(A) is set so that the frequency difference FD between two time positions separated by a time ΔT is always larger than the bandwidth B.

[0021] Fig. 6 is a diagram showing an example of the configuration of an optical receiving device according to this embodiment. Fig. 6(A) shows an example of the configuration when direct detection is performed. The conversion unit 22 performs photoelectric conversion on the received light including the direct light from the optical transmission line 3 and the reflected light, and outputs an electric signal. The electric signal includes a beat component between the direct lights, a beat component between the reflected lights, and a beat component between the direct light and the reflected light. Note that what is required for demodulation is the beat component between the direct lights. 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 lights is the highest, and the level of the beat component between the reflected lights is the lowest.

[0022] For example, if the frequency of the carrier light is not changed in the optical transmission device 1, the beat component between reflected lights and the beat component between direct light and reflected light will both be in the same band as the beat component between direct lights. Therefore, both the beat component between reflected lights and the beat component between direct light and reflected light will affect demodulation. However, in this embodiment, the reflected light is outside the band of the direct light, so the beat component between direct light and reflected light will be outside the band of the beat component between direct lights. 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 lights will be in the same band as the beat component between direct lights, but the beat component between reflected lights is sufficiently small compared to the beat component between direct lights, so it does not pose a problem to demodulation.

[0023] FIG. 6B shows another example of the configuration when performing direct detection. In the example of FIG. 6B, the received light including the direct light and the reflected light from the optical transmission line 3 is input to the tunable optical filter 21. The pass band of the tunable optical filter 21 is controlled by the control unit 23. The control unit 23 stores in advance the same information as the time change pattern stored in the control unit 13. Then, the control unit 23 controls the pass band of the tunable optical filter 21 according to the frequency change of the carrier light 80 of the direct light indicated by the time change pattern so that the pass band of the tunable optical filter 21 includes the direct light and does not include the reflected light. Therefore, the tunable optical 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 the reflected lights can be further suppressed compared to the configuration of FIG. 6A.

[0024] It should be noted that the control unit 13 and the control unit 23 need to operate in synchronization. For this reason, the control unit 13 and the control unit 23 may be configured to transmit and receive a synchronization signal. The synchronization signal may be transmitted and received, for example, via the optical transmission path 3. Alternatively, the synchronization signal may be transmitted and received via a transmission path other than the optical transmission path 3. Furthermore, instead of transmitting and receiving a synchronization signal between the control unit 13 and the control unit 23, the control unit 13 and the control unit 23 may be configured to operate in synchronization based on the time or the like obtained from a GPS system or the like.

[0025] FIG. 6C shows a configuration example when coherent detection is performed. In the example of FIG. 6C, the received light including the direct light and the reflected light from the optical transmission line 3 is input to the demodulator 24. The demodulator 24 performs coherent detection of the received light using the local light from the light source 25. The control unit 23 stores in advance the same information as the time change pattern stored in the control unit 13. Then, the control unit 23 controls the light source 25 to change the frequency of the local light according to the frequency change of the carrier light 80 of the direct light indicated by the time change pattern. In the case of homodyne detection, the frequency of the local light changes in the same manner as the frequency of the carrier light 80 of the direct light. On the other hand, in the case of heterodyne detection, the frequency of the local light changes with a certain frequency difference from the frequency of the carrier light 80 of the direct light. The beat component of the local light and the reflected light is outside the band of the beat component of the local light and the direct light required for demodulation, so the beat component of the local light and the reflected light does not affect the demodulation.

[0026] In Fig. 3(A), the frequency of the carrier light only decreases or increases within one period. However, the time change pattern of the frequency of the carrier light is not limited to that shown in Fig. 3(A), and may be, for example, as shown in Fig. 3(B), in which the frequency of the carrier light decreases and increases within one period. According to Fig. 3(B), the frequency of the carrier light decreases from frequency f2 to frequency f1, increases to frequency f3 when it reaches frequency f1, then decreases to frequency f2, and further increases to frequency f4 when it reaches frequency f2, and then decreases to frequency f3.

[0027] FIG. 4(B) shows a configuration example of the light source unit 11 for changing the frequency of the carrier light as in FIG. 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 one period, 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 allows the frequency of the carrier light to be changed as in FIG. 3(B).

[0028] Also, for example, in the case of time division duplexing (TDD), when a transmission period in which a signal light is transmitted from the optical transmitter 1 to the optical receiver 2 and a non-transmission period in which a signal light is not transmitted from the optical transmitter 1 to the optical receiver 2 alternate, a period in which the light source unit 11 does not output a carrier light in one period can be provided as shown in FIG. 3(C). In FIG. 3(C), a period in which a carrier light is output in one period corresponds to a transmission period, and a period in which a carrier light is not output in one period corresponds to a non-transmission period. In this case, the time change pattern is set so that the frequency difference when two time positions separated by a period ΔT are both within the transmission period is always larger than FD. Note that, although the light source 11 does not generate a carrier light in the non-transmission period in FIG. 3(C), a configuration in which a carrier light that changes from a frequency f2 to a frequency f1 in the non-transmission period may be generated.

[0029] Second Embodiment Next, the second embodiment will be described. FIG. 7 shows an optical communication system according to this embodiment. The optical communication system according to the first embodiment is 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 an optical receiver 2 communicate one-to-many. As shown in FIG. 7, an optical transmitter 1 transmits signal light to a plurality of optical receivers 2 via an optical transmission path 3 for P2MP, for example. Note that, although the number of optical receivers 2 is three in FIG. 7, the number of optical receivers 2 may be any number equal to or greater than two. Hereinafter, this embodiment will be described with a focus on the differences from the first embodiment.

[0030] FIG. 8 is an explanatory diagram of carrier light generated by the light source unit 11 according to this embodiment. The optical transmitting device 1 has at least the same number of light source units 11 as the number of optical receiving devices 2. In FIG. 8, the number of light source units 11 that the optical transmitting device 1 has is three, and the light source units 11 are denoted as light source units #1, #2, and #3. Also, P#1 to P#4 in FIG. 8 each indicate one period in which the carrier light is changed. As shown in FIG. 8, the light source unit #1, the light source unit #2, and the 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] FIG. 9 shows which light source unit 11 generates a carrier light and uses it for the signal light to be transmitted to which optical receiver 2 in each of the periods P#1 to P#4. According to FIG. 9, in the period P#1, the carrier light generated by the light source unit #1 is used for the signal light to the optical receiver #1, in the period P#2, the carrier light generated by the light source unit #2 is used for the signal light to the optical receiver #1, and in the period P#3, the carrier light generated by the light source unit #3 is used for the signal light to the optical receiver #1. After the period P#4, the period P#1 to the period P#3 are repeated. Therefore, the frequency of the carrier light used for the signal light to the optical receiver #1 changes as shown in FIG. 3(B). The same applies to the optical receivers #2 and #3. Therefore, as in the first embodiment, the influence of the reflected light in the optical receivers #1 to #3 can be suppressed.

[0032] FIG. 10 shows a configuration example of the optical transmission device 1 according to this embodiment. The carrier light generated by the light source unit #1 to the light source unit #3 is output to the optical switch 14. The control unit 13 controls the frequency of the carrier light generated by the light source unit #1 to the light source unit #3 according to the time change pattern of the frequency. The modulation unit 12 has modulators #1 to #3. The modulator #1 modulates the carrier light from the optical switch 14 based on the electrical signal #1 to generate a signal light to be transmitted to the optical receiving device #1, the modulator #2 modulates the carrier light from the optical switch 14 based on the electrical signal #2 to generate a signal light to be transmitted to the optical receiving device #2, and the modulator #3 modulates the carrier light from the optical switch 14 based on the electrical signal #3 to generate a signal light to be transmitted to the optical receiving device #3.

[0033] The control unit 13 outputs the carrier light from the three light source units 11 input to the optical switch 14 to an appropriate modulator based on switching information as shown in Fig. 9. In this case, the optical receiving device #1 to the optical receiving device #3 in Fig. 9 are replaced with modulators #1 to #3. The multiplexing unit multiplexes the signal light from modulators #1 to #3 and outputs the multiplexed signal to the optical transmission line 3.

[0034] FIG. 11 shows another example of the configuration of the optical transmission device 1 according to the present embodiment. The carrier lights generated by the light source unit #1 to the light source unit #3 are output to the modulator #1 to the modulator #3, respectively. In addition, the electric signals #1 to #3 to be transmitted to the optical receiving device #1 to the optical receiving device #3 are input to the electric switch. The control unit 13 performs control based on the switching information as shown in FIG. 9 so that in the period P#1, the electric signal #1 is output to the modulator #1, in the period P#2, the electric signal #1 is output to the modulator #2, in the period P#3, the electric signal #1 is output to the modulator #3, and in the period P#4, the electric signal #1 is output to the modulator #1. In this case, the optical receiving device #1 to the optical receiving device #3 in FIG. 9 are replaced with the electric signals #1 to #3, and the light source unit #1 to the light source unit #3 are replaced with the modulator #1 to the modulator #3.

[0035] The optical receiving devices #1 to #3 may be configured as shown in FIG. 6(B) or FIG. 6(C), for example, so that signal light different from the signal light they receive does not affect demodulation.

[0036] <Third embodiment> Next, the third embodiment will be described with respect to differences from the first and second embodiments. In the first and second embodiments, the time change pattern of the frequency of the carrier light generated by the light source unit 11 is set based on the propagation delay difference ΔT between the reflected light with the smallest propagation delay and the direct light so that the frequency difference at a time position separated by a period ΔT is larger than the bandwidth B of the signal light. However, the level of the reflected light in the optical receiver 2 also changes depending on the amount of reflection at each reflection point. Therefore, even if the reflected light has the second or third smallest propagation delay difference with the direct light, it may affect the demodulation of the direct light.

[0037] For this reason, in this embodiment, one or more reflected lights 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 lights and the direct light is determined. Then, according to the maximum propagation delay difference ΔTmax of each of the one or more reflected lights, a time change pattern of the frequency of the carrier light generated by the light source unit 11 is set so that the frequency difference at a time position separated by a period ΔTmax is larger than the bandwidth B of the signal light.

[0038] For example, the predetermined criterion may be a criterion based on the level of reflected light at the optical receiving device 2. In this case, reflected light whose level at the optical receiving device 2 is greater than a threshold value is regarded as reflected light that directly affects the demodulation of light. The predetermined criterion may also be the number of reflections. As an example, reflected light that has been reflected twice may be regarded as reflected light that directly affects the demodulation of light. For example, in the case of an optical transmission path 3 as shown in FIG. 1, of the first to third reflected lights, the third reflected light has the greatest delay, and in this case, the maximum value ΔTmax is 2×(D1+D2).

[0039] The above configuration can reduce the effects of reflected light without dithering, making it possible to contribute to Goal 9 of the United Nations Sustainable Development Goals (SDGs), which is to "build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0040] 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]

[0041] 11: light source unit, 13: control unit, 12: modulation unit

Claims

1. An optical transmitter connected to an optical receiver via an optical transmission line, A generating means for generating carrier light; a control means for controlling the frequency of the carrier light in accordance with a time-varying pattern of the frequency; a generating means for generating a 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 transmission device, wherein a difference between frequencies separated by a predetermined period in the time change pattern is greater than a bandwidth of the signal light.

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

3. The optical transmission device according to claim 2 , wherein the first reflected light is a reflected light having a smallest propagation delay difference with respect to the direct light among the one or more reflected lights.

4. 3. The optical transmission device according to claim 2, wherein the first reflected light is a reflected light having the largest propagation delay difference with respect to the direct light among reflected lights whose level in the optical reception device is greater than a threshold value.

5. 3. The optical transmitting device according to claim 2, wherein the first reflected light is a reflected light having the largest propagation delay difference with respect to the direct light among reflected lights that have been reflected only twice in the optical transmission path and have reached the optical receiving device.

6. the optical transmitter communicates with the optical receiver by time division duplex; 2. The optical transmitting device according to claim 1, wherein the control means controls the frequency of the carrier light so that the difference in frequency of the carrier light separated by the predetermined period during the period in which the optical transmitting device transmits the signal light to the optical receiving device is greater than the bandwidth of the signal light.

7. 2. The optical transmission device according to claim 1, wherein said optical transmission device communicates with a plurality of said optical receiving devices via said optical transmission line, and said generating means are at least as many in number as said optical receiving devices.

8. 8. An optical receiving device connected to the optical transmitting device according to claim 1 via the optical transmission line, a tunable optical filter to which received light from the optical transmission line is input; a control means for controlling a passband of the tunable optical filter in accordance with the time-varying pattern; A conversion means for photoelectrically converting the received light that has passed through the tunable optical filter; An optical receiving device comprising:

9. 8. An optical receiving device connected to the optical transmitting device according to claim 1 via the optical transmission line, a local light generating means for generating local light whose frequency changes according to the time change pattern; a demodulation unit that demodulates a received light from the optical transmission line based on the local light to generate the electrical signal; An optical receiving device comprising:

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