Optical communication equipment

Abstract

To provide a technique capable of suppressing an influence of a reflection light even if a state of an optical communication system is changed.SOLUTION: An optical communication device performs a communication with an opposite optical communication device that is operated in any one of a first mode that makes a rotational amount of a polarization of a first modulation light change in accordance with a time course and a second mode for fixing the rotational amount to a setting value via an optical transmission path. The optical communication device comprises: demodulation means of receiving the first modulation light for transmitting data in a first period from the opposite optical communication device, and receiving the first modulation light for transmitting a measurement signal from the opposite optical communication device in a second period for modulation; and control means of determining an operation mode of the opposite optical communication device, transmitting a control signal to the opposite optical communication device so that it is operated in an operation mode determined by the opposite optical communication device. The control means changes the operation mode of the opposite optical communication device to the first mode when a transmission quality in the second mode determined based on the measurement signal is deteriorated than a threshold value in the case the optical communication device is operated in the second mode.SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

[0002] An optical transmission line has multiple connection points for connecting optical fibers. Optical fibers are connected to each other using optical connectors or fusion splices. At these connection points, a portion of the modulated light transmitted from a transmitter of an optical communication device to a receiver of the optical communication device is reflected back toward the transmitter. The portion of the modulated light reflected back toward the transmitter may then be reflected back toward the receiver at another connection point. Because the transmitter has an isolator or other device that blocks light propagating in the opposite direction to the transmitted modulated light, there is no problem even if the reflected modulated light reaches the transmitter. Meanwhile, the receiver receives received light that includes modulated light that reaches the receiver without being reflected in the optical transmission line (hereinafter referred to as direct light) and modulated light that reaches the receiver after being reflected an even number of times in the optical transmission line (hereinafter referred to as reflected light). Because the propagation delays of the direct light and 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 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, thereby generating transmission light including modulated light and dithering light. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-232764 [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] To reduce the influence of reflected light by dithering, the dithering light must be adjusted for each optical communication system. Therefore, if the status of the optical communication system changes after service starts, the dithering light may no longer be suitable for the optical communication system.

[0007] The present disclosure provides a technique that can suppress the influence of reflected light even when the state of an optical communication system changes. [Means for solving the problem]

[0008] According to one aspect of the present invention, an optical communication device that communicates via an optical transmission path with a counterpart optical communication device that operates in either a first mode in which the amount of rotation of the polarization of first modulated light is changed over time, or a second mode in which the amount of rotation is fixed to a set value, comprises: a demodulation means that receives the first modulated light carrying data from the counterpart optical communication device during a first period, and receives and demodulates the first modulated light carrying a measurement signal from the counterpart optical communication device during a second period, wherein the first period and the second period are repeated alternately; and a control means that determines the operation mode of the counterpart optical communication device and controls the counterpart optical communication device to operate in the determined operation mode by transmitting a control signal to the counterpart optical communication device, and when the counterpart optical communication device is operating in the second mode, the control means changes the operation mode of the counterpart optical communication device to the first mode if the transmission quality in the second mode determined based on the measurement signal deteriorates below a threshold value. [Effects of the Invention]

[0009] According to the present disclosure, the influence of reflected light can be suppressed even if the state of the optical communication system changes. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a block diagram of an optical communication system according to some embodiments. [Figure 2] 4A and 4B illustrate signals transmitted during each of the upstream and downstream periods, according to some embodiments. [Figure 3] FIG. 10 illustrates a configuration for reducing the effect of reflected light in the downstream direction, according to some embodiments. [Figure 4] 4 is a sequence diagram of a process for determining a downstream operating mode according to some embodiments. [Figure 5] 4 is a state transition diagram for downstream operating modes according to some embodiments. [Figure 6] 10 is a flowchart of the processing performed by a mode control unit according to some embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0011] 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.

[0012] First Embodiment FIG. 1 is a configuration diagram of an optical communication system according to this embodiment. Two optical communication devices, 1 and 2, are connected via an optical transmission path 3. From the perspective of optical communication device 1, optical communication device 2 is the opposing optical communication device, and from the perspective of optical communication device 2, optical communication device 1 is the opposing optical communication device. The optical transmission path 3 has multiple connection points (reflection points). In this embodiment, optical communication device 1 and optical communication device 2 perform bidirectional communication using time division duplex (TDD) and frequency division duplex (FDD). Hereinafter, the direction from optical communication device 1 to optical communication device 2 will be referred to as the downstream direction, and the direction from optical communication device 2 to optical communication device 1 will be referred to as the upstream direction. Furthermore, the frequency of the optical carrier wave used in downstream communication will be referred to as f1, and the frequency of the optical carrier wave used in upstream communication will be referred to as f2.

[0013] As shown in FIG. 2, during the upstream period, the optical communication device 2 transmits upstream data to the optical communication device 1 using an optical carrier wave of frequency f2. During this upstream period, the optical communication device 1 transmits a measurement signal for measuring downstream transmission quality and a control signal for controlling the upstream operation mode (hereinafter referred to as the upstream control signal) to the optical communication device 2 using an optical carrier wave of frequency f1. The upstream operation mode is also the operation mode of the optical communication device 2. Details of the operation mode will be described later. Similarly, during the downstream period following the upstream period, the optical communication device 1 transmits downstream data to the optical communication device 2 using an optical carrier wave of frequency f1. During this downstream period, the optical communication device 2 transmits a measurement signal for measuring upstream transmission quality and a control signal for controlling the downstream operation mode (hereinafter referred to as the downstream control signal) to the optical communication device 1 using an optical carrier wave of frequency f2. The downstream operation mode is also the operation mode of the optical communication device 1. Following the downstream period, the upstream period in FIG. 2 begins. As shown in FIG. 2, the downstream processing and the upstream processing are the same except for the direction, so only the downstream processing will be described below.

[0014] 3 is a configuration diagram of an optical communication system for suppressing the effects of reflected light in the downstream direction. A modulator 10 generates modulated light based on an optical carrier wave of frequency f1 and outputs the modulated light to a polarization controller 11. The modulated light carries downstream data during the downstream period, and carries a downstream measurement signal and an upstream control signal during the upstream period. The downstream measurement signal and the upstream control signal are multiplexed using any multiplexing method, such as time division multiplexing or frequency division multiplexing, in the electrical domain, and are carried by an optical carrier wave of frequency f1. The upstream control signal is not relevant to the process for suppressing the effects of reflected light in the downstream direction, so its description will be omitted below.

[0015] The polarization control unit 11 changes (rotates) the polarization of the modulated light from the modulation unit 10 according to the operating mode of the optical communication device 1 and outputs the changed polarization. Hereinafter, the amount of change in polarization that the polarization control unit 11 applies to the polarization of the modulated light from the modulation unit 10 is referred to as the amount of rotation. In this embodiment, the operating mode of the optical communication device 1 is either a first mode or a second mode. When the optical communication device 1 is set to the first mode, the polarization control unit 11 changes the amount of rotation over time within one uplink period or one downlink period. The amount of rotation can be changed, for example, randomly over time. Alternatively, the amount of rotation can be changed according to rotation information indicating the relationship between the amount of time elapsed and the amount of rotation. On the other hand, when the optical communication device 1 is set to the second mode, the polarization control unit 11 keeps the amount of rotation constant at a set value. In the second mode, the amount of rotation is not changed except when the set value of the amount of rotation is changed. The set value can be, for example, a value greater than or equal to 0 degrees and less than 360 degrees. When the set value is 0 degrees, the polarization control unit 11 outputs the modulated light from the modulation unit 10 without rotating the polarization. The modulated light that has passed through the polarization control unit 11 is transmitted to the optical communication device 2 via the optical transmission path 3.

[0016] The optical communication device 2 receives received light, including direct light and reflected light, via the optical transmission path 3. The direct light is modulated light that reaches the optical communication device 2 without being reflected on the optical transmission path 3, and the reflected light is modulated light that reaches the optical communication device 2 after being reflected on the optical transmission path 3 an even number of times. During the downstream period, the demodulator 25 demodulates the received light and outputs downstream data to a processing unit (not shown). During the upstream period, the demodulator 25 demodulates the received light and outputs a downstream measurement signal to the mode controller 26. The downstream measurement signal is a signal known to the optical communication device 2, and the mode controller 26 determines the downstream transmission quality by comparing the measurement signal demodulated by the demodulator 25 with the known measurement signal. The transmission quality can be evaluated, for example, by the signal-to-noise ratio (SNR). Alternatively, the transmission quality can be evaluated by the error vector magnitude (EVM). The EVM is a value based on a vector pointing from the coordinates of the known measurement signal on the complex plane to the coordinates of the received measurement signal on the complex plane. Furthermore, the transmission quality can be evaluated by the bit error rate of the measurement signal.

[0017] The mode control unit 26 determines the operating mode of the optical communication device 1 according to a predetermined determination criterion. This determination criterion uses downstream transmission quality determined based on the measurement signal. During the downstream period, the mode control unit 26 transmits the operating mode of the optical communication device 1 to the optical communication device 1 by a downstream control signal. Note that the downstream control signal is actually input to the polarization control unit 11 of the optical communication device 1 via a modulation unit of the optical communication device 2 and a demodulation unit of the optical communication device 1, but for simplicity of the diagram, the modulation unit of the optical communication device 2 and the demodulation unit of the optical communication device 1 are omitted from the diagram.

[0018] FIG. 4 is a sequence diagram of a process for controlling the downstream operation mode, i.e., the operation mode of the optical communication device 1. In S1 during the upstream period, the optical communication device 1 transmits a downstream measurement signal to the optical communication device 2. When the optical communication device 1 is in the first mode, the polarization control unit 11 rotates (changes) the polarization of the modulated light carrying the downstream measurement signal over time. On the other hand, when the optical communication device 1 is in the second mode, the polarization control unit 11 does not rotate the polarization of the modulated light carrying the downstream measurement signal over time. In S2 during the upstream period, the mode control unit 26 of the optical communication device 2 determines the operation mode of the optical communication device 1 from the next downstream period. In S3 during the downstream period, the mode control unit 26 of the optical communication device 2 notifies the optical communication device 1 of the determined operation mode using a downstream control signal. If the operation mode notified by the optical communication device 2 differs from the current operation mode, the polarization control unit 11 of the optical communication device 1 switches the operation mode. At S4 in the downstream period, the optical communication device 1 transmits modulated light carrying downstream data to the optical communication device 2 in accordance with the current operation mode.

[0019] For example, the downstream control signal in S3 is transmitted at the start of the downstream period, and the optical communication device 1 starts downstream data transmission after receiving the downstream control signal and setting the operating mode. However, this embodiment is not limited to such a configuration. For example, the downstream control signal in S3 may be transmitted within the downstream period. If the operating mode notified by the downstream control signal differs from the current operating mode, the optical communication device 1 switches the operating mode in accordance with the notification by the downstream control signal. Therefore, although FIG. 4 shows that the mode control unit 26 of the optical communication device 2 determines the operating mode of the optical communication device 1 within the upstream period (S2), the processing of S2 may be performed within the downstream period as long as the operating mode of the optical communication device 1 is controlled by the downstream control signal within the following downstream period.

[0020] Furthermore, the mode control unit 26 of the optical communication device 2 can be configured to send a downstream control signal to the optical communication device 1 only when the operating mode of the optical communication device 1 is switched, and not to send a downstream control signal to the optical communication device 1 when the operating mode of the optical communication device 1 is not switched.

[0021] 5 is an explanatory diagram of the process in which the mode control unit 26 determines the operation mode of the optical communication device 1. The mode control unit 26 holds a reference value for transmission quality. The initial value of the reference value for transmission quality is, for example, the transmission quality actually measured before the start of service when the operation mode of the optical communication device 1 is the first mode. The mode control unit 26 also holds a predetermined value α corresponding to the amount of degradation from the reference value. The mode control unit 26 sets the transmission quality lower than the reference value by the predetermined value α as the threshold value Th.

[0022] As shown in FIG. 5 , the mode control unit 26 transitions the operating mode of the optical communication device 1 to the second mode after a predetermined period T0 has elapsed since the first mode was selected. In this embodiment, the polarization control unit 11 resets the rotation amount setting to an initial value upon transition from the first mode to the second mode. The initial value may be a predetermined value, such as 0 degrees. Alternatively, the initial value may be the rotation amount upon transition from the first mode to the second mode. The mode control unit 26 determines the transmission quality based on the downstream measurement signal while the optical communication device 1 is operating in the second mode. If the determined transmission quality is not degraded below a threshold, the mode control unit 26 maintains the operating mode of the optical communication device 1 in the second mode. On the other hand, if the determined transmission quality is degraded below a threshold, the mode control unit 26 transitions the operating mode of the optical communication device 1 to the first mode. Optionally, the mode control unit 26 may transition the operating mode of the optical communication device 1 to the first mode after a predetermined period T1 has elapsed since the second mode was selected. After switching the optical communication device 1 to the first mode by the downstream control signal, the mode control unit 26 updates the reference value to the transmission quality determined based on the downstream measurement signal received during the upstream period, and updates the threshold value based on the updated reference value.

[0023] As described above, the received light received by the optical communication device 2 from the optical communication device 1 includes direct light and reflected light. The received light may contain not just one reflected light but multiple reflected lights reflected at different positions. For example, if the optical transmission path 3 has three reflection points, a first reflection point, a second reflection point, and a third reflection point, the received light may contain three reflected lights: a first reflected light reflected at the first and second reflection points, a second reflected light reflected at the first and third reflection points, and a third reflected light reflected at the second and third reflection points. The propagation delays of the first, second, and third reflected lights may be different. Furthermore, if the optical transmission path 3 has four reflection points, the received light may also contain reflected lights reflected at the four reflection points.

[0024] Here, the influence of reflected light is greater when the polarization planes of the direct light and the reflected light are aligned, and is smaller as the polarization planes of the direct light and the reflected light are closer to being orthogonal. Therefore, when the direct light is orthogonal to all of the one or more reflected lights, the influence of reflected light is minimized, and when the direct light has the same polarization as all of the one or more reflected lights, the influence of reflected light is maximized. In this embodiment, when operating in the first mode, the polarization control unit 11 rotates the polarization of the modulated light over time, thereby changing the polarization relationship between the direct light and each of the one or more reflected lights in the time axis direction. Therefore, the influence of reflected light is approximately halfway between the minimum and maximum, and the transmission quality measured at this time is used as the reference value.

[0025] However, when the level of most of the reflected light is low and there are only one or two reflected lights that have a level that affects the direct light, and when the polarization control unit 11 is operating in the second mode, it may happen that the direct light and the reflected light that has a level that affects the direct light are orthogonal to each other at the receiving point of the demodulation unit 25. In such a case, the influence of the reflected light is reduced by operating the polarization control unit 11 in the second mode rather than in the first mode.

[0026] Here, the state of the optical communication system may change over time. For example, when used outdoors, the fiber may vibrate or rotate due to the influence of wind, etc. The optical transmission line 3 constituting the optical communication system may be partially replaced or partially rerouted for various reasons during operation. The modulated light may also change due to replacement of the modulator 10, etc. Therefore, the polarization relationship between the direct light and the reflected light may also change over time. For this reason, in this embodiment, by switching the operating mode depending on the comparison result between the threshold value set based on the reference value and the transmission quality in the second mode and the passage of time, it is possible to prevent the reflected light from constantly having a strong influence on the direct light, even if the polarization relationship between the direct light and the reflected light changes.

[0027] Note that this embodiment can also use dithering described in Patent Document 1 and Non-Patent Document 1. That is, the modulator 10 can be configured to output transmission light including modulated light and dithering light to the polarization controller 11.

[0028] Furthermore, in this embodiment, when the optical communication device 1 is operating in the second mode, the mode control unit 26 determines the transmission quality based on the measurement signal within one upstream period to determine the operating mode of the optical communication device 1, and notifies the optical communication device 1 of the operating mode using a downstream control signal during the downstream period following the upstream period. However, instead of determining the transmission quality based on the measurement signal within one upstream period, the mode control unit 26 may be configured to determine the transmission quality based on the measurement signals within multiple consecutive upstream periods. Furthermore, when changing the operating mode of the optical communication device 1, the mode control unit 26 may be configured to notify the optical communication device 1 of the change in operating mode at any timing within the downstream period. Note that the mode control unit 26 may be configured to determine, for example, that the timing at which the polarization control unit 11 of the optical communication device 1 switches the operating mode is a timing a pre-measured period after the timing at which the downstream control signal is transmitted. Alternatively, the optical communication device 1 may be configured to notify the optical communication device 2 of the switch in operating mode using an upstream control signal.

[0029] Furthermore, in this embodiment, the optical communication device 1 and the optical communication device 2 transmit and receive upstream control signals and downstream control signals via the optical transmission path 3, but the control signals can also be transmitted and received via a network (not shown). In this case, the timing of transmitting the upstream control signals and downstream control signals can be made unrelated to the upstream period and downstream period in the optical transmission path 3. Furthermore, the optical communication device 1 can transmit the upstream control signal via the optical transmission path 3 using an optical carrier wave with a frequency f3, which is different from the frequencies f1 and f2, and the optical communication device 2 can transmit the downstream control signal via the optical transmission path 3 using an optical carrier wave with a frequency f4, which is different from the frequencies f1 to f3.

[0030] Second Embodiment Next, the second embodiment will be described, focusing on the differences from the first embodiment. In the first embodiment, the amount of polarization rotation in the polarization control unit 11 in the second mode was constant at the initial value set when transitioning to the second mode. In this embodiment, the mode control unit 26 also controls the amount of rotation in the polarization control unit 11 after transitioning the optical communication device 1 to the second mode.

[0031] Fig. 6 is a flowchart of the process executed by the mode control unit 26 in this embodiment. The process in Fig. 6 is started in response to the optical communication device 1 being transitioned to the first mode as a result of the lapse of the period T0 shown in Fig. 5.

[0032] In S10, the mode control unit 26 determines the transmission quality based on the downstream measurement signal during the first upstream period after transitioning the optical communication device 1 to the second mode. Note that the amount of polarization rotation in the polarization control unit 11 at this time is an initial value. In S11, the mode control unit 26 determines whether the transmission quality is equal to or greater than a threshold. If the transmission quality is not equal to or greater than the threshold, in S18, the mode control unit 26 instructs the optical communication device 1 to transition to the first mode using a downstream control signal, and the processing of FIG. 6 ends.

[0033] On the other hand, if the transmission quality is equal to or greater than the threshold, the mode control unit 26 instructs the increase of the rotation amount by a downlink control signal in S12. In response to the downlink control signal instructing the increase of the rotation amount, the polarization control unit 11 increases the rotation amount by a predetermined value. In S13, the mode control unit 26 determines the transmission quality based on the measurement signal in the downlink direction in the next uplink period. In S14, the mode control unit 26 determines whether the transmission quality has deteriorated from the transmission quality at the time of the previous measurement. If the transmission quality has not deteriorated from the transmission quality at the time of the previous measurement, the mode control unit 26 repeats the process from S12.

[0034] On the other hand, if the transmission quality is worse than the transmission quality at the time of the previous measurement, the mode control unit 26 instructs the reduction of the rotation amount by a downstream control signal in S15. In response to the downstream control signal instructing the reduction of the rotation amount, the polarization control unit 11 reduces the rotation amount by a predetermined value. In other words, in response to the downstream control signal instructing the reduction of the rotation amount, the polarization control unit 11 returns the rotation amount to the rotation amount at the time of the previous measurement.

[0035] Thereafter, the mode control unit 26 measures the transmission quality in S16 without changing the rotation amount of the polarization control unit 11, and determines in S17 whether the transmission quality is equal to or greater than a threshold, and repeats this process until the transmission quality deteriorates below the threshold. If the transmission quality deteriorates below the threshold, the mode control unit 26 instructs the optical communication device 1 to transition to the first mode using a downstream control signal in S18, and ends the processing of Fig. 6. Although not shown in the flowchart of Fig. 6, similar to the first embodiment, when a predetermined period T1 has elapsed since the optical communication device 1 was transitioned to the second mode, the optical communication device 1 may be configured to transition to the first mode regardless of the transmission quality.

[0036] As described above, in this embodiment, after transitioning to the second mode, the rotation amount in the polarization control unit 11 is gradually changed repeatedly as long as the transmission quality does not deteriorate from the previous transmission quality. Then, by detecting that the transmission quality has deteriorated from the previous transmission quality, the rotation amount is fixed to the amount that maximizes the transmission quality within the range in which the transmission quality was measured. With this configuration, the transmission quality in the second mode can be improved compared to the configuration of the first embodiment.

[0037] 6, the measurement of transmission quality and the transmission of downstream control signals are performed in each upstream period and each downstream section, but the configuration may be such that the transmission quality is determined based on measurement signals received in multiple downstream periods, and the transmission of downstream control signals is also performed at any timing during the downstream period. Furthermore, as described in the first embodiment, the configuration may be such that the control signals are transmitted and received via a network (not shown), or that optical carrier waves different from the frequencies f1 and f2 are transmitted and received via the optical transmission path 3.

[0038] This configuration can reduce the effects of reflected light even when the state of the optical communication system changes, 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."

[0039] 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]

[0040] 25: Demodulation section, 26: Mode control section

Claims

1. an optical communication device that communicates with an opposing optical communication device via an optical transmission path, the opposing optical communication device operating in either a first mode in which an amount of rotation of polarization of first modulated light is changed over time, or a second mode in which the amount of rotation is fixed to a set value, a demodulation means for receiving the first modulated light carrying data from the opposing optical communication device during a first period, and for receiving and demodulating the first modulated light carrying a measurement signal from the opposing optical communication device during a second period, the first period and the second period being repeated alternately; a control means for determining the operation mode of the opposing optical communication device and transmitting a control signal to the opposing optical communication device, thereby controlling the opposing optical communication device to operate in the determined operation mode; Equipped with The control means changes the operating mode of the opposing optical communication device to the first mode when the transmission quality in the second mode determined based on the measurement signal deteriorates below a threshold value when the opposing optical communication device is operating in the second mode.

2. 2. The optical communication device according to claim 1, wherein the control signal is transmitted to the opposing optical communication device via the optical transmission line by a second modulated light having a frequency different from that of the first modulated light during the first period.

3. 2. The optical communication device according to claim 1, wherein the control means changes the operating mode of the opposing optical communication device to the second mode when a first period has elapsed since the operating mode of the opposing optical communication device was transitioned from the second mode to the first mode.

4. 2. The optical communication device according to claim 1, wherein the control means changes the operating mode of the opposing optical communication device to the first mode when a second period has elapsed since the control means transitioned the operating mode of the opposing optical communication device from the first mode to the second mode.

5. 2. The optical communication device according to claim 1, wherein when the control means transitions the operating mode of the opposing optical communication device from the second mode to the first mode, the control means updates the threshold based on the transmission quality in the first mode determined based on the measurement signal.

6. 2. The optical communication device according to claim 1, wherein, when the control means transitions the operating mode of the opposing optical communication device from the first mode to the second mode, the control means increases the set value of the rotation amount in the opposing optical communication device by a predetermined value using the control signal and determines the transmission quality in the second mode based on the measurement signal, repeating this process until the transmission quality deteriorates from the transmission quality determined previously, and when the transmission quality deteriorates from the transmission quality determined previously, notifies the opposing optical communication device by the control signal that the rotation amount will be fixed to the amount that resulted in the best transmission quality among the determined transmission qualities.

7. An optical communication device that transmits a first modulated light to an opposing optical communication device via an optical transmission path, a modulation means for generating the first modulated light carrying data during a first period and generating the first modulated light carrying a measurement signal during a second period, the first period and the second period being repeated alternately; a polarization control means for changing an amount of rotation of polarization of the first modulated light over time when the operation mode is a first mode, and for fixing the amount of rotation to a set value when the operation mode is a second mode; a receiving means for receiving a control signal specifying the operation mode from the opposing optical communication device; An optical communication device comprising:

8. 8. The optical communication device according to claim 7, wherein the control signal is received from the opposing optical communication device via the optical transmission line by second modulated light having a frequency different from that of the first modulated light during the first period.

9. The optical communication device according to claim 7 , wherein the control signal is used to change the set value of the amount of rotation when the operation mode is the second mode.

Images