JP7788989B2 - Optical communication equipment - Google Patents

Description

This disclosure relates to technology for reducing the effects of reflected light in optical communication systems.

Optical transmission paths have multiple connection points for connecting optical fibers. Optical fibers are connected using optical connectors or fusion splices. At these connection points, a portion of the modulated light transmitted from the transmitter of an optical communication device to the 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 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 path (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 path (hereinafter referred to as "reflected light"). Because the propagation delays of direct light and reflected light are different, the reflected light acts as interference light for the direct light, affecting the demodulation of the direct light.

Patent Document 1 and Non-Patent Document 1 disclose configurations that perform dithering to reduce the effects of reflected light. Specifically, Non-Patent Document 1 discloses a configuration that generates dithering light using a phase modulator dedicated to dithering. Patent Document 1 also discloses a configuration that generates transmitted light that includes modulated light and dithering light by driving a light source with both a signal carrying information and a signal for dithering.

JP 2010-232764 A

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

To reduce the effects of reflected light using dithering, the dithering light must be adjusted for each optical communication system. Therefore, changes in the state of the optical communication system after service begins may cause the dithering light to no longer be suitable for the optical communication system.

This disclosure provides technology that can reduce the effects of reflected light even when the state of the optical communication system changes.

According to one aspect of the present invention, an optical communication device that communicates via an optical transmission path with a counter optical communication device operating 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: demodulation means that receives the first modulated light carrying data from the counter optical communication device during a first period and receives and demodulates the first modulated light carrying a measurement signal from the counter optical communication device during a second period, the first period and the second period being repeated alternately; and control means that determines the operating mode of the counter optical communication device and controls the counter optical communication device to operate in the determined operating mode by transmitting a control signal to the counter optical communication device; and when the counter optical communication device is operating in the second mode, if the transmission quality in the second mode determined based on the measurement signal deteriorates below a threshold, the control means changes the operating mode of the counter optical communication device to the first mode.

This disclosure makes it possible to reduce the effects of reflected light even when the state of the optical communication system changes.

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

The following embodiments are described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the claimed invention, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be combined in any desired manner. Furthermore, the same reference numbers are used for identical or similar components, and duplicate descriptions will be omitted.

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 duplexing (TDD) and frequency division duplexing (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.

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

Figure 3 is a configuration diagram of an optical communication system for suppressing the effects of reflected light in the downstream direction. The modulator 10 generates modulated light based on an optical carrier wave of frequency f1 and outputs it to the 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 in the electrical domain using any multiplexing method, such as time division multiplexing or frequency division multiplexing, and are carried on an optical carrier wave of frequency f1. Note that 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.

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 it. Hereinafter, the amount of polarization change that the polarization control unit 11 imparts to the polarization of the modulated light from the modulation unit 10 is referred to as the rotation amount. In this embodiment, the operating mode of the optical communication device 1 is either the first mode or the second mode. When the optical communication device 1 is set to the first mode, the polarization control unit 11 changes the rotation amount over time within one uplink period or one downlink period. The rotation amount can be changed, for example, randomly over time. Alternatively, the rotation amount can be changed according to rotation information indicating the relationship between the amount of time elapsed and the rotation amount. On the other hand, when the optical communication device 1 is set to the second mode, the polarization control unit 11 keeps the rotation amount constant at a set value. In the second mode, the rotation amount is not changed except when the set value of the rotation amount 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 passes through the polarization control unit 11 is transmitted to the optical communication device 2 via the optical transmission path 3.

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, while 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 control unit 26. The downstream measurement signal is a signal known to the optical communication device 2, and the mode control unit 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 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, transmission quality can be evaluated by the bit error rate of the measured signal.

The mode control unit 26 determines the operating mode of the optical communication device 1 according to predetermined criteria. These criteria use 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 using 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 the modulation unit of the optical communication device 2 and the demodulation unit of the optical communication device 1; however, to simplify 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.

Figure 4 is a sequence diagram of the process for controlling the downstream operating mode, i.e., the operating mode of the optical communication device 1. At 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. At S2 during the upstream period, the mode control unit 26 of the optical communication device 2 determines the operating mode of the optical communication device 1 from the next downstream period. At 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 operating mode via a downstream control signal. If the operating mode notified by the optical communication device 2 differs from the current operating mode, the polarization control unit 11 of the optical communication device 1 switches the operating mode. During S4 in the downstream period, optical communication device 1 transmits modulated light carrying downstream data to optical communication device 2 in accordance with the current operating mode.

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 this configuration. For example, the downstream control signal in S3 may be transmitted within the downstream period. If the operating mode notified in the downstream control signal differs from the current operating mode, the optical communication device 1 switches the operating mode in accordance with the notification in 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 during 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 during the following downstream period.

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.

Figure 5 is an explanatory diagram of the process by which the mode control unit 26 determines the operating 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 operating 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 that is lower than the reference value by the predetermined value α as the threshold value Th.

As shown in FIG. 5, the mode control unit 26 transitions the operating mode of the optical communication device 1 to the second mode when a predetermined period T0 has elapsed since the first mode was selected. In this embodiment, when the polarization control unit 11 transitions from the first mode to the second mode, it resets the rotation amount setting to an initial value. The initial value may be a predetermined value, such as 0 degrees. Alternatively, the initial value may be the rotation amount at the time of transition from the first mode to the second mode. The mode control unit 26 determines the transmission quality based on the downstream measurement signal when the optical communication device 1 is operating in the second mode. If the determined transmission quality does not deteriorate below the 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 deteriorates below the 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 when a predetermined period T1 has elapsed since the second mode was selected. After switching the optical communication device 1 to the first mode using 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.

As described above, the received light received by optical communication device 2 from optical communication device 1 includes direct light and reflected light. The received light may contain more than one reflected light, but multiple reflected lights reflected at different positions. For example, if there are three reflection points, first to third reflection points, on optical transmission path 3, the received light may contain three reflected lights: first reflected light reflected at the first and second reflection points, second reflected light reflected at the first and third reflection points, and third reflected light reflected at the second and third reflection points. The propagation delays of the first to third reflected lights may differ. Furthermore, if there are four reflection points on optical transmission path 3, the received light may also contain reflected light reflected at all four reflection points.

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 the closer the polarization planes of the direct light and the reflected light are to being orthogonal. Therefore, when the direct light is orthogonal to all of the one or more reflected lights, the influence of the 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 the 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 somewhere between minimum and maximum, and the transmission quality measured at this time is used as the reference value.

However, when the level of most of the reflected light is low, and there are only one or two reflected lights that are at a level that affects the direct light, and the polarization control unit 11 is operating in the second mode, it is possible that the direct light and the reflected light at a level that affects the direct light may be orthogonal at the receiving point of the demodulation unit 25. In such cases, the influence of the reflected light is reduced by operating the polarization control unit 11 in the second mode rather than the first mode.

The state of the optical communication system can change over time. For example, when used outdoors, the fiber can vibrate or rotate due to factors such as wind. The optical transmission path 3 that constitutes the optical communication system can undergo partial replacement or partial rerouting for various reasons during operation. Modulated light can also change depending on factors such as replacement of the modulator 10. Therefore, the polarization relationship between direct light and reflected light can also change over time. For this reason, in this embodiment, by switching the operating mode based on the comparison result between a threshold value set based on a reference value and the transmission quality in the second mode and the passage of time, it is possible to prevent reflected light from constantly having a strong effect on direct light, even if the polarization relationship between direct light and reflected light changes.

Note that this embodiment can also use dithering, as described in Patent Document 1 and Non-Patent Document 1. In other words, the modulator 10 can be configured to output transmission light including modulated light and dithering light to the polarization controller 11.

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 can also 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 can 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 can 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 can be configured to notify the optical communication device 2 that it has switched the operating mode using an upstream control signal.

Furthermore, in this embodiment, optical communication device 1 and optical communication device 2 send and receive upstream control signals and downstream control signals via optical transmission path 3, but they can also be configured to send and receive control signals via a network (not shown). In this case, the timing of sending the upstream control signals and downstream control signals can be unrelated to the upstream and downstream periods on optical transmission path 3. Furthermore, optical communication device 1 can be configured to send upstream control signals via optical transmission path 3 using an optical carrier wave with frequency f3, which is different from frequencies f1 and f2, and optical communication device 2 can be configured to send downstream control signals via optical transmission path 3 using an optical carrier wave with frequency f4, which is different from frequencies f1 to f3.

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 the optical communication device 1 transitioned 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.

Figure 6 is a flowchart of the processing executed by the mode control unit 26 in this embodiment. The processing in Figure 6 is initiated in response to the optical communication device 1 transitioning to the first mode due to the lapse of the period T0 shown in Figure 5.

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 the 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.

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

On the other hand, if the transmission quality has deteriorated compared to the transmission quality at the time of the previous measurement, the mode control unit 26 instructs the reduction of the rotation amount using 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.

Then, without changing the amount of rotation of the polarization control unit 11, the mode control unit 26 measures the transmission quality in S16 and determines in S17 whether the transmission quality is equal to or greater than the threshold, repeating 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, thereby ending the processing of FIG. 6. Although not shown in the flowchart of FIG. 6, similar to the first embodiment, the optical communication device 1 can be configured to transition to the first mode after a predetermined period T1 has elapsed since the optical communication device 1 transitioned to the second mode, regardless of the transmission quality.

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 previously measured 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. This configuration makes it possible to improve the transmission quality in the second mode compared to the configuration of the first embodiment.

In the explanation of Figure 6, it was stated that measurement of transmission quality and transmission of downstream control signals are performed in each upstream period and each downstream section, but it is also possible to configure the system so that transmission quality is determined based on measurement signals received in multiple downstream periods, and so that downstream control signals are transmitted at any timing during the downstream period. Furthermore, as described in the first embodiment, it is also possible to configure the system so that control signals are transmitted and received via a network (not shown), or so that control signals are transmitted and received via optical transmission path 3 using optical carrier waves different from frequencies f1 and f2.

The above configuration makes it possible to reduce the effects of reflected light even when the state of the optical communications system changes. This makes it possible to contribute to Goal 9 of the United Nations' Sustainable Development Goals (SDGs), which states, "Build resilient infrastructure, promote sustainable industrialization and foster innovation."

The invention is not limited to the above-described embodiments, and various modifications and variations are possible within the scope of the invention.

25: Demodulation section, 26: Mode control section

Claims (9)

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. The optical communication device of claim 1, wherein the control signal is transmitted to the opposing optical communication device via the optical transmission path during the first period using second modulated light having a different frequency from the first modulated light. The optical communication device described in 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. The optical communication device described in 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 operating mode of the opposing optical communication device was transitioned from the first mode to the second mode. The optical communication device described in claim 1, wherein the control means, when transitioning the operating mode of the opposing optical communication device from the second mode to the first mode, updates the threshold based on the transmission quality in the first mode determined based on the measurement signal. The optical communication device of claim 1, wherein the control means, when transitioning the operating mode of the opposing optical communication device from the first mode to the second mode, 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 previously determined transmission quality; and, if the transmission quality deteriorates from the previously determined transmission quality, notifies the opposing optical communication device using the control signal to fix the rotation amount to the amount that resulted in the best transmission quality among the determined transmission qualities. 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: The optical communication device described in claim 7, wherein the control signal is received from the opposing optical communication device via the optical transmission path during the first period using second modulated light having a different frequency from the first modulated light. The optical communication device of claim 7, wherein the control signal is used to change the set value of the rotation amount when the operating mode is the second mode.