Optical transmitter

Abstract

To provide a technique for suppressing an influence of reflected light in a simple way.SOLUTION: An optical transmitter connected to an optical receiver via an optical transmission path includes: light modulation means which generates modulated light on the basis of a transmission signal transmitted to the optical receiver; polarization adjustment means which adjusts a polarized wave of the modulated light to a first polarized wave or a second polarized wave which is orthogonal to the first polarized wave; and control means which controls the polarization adjustment means so that the polarized wave of the modulated light is switched between the first polarized wave and the second polarized wave at every switching period. The control means sets the switching period on the basis of a delay difference between direct light that reaches the optical receiver without being reflected in the optical transmission path and first reflected light which is one of one or more reflected light beams that are reflected only an even number of times in the optical transmission path and reaches the optical receiver.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present disclosure relates to an optical transmitter in an optical communication system. [Background technology]

[0002] An optical transmission path 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 an optical transmitter to an optical receiver is reflected toward the optical transmitter. The portion of the modulated light reflected toward the optical transmitter may be further reflected toward the optical receiver at another connection point. Because the optical transmitter includes an isolator or the like 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 optical transmitter. On the other hand, the optical receiver receives both modulated light that reaches the optical receiver without being reflected in the optical transmission path (hereinafter referred to as "direct light") and modulated light that reaches the optical 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 the direct light and the reflected light are different, the reflected light becomes interference light of the direct light and affects the demodulation of the direct light.

[0003] Patent Document 1 and Non-Patent Document 1 disclose a configuration in which dithering is performed to suppress the influence of interference light. Specifically, Non-Patent Document 1 discloses a configuration in which a phase modulator dedicated to dithering is used. 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 modulated 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] However, there is a need for a simpler method to suppress the influence of reflected light without using dithering.

[0007] The present disclosure provides a technique for suppressing the influence of reflected light in a simple manner. [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 an optical modulation means for generating modulated light based on a transmission signal to be transmitted to the optical receiving device, a polarization adjustment means for changing the polarization of the modulated light to a first polarization or a second polarization orthogonal to the first polarization, and a control means for controlling the polarization adjustment means so that the polarization of the modulated light is switched between the first polarization and the second polarization for each switching period, and the control means sets the switching period based on the delay difference between direct 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 that are reflected an even number of times in the optical transmission path and reach the optical receiving device. [Effects of the Invention]

[0009] According to the present disclosure, the influence of reflected light can be suppressed using a simple method. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a block diagram of an optical communication system according to one embodiment. [Figure 2]FIG. 1 is a diagram illustrating the configuration of an optical transmitting device according to an embodiment. [Figure 3] 10A and 10B are diagrams illustrating reflection point information and reflected light information according to one embodiment. [Figure 4] 1 illustrates the paths of reflected and direct light, according to one embodiment. [Figure 5] 1 is an illustration of a polarization switching period according to one embodiment. 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 1 is a diagram showing the configuration of an optical communication system according to this embodiment. An optical transmitter 1 and an optical receiver 2 are connected via an optical transmission line 3. The optical transmission line 3 has a plurality of connection points (reflection points).

[0013] FIG. 2 is a configuration diagram of the optical transmitter 1 according to this embodiment. The optical modulator 11 modulates light based on a transmission signal corresponding to transmission information and outputs the modulated light to the polarization adjuster 14. The memory 12 stores reflection point information related to connection points (reflection points) of the optical transmission line 3. FIG. 3(A) is an example of the reflection point information. According to FIG. 3(A), the reflection point information indicates that three reflection points #1 to #3 exist in the optical transmission line 3. Furthermore, the reflection point information indicates the position of each reflection point and the amount of reflection at the reflection point. The position of the reflection point is indicated, for example, by the distance from the end of the optical transmission line 3 on the optical transmitter 1 side. The amount of reflection is indicated by the ratio of the power of the reflected light to the power of the light incident on the reflection point.

[0014] Based on the reflection point information shown in Fig. 3(A), the reflected light information shown in Fig. 3(B) can be generated. According to Fig. 3(B), the reflected light information indicates that three reflected light beams O#1 to O#3 reach the optical receiving device due to the reflection points indicated by the reflection point information. The reflected light information indicates that the reflected light beam O#1 is generated by reflection at reflection points X#1 and X#2, the reflected light beam O#2 is generated by reflection at reflection points X#2 and X#3, and the reflected light beam O#3 is generated by reflection at reflection points X#1 and X#3. Fig. 4 shows the paths that the direct light and reflected light beams O#1 to O#3 take to reach the optical receiving device 2.

[0015] It is also possible for reflected light to be reflected from an even number of reflection points (four or more), such as light reflected in the order of reflection points X#2, X#1, X#3, and X#2. However, since the power of such reflected light at the position of the optical receiving device 2 is very small, this embodiment only considers reflected light reflected from two of the multiple reflection points.

[0016] The reflected light information further indicates the delay difference between the direct light and the reflected light that arrive at the optical receiving device, and the power of the reflected light (the ratio of the power of the reflected light to the power of the direct light). The delay difference between the direct light and the reflected light can be calculated based on the distance between the reflection positions and the propagation delay per unit distance in the optical transmission path 3. The ratio of the power of the reflected light to the power of the direct light can be calculated based on the amount of reflection indicated by the reflection information, the distance between each reflection point, and the amount of attenuation per unit distance in the optical transmission path 3. The reflected light information shown in FIG. 3(B) is obtained based on the reflection point information shown in FIG. 3(A). However, the reflected light information may be stored in the storage unit 12 instead of or in addition to the reflection point information.

[0017] Returning to FIG. 2, when only reflection point information is stored in the memory unit 12, the polarization control unit 13 has the function of generating reflected light information based on the reflection point information. Note that when reflected light information is stored in the memory unit 12, the polarization control unit 13 uses the reflected light information stored in the memory unit 12 as is. Based on the reflected light information, the polarization control unit 13 determines the reflected light with the highest power at the reception point of the optical receiving device 2. In the following description, it is assumed that the power P#1 of the reflected light O#1 is the highest. In this case, based on the delay difference T#1 between the direct light and the reflected light O#1, the polarization control unit 13 generates a control signal for the polarization adjustment unit 14 and outputs it to the polarization adjustment unit 14 so that the polarization of the modulated light output to the optical transmission line 3 is the first polarization or a second polarization orthogonal to the first polarization. The polarization adjustment unit 14 adjusts the polarization of the input modulated light in accordance with the control signal.

[0018] FIG. 5A is an explanatory diagram of the polarization of the modulated light output by the polarization adjustment unit 14. The polarization control unit 13 controls the polarization adjustment unit 14 so that the polarization of the modulated light switches between the first polarization and the second polarization every switching period ΔTp. According to FIG. 5A, the switching period Tp is the same period T#1 as the delay difference between the direct light and the reflected light O#1. As shown in FIG. 5A, the reflected light O#1 arrives at the optical receiving device 2 T#1 later than the direct light. Therefore, while the optical receiving device 2 receives the reflected light O#1 of the first polarization, the optical receiving device 2 receives the direct light of the second polarization. Meanwhile, while the optical receiving device 2 receives the reflected light O#1 of the second polarization, the optical receiving device 2 receives the direct light of the first polarization. In other words, the polarizations of the direct light and the reflected light O#1 received by the optical receiving device 2 are orthogonal. Therefore, it is possible to demodulate the direct light while suppressing the influence of the reflected light O#1.

[0019] Note that this embodiment is not limited to setting the switching period ΔTp to the delay difference T#1 between the direct light and the reflected light O#1. For example, as shown in FIG. 5B, the switching period ΔTp can be set to (T#1) / 3. More generally, the switching period ΔTp can be set to a period obtained by dividing the delay difference T#1 between the direct light and the reflected light O#1 by an odd number.

[0020] As described above, by switching the polarization of the modulated light between the first polarization and the second polarization at intervals set based on the delay difference between the direct light and the reflected light, the effect of the reflected light on the demodulation of the modulated light (direct light) in the optical receiving device 2 can be reduced.

[0021] Second Embodiment Next, the second embodiment will be described, focusing on the differences from the first embodiment. In the first embodiment, the switching period ΔTp was determined based on the delay of the reflected light with the highest power among the reflected lights received by the optical receiving device 2. In this embodiment, the switching period ΔTp is determined based on the delay of one or more reflected lights whose power at the receiving position of the optical receiving device 2 is greater than a threshold among the reflected lights. In the following, it is assumed that the powers P#1 to P#3 of the three reflected lights O#1 to O#3 shown in FIG. 3(B) are greater than a threshold.

[0022] First, the condition for making the polarizations of reflected light O#1 and direct light orthogonal is to switch polarization at a period of ΔT1=T#1 / (2n-1), as explained in the first embodiment, where n is a natural number. Similarly, the condition for making the polarizations of reflected light O#2 and direct light orthogonal is to switch polarization at a period of ΔT2=T#2 / (2m-1), where m is a natural number. Furthermore, the condition for making the polarizations of reflected light O#3 and direct light orthogonal is to switch polarization at a period of ΔT3=T#3 / (2k-1), where k is a natural number.

[0023] Therefore, for example, when n=n1, m=m1, and k=k1, and ΔT1=ΔT2=ΔT3=ΔTp, if the polarization of the modulated light is switched every switching period ΔTp, the polarization of the reflected light O#1 to O#3 can be made orthogonal to the polarization of the direct light in the optical receiving device 2. However, generally, there may be no n, m, and k such that ΔT1=ΔT2=ΔT3.

[0024] In this embodiment, the polarization control unit 13 approximately calculates the switching period ΔTp. Specifically, the polarization control unit 13 calculates the switching period ΔTp so that the sum of the error |ΔTp-ΔT1|, the error |ΔTp-ΔT2|, and the error |ΔTp-ΔT3| is minimized.

[0025] With the above configuration, the polarization of the reflected light and the direct light is the same during the period determined in accordance with the error within the switching period ΔTp, but because the error is minimized, the influence of the reflected light can be suppressed.

[0026] Third Embodiment Next, the third embodiment will be described, focusing on the differences from the first and second embodiments. In this embodiment, the polarization control unit 13 receives feedback on communication quality from the optical receiving device 2 and adjusts the switching period ΔTp based on the communication quality.

[0027] The optical receiving device 2 may determine the communication quality by any method. For example, if the transmission signal input to the optical modulation unit 11 is encoded with an error correction code or an error detection code, the optical receiving device 2 can determine the communication quality by inspecting the error correction code or the error detection code of the signal obtained by demodulation. Alternatively, the optical modulation unit 11 may periodically insert a predetermined inspection signal into the transmission signal, and the optical receiving device 2 may inspect this inspection signal to determine the communication quality. The polarization control unit 13 may, for example, change the switching period ΔTp so that the communication quality received from the optical receiving device 2 is improved.

[0028] As described above, by controlling the switching period ΔTp based on feedback from the optical receiving device 2, the influence of reflected light can be suppressed.

[0029] The above configuration provides a simple method for reducing the effects of reflected light, thereby contributing to Goal 9 of the United Nations' Sustainable Development Goals (SDGs), which is to "Build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0030] 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]

[0031] 11: Optical modulation unit, 13: Polarization control unit, 14: Polarization adjustment unit

Claims

1. An optical transmitting device connected to an optical receiving device via an optical transmission line, an optical modulation means for generating modulated light based on a transmission signal transmitted to the optical receiving device; a polarization adjusting means for adjusting the polarization of the modulated light to a first polarization or a second polarization orthogonal to the first polarization; a control means for controlling the polarization adjustment means so that the polarization of the modulated light is switched between the first polarization and the second polarization for each switching period; Equipped with An optical transmitting device in which the control means sets the switching period based on the delay difference between direct light that reaches the optical receiving device without reflecting on the optical transmission path and a first reflected light among one or more reflected lights that reach the optical receiving device after being reflected an even number of times on the optical transmission path.

2. 2. The optical transmitting apparatus according to claim 1, wherein said control means sets said switching period to a period in which said delay difference is divided by an odd number.

3. 3. The optical transmitter according to claim 2, wherein the first reflected light is the reflected light having the greatest power among the one or more reflected lights.

4. 2. The optical transmitter according to claim 1, wherein the control means sets the switching period based on a period in which the delay difference between the plurality of first reflected lights is divided by an odd number.

5. 5. The optical transmitting device according to claim 4, wherein the control means sets the switching period so that the sum of the difference between the period in which the delay difference of each of the plurality of first reflected lights is divided by an odd number and the switching period is smallest.

6. the optical transmission line has a plurality of connection points; 2. The optical transmitting device according to claim 1, wherein the one or more reflected beams are generated by reflection at two of the plurality of connection points.

7. a means for storing reflection point information indicating positions of the plurality of connection points and amounts of reflection at the plurality of connection points; 7. The optical transmitting device according to claim 6, wherein the control means determines the first reflected light among the one or more reflected lights and the delay difference between the first reflected light and the direct light based on the reflection point information.

8. 8. The optical transmitting device according to claim 1, wherein the control means changes the switching period based on the quality of a result of demodulating the modulated light received from the optical receiving device.

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