Optical receiving device

Abstract

To provide a technique for suppressing the influence of reflected light by a simple method.SOLUTION: An optical receiving device receives modulated light, which is generated by an optical transmitting device intensity-modulating carrier light with an electrical signal carrying information and transmitting it to an optical transmission line and includes a carrier component corresponding to the carrier light and a sideband component corresponding to the electrical signal, as received light from the optical transmission line. The optical receiving device includes: output means that outputs first light in which the carrier component included in the received light is suppressed; a light source that generates local light; multiplexing means for multiplexing the local light and the first light and outputting second light; and photoelectric conversion means for performing photoelectric conversion on the second light.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to an optical receiving device 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 received light that includes 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 invention, an optical receiving device receives modulated light from an optical transmission line as received light, the modulated light being generated by an optical transmitting device by intensity-modulating carrier light with an electrical signal carrying information and transmitted to the optical transmission line, the modulated light including a carrier component corresponding to the carrier light and a sideband component corresponding to the electrical signal. The optical receiving device includes: output means for outputting first light in which the carrier component included in the received light is suppressed; a light source for generating local light; multiplexing means for multiplexing the local light and the first light and outputting second light; and photoelectric conversion means for performing photoelectric conversion of the second light. [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 some embodiments. [Figure 2] 1 illustrates modulated light according to some embodiments. [Figure 3] FIG. 1 is a block diagram of an optical receiving device, according to some embodiments. [Figure 4] 4 is an illustration of processing in an optical receiving device, according to some embodiments. [Figure 5] 4 is an illustration of processing in an optical receiving device, according to some embodiments. [Figure 6] FIG. 1 is a block diagram of an optical receiving device, 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. 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). The optical transmitter 1 generates modulated light by intensity-modulating carrier light with an electrical signal carrying information (hereinafter referred to as an information signal), and transmits the modulated light to the optical receiver 2. The optical receiver 2 performs direct detection of the received light received via the optical transmission line 3 in order to demodulate the modulated light and generate an information signal. That is, the optical receiver 2 performs photoelectric conversion of the received light to generate the information signal, and outputs an electrical signal.

[0013] Fig. 2 shows the frequency components of modulated light generated by the optical transmitter 1 and output to the optical transmission line 3. Hereinafter, the frequency of the carrier light is represented by fc, and the center frequency of the information signal is represented by fd. In Fig. 2, reference numeral 90 denotes a carrier component corresponding to the carrier light, reference numeral 91 denotes a lower sideband component corresponding to the information signal, and reference numeral 92 denotes an upper sideband component corresponding to the information signal.

[0014] When multiple reflection points exist in the optical transmission path 3, the received light received by the optical receiving device 2 is a combination of direct light, which is modulated light that reaches the optical receiving device 2 without being reflected on the optical transmission path 3, and reflected light, which is modulated light that reaches the optical receiving device 2 after being reflected at an even number of reflection points. The received light may contain not just one reflected light, but multiple reflected lights reflected at different positions. For example, when three reflection points, first to third reflection points, exist in the optical transmission path 3, 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 to third reflected lights may be different. Furthermore, when four reflection points exist in the optical transmission path 3, the received light may also contain reflected lights reflected at all four reflection points.

[0015] Therefore, the electrical signal output by direct detection of the received light in the optical receiving device 2 includes signals corresponding to beats between the components of the modulated light, each of which has a different propagation delay. The information signal is a signal corresponding to beats between the carrier component 90 contained in the direct light and the sideband components contained in the direct light, and signals corresponding to beats between other components are noise components (or interference components) relative to the information signal. Here, the level of the carrier component 90 contained in the modulated light is usually greater than the levels of the lower sideband component 91 and the upper sideband component 92, so the dominant noise component is a component corresponding to beats involving the carrier component 90 contained in each of one or more reflected lights.

[0016] Therefore, in this embodiment, the optical receiving device 2 removes the carrier component contained in the received light and suppresses the noise component by performing photoelectric conversion of light including the component of local light, which is the carrier light generated in the optical receiving device, and the lower sideband component 91 and upper sideband component 92 contained in the received light.

[0017] FIG. 3 is a configuration diagram of an optical receiving device 2 according to this embodiment. Received light from the optical transmission line 3 is input to the filter unit 21. FIG. 4(A) shows the frequency components of the received light. The received light includes a carrier component 80, a lower sideband component 81, and an upper sideband component 82. The carrier component 80 corresponds to a combination of a carrier component 90 included in the direct light and a carrier component 90 included in one or more reflected lights. Similarly, the lower sideband component 81 corresponds to a combination of a lower sideband component 91 included in the direct light and a lower sideband component 91 included in one or more reflected lights, and the upper sideband component 82 corresponds to a combination of an upper sideband component 92 included in the direct light and an upper sideband component 92 included in one or more reflected lights.

[0018] The filter unit 21 is a band-elimination filter that suppresses a carrier component 80 contained in the received light and outputs light containing a lower sideband component 81 and an upper sideband component 82. FIG. 4(B) shows the frequency components of the light output by the filter unit 21. The light output by the filter unit 21 is input to the adder 23. If the optional polarization control unit 24 is provided, the light output by the filter unit 21 is input to the adder 23 via the polarization control unit 24. The optional polarization control unit 24 adjusts the polarization of the light output by the filter unit 21.

[0019] Meanwhile, the light source 22 generates local light of frequency fc. The local light of frequency fc is input to the adder 23. If an optional polarization controller 25 is provided, the local light of frequency fc is input to the adder 23 via the polarization controller 25. The optional polarization controller 25 adjusts the polarization of the local light.

[0020] The adder 23 multiplexes the light output by the filter unit 21 and the local light. Fig. 4(C) shows the frequency components of the light output by the adder 23. In Fig. 4(C), reference numeral 70 denotes a carrier component 70 corresponding to the local light. The photoelectric converter 26 performs photoelectric conversion on the light shown in Fig. 4(C) and outputs an electrical signal.

[0021] The optional polarization control units 24 and 25 are provided to match the polarization plane of the local light input to the adder 23 with that of the light from the filter unit 21. Matching the polarization plane of the local light input to the adder 23 with that of the light from the filter unit 21 prevents a decrease in the level of the signal output from the photoelectric conversion unit 23, corresponding to the beat between the carrier component 70 and the lower sideband component 81 / upper sideband component 82. Even when controlling to match the polarization planes, a configuration may be adopted in which only one of the polarization control units 24 and 25 is provided.

[0022] As described above, in the optical receiving device 2, the carrier component 80 of the received light containing one or more reflected lights is suppressed, and direct detection is performed on light containing the local light and the lower sideband component 81 and upper sideband component 82 contained in the received light. With this configuration, it is possible to suppress noise components generated by the carrier component 90 of one or more reflected lights contained in the carrier component 80.

[0023] In this embodiment, the filter unit 21 suppresses only the carrier component 80. However, the filter unit 21 may be configured to further suppress either the lower sideband component 81 or the upper sideband component 82. In this case, the filter unit 21 is a low-pass filter or a high-pass filter, rather than a band-stop filter. Furthermore, in this embodiment, the modulated light output from the optical transmitter 1 to the optical transmission line 3 includes both sideband components, i.e., the lower sideband component 91 and the upper sideband component 92, in addition to the carrier component 90. However, the modulated light may include either the lower sideband component 91 or the upper sideband component 92 and the carrier component 90.

[0024] Second Embodiment Next, the second embodiment will be described, focusing on the differences from the first embodiment. In the first embodiment, the light source 22 generated local light having the same frequency fc as the carrier light generated by the optical transmission device 1. When the filter unit 21 is a band-stop filter and light including a lower sideband component 81 and an upper sideband component 82 is output to the adder unit 23, the frequency of the local light generated by the light source 22 must be the same frequency fc as the carrier light generated by the optical transmission device 1. However, when outputting either the lower sideband component 81 or the upper sideband component 82 to the adder unit 23 as described in the first embodiment, the frequency of the local light generated by the light source 22 can be set arbitrarily.

[0025] For example, FIG. 5A is the same as FIG. 4A and shows the frequency components of the received light. As shown in FIG. 5B, when the filter unit 21 is a high-pass filter and the upper sideband component 82 is output to the adder 23, the frequency of the local light generated by the light source 22 can be set to a frequency fl, which is different from the frequency fc. Therefore, the photoelectric conversion unit 26 performs photoelectric conversion on light having the frequency components shown in FIG. 5C. In this case, the center frequency of the signal generated by the beat between the carrier component 71 and the upper sideband component 82 corresponds to the difference between the frequency fl and the frequency (fc+fd), and the center frequency of the electrical signal can be controlled by the frequency fl. For example, by setting the center frequency of the electrical signal to a radio frequency band signal, a radio-over-fiber (RoF) system can be realized. In this embodiment, the filter unit 21 suppresses one of the two sideband components in addition to the carrier component 80. However, the optical transmitting device 1 may also be configured to transmit a signal by suppressing one of the two sideband components.

[0026] Third Embodiment Next, the third embodiment will be described, focusing on the differences from the first embodiment. In the first embodiment, the light source 22 generated the local light independently of the carrier light generated by the optical transmitter 1. In this embodiment, the light source 22 generates the local light by injection locking the carrier light from the optical transmitter 1.

[0027] Fig. 6 is a configuration diagram of an optical receiving device 2 according to this embodiment. Note that in Fig. 6, components similar to those in the configuration shown in Fig. 3 are assigned the same reference numerals, and descriptions thereof will basically be omitted. The separator 27 separates the components contained in the received light, outputs light corresponding to the carrier component 80 to the light source 22, and outputs light corresponding to the sideband components to the adder 23 (or the polarization controller 24 if the optional polarization controller 24 is provided). Note that the sideband components contained in the light output to the adder 23 may be both the lower sideband component 81 and the upper sideband component 82, as described in the first embodiment, or only one of them.

[0028] The light source 22 is an injection-locked light source that generates local light of frequency fc by locking to light containing a carrier component 80. In this way, by generating the local light generated by the light source 22 based on the carrier component 80, it is possible to suppress noise components caused by fluctuations in the frequency of the local light relative to the frequencies of the sideband components. Note that the carrier component 80 includes a carrier component 90 of the direct light and one or more carrier components 90 of the reflected light, but this does not affect injection locking.

[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] 21: filter section, 22: light source, 23: adder section, 26: photoelectric conversion section

Claims

1. An optical receiving device receives modulated light from an optical transmission line as received light, the modulated light being generated by an optical transmitting device by intensity-modulating a carrier light with an electrical signal carrying information and transmitted to an optical transmission line, the modulated light including a carrier component corresponding to the carrier light and a sideband component corresponding to the electrical signal, an output unit for outputting a first light in which the carrier component contained in the received light is suppressed; a light source that generates local light; a multiplexing unit that multiplexes the local light and the first light and outputs a second light; a photoelectric conversion means for performing photoelectric conversion of the second light; An optical receiving device comprising:

2. 2. The optical receiving device according to claim 1, wherein the frequency of the local light is equal to the frequency of the carrier light.

3. the output means is a separation means that separates the first light from the third light containing the carrier component, 3. The optical receiving device according to claim 2, wherein the light source is a light source that performs injection locking using the third light as an input.

4. the sideband component included in the modulated light is only one of an upper sideband component and a lower sideband component, 3. The optical receiving device according to claim 2, wherein the first light output by the output means includes the sideband component.

5. the sideband components included in the modulated light are an upper sideband component and a lower sideband component, 3. The optical receiving device according to claim 2, wherein the first light output by the output means includes the upper sideband component and the lower sideband component.

6. the sideband components included in the modulated light are an upper sideband component and a lower sideband component, 3. The optical receiving device according to claim 2, wherein the first light output by the output means includes only one of the upper sideband component and the lower sideband component.

7. the sideband component included in the modulated light is one of an upper sideband component and a lower sideband component, the first light output by the output means includes the sideband component, 2. The optical receiving device according to claim 1, wherein the frequency of the local light is different from the frequency of the carrier light.

8. the sideband components included in the modulated light are an upper sideband component and a lower sideband component, the first light output by the output means includes only one of the upper sideband component and the lower sideband component, 2. The optical receiving device according to claim 1, wherein the frequency of the local light is different from the frequency of the carrier light.

9. 9. The optical receiving device according to claim 1, further comprising a polarization control unit that controls a polarization plane of at least one of the first light and the local light.

10. 9. The optical receiving device according to claim 1, wherein the optical transmission line has a plurality of reflection points.

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