Communication device, communication system, and communication method

Abstract

This communication device comprises: a variable splitter which acquires, from another communication device, an optical signal corresponding to a main signal on which a control signal is superimposed with a notified superimposition ratio, and, in accordance with a designated splitting ratio, splits the main signal into a first split signal and a second split signal; an intensity measurement unit which measures the optical power of the control signal in the second split signal; and a control unit which, on the basis of a minimum reception sensitivity of the optical power of the control signal and the result of measurement of the optical power of the control signal, selects, from candidates of combinations of superimposition ratios and splitting ratios, a combination that minimizes the total of a main signal loss due to the superimposition and the splitting, notifies the other communication device of the superimposition ratio of the selected combination, and designates the splitting ratio of the selected combination for the variable split unit.

Description

[Technical Field]

[0001] The present invention relates to a communication device, a communication system, and a communication method. [Background technology]

[0002] The International Telecommunication Union Telecommunication Standardization sector (ITU-T) G.989.2 recommendation defines a PtP (Point to Point) WDM (Wavelength Division Multiplexing)-PON (Passive Optical Network) system as one of the communication systems (see Non-Patent Document 1).

[0003] A PtP WDM-PON system comprises an optical line terminal (OLT) and an optical network unit (ONU). Hereinafter, the direction from the ONU to the OLT is referred to as "upstream." Hereinafter, the direction from the OLT to the ONU is referred to as "downstream."

[0004] The PtP WDM-PON system performs wavelength multiplexing of optical signals, and in the upstream and downstream, each ONU communicates using an optical signal with a different wavelength.

[0005] In a PtP WDM-PON system, an Auxiliary Management and Control Channel (AMCC) signal is specified as a control signal containing control information transmitted and received between an optical line terminal (OLT) and optical network units (ONUs). The AMCC signal is superimposed on a main signal containing transmission data modulated using a predetermined method. Because the AMCC signal is superimposed on the main signal, the wavelength range of the control signal falls within the wavelength range of the main signal. This allows a PtP WDM-PON system to manage and control communication devices such as an OLT and optical network units (ONUs) without using a wavelength range dedicated to control signals. In a PtP WDM-PON system, the wavelength determination process, which determines the wavelengths of optical signals in each of the upstream and downstream directions, is performed using an AMCC signal.

[0006] FIG. 11 is a diagram illustrating an example of a PtP WDM-PON system. In FIG. 11, the PtP WDM-PON system includes multiple "PtP WDM OLTs," a wavelength multiplexing / demultiplexing unit, and multiple ONUs. The ONUs include an ONU management and control unit, a transmitter, a receiver, and a multiplexing / demultiplexing unit. The PtP WDM OLT includes an OLT management and control unit, a transmitter, a receiver, and a multiplexing / demultiplexing unit. The AMCC signal is superimposed on the main signal at the optical signal stage. The AMCC signal is separated from the main signal at the electrical signal stage.

[0007] 12 is a diagram showing an example of the optical power of an optical signal on which a control signal (AMCC signal) is superimposed. The AMCC signal is transmitted, for example, from an ONU or an OLT. The control signal is superimposed on the transmitted optical signal, and intensity modulation is performed on the envelope of the optical power of the main signal on which the control signal is superimposed. The data rate of the main signal is high (Gb / s). In contrast, the data rate of the control signal is low (kb / s) (see Non-Patent Document 2). [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] ITU-T G.989.2 Recommendation, “40-Gigabit-capable-passive optical networks (NG-PON2): Physical media dependent (PMD) layer specification,” Feb. 2019. [Non-patent document 2] Y. Luo, H. Roberts, K. Grobe, M. Valvo, D. Nesset, K. Asaka, H. Rohde, J. Smith, JS Wey, and F. Effenberger, “Physical Layer Aspects of NG-PON2 Standards-Part 2: System Design and Technology Feasibility,” J. Opt. Commun. Netw., 8(1), pp.43-52, Jan. 2016. Summary of the Invention [Problem to be solved by the invention]

[0009] In this way, the data rate of the AMCC signal is slower than that of the main signal. In communication systems such as PtP WDM-PON systems, such AMCC signals are superimposed on the main signal, making it possible to transmit and receive control information without relying on protocols.

[0010] Fig. 13 is a diagram showing a first example of the configuration of a communication system. In Fig. 13, communication devices (master station and slave stations) that receive a main signal transmitted using an optical signal acquire an AMCC signal superimposed on the main signal. Fig. 14 is a diagram showing a second example of the configuration of a communication system. In Fig. 14, a communication device (master station that manages the network) that is different from the communication devices (slave stations) that receive the main signal transmitted using an optical signal acquires the AMCC signal superimposed on the main signal.

[0011] The communication system includes a branching device along the optical signal path. The communication devices (master station and slave station) that acquire the AMCC signal include an AMCC signal acquisition unit. The branching device branches the optical signal along the optical signal path to a transmitter / receiver and an AMCC signal acquisition unit. The AMCC signal acquisition unit acquires one of the branched AMCC signals.

[0012] However, branching the optical signal attenuates the optical power of the main signal, shortening the transmission distance of the main signal. Furthermore, because the AMCC signal superimposed on the main signal acts as noise for the main signal, the higher the superimposition ratio of the AMCC signal's optical power to the main signal's optical power, the more the signal quality of the main signal deteriorates, shortening the transmission distance of the main signal. These problems pose a problem: while enabling reception of the control signal, it is not possible to maximize the amount of light (power budget) available for the main signal superimposed with the control signal.

[0013] In view of the above circumstances, the present invention aims to provide a communication device, a communication system, and a communication method that enable reception of a control signal while maximizing the amount of light available for a main signal on which the control signal is superimposed. [Means for solving the problem]

[0014] One aspect of the present invention is a communication device comprising: a variable branching unit that acquires, from another communication device, an optical signal corresponding to a main signal on which a control signal is superimposed at a notified superposition ratio, and branches the main signal into a first branch signal and a second branch signal according to a specified branching ratio; an intensity measurement unit that measures the optical power of the control signal in the second branch signal; and a control unit that selects, from among candidate combinations of the superposition ratio and the branching ratio, a combination that minimizes the total main signal loss due to superposition and branching, based on a minimum receiving sensitivity of the optical power of the control signal and the measurement result of the optical power of the control signal, notifies the other communication device of the superposition ratio of the selected combination, and specifies the branching ratio of the selected combination to the variable branching unit.

[0015] One aspect of the present invention is a communication system comprising a first communication device and a second communication device, wherein the first communication device comprises a transmitter that superimposes a control signal onto a main signal at a notified superposition ratio and transmits an optical signal corresponding to the main signal on which the control signal is superimposed, and the second communication device comprises a variable branching unit that acquires the optical signal from the transmitter and branches the main signal into a first branched signal and a second branched signal according to a specified branching ratio, an intensity measurement unit that measures the optical power of the control signal in the second branched signal, and a control unit that selects, from among candidate combinations of the superposition ratio and the branching ratio, a combination that minimizes the total main signal loss due to superposition and branching based on the minimum receiving sensitivity of the optical power of the control signal and the measurement result of the optical power of the control signal, notifies the first communication device of the superposition ratio of the selected combination, and specifies the branching ratio of the selected combination to the variable branching unit.

[0016] One aspect of the present invention is a communication method executed by a communication device, the communication method including the steps of: acquiring, from another communication device, an optical signal corresponding to a main signal on which a control signal has been superimposed at a notified superposition ratio; branching the main signal into a first branch signal and a second branch signal according to a specified branching ratio; measuring the optical power of the control signal in the second branch signal; selecting, from among candidate combinations of the superposition ratio and the branching ratio, a combination that minimizes the total main signal loss due to superposition and branching based on a minimum receiving sensitivity of the optical power of the control signal and a measurement result of the optical power of the control signal; notifying the other communication device of the superposition ratio of the selected combination; and specifying the branching ratio of the selected combination to a variable branching unit that branches the main signal into the first branch signal and the second branching signal. [Effects of the Invention]

[0017] According to the present invention, it is possible to maximize the amount of light available for the main signal on which the control signal is superimposed, while still enabling reception of the control signal. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of a communication system in a first embodiment. [Figure 2] FIG. 2 is a diagram illustrating a configuration example of a variable branching unit in the first embodiment. [Figure 3] FIG. 10 is a diagram showing an example of a data table for "branching ratio=1.0" in the first embodiment. [Figure 4] FIG. 10 is a diagram showing an example of a data table for "branching ratio=0.9" in the first embodiment. [Figure 5] FIG. 10 is a diagram showing an example of a data table for "branching ratio=0.8" in the first embodiment. [Figure 6] 4 is a flowchart showing an example of the operation of the communication system in the first embodiment. [Figure 7] FIG. 10 is a diagram illustrating an example of the configuration of a communication system according to a modified example of the first embodiment. [Figure 8] FIG. 10 is a diagram showing an example of a data table for "branching ratio=1.0" in a modified example of the first embodiment. [Figure 9] FIG. 10 is a diagram illustrating an example of the configuration of a communication system in a second embodiment. [Figure 10] FIG. 2 is a diagram illustrating an example of a hardware configuration of a communication device in each embodiment. [Figure 11] FIG. 1 illustrates an example of a PtP WDM-PON system. [Figure 12] 10A and 10B are diagrams illustrating examples of optical power of an optical signal on which a control signal is superimposed. [Figure 13] FIG. 1 is a diagram illustrating a first example of the configuration of a communication system. [Figure 14] FIG. 10 is a diagram illustrating a second example of the configuration of a communication system. DETAILED DESCRIPTION OF THE INVENTION

[0019] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the drawings. (First embodiment) FIG. 1 is a diagram showing an example of the configuration of a communication system 1a in the first embodiment. The communication system 1a is a system (optical communication system) that communicates using optical signals. The optical signals are used to transmit main signals and control signals. The control signals are, for example, AMCC signals. The data rate of the control signals is slower than the data rate of the main signals. At the stage of the transmitted optical signal, the control signals are superimposed on the main signals at a predetermined superimposition ratio. The superimposition ratio is, for example, the ratio (proportion) of the optical power of the control signals to the optical power of the optical signal. Furthermore, at the stage of the electrical signals converted from the received optical signals, the control signals are separated from the electrical signals including the main signals and the control signals.

[0020] The communication system 1a includes a master station 2a and a slave station 3a. The master station 2a faces the slave station 3a. The master station 2a is, for example, an optical line terminal (OLT). The slave station 3a is, for example, an optical line termination (ONU). The communication system 1a may also include another slave station (not shown) facing the slave station 3a.

[0021] The master station 2 a includes a master station control unit 20 , a transmitter / receiver unit 21 a , a signal circulator 22 , a variable branching unit 23 , a control signal acquisition unit 24 , and an intensity measurement unit 25 .

[0022] The slave station 3 a includes a signal circulator 30 , a variable branching unit 31 , a transmitter / receiver 32 a , a control signal acquirer 33 , an intensity measuring unit 34 , and a slave station controller 35 .

[0023] Hereinafter, the value where "optical power of optical signal branched to the transmitter / receiver" is the numerator and the sum of "optical power of optical signal branched to the transmitter / receiver" and "optical power of optical signal branched to the control signal acquirer" is the denominator is referred to as "branching ratio." For example, if "optical power of optical signal branched to the transmitter / receiver:optical power of optical signal branched to the control signal acquirer" is "10:0," the branching ratio is expressed as "1.0" or "10:0." For example, if "optical power of optical signal branched to the transmitter / receiver:optical power of optical signal branched to the control signal acquirer" is "9:1," the branching ratio is expressed as "0.9" or "9:1." For example, if "optical power of optical signal branched to the transmitter / receiver:optical power of optical signal branched to the control signal acquirer" is "0:10," the branching ratio is expressed as "0.0" or "0:10."

[0024] First, the master station 2a will be described. The master station control unit 20 (control unit) includes a data table. The master station control unit 20 controls the operation of each functional unit of the master station 2a. For example, during the initial setting stage before the master station 2a starts operating, the master station control unit 20 sets the branching ratio of the upstream optical signal transmitted in the variable branching unit 23 to "0.0." As a result, almost all of the optical power of the upstream optical signal received by the signal circulator 22 is input to the control signal acquisition unit 24 via the variable branching unit 23. Here, the master station control unit 20 acquires the measurement result of the optical power of the control signal in the control signal acquisition unit 24 from the intensity measurement unit 25. Because almost all of the optical power of the upstream optical signal received by the signal circulator 22 is input to the control signal acquisition unit 24, this measurement result represents the upper limit (threshold) of the optical power of the control signal.

[0025] The parent station control unit 20 stores in advance a data table for each branching ratio regarding the optical power and loss of the optical signal. The details of the data table for each branching ratio will be described later. Based on the optical power of the optical signal in the control signal acquisition unit 24, the parent station control unit 20 uses the data table for each branching ratio to select, for the upstream transmitted optical signal, a combination of branching ratio and superposition ratio that minimizes the total main signal loss due to superposition and branching.

[0026] Based on the selected combination, the parent station control unit 20 notifies the child station control unit 35 of a request signal for the superposition ratio of the control signal in the optical signal to be transmitted upstream. In addition, the parent station control unit 20 controls the operation of the variable branching unit 23 so that the variable branching unit 23 branches the optical signal at the branching ratio in the selected combination.

[0027] The master station control unit 20 acquires a request signal for the superposition ratio of the control signal in the optical signal to be transmitted downstream from the slave station control unit 35. For example, the master station control unit 20 acquires the request signal for the superposition ratio of the control signal in the optical signal to be transmitted downstream from the slave station control unit 35 at the stage of initial setup before the master station 2a starts operating. Based on the request signal for the superposition ratio acquired from the slave station control unit 35, the master station control unit 20 controls the superposition ratio of the control signal in the optical signal to be transmitted downstream from the transceiver unit 21a.

[0028] When the master station 2a is in operation (when initial settings have been completed), the master station control unit 20 acquires the main signal transmitted upstream from the slave station 3a from the transceiver unit 21a. The master station control unit 20 acquires the control signal transmitted upstream from the slave station 3a from the control signal acquisition unit 24. Here, the master station control unit 20 may acquire the control signal from the control signal acquisition unit 24 via the transceiver unit 21a. Note that the master station control unit 20 may also acquire the main signal transmitted upstream from the slave station 3a at the initial setting stage before the master station 2a starts operating.

[0029] Transmitting and receiving unit 21a (TR x ) (master station transmitter) (master station receiver) acquires the optical signal branched to the transmitter / receiver 21a from the variable brancher 23. The transmitter / receiver 21a converts the acquired optical signal into an electrical signal. The transmitter / receiver 21a outputs the main signal (electrical signal) to the master station controller 20.

[0030] The transmitter / receiver 21a superimposes a control signal on the main signal of the optical signal to be transmitted downstream in accordance with the control of the parent station control unit 20. That is, the transmitter / receiver 21a superimposes a control signal on the main signal of the optical signal to be transmitted downstream at a superimposition ratio requested by the child station control unit 35. The transmitter / receiver 21a outputs an optical signal corresponding to the main signal on which the control signal has been superimposed to the signal circulator 22.

[0031] The signal circulator 22 is, for example, a circulator or an upstream / downstream separation filter. The signal circulator 22 outputs the optical signal transmitted upstream from the slave station 3a to the variable splitter 23. The signal circulator 22 transmits the optical signal transmitted downstream from the transceiver 21a downstream to the slave station 3a.

[0032] The variable branching unit 23 branches the optical signal input from the signal circulator 22 to the transmitter / receiver 21a and the control signal acquiring unit 24 at a variable branching ratio according to the control of the parent station control unit 20. The variable branching unit 23 is, for example, an evanescent coupling type optical coupler or a planar lightwave circuit (PLC).

[0033] The control signal acquirer 24 acquires the optical signal branched to the control signal acquirer 24 from the variable brancher 23. The control signal acquirer 24 has a predetermined minimum receiving sensitivity for the optical power of the control signal. The control signal acquirer 24 converts the optical signal acquired from the variable brancher 23 into an electrical signal. The control signal acquirer 24 outputs the control signal (electrical signal) transmitted upstream to the parent station control unit 20. The control signal acquirer 24 may output the control signal to the parent station control unit 20 via the transmitter / receiver 21a.

[0034] The intensity measurement unit 25 measures the optical power of the control signal in the control signal acquisition unit 24. The intensity measurement unit 25 outputs the measurement result of the optical power of the control signal to the parent station control unit 20.

[0035] Next, the slave station 3a will be described. The signal circulator 30 is, for example, a circulator or an upstream / downstream separation filter. The signal circulator 30 outputs the optical signal transmitted downstream from the master station 2a to the variable splitter 31. The signal circulator 30 outputs the optical signal transmitted upstream from the transmitter / receiver 32a to the master station 2a.

[0036] The variable splitter 31 splits the optical signal input from the signal circulator 30 to the transceiver 32a and the control signal acquirer 33 at a variable split ratio in response to control by the slave station controller 35. That is, the variable splitter 31 outputs a first split signal of the optical signal input from the signal circulator 30 to the transceiver 32a at a variable split ratio in response to control by the slave station controller 35. The variable splitter 31 outputs a second split signal of the optical signal input from the signal circulator 30 to the control signal acquirer 33 at a variable split ratio in response to control by the slave station controller 35. The variable splitter 31 is, for example, an evanescent coupling type optical coupler or a planar optical circuit.

[0037] Transmitting and receiving unit 32a (TR x ) (slave station transmitter) (slave station receiver) acquires the optical signal branched to the transmitter / receiver 32a from the variable brancher 31. The transmitter / receiver 32a converts the acquired optical signal into an electrical signal. The transmitter / receiver 32a outputs the main signal (electrical signal) to the slave station controller 35.

[0038] The transceiver 32a superimposes a control signal on the main signal of the optical signal to be transmitted upstream in accordance with the control of the slave station controller 35. That is, the transceiver 32a superimposes a control signal on the main signal of the optical signal to be transmitted upstream at a superimposition ratio requested by the master station controller 20. The transceiver 32a outputs an optical signal corresponding to the main signal on which the control signal has been superimposed to the signal circulator 30.

[0039] The control signal acquirer 33 acquires the optical signal branched to the control signal acquirer 33 from the variable brancher 23. The control signal acquirer 33 converts the optical signal acquired from the variable brancher 31 into an electrical signal. The control signal acquirer 33 outputs the control signal (electrical signal) transmitted downstream to the slave station control unit 35. The control signal acquirer 33 may output the control signal to the slave station control unit 35 via the transmitter / receiver 32a.

[0040] The intensity measurement unit measures the optical power of the control signal in the control signal acquisition unit 33. The intensity measurement unit outputs the measurement result of the optical power of the control signal to the slave station control unit .

[0041] The slave station control unit 35 (control unit) includes a data table. The slave station control unit 35 controls the operation of each functional unit of the slave station 3a. For example, during the initial setting stage before the slave station 3a starts operating, the slave station control unit 35 sets the branching ratio of the downstream optical signal transmitted in the variable branching unit 31 to "0.0." As a result, almost all of the optical power of the downstream optical signal received by the signal circulator 30 is input to the control signal acquiring unit 33 via the variable branching unit 31. Here, the slave station control unit 35 acquires the measurement result of the optical power of the control signal in the control signal acquiring unit 33 from the intensity measuring unit 34. Because almost all of the optical power of the downstream optical signal received by the signal circulator 30 is input to the control signal acquiring unit 33, this measurement result represents the upper limit (threshold) of the optical power of the control signal.

[0042] The slave station control unit 35 stores in advance a data table for each branching ratio regarding the optical power and loss of the optical signal. The details of the data table for each branching ratio will be described later. Based on the optical power of the optical signal in the control signal acquisition unit 33, the slave station control unit 35 uses the data table for each branching ratio to select, for the downstream transmitted optical signal, a combination of branching ratio and superposition ratio that minimizes the total main signal loss due to superposition and branching.

[0043] Based on the selected combination, the slave station control unit 35 notifies the master station control unit 20 of a request signal for the superposition ratio of the control signal in the optical signal to be transmitted downstream. In addition, the slave station control unit 35 controls the operation of the variable splitter 31 so that the variable splitter 31 splits the optical signal at the splitting ratio in the selected combination.

[0044] The slave station control unit 35 acquires a request signal for the superposition ratio of the control signal in the optical signal to be transmitted upstream from the master station control unit 20. For example, the slave station control unit 35 acquires the request signal for the superposition ratio of the control signal in the optical signal to be transmitted upstream from the master station control unit 20 at the initial setting stage before the operation of the slave station 3a starts. The slave station control unit 35 controls the superposition ratio of the control signal in the optical signal to be transmitted upstream from the transceiver unit 32a based on the request signal for the superposition ratio acquired from the slave station control unit 35.

[0045] At the operation stage of the slave station 3a (stage where initial settings are completed), the slave station control unit 35 acquires the main signal transmitted downstream from the master station 2a from the transceiver unit 32a. The slave station control unit 35 acquires the control signal transmitted downstream from the master station 2a from the control signal acquisition unit 33. Here, the slave station control unit 35 may acquire the control signal from the control signal acquisition unit 33 via the transceiver unit 32a. Note that at the initial setting stage before the operation of the slave station 3a starts, the slave station control unit 35 may acquire the main signal transmitted downstream from the master station 2a.

[0046] 2 is a diagram showing an example of the configuration of the variable branching device 31 in the first embodiment. The configuration of the variable branching device 31 is not limited to a specific configuration as long as it is capable of branching an optical signal at a variable branching ratio. The configuration of the variable branching device 23 may be the same as the configuration of the variable branching device 31.

[0047] 2, the variable splitter 31 includes, for example, a base 310-1 and a base 310-2. The base 310-1 includes an optical fiber 311-1 as a path for an optical signal branched to the transceiver 32a. The base 310-2 includes an optical fiber 311-2 as a path for an optical signal branched to the control signal acquirer 33.

[0048] When the distance "L" between the core of the optical fiber 311-1 and the core of the optical fiber 311-2 becomes equal to or less than a predetermined distance, the optical fiber 311-1 comes into contact with the optical fiber 311-2. For example, a part of the optical signal propagating through the optical fiber 311-1 is branched to the optical fiber 311-2 at a branching ratio according to the length of the contact portion between the optical fibers 311-1.

[0049] The variable splitter 31 adjusts the branching ratio of the optical signal of the optical fiber 311-1 by changing the distance "L" between the core of the optical fiber 311-1 and the core of the optical fiber 311-2 under the control of the slave station control unit 35. The slave station control unit 35 moves the platform 310-2 closer to the platform 310-1 so that the distance "L" between the cores becomes a predetermined distance (a distance corresponding to the derived branching ratio). The slave station control unit 35 may move the platform 310-2 in the y-axis direction or may move the platform 310-2 in the z-axis direction.

[0050] Next, a data table for each branching ratio will be described. Each measurement value in the data table is measured in advance. The branching ratio is arbitrary and need not be limited to the following examples of "branching ratio = 1.0", "branching ratio = 0.9", and "branching ratio = 0.8". Similarly, the superposition ratio is also arbitrary. Furthermore, the units of loss, sensitivity, and optical power are arbitrary. The units of loss, sensitivity, and optical power may be expressed logarithmically, for example, "dB" or "dBm".

[0051] Fig. 3 is a diagram showing an example of a data table for "branching ratio = 1.0" in the first embodiment. Fig. 4 is a diagram showing an example of a data table for "branching ratio = 0.9" in the first embodiment. Fig. 5 is a diagram showing an example of a data table for "branching ratio = 0.8" in the first embodiment.

[0052] In the data table for each branching ratio, the superposition ratio, the main signal loss due to superposition (the penalty to the quality of the main signal), the sum of the main signal losses due to superposition and branching, the minimum receiving sensitivity of the optical power of the control signal, the control signal loss due to branching, and the optical power of the control signal before branching are pre-associated.

[0053] Next, a method for selecting a combination of superposition ratios and branching ratios will be described. Here, since the selection method is the same for upstream and downstream, a method for selecting a combination of superposition ratios and branching ratios of optical signals will be described for downstream as an example.

[0054] The slave station control unit 35 sets the branching ratio of the optical signal in the variable branching unit 31 to "0.0." As a result, all of the optical power of the optical signal received downstream by the signal circulator 30 is input to the control signal acquisition unit 33 via the variable branching unit 31. Here, the intensity measurement unit 34 measures the optical power of the control signal in the control signal acquisition unit 33. Since all of the optical power of the optical signal received downstream by the signal circulator 30 is input to the control signal acquisition unit 33, this measurement result represents the upper limit (threshold) of the optical power of the control signal. In the following, the upper limit of the optical power of the control signal is "-13.00," as an example.

[0055] Therefore, the slave station control unit 35 selects a row (range) in which the optical power of the control signal before branching is less than "-13.00" from the data table for each branching ratio. For example, in the data table illustrated in Fig. 4, the slave station control unit 35 selects the row in which the "optical power of the optical signal received by the transmitter / receiver" is "-14.00" as a candidate combination. For example, in the data table illustrated in Fig. 5, the slave station control unit 35 selects the rows in which the "optical power of the optical signal received by the transmitter / receiver" is "-15.01," "-16.01," and "-17.01" as candidate combinations.

[0056] The slave station control unit 35 selects from among the candidates a combination of branching ratio and superposition ratio that minimizes the total "A+B" of main signal losses due to superposition and branching, while satisfying the condition that the sum of "D" control signal loss due to branching and the minimum receiving sensitivity "C" is the optical power "C+D" of the control signal before branching.

[0057] 3, 4, and 5, the slave station control unit 35 selects a combination of a superposition ratio of 5% and a branching ratio of 0.8 so that the total main signal loss due to superposition and branching is the minimum value of 1.97. That is, the slave station control unit 35 selects the superposition ratio M=5% in the data table for the branching ratio of 0.8.

[0058] The slave station control unit 35 outputs a request signal indicating the superposition ratio "M=5%" to the transceiver unit 32a. The transceiver unit 32a transmits the request signal indicating the superposition ratio to the master station 2a. The slave station control unit 35 sets the branching ratio of the downstream transmitted optical signal in the variable branching unit 31 to "0.8". The variable branching unit 31 branches the optical signal input from the signal circulator 30 to the transceiver unit 32a and the control signal acquisition unit 33 at a branching ratio of "0.8" that is variable according to the control of the slave station control unit 35. This makes it possible to receive the downstream transmitted control signal and maximize the amount of light (power budget) available for the main signal on which the control signal is superimposed.

[0059] Next, an example of the operation of the communication system 1a will be described. 6 is a flowchart showing an example of operation of the communication system 1a in the first embodiment. The variable splitter 31 obtains an optical signal corresponding to a main signal on which a control signal is superimposed at a superimposition ratio notified to the parent station control unit 20 from the transceiver unit 21a via the signal circulator 22 and the signal circulator 30 (step S101).

[0060] The variable branching unit 31 branches the main signal into a first branch signal and a second branch signal in accordance with a branching ratio specified by the slave station control unit 35 (step S102). The intensity measurement unit 34 measures the optical power of the control signal in the second branch signal input to the control signal acquisition unit 33 (step S103).

[0061] The slave station control unit 35 selects, from among candidate combinations of superposition ratios and branching ratios, a combination that minimizes the total main signal loss due to superposition and branching, based on the minimum receiving sensitivity of the optical power of the control signal and the measurement result of the optical power of the control signal (step S104). The slave station control unit 35 notifies the parent station control unit 20 of the superposition ratio of the selected combination (step S105). The slave station control unit 35 specifies the branching ratio of the selected combination to the variable branching unit 31 (step S106).

[0062] The transmission direction of the control signal is not limited to either uplink or downlink. That is, the control signal may be transmitted from the master station to the slave station, or from the slave station to the master station. This bidirectional transmission of the control signal is common to all the embodiments and their modifications. For example, in the flowchart illustrated in FIG. 6, the same steps can be executed even if the terms "master station" and "slave station" are interchanged.

[0063] Since control signals can be transmitted in both directions, the master station 2a may be referred to as a "first communication device" or "other communication device," and the slave station 3a may be referred to as a "second communication device" or "own communication device." The master station 2a may be referred to as a "second communication device" or "own communication device," and the slave station 3a may be referred to as a "first communication device" or "other communication device."

[0064] As described above, the master station 2a superimposes the control signal onto the main signal at the superimposition ratio notified by the slave station 3a. The variable splitter 31 receives from the master station 2a an optical signal corresponding to the main signal on which the control signal is superimposed. The variable splitter 31 splits the main signal into a first split signal and a second split signal according to the specified split ratio. The intensity measurement unit 34 measures the optical power of the control signal in the second split signal. Based on the minimum receiving sensitivity of the optical power of the control signal and the measurement results of the optical power of the control signal, the slave station control unit 35 selects a combination of superimposition ratios and splitting ratios from among candidate combinations (data table) that minimizes the total main signal loss due to superimposition and splitting. The slave station control unit 35 notifies the master station control unit 20 of the superimposition ratio of the selected combination. The slave station control unit 35 specifies the splitting ratio of the selected combination to the variable splitter 31.

[0065] In this way, a low-data-rate control signal is superimposed on a high-data-rate main signal, and the main signal and the control signal are transmitted using an optical signal. In such a communication system, the superimposition ratio of the main signal and the control signal in the optical signal and the branching ratio of the splitter that branches off a part of the optical signal along the path are variable depending on the transmission distance of the low-data-rate control signal. The communication system 1a performs communication based on the combination of the superimposition ratio and the branching ratio that minimizes the total value of the penalty of the main signal caused by the superimposition of the control signal and the branching loss within the range in which the low-data-rate control signal can be received.

[0066] This makes it possible to maximize the amount of light (power budget) available for the main signal on which the control signal is superimposed, while still allowing for the reception of the control signal. It also makes it possible to extend the transmission distance of the optical signal. By taking into account the loss of each wavelength of the optical signal in the optical component, it is possible to maximize the budget according to the wavelength.

[0067] At the time of initial connection, the opposing communication devices communicate using AMCC signals, which allows communication setup regardless of the type of main signal. If the types of main signals are unified in the opposing communication devices, the opposing communication devices may communicate using the main signal.

[0068] In the above example, for example, in the initial setting stage before the operation of the slave station 3a is started, the slave station control unit 35 sets the branching ratio of the downstream optical signal transmitted in the variable branching unit 31 to "0.0." The reason for setting the branching ratio to "0.0" is also for the sake of simplicity. For example, the slave station control unit 35 may acquire information on the branching ratio of the downstream optical signal transmitted in the variable branching unit 31, and then the intensity measurement unit 34 may measure the optical power of the control signal in the control signal acquisition unit 33.

[0069] Instead of having the intensity measurement unit 25 measure the optical power of the control signal received in the upstream direction by the control signal acquisition unit 24, the master station control unit 20 may acquire information on the optical power of the optical signal or control signal transmitted in the upstream direction from the transceiver unit 32a from the slave station control unit 35 using the transceiver unit 21a. The master station control unit 20 stores in advance information indicating the loss of optical power due to each optical component of the transmission path in the communication system 1a. The master station control unit 20 transmits information indicating the distance between a predetermined position and the master station 2a (hereinafter referred to as the "master station user distance") to the slave station control unit 35 using a control signal. The master station control unit 20 acquires from the slave station control unit 35 a control signal indicating the distance between the predetermined position and the slave station 3a (hereinafter referred to as the "slave station user distance"). The master station control unit 20 derives the transmission distance of the optical signal between the slave station 3a and the master station 2a based on the predetermined position, the master station user distance, and the slave station user distance. The master station control unit 20 derives the optical power loss of the optical signal or control signal due to the transmission path in the communication system 1a, based on the transmission distance of the optical signal between the slave station 3a and the master station 2a. The master station control unit 20 may derive the optical power of the control signal received upstream in the control signal acquisition unit 24, based on the optical power loss of the optical signal or control signal due to the transmission path in the communication system 1a and information on the optical power of the optical signal or control signal transmitted upstream from the transceiver 32a.

[0070] Instead of the intensity measurement unit 34 measuring the optical power of the control signal received downstream by the control signal acquisition unit 33, the slave station control unit 35 may acquire information on the optical power of the optical signal or control signal transmitted downstream from the transceiver unit 21a from the master station control unit 20 using the transceiver unit 32a. The slave station control unit 35 previously stores information indicating the optical power loss due to each optical component of the transmission path in the communication system 1a. The slave station control unit 35 transmits information indicating the inter-user distance of the slave station to the master station control unit 20 using a control signal. The slave station control unit 35 acquires a control signal indicating the inter-user distance of the master station from the master station control unit 20. The slave station control unit 35 derives the transmission distance of the optical signal between the slave station 3a and the master station 2a based on the predetermined position, the inter-user distance of the master station, and the inter-user distance of the slave station. The slave station control unit 35 derives the optical power loss of the optical signal or control signal due to the transmission path in the communication system 1a based on the transmission distance of the optical signal between the slave station 3a and the master station 2a. The slave station control unit 35 may derive the optical power of the control signal received downstream at the control signal acquisition unit 33 based on the loss of optical power of the optical signal or control signal due to the transmission path in the communication system 1a and information on the optical power of the optical signal or control signal transmitted downstream from the transceiver unit 21a.

[0071] (Modification of the first embodiment) In the modified example of the first embodiment, the main difference from the first embodiment is that the transmitter / receiver acquires not only the main signal but also the control signal. That is, the main difference from the first embodiment is that the transmitter / receiver is provided with a control signal acquisition unit. In the modified example of the first embodiment, the differences from the first embodiment will be mainly described.

[0072] FIG. 7 is a diagram showing an example of the configuration of a communication system 1b in a modified example of the first embodiment. The communication system 1b includes a master station 2b and a slave station 3b. The master station 2b includes a master station control unit 20, a transceiver unit 21b, a signal circulator 22, and an intensity measurement unit 25. The transceiver unit 21b is integrated with the control signal acquisition unit of the master station 2b. The slave station 3a includes a signal circulator 30, a transceiver unit 32b, an intensity measurement unit 34, and a slave station control unit 35. The transceiver unit 32b is integrated with the control signal acquisition unit of the slave station 3a. The transceiver unit 32b includes, for example, a low-pass filter. The transceiver unit 32b acquires (extracts) the control signal from the optical signal using the low-pass filter. The transceiver unit 32b acquires (extracts) the main signal from the optical signal without using this low-pass filter.

[0073] Since the transceiver 21b acquires (extracts) the main signal and control signal from the optical signal, the master station 2b does not include a variable splitter. Therefore, the master station control unit 20 does not need to control the split ratio of the optical signal transmitted upstream in the variable splitter. In a modification of the first embodiment, the split ratio is "1.0." All of the optical power of the optical signal received upstream by the signal circulator 22 is input to the transceiver 21b. The master station control unit 20 acquires the measurement result of the optical power of the control signal in the transceiver 21b from the intensity measurement unit 25.

[0074] Based on the optical power of the optical signal or control signal at the transceiver 21b, the master station control unit 20 uses a data table for a branching ratio of "1.0" to select, for the upstream transmitted optical signal, a combination of branching ratio and superposition ratio that minimizes the total main signal loss due to superposition and branching. Based on the selected combination, the master station control unit 20 notifies the slave station control unit 35 of a request signal for the superposition ratio of the control signal in the upstream transmitted optical signal. Furthermore, during the operation stage of the master station 2b (stage where initial settings have been completed), the master station control unit 20 acquires the main signal and control signal transmitted upstream from the slave station 3b from the transceiver 21b.

[0075] Transmitting / receiving unit 21b (TR x) acquires the optical signal received by the signal circulator 22 from the signal circulator 22. The transceiver 21b converts the acquired optical signal into an electrical signal. The transceiver 21b outputs the main signal and the control signal (electrical signal) to the parent station control unit 20.

[0076] The signal circulator 22 outputs the optical signal transmitted upstream from the remote station 3b to the transceiver 21b. The signal circulator 22 transmits the optical signal transmitted downstream from the transceiver 21b downstream to the remote station 3b. The intensity measurement unit 25 measures the optical power of the control signal in the transceiver 21b. The intensity measurement unit 25 outputs the measurement result of the optical power of the control signal to the parent station control unit 20.

[0077] The signal circulator 30 outputs the optical signal transmitted downstream from the master station 2b to the transceiver 32b, and transmits the optical signal transmitted upstream from the transceiver 32b downstream to the master station 2a.

[0078] Transmitting / receiving unit 32b (TR x ) acquires from the signal circulator 30 the optical signal received downstream by the signal circulator 30. The transceiver 32b converts the acquired optical signal into an electrical signal. The transceiver 32b outputs the main signal and the control signal (electrical signal) to the slave station control unit 35. The intensity measurement unit 34 measures the optical power of the control signal in the transceiver 32b. The intensity measurement unit 25 outputs the measurement result of the optical power of the control signal to the slave station control unit 35.

[0079] Since the transceiver 32b acquires (extracts) the main signal and control signal from the optical signal, the slave station 3b does not include a variable splitter. Therefore, the slave station control unit 35 does not need to control the split ratio of the downstream transmitted optical signal in the variable splitter. All of the optical power of the downstream received optical signal by the signal circulator 30 is input to the transceiver 32b. The slave station control unit 35 acquires the measurement result of the optical power of the control signal in the transceiver 32b from the intensity measurement unit 34.

[0080] The slave station control unit 35 uses a data table for a branching ratio of "1.0" based on the optical power of the optical signal or control signal in the transceiver 32b to select, for the downstream transmitted optical signal, a combination of branching ratio and superposition ratio that minimizes the total main signal loss due to superposition and branching. Based on the selected combination, the slave station control unit 35 notifies the parent station control unit 20 of a request signal for the superposition ratio of the control signal in the downstream transmitted optical signal. Furthermore, during the operation stage of the slave station 3b (stage where initial settings have been made), the slave station control unit 35 acquires the main signal and control signal downstream transmitted from the parent station 2b from the transceiver 32b.

[0081] 8 is a diagram showing an example of a data table for "branching ratio=1.0" in a modified example of the first embodiment. In the data table for each branching ratio, the superposition ratio, the main signal loss due to superposition (penalty on the quality of the main signal), the sum of the main signal losses due to superposition and branching, the minimum receiving sensitivity of the optical power of the control signal, the control signal loss due to branching, and the optical power of the control signal before branching are associated in advance.

[0082] Here, since the transceiver 32b acquires the main signal and the control signal from the optical signal and the branching ratio is 1.0, the control signal loss D due to branching is 0.00. Therefore, in the modification of the first embodiment, the minimum receiving sensitivity C of the optical power of the control signal is equal to the optical power C+D of the control signal before branching.

[0083] The slave station control unit 35 selects rows (ranges) in which the optical power of the control signal before branching is less than "-13.00" from the data table with a branching ratio of "1.0." For the data table illustrated in Fig. 8, the slave station control unit 35 selects rows in which the "optical power of the optical signal received by the transmitter / receiver" is "-22.00," "-23.00," and "-24.00" as combination candidates.

[0084] The slave station control unit 35 selects from among the candidates a combination of branching ratio and superposition ratio that minimizes the total "A+B" of main signal losses due to superposition and branching, while satisfying the condition that the sum of "D" control signal loss due to branching and the minimum receiving sensitivity "C" is the optical power "C+D" of the control signal before branching.

[0085] 8, the slave station control unit 35 selects a combination of a superposition ratio of 5% and a branching ratio of 1.0 so that the sum of the main signal losses due to superposition and branching is the minimum value of 1.00. That is, the slave station control unit 35 selects the superposition ratio M=5% in the data table for the branching ratio of 1.0.

[0086] As described above, the transceiver 21a may acquire not only the main signal but also the control signal from the optical signal. Also, the transceiver 32a may acquire not only the main signal but also the control signal from the optical signal. This makes it possible to miniaturize the communication device, enable reception of the control signal, and maximize the amount of light available for the main signal on which the control signal is superimposed. It also makes it possible to increase the transmission distance of the optical signal.

[0087] (Second embodiment) The second embodiment is different from the first embodiment mainly in that only the master station has a control signal acquisition unit and a data table. The second embodiment will be described focusing on the differences from the first embodiment.

[0088] 9 is a diagram showing an example of the configuration of a communication system 1c according to the second embodiment. The communication system 1c includes a master station 2c and multiple slave stations 3c. The communication system 1c includes the multiple master stations 2c, multiple optical switches 4, a signal circulator 26, a variable splitter 27, a signal circulator 28, and a wavelength multiplexer / demultiplexer 29 on the path of an optical signal between slave stations 3c-1 and 3c-2.

[0089] The slave station 3c-1 faces the slave station 3c-2. The optical switch 4-1 faces the optical switch 4-2. The signal circulator 26-1 faces the signal circulator 26-2. The variable branching device 27-1 faces the variable branching device 27-2. The signal circulator 28-1 faces the signal circulator 28-2. The wavelength multiplexing / demultiplexing unit 29-1 faces the wavelength multiplexing / demultiplexing unit 29-2.

[0090] The master station 2c includes a master station control unit 20, multiple transceivers 21c, and an intensity measurement unit 25. The transceiver 21c includes a control signal acquisition unit. The slave station 3c includes a signal circulator 30, a transceiver 32c, and a slave station control unit 35. The optical switch 4 includes multiple slave station ports 40 and multiple master station ports 41.

[0091] A method for selecting a combination of superposition ratio and branching ratio will be described. The transceiver 32c superimposes a control signal on the main signal of the optical signal to be transmitted in accordance with control by the slave station control unit 35. The transceiver 32c outputs an optical signal corresponding to the main signal on which the control signal is superimposed to the signal circulator 30. The signal circulator 30 transmits the optical signal transmitted from the transceiver 32c to the optical switch 4.

[0092] The optical switch 4 switches the path of the optical signal under the control of the parent station control unit 20. For example, in the initial setting stage before the operation of the child station 3c-1 begins, the optical switch 4-1 transmits the optical signal transmitted from the child station 3c-1 to the transceiver unit 21c-1 of the parent station 2c-1. The optical signal transmitted from the child station 3c-1 includes a control signal. Note that in the operation stage of the child station 3b (stage after initial setting), the optical switch 4-1 transmits the optical signal transmitted from the child station 3c-1 to the signal circulator 26-1.

[0093] The signal circulator 26 transmits the optical signal transmitted from the optical switch 4 to the variable branching unit 27. The signal circulator 26 transmits the optical signal transmitted from the signal circulator 28 to the optical switch 4. The variable branching unit 27 branches the optical signal transmitted from the signal circulator 26 to the master station 2c and the signal circulator 28 in accordance with the control of the master station control unit 20. The signal circulator 28 transmits the optical signal transmitted from the variable branching unit 27 to the wavelength multiplexing / demultiplexing unit 29. The signal circulator 28 transmits the optical signal transmitted from the wavelength multiplexing / demultiplexing unit 29 to the signal circulator 26.

[0094] The wavelength multiplexing / demultiplexing unit 29 multiplexes optical signals of multiple wavelengths. For example, the wavelength multiplexing / demultiplexing unit 29-1 multiplexes optical signals of multiple wavelengths transmitted from multiple signal circulators 28-1. The wavelength multiplexing / demultiplexing unit 29-1 transmits the multiplexed optical signal to the wavelength multiplexing / demultiplexing unit 29-2.

[0095] The wavelength multiplexing / demultiplexing unit 29 demultiplexes the multiplexed optical signal into optical signals of multiple wavelengths. For example, the wavelength multiplexing / demultiplexing unit 29-1 demultiplexes the multiplexed optical signal transmitted from the wavelength multiplexing / demultiplexing unit 29-2 into optical signals of multiple wavelengths. The wavelength multiplexing / demultiplexing unit 29-1 transmits the demultiplexed optical signals to multiple signal circulators 28-1.

[0096] The master station control unit 20 controls the operation of each functional unit of the master station 2c. For example, when the slave station 3c-1 transmits a control signal during the initial setting stage before the slave station 3c-1 starts operating, the master station control unit 20-1 connects the master station side port 41-1 of the optical switch 4-1 to the transmitter / receiver 21c-1. As a result, the optical signal transmitted from the slave station 3c-1 is input to the transmitter / receiver 21c-1 via the optical switch 4. Here, the master station control unit 20-1 obtains the measurement result of the optical power of the control signal in the transmitter / receiver 21c-1 from the intensity measurement unit 25-1.

[0097] The selection method in the second embodiment is similar to that in the first embodiment or the modified example of the first embodiment, but in the second embodiment, losses due to optical components in the path of the optical signal are taken into consideration in advance in each data table. The optical components are, for example, optical switches 4.

[0098] The parent station control unit 20 notifies the child station control unit 35 of the superposition ratio for the selected combination. The parent station control unit 20 notifies the optical switch 4 of the path of the optical signal. The parent station control unit 20 sets the branching ratio for the selected combination in the variable branching unit 27. The child station 3c-1 communicates with the opposing child station 3c-2 using an optical signal corresponding to the main signal on which the control signal is superimposed.

[0099] As described above, the fact that control signals can be transmitted bidirectionally is a common feature among the embodiments and their modifications. Since control signals can be transmitted bidirectionally, the master station 2c may be referred to as a "first communication device" or "other communication device," and the slave station 3c may be referred to as a "second communication device" or "own communication device." The master station 2c may be referred to as a "second communication device" or "own communication device," and the slave station 3c may be referred to as a "first communication device" or "other communication device."

[0100] As described above, only the master station 2c may be provided with a control signal acquisition unit (transmitter / receiver 21c) and a data table. This allows the reception of control signals and maximizes the amount of light available for the main signal on which the control signals are superimposed. It also makes it possible to increase the transmission distance of optical signals.

[0101] (Modification of the second embodiment) In the modified example of the second embodiment, one of the differences from the second embodiment is that only the slave station has a data table and only the master station has a control signal acquisition unit. The modified example of the second embodiment will be described focusing on the differences from the second embodiment.

[0102] During the initial setup stage before the slave station 3a begins operation, the slave station control unit 35 communicates with the master station control unit 20. For example, the slave station control unit 35 and the master station control unit 20 communicate information about the optical power of the received optical signal and the superposition ratio. Based on a data table for each branching ratio, the slave station control unit 35 derives a combination of branching ratios and superposition ratios that minimizes the total main signal loss due to superposition and branching. The slave station control unit 35 notifies the master station control unit 20 of a request signal for the branching ratio in the derived combination. The master station control unit 20 controls the operation of the variable branching unit 27 in the optical signal path of the slave station 3a so that the variable branching unit 27 branches the optical signal at the requested branching ratio.

[0103] As described above, only the master station 2c may have a control signal acquisition unit (transmitter / receiver 21c). Only the slave station 3c may have a data table. The master station 2c and the slave station 3c may communicate information about the optical power of the received optical signal and the superposition ratio. This makes it possible to receive the control signal and maximize the amount of light available for the main signal on which the control signal is superimposed. It also makes it possible to increase the transmission distance of the optical signal.

[0104] (Example of hardware configuration) FIG. 10 is a diagram illustrating an example of the hardware configuration of a communication device in each embodiment. The communication device 5 corresponds to at least one of the master station and slave station in each embodiment. The communication device 5 includes a processor 50. The processor 50, such as a CPU (Central Processing Unit), executes a program stored in a storage device 52 having a non-volatile recording medium (non-transitory recording medium) and a memory 51, thereby realizing the communication device 5 as software. The program may be recorded on a computer-readable non-transitory recording medium. The program may be a multi-threaded program. Examples of computer-readable non-transitory recording media include portable media such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), and a CD-ROM (Compact Disc Read Only Memory), and storage devices such as a hard disk built into a computer system. The communication unit 53 executes predetermined communication processing.

[0105] At least a part of each functional unit of the communication device 5 may be an analog circuit or a digital circuit. The signal processing device may be realized using hardware including an electronic circuit or circuitry using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), an FPGA (Field Programmable Gate Array), or the like.

[0106] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention. [Industrial Applicability]

[0107] The present invention is applicable to optical communication systems. [Explanation of symbols]

[0108] 1a, 1b, 1c...communication system, 2a, 2b, 2c...master station, 3a, 3b, 3c...slave station, 4...optical switch, 5...communication device, 20...master station control unit, 21a, 21b...transmitter / receiver, 22...signal circulator, 23...variable splitter, 24...control signal acquisition unit, 25...intensity measurement unit, 26...signal circulator, 27...variable splitter, 28...signal circulator, 29...wavelength multiplexing / demultiplexing unit, 30...signal circulator, 31...variable splitter, 32a, 32b...transmitter / receiver, 33...control signal acquisition unit, 34...intensity measurement unit, 35...slave station control unit, 40...slave station side port, 41...master station side port, 50...processor, 51...memory, 52...storage device, 53...communication unit, 100...main signal, 101...control signal, 310...unit, 311...optical fiber

Claims

1. a variable branching device that acquires, from another communication device, an optical signal corresponding to a main signal on which a control signal is superimposed at the notified superposition ratio, and branches the optical signal into a first branch signal and a second branch signal according to a specified branching ratio; an intensity measurement unit that measures the optical power of the control signal in the second branch signal; a control unit that selects, from among candidate combinations of the superposition ratio and the branching ratio, a combination that minimizes the total of main signal losses caused by superposition and branching, based on a minimum receiving sensitivity of the optical power of the control signal and a measurement result of the optical power of the control signal, notifies the other communication device of the superposition ratio of the selected combination, and specifies the branching ratio of the selected combination to the variable branching device; A communication device comprising:

2. The communication device according to claim 1 , wherein the control unit selects a combination in which the optical power of the control signal in the second branch signal is equal to or greater than the minimum receiving sensitivity and the total main signal loss is minimum.

3. A communication system comprising a first communication device and a second communication device, The first communication device a transmitter that superimposes a control signal on a main signal at the notified superimposition ratio and transmits an optical signal corresponding to the main signal on which the control signal is superimposed, the second communication device, a variable branching unit that receives the optical signal from the transmitting unit and branches the optical signal into a first branched signal and a second branched signal according to a specified branching ratio; an intensity measurement unit that measures the optical power of the control signal in the second branch signal; a control unit that selects, from among candidate combinations of the superposition ratio and the branching ratio, a combination that minimizes the total of main signal losses caused by superposition and branching, based on a minimum receiving sensitivity of the optical power of the control signal and a measurement result of the optical power of the control signal, notifies the first communication device of the superposition ratio of the selected combination, and specifies the branching ratio of the selected combination to the variable branching unit; A communication system comprising:

4. A communication method performed by a communication device, comprising: acquiring, from another communication device, an optical signal corresponding to a main signal on which a control signal has been superimposed at the notified superimposition ratio, and branching the optical signal into a first branch signal and a second branch signal in accordance with a specified branch ratio; measuring the optical power of the control signal in the second branch signal; selecting a combination of the superposition ratio and the branching ratio that minimizes the total of main signal losses due to superposition and branching from among candidate combinations of the superposition ratio and the branching ratio based on a minimum receiving sensitivity of the optical power of the control signal and a measurement result of the optical power of the control signal, notifying the other communication device of the superposition ratio of the selected combination, and specifying the branching ratio of the selected combination to a variable branching device that branches the optical signal into a first branched signal and a second branched signal; A communication method including:

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