Patent Publication Number: US-2021194597-A1

Title: Submarine optical communication system and submarine branching apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a submarine optical communication system and a submarine branching apparatus. 
     BACKGROUND ART 
     A submarine optical communication system that enables optical communication by connecting base stations on land via a submarine cable is being widely used. A submarine relay apparatus that amplifies an attenuation of a transmission signal, a submarine branching apparatus that enables optical communication among three or more base stations disposed on land, etc. are inserted in the submarine cable. 
     Generally, the submarine branching apparatus is inserted in-between a submarine cable (a trunk path) that connects two trunk stations, and a submarine cable that configures a branch path branched from the submarine branching apparatus is connected to a branch station. By this configuration, the optical signal transmitted from the trunk station is transmitted through the trunk path, the submarine branching apparatus, and the branch path and received by the branch station. 
     For this kind of submarine optical communication system, a system capable of handling failures that occur in the paths through which an optical signal is transmitted is proposed (Patent Literature 1). In this system, power is supplied to a submarine branching apparatus from a branch terminal (i.e., a branch station) through a branch path. When a failure occurs in the branch path, power supply from the branch terminal to the submarine branching apparatus stops, and when the power supply stops, an optical switch inside the submarine branching apparatus is switched so as to switch the output destination of the optical signal transmitted from one of the trunk stations from the branch path to the trunk path. By this configuration, even when a failure occurs in the branch path, it is possible to switch to an optical transmission between the trunk stations. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 5892950 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, it is conceivable to provide a failure detection mechanism to a branch station in order to detect a failure in a branch path. In this case, there is a problem in realizing switching of a transmission path of an optical signal in accordance with the failure detected by the branch station. 
     For example, in the system described above, the terminal (station) gives an instruction to the submarine branching apparatus to switch the path, however, when the branch station detects a failure in the branch path, it is not possible for the terminal (station) to give an instruction to the submarine branching apparatus to switch the path through the branch path. 
     The present disclosure has been made in view of the matters mentioned above and has an object to detect a failure in a branch path between a submarine branching apparatus and an optical transmission apparatus by the optical transmission apparatus and to switch a transmission path of an optical signal in accordance with the result of the detection. 
     Solution to Problem 
     A submarine optical communication system according to a first example aspect includes: a first optical transmission apparatus configured to output a first optical signal; a second optical transmission apparatus; a third optical transmission apparatus configured to output an optical signal and a monitoring signal; and a submarine branching apparatus including a first return unit configured to return the monitoring signal received from the third optical transmission apparatus and configured to switch an output destination of the first optical signal received from the first optical transmission apparatus to the second optical transmission apparatus or the third optical transmission apparatus, in which the third optical transmission apparatus is further configured to detect the monitoring signal returned from the first return unit and notify the first optical transmission apparatus of a result of the detection, and the first optical transmission apparatus is further configured to instruct the submarine branching apparatus to switch the output destination of the first optical signal in accordance with the notification. 
     A submarine branching apparatus according to a second example aspect includes: a first optical switch configured to receive a first optical signal from a first optical transmission apparatus; a second optical switch configured to output the received optical signal to a second optical transmission apparatus; a control unit configured to control the first and the second optical switches in accordance with an instruction from the first optical transmission apparatus; and a return unit configured to return a monitoring signal received from a third optical transmission apparatus to the third optical transmission apparatus, in which when an optical intensity of the monitoring signal received by the third optical transmission apparatus is smaller than a prescribed value, the control unit switches: an output destination of the first optical signal of the first optical switch from the third optical transmission apparatus to the second optical switch; and an input source of the optical signal received by the second optical switch from the third optical transmission apparatus to the first optical switch. 
     Advantageous Effects of Invention 
     According to the present disclosure, a failure in a branch path between a submarine branching apparatus and an optical transmission apparatus is detected by the optical transmission apparatus and a transmission path of an optical signal is switched in accordance with the result of the detection. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a basic configuration of a submarine optical communication system according to a first example embodiment; 
         FIG. 2  is a diagram showing a configuration of the submarine optical communication system according to the first example embodiment in more detail; 
         FIG. 3  is a diagram showing an example of a configuration of an optical transmission apparatus according to an example embodiment; 
         FIG. 4  is a diagram showing a transmission path of a main signal after switching to a trunk path in the submarine optical communication system according to the first example embodiment; 
         FIG. 5  is a diagram showing a modified example of the submarine optical communication system according to the first example embodiment; 
         FIG. 6  is a diagram showing a modified example of the submarine optical communication system according to the first example embodiment; 
         FIG. 7  is a diagram showing a configuration of a submarine optical communication system according to a second example embodiment; 
         FIG. 8  is a diagram showing a configuration of a submarine optical communication system according to a third example embodiment; 
         FIG. 9  is a diagram showing a modified example of the submarine optical communication system according to the third example embodiment; 
         FIG. 10  is a diagram showing a configuration of a submarine optical communication system according to a fourth example embodiment; and 
         FIG. 11  is a diagram showing a modified example of the submarine optical communication system according to the fourth example embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinbelow, example embodiments of the present disclosure are explained with reference to the drawings. The same symbols are assigned to the same elements throughout the drawings and duplicated explanations are omitted as necessary. 
     First Example Embodiment 
     A submarine optical communication system  100  according to a first example embodiment is explained.  FIG. 1  is a diagram showing a basic configuration of the submarine optical communication system  100  according to the first example embodiment. The submarine optical communication system  100  includes optical transmission apparatuses  1  to  3  and a submarine branching apparatus  4 . Note that here, the optical transmission apparatuses  1  to  3  are also referred to as the first to third optical transmission apparatuses, respectively. 
     Each of the optical transmission apparatuses  1  and  2  is provided within a trunk station. The optical transmission apparatus  1  and the submarine branching apparatus  4  are connected with each other by an optical fiber FT 1  incorporated within the submarine cable. The optical transmission apparatus  2  and the submarine branching apparatus  4  are connected with each other by an optical fiber FT 2  incorporated within the submarine cable. As described above, a trunk path TL to which a main signal that is an optical signal is transmitted is configured of the optical transmission apparatus  1 , the optical transmission apparatus  2 , the submarine branching apparatus  4 , and the optical fibers FT 1  and FT 2 . 
     The optical transmission apparatus  3  is provided within a branch station. The optical transmission apparatus  3  and the submarine branching apparatus  4  are connected with each other by an optical fiber FB 1  (also referred to as a first optical fiber) for downlink communication and an optical fiber FB 2  (also referred to as a second optical fiber) for uplink communication that are incorporated in the submarine cable. As described above, a branch path is configured of the optical transmission apparatus  3 , the submarine branching apparatus  4 , and the optical fibers FB 1  and FB 2 . 
     Next, a configuration of the submarine optical communication system  100  is described in more detail.  FIG. 2  is a diagram showing the submarine optical communication system  100  according to the first example embodiment. 
     The optical transmission apparatuses  1  and  2  are installed inside a trunk station T 1  and a trunk station T 2 , respectively. The optical transmission apparatus  3  is installed inside a branch station B 1 . 
     The optical transmission apparatus  1  installed in the trunk station T 1  includes an optical transmission/reception unit  11 , a notification signal reception unit  12 , and an instruction unit  13 . 
     The optical transmission/reception unit  11  outputs a main signal (also referred to as a first optical signal) to the submarine branching apparatus  4  as well as an instruction signal INS for instructing switching of a transmission path of the main signal in accordance with a request from the instruction unit  13  to the submarine branching apparatus  4 . Further, the optical transmission/reception unit  11  can receive the optical signal. 
     Here, a wavelength-division multiplexed signal that has been modulated by a modulation technique such as the DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) is used as the main signal. 
     The notification signal reception unit  12  receives a notification signal ALM from the optical transmission apparatus  3  and informs the instruction unit  13  of the result of the notification. The notification signal reception unit  12  may be configured of an EMS (Element Management System) provided to the optical transmission apparatus  1 . 
     The instruction unit  13  refers to the result of the notification received from the notification signal reception unit  12  and when there is a failure in the branch path, instructs the optical transmission/reception unit  11  to output the instruction signal INS for switching the transmission path of the main signal from the branch path to the trunk path. 
     The instruction signal INS may be superimposed on the main signal and output to the submarine branching apparatus  4 . The instruction signal INS may be a signal having a wavelength other than a wavelength used for the main signal. Further, the instruction signal INS may be superimposed on the main signal by performing full-wave modulation (for example, amplitude modulation) of the main signal. 
     Here, an example of a configuration of the optical transmission apparatus  1  is explained.  FIG. 3  shows an example of a configuration of the optical transmission apparatus  1  according to the example embodiment. As shown in  FIG. 3 , the optical transmission/reception unit  11  may be configured so as to have a plurality of optical transmitters/receivers (transponders)  14  connectable thereto. The optical transmitters/receivers  14  are manufactured by various venders and an arbitrary transponder which conforms to various standards may be used. Further, the optical transmission/reception unit  11  may be configured of an OCI (Open Cable Interface). 
     For example, the optical signals output from the plurality of optical transmitters/receivers  14  are multiplexed by a multiplexer  11 A of the optical transmission/reception part  11  to become a main signal which is output to a multiplexer  11 C. The instruction unit  13  controls a signal generation unit  11 B of the optical transmission/reception unit  11  whereby the instruction signal INS is output from the signal generation unit  11 B to the multiplexer  11 C. The main signal and the instruction signal INS are multiplexed by the multiplexer  11 C and output to the optical fiber FT 1 . 
     The optical transmission apparatus  2  installed in the trunk station T 2  can receive the main signal output from the optical transmission apparatus  1  and transmitted through the trunk path or the branch path. For example, the optical transmission apparatus  2  may include an optical transmission/reception unit that is the same as the optical transmission/reception unit  11  of the optical transmission reception apparatus  1 , or may have the same configuration as that of the optical transmission apparatus  1 . 
     The submarine branching apparatus  4  is an apparatus that is installed at the bottom of the sea for branching an optical signal that is transmitted through the submarine cable laid at the bottom of the sea. The submarine branching apparatus  4  includes a return unit  41 , optical switches  42  and  43 , and a control unit  44 . 
     The return unit  41  (also referred to as a first return unit) can selectively return a monitoring signal MON output from the optical transmission apparatus  3  to the optical transmission apparatus  3 . For example, the return unit  41  includes a filer  41 A and a coupler  41 B. The filter  41 A is a wavelength filter and selectively separates only a light having a wavelength of the monitoring signal MON. Therefore, the monitoring signal MON output from the optical transmission apparatus  3  to the optical fiber FB 2  is selectively separated by the filter  41 A, and the separated monitoring signal MON is output to the optical transmission apparatus  3  through the optical fiber FB 1  by the coupler  41 B. 
     The optical switch  42  (also referred to as a first optical switch) can switch an output path (output destination) of the main signal output from the optical transmission apparatus  1  between the branch path (i.e., a path through which the main signal is output to the optical fiber FB 1 ) and the trunk path (i.e., a path through which the main signal is output to the optical switch  43 ) in accordance with a control signal CON 1  provided from the control unit  44 . 
     The optical switch  43  (also referred to as a second optical switch) can switch an input path (an input destination) of the received optical signal to the branch path (i.e., a path through which the main signal is received via the optical fiber FB 2 ) or the trunk path (i.e., a path through which the main signal is received from the optical switch  42 ) in accordance with a control signal CON 2  provided from the control unit  44 . 
     The control unit  44  can receive the instruction signal INS input from the optical transmission apparatus  1  and controls switching between the optical switches  42  and  43  in accordance with the instruction signal. 
     The branch station B 1  provided to the optical transmission apparatus  3  includes an optical transmission/reception unit  31 , a detection unit  32 , and a notification unit  33 . 
     The optical transmission/reception unit  31  can receive the main signal output from the optical transmission apparatus  1  via the optical fiber FB 1  and the detection unit  32  and can output an optical signal having a prescribed wavelength to the submarine branching apparatus  4  via the optical fiber FB 2 . Further, the optical transmission/reception unit  31  can output the monitoring signal MON having a prescribed wavelength to the submarine branching apparatus  4  via the optical fiber FB 2 . 
     The monitoring signal MON may be superimposed on the main signal and output to the submarine branching apparatus  4 . The monitoring signal MON may be a signal having a wavelength other than a wavelength used for the main signal. For example, when the main signal, which is an object of reception by the optical transmission apparatus  3 , is a signal of the C-band (1530 nm to 1565 nm), a signal of the L-band (1565 nm to 1625 nm) may be used as the monitoring signal MON. 
     The detection unit  32  detects the monitoring signal MON output from the optical transmission/reception unit  31  to the submarine branching apparatus  4  and returned to the optical transmission apparatus  3  from the return unit  41  of the submarine branching apparatus  4  and outputs a detection signal DET that indicates the result of the detection. For example, the detection unit  32  detects an optical intensity of the monitoring signal MON and outputs a signal that indicates the optical intensity as the detection signal DET. 
     The detection unit  32  includes, for example, a filter  32 A and an optical reception unit  32 B. The filter  32 A is a wavelength filter and selectively separates only a light having a wavelength of the monitoring signal MON and transmits lights having other wavelengths. Therefore, the control signal MON returned from the return unit  41  of the submarine branching apparatus  4  is selectively separated by the filter  32 A and the separated monitoring signal MON is detected by the optical reception unit  32 B. The optical reception unit  32 B is configured using, for example, a photodiode, and can generate the detection signal DET by converting the monitoring signal MON into an electric signal. 
     The notification unit  33  outputs the notification signal ALM indicating whether or not the monitoring signal MON has been detected to the notification signal reception unit  12  of the optical transmission apparatus  1  in accordance with the detection signal DET. The notification unit  33  informs the notification signal reception unit  12  that a failure has occurred in the branch path configured of the optical fibers FB 1  and FB 2  through the notification signal ALM when the optical intensity of the monitoring signal MON is smaller than a prescribed threshold value. The notification unit  33  may be configured as an EMS (Element Management System) provided to the optical transmission apparatus  3 . 
     The notification signal ALM may be transmitted from the notification unit  33  to the notification signal reception unit  12  of the optical transmission apparatus  1  using either one of the radio communication or the cable communication, or may be transmitted, for example, via a network that is controlled by a UMS (Unified Management System). 
     As described above, according to the present configuration, it is possible to monitor the monitoring signal MON that is output from the optical transmission apparatus  3  and returned from the return unit  41  of the submarine branching apparatus  4  and to detect occurrence of a failure in the branch path by detecting a change in the monitoring signal MON. Further, it becomes possible for the optical transmission apparatus  1  to instruct the submarine branching apparatus  4  to switch the transmission path of the optical signal from the branch path to the trunk path by having the optical transmission apparatus  3  inform the notification signal reception unit  12  of the optical transmission apparatus  1  of the occurrence of the failure.  FIG. 4  shows the transmission path of the main signal after switching to the trunk path in the submarine optical communication system  100  according to the first example embodiment. 
     As described above, by detecting occurrence of a failure in the branch path by the optical transmission apparatus connected to the branch path and the branch station and giving an instruction to the submarine branching apparatus  4 , it is possible to switch the transmission path in an ideal manner so that the transmission of the main signal is not interrupted. 
     Further, according to the present configuration, it is possible to detect occurrence of a failure in the branch path as described above even in the configuration in which power is not supplied from the optical transmission apparatus of the branch path to the submarine branching apparatus. 
     Next, a modified example of the submarine optical communication system  100  is considered.  FIG. 5  is a diagram showing a modified example of the submarine optical communication system according to the first example embodiment. A submarine optical communication system  101  shown in  FIG. 5  has three submarine branching apparatuses inserted between the optical transmission apparatuses  1  and  2 , configuring three branching paths. 
     As shown in  FIG. 5 , the two submarine branching apparatuses  4  described above and another submarine branching apparatus  101 B are inserted on the trunk path from the optical transmission apparatus  1 . Each of the submarine branching apparatuses  4  is connected to the optical transmission apparatus  3  via the branch path. Each of the optical transmission apparatuses  3  can output the notification signal ALM to the optical transmission apparatus  1 . The submarine branching apparatus  101 B is connected to the optical transmission apparatus  101 A via the branch path. A repeater  101 C that amplifies the main signal is inserted in the branch path between the optical transmission apparatus  101 A and the submarine branching apparatus  101 B. Note that power is supplied from the optical transmission apparatus  101 A to the submarine branching apparatus  101 B and the repeater  101 C via a power supply line SUP. Note that there is no need to mention that a failure in the branch path can be detected as described above by having the submarine branching apparatus detect that the power supply has stopped when there is a submarine branching apparatus that has power supplied from the optical transmission apparatus as shown in  FIG. 5 . 
     As described above, the optical transmission apparatus  3  and the submarine branching apparatus  4  according to the first example embodiment can configure a submarine optical communication system along with another optical transmission apparatus of a general configuration and a submarine branching apparatus as shown in  FIG. 5 . That is, even when the repeater to which power is supplied from the optical transmission apparatus is not inserted in the branch path, it is possible to detect a failure in the branch path. Further, when the repeater is not inserted in the branch path, by selecting a wavelength at which the main signal transmitted through the branch path can be amplified by Raman amplification as the wavelength of the monitoring signal, it is possible to extend the length of the optical fiber of the branch path. 
     Next, other structural example of the submarine optical communication system  100  is explained.  FIG. 6  is a diagram showing a modified example of the submarine optical communication system according to the first example embodiment. In the explanation given above, the notification signal reception unit  12  and the notification unit  33  are described to be configurable as an EMS. Here, an example in which the EMS provided to the optical transmission apparatus performs communication via a cloud network is explained.  FIG. 6  is a diagram showing a modified example of the submarine optical communication system according to the first example embodiment. A submarine optical communication system  102  shown in  FIG. 6  includes the optical transmission apparatuses  1  to  3  including the EMS  1 A,  2 A, and  3 A, respectively. Each of the EMS  1 A,  2 A, and  3 A can communicate with each other via the cloud network that is controlled by the UMS  102 A. For example, the optical transmission apparatus  3  can transmit the notification signal ALM to the optical transmission apparatus  1  via the cloud network. 
     Further, a SDN (Software Defined Network) controller may be used in place of the UMS and the SDN controller may be set on the cloud. The SDN controller can control the transmission path and the wavelength of the optical signal of the submarine optical communication system in accordance with the state of the network or an instruction from an operator. Further, the SDN controller virtually manages the physical network topology and the resource of the submarine optical communication system and can realize sharing of the system by several users or operators. 
     As described above, it can be understood that according to the present configuration, the submarine optical communication system can be configured by using not only an exclusive line but also an exclusive or an open cloud network. 
     Second Example Embodiment 
     A submarine optical communication system  200  according to a second example embodiment is explained.  FIG. 7  is a diagram showing a configuration of the submarine optical communication system  200  according to the second example embodiment. The submarine optical communication system  200  has a configuration in which the optical transmission apparatus  3  of the submarine optical communication system  100  is replaced by an optical transmission apparatus  5 . 
     The optical transmission apparatus  5  has a configuration in which a return unit  51  (hereinafter also referred to as a second return unit) is added to the optical transmission apparatus  3 . Among optical signals that are made incident from the submarine branching apparatus  4  via the optical fiber FB 1  of the branch path (L+LT in  FIG. 7 ), the optical signal having a wavelength that is the object of reception by the optical transmission apparatus  5  (LT in  FIG. 7 ) passes through the return unit  51  and is made incident on the optical transmission/reception unit  31 , and the optical signal having a wavelength that is not the object of reception is returned from the return unit  51  and output to the optical switch  43  of the submarine branching apparatus  4  via the optical fiber FB 2  of the branch path. 
     By this configuration, the optical transmission apparatus  5  can receive only the optical signal having the wavelength which is the object of reception (LT in  FIG. 7 ) and the optical signals having other wavelengths (L in  FIG. 7 ) can be transmitted to the optical transmission apparatus  2  by being returned to the submarine branching apparatus  4  via the trunk path. In other words, the branch station receives only the optical signal having the wavelength which is the object of reception (LT in  FIG. 7 ) and the optical signals having other wavelengths (L in  FIG. 7 ) are substantially not branched and transmitted through the trunk path. 
     As described above, in the present configuration, the optical signals having the wavelengths other than the wavelength that is the object of reception at the branch station are also transmitted through the branch path temporarily. When a failure occurs in the branch path, the optical signals having the wavelengths other than the wavelength which is the object of reception at the branch station cannot be returned to the submarine branching apparatus  4  and thus, these optical signals cannot be output to the trunk path. 
     However, in the present configuration, when the failure in the branch path is detected, the transmission path of the optical signal can be switched to the trunk path whereby the optical signals having the wavelengths other than the wavelength which is the object of reception can be continuously transmitted through the trunk path. 
     Third Example Embodiment 
     A submarine optical communication system  300  according to a third example embodiment is explained.  FIG. 8  shows the submarine optical communication system  300  according to the third example embodiment. The submarine optical communication system  300  has a configuration in which the optical transmission apparatus  3  and the submarine branching apparatus  4  of the submarine optical communication system  100  are replaced by an optical transmission apparatus  6  and a submarine branching apparatus  7 , respectively. The submarine optical communication system  300  is configured so as to be capable of further transmitting the main signal (also referred to as a second optical signal) from the optical transmission apparatus  2  towards the optical transmission apparatus  1 . 
     The submarine optical branching apparatus  7  has a configuration in which a return unit  71  and optical switches  72  and  73  are added to the submarine branching apparatus  4 . 
     The control unit  44  controls the optical switches  42 ,  43 ,  72 , and  73  according to the control signals CON 1  to CON 4 , respectively. 
     The return unit  71  (also referred to as a third return unit) can selectively return the monitoring signal MON output from the optical transmission apparatus  6  to the optical transmission apparatus  6 . 
     The optical transmission apparatus  2  and the optical switch  72  are connected with each other by an optical fiber FT 3  of the trunk path. The optical switch  72  (also referred to as a third optical switch) can switch an output path (output destination) of the main signal received from the optical transmission apparatus  2  via the optical fiber FT 3  to the branch path (i.e., a path through which the main signal is output to the optical fiber FB 3 ) or the trunk path (i.e., a path through which the main signal is output to the optical switch  73 ) in accordance with the control signal CON 3 . 
     The optical switch  73  (also referred to as a fourth optical switch) can switch an input path (input destination) of the received optical signal output to the branch path (i.e., a path through which the main signal is received via the optical fiber FB 4 ) or the trunk path (i.e., a path through which the main signal is received from the optical switch  72  and output to the optical transmission apparatus  1  via the optical fiber FT 4 ) in accordance with the control signal CON 4 . 
     The optical transmission apparatus  6  has a configuration in which a return unit  61  is added to the optical transmission apparatus  3  and the detection unit  32  is replaced by a detection unit  62  having a similar configuration. 
     The return unit  61  (also referred to as a fourth return unit) is inserted between the optical fiber FB 1  of the branch path (the first branch path) on the return unit  41  side and the optical transmission/reception unit  31  and between the optical fiber FB 4  of the branch path (the second branch path) on the return unit  71  side and the optical transmission/reception unit  31 , and selectively returns only the monitoring signal MON made incident from the optical fiber FB 1  to the optical fiber FB 4 . For example, the return unit  61  includes a filter  61 A and a coupler  61 B. The filter  61 A is a wavelength filter and selectively separates only a light having a wavelength of the monitoring signal MON. The separated monitoring signal MON is output to the optical fiber FB 4  by the coupler  61 B. 
     The detection unit  62  detects the monitoring signal MON made incident from the optical fiber FB 3  (also referred to as a third optical fiber) and outputs the detection signal DET that indicates the result of the detection to the notification unit  33 . 
     Next, the transmission path of the monitoring signal MON is explained. The monitoring signal MON output from the optical transmission apparatus  6  is transmitted through the uplink optical fiber FB 2  of the branch path and arrives at the return unit  41  and then, it is returned to be transmitted through the downlink optical fiber FB 1  of the branch path to thereby arrive at the return unit  61 . Then, the monitoring signal MON is returned from the return unit  61  and output to the uplink optical fiber FB 4  (the fourth optical fiber) of the adjacent branch path. Thereafter, the monitoring signal MON is transmitted through the uplink optical fiber FB 4  of the branch path and arrives at the return unit  71 , and then it is returned to be transmitted through the downlink optical fiber FB 3  of the branch path and arrives at the detection unit  62 . 
     As described above, according to the present configuration, by providing the return unit for the monitoring signal to the optical transmission apparatus of the branch station, it is possible to collectively detect failures that may occur in the two branch paths. Further, since the failures that may occur in the two branch paths can be detected without having to provide a detection unit for each branch path, it is possible to realize downsizing of the optical transmission apparatus. 
     Next, a modified example of the submarine optical communication system  300  is explained.  FIG. 9  shows a modified example of the submarine optical communication system according to the third example embodiment. A submarine optical communication system  301  shown in  FIG. 9  has a configuration in which the optical transmission apparatus  6  is replaced by an optical transmission apparatus  6 A. The optical transmission apparatus  6 A has a configuration in which the return unit  61  of the optical transmission apparatus  6  is removed and the monitoring signal MON is returned inside the optical transmission/reception unit  31 . 
     The optical transmission/reception unit  31  includes, for example, a Wavelength Selectable Switch (hereinbelow referred to as the WSS) for separating and synthesizing each wavelength of the wavelength-division multiplexed signal. In this example, WSS  31 A is provided on the first branch path side and WSS  31 B is provided on the second branch path side. As described below, the WSS  31 A and the WSS 31 B function as a return unit  63 . The monitoring signal MON is input from an external signal generation unit (not shown) to the WSS  31 A. The WSS  31 A selectively outputs the monitoring signal MON to the optical fiber FB 2  and selectively outputs the monitoring signal MON that is returned from the return unit  41  and made incident via the optical fiber FB 1  to the WSS  31 B. The WSS  31 B selectively outputs the monitoring signal MON that is made incident from the WSS  31 A to the optical fiber FB 4 . The monitoring signal MON output from the WSS  31 B is returned from the return unit  71  and detected by the detection unit  62 . 
     As described above, the submarine optical communication system  301  of the present configuration can collectively detect failures that may occur in the two branch paths by providing a return unit for the monitoring signal to the optical transmission apparatus of the branch station in the same manner as in the submarine optical communication system  300 . Further, since the failures that may occur in the two branch paths can be detected without having to provide a detection unit for each branch path, it is possible to realize downsizing of the optical transmission apparatus. 
     Further, according the present configuration, it is possible to configure the return unit for the monitoring signal by utilizing the WSS provided to the optical transmission/reception unit  31 , which can be said as being advantageous in terms of downsizing the optical transmission apparatus compared to the submarine optical communication system  300  in which a return unit is provided externally to the optical transmission/reception unit  31  using an additional optical component. 
     Fourth Example Embodiment 
     A submarine optical communication system  400  according to a fourth example embodiment is explained.  FIG. 10  is a diagram showing a configuration of the submarine optical communication system  400  according to the fourth example embodiment. The submarine optical communication system  400  has a configuration in which the optical transmission apparatus  6  of the submarine optical communication system  300  is replaced by an optical transmission apparatus  8 . 
     The optical transmission apparatus  8  has a configuration in which the return unit  61  of the optical transmission apparatus  6  is removed and a branching unit  81  and the detection unit  32  are added. The branching unit  81  branches the monitoring signal MON output from the optical transmission/reception unit  31  into two. One of the branched monitoring signal MON is input to the uplink optical fiber FB 2  of the first branch path and the other one of the branched monitoring signal MON is input to the uplink optical fiber FB 4  of the second branch path. 
     The monitoring signal MON input to the optical fiber FB 2  is returned from the return unit  41  and is made incident on the detection unit  32  via the downlink optical fiber FB 1  of the first branch path. The detection unit  32  outputs a detection signal DET 1  that indicates the result of the detection by the monitoring signal MON to the notification unit  33 . 
     The monitoring signal MON input to the optical fiber FB 4  is returned from the return unit  71  and is made incident on the detection unit  62  via the downlink optical fiber FB 3  of the second branch. The detection unit  62  outputs a detection signal DET 2  that indicates the result of the detection by the monitoring signal MON to the notification unit  33 . 
     As described above, according to the present configuration, it is possible to collectively detect failures that may occur in the two branch paths in the same manner as in the submarine optical communication system  300  according to the third example embodiment. Further, since the failures that may occur in the two branch paths can be detected without having to generate a monitoring signal for each branch path, it is possible to realize downsizing of the optical transmission apparatus. 
     Further, according to the present configuration, a detection unit dedicated to each branch path is provided whereby it is possible to detect whether or not a failure has occurred in either one of the two branch paths. 
     Next, a modified example of the submarine optical communication system  400  described above is explained.  FIG. 11  shows a modified example of the submarine optical communication system according to the fourth example embodiment. A submarine optical communication system  401  shown in  FIG. 11  has a configuration in which the optical transmission apparatus  8  of the submarine optical communication system  400  is replaced by an optical transmission apparatus  9 . 
     The optical transmission apparatus  9  has a configuration in which the detection unit  32  of the optical transmission apparatus  8  is removed and a branching unit  91  is added. 
     Like in the case of the optical transmission apparatus  8 , the branching unit  81  branches the monitoring signal MON output from the optical transmission/reception unit  31  into two. One of the branched monitoring signal MON is input to the uplink optical fiber FB 2  of the first branch path and the other one of the branched monitoring signal MON is input to the uplink optical fiber FB 4  of the second branch path. 
     The monitoring signal MON input to the optical fiber FB 2  is returned from the return unit  41  and is made incident on the branching unit  91  via the downlink optical fiber FB 1  of the first branch path. The branching unit  91  selectively branches the light having a wavelength of the monitoring signal MON and transmits lights having other wavelengths toward the optical transmission/reception unit  31 . Accordingly, the monitoring signal MON is branched by the branching unit  91  and output to the detection unit  62 . 
     The monitoring signal MON input to the optical fiber FB 4  is returned from the return unit  71  and is made incident on the detection unit  62  via the downlink optical fiber FB 3  of the second branch. 
     The detection unit  62  outputs the detection signal DET that indicates the result of the detection by the monitoring signal MON to the notification unit  33 . 
     In the present configuration, since the first and the second branch paths are monitored using one detection unit, when a failure occurs in either one of the two branch paths, the optical intensity of the monitoring signal MON detected by the detection unit  62  lowers. Therefore, in the present configuration, by having the detection unit  62  compare the optical intensity of the monitoring signal MON with the prescribed threshold value, it is possible to detect that a failure has occurred in at least one of the first or the second branch path. 
     As described above, the submarine optical communication system  401  can eliminate the detection unit provided to the optical transmission apparatus at the branch station when compared with the submarine optical communication system  400 . Therefore, it is advantageous in terms of realizing the downsizing of the optical transmission apparatus. 
     Other Example Embodiments 
     Note that the present disclosure is not limited to the example embodiments mentioned above and can be modified as appropriate without departing from the gist of the present disclosure. For example, in the example embodiments mentioned above, the monitoring signal was output to the uplink optical fiber of the branch path and the monitoring signal which was received via the downlink optical fiber has been detected, however, this is merely an example. In other words, a configuration in which the monitoring signal is output to the downlink optical fiber of the branch path and the monitoring signal which was received via the uplink optical fiber is detected as the monitoring signal may be adopted. 
     In the example embodiments described above, the receiver has been described as one that receives the DP-QPSK optical signal, however, this is merely an example. For example, it may be configured such that optical signals of other modulation techniques can be transmitted. 
     The present disclosure has been described above with reference to the example embodiments, however, the present disclosure is not to be limited to the aforementioned disclosure. The configuration and the details of the present disclosure can be modified in various ways within the scope that is understandable by a person skilled in the art. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2017-208874, filed on Oct. 30, 2017, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
         ALM NOTIFICATION SIGNAL 
         B 1  BRANCH STATION 
         CON 1 -CON 4  CONTROL SIGNAL 
         DET, DET 1 , DET  2  DETECTION SIGNAL 
         FB 1  to FB 4 , FT 1  to FT 4  OPTICAL FIBER 
         INS INSTRUCTION SIGNAL 
         MON MONITORING SIGNAL 
         SUP POWER SUPPLY LINE 
         T 1 , T 2  TRUNK STATION 
         TL TRUNK PATH 
           1 - 3 ,  5 ,  6 ,  6 A,  8 ,  9  OPTICAL TRANSMISSION APPARATUS 
           4 ,  7  SUBMARINE BRANCHING APPARATUS 
           11  OPTICAL TRANSMISSION/RECEPTION UNIT 
           11 A,  11 C MUTIPLEXER 
           11 B SIGNAL GENERATION UNIT 
           12  NOTIFICATION SIGNAL RECEPTION UNIT 
           13  INSTRUCTION UNIT 
           14  OPTICAL TRANSMITTER/RECEIVER 
           31  OPTICAL TRANSMISSION/RECEPTION UNIT 
           32 ,  62  DETECTION UNIT 
           32 A,  41 A,  61 A FILTER 
           32 B OPTICAL RECEPTION UNIT 
           33  NOTIFICATION UNIT 
           41 ,  51 ,  61 ,  63 ,  71  RETURN UNIT 
           41 B,  61 B COUPLER 
           42 ,  43 ,  72 ,  73  OPTICAL SWITCH 
           44  CONTROL UNIT 
           81 ,  91  BRANCHING UNIT 
           100 ,  101 ,  102 ,  200 ,  300 ,  301 ,  400 ,  401  SUBMARINE OPTICAL COMMUNICATION SYSTEM 
           101 A OPTICAL TRANSMISSION APPARATUS 
           101 B SUBMARINE BRANCHING APPARATUS 
           101 C REPEATER