Patent Publication Number: US-2022236141-A1

Title: Optical transmission path inspecting system, and optical transmission path inspecting device

Description:
TECHNICAL FIELD 
     The present disclosure relates to an optical transmission path inspecting system and an optical transmission path inspecting device. This application claims the benefit of the priority based on Japanese Patent Application No. 2019-108603, filed on Jun. 11, 2019, the entire contents disclosed in the application is incorporated herein by reference. 
     BACKGROUND ART 
     Patent Literature 1 discloses a technology related to an optical path loss measuring system. This system has an optical path switching means, a storage means, an optical path loss calculating means. The optical path switching means switches an optical path such that light from a light source is emitted to one of a measurement target optical path to which a first optical power meter is connected or a second optical power meter. The storage means stores characteristics of an optical loss of the optical path switching means in advance. The optical path loss calculating means calculates the amount of an optical loss of the measurement target optical path on a basis of the characteristics of an optical loss of the optical path switching means stored in the storage means, measurement values of the first optical power meter, and measurement values of the second optical power meter. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] Japanese Unexamined Patent Publication No. 2000-206004 
     SUMMARY OF INVENTION 
     An optical transmission path inspecting system of the present disclosure is a system for inspecting an optical transmission path constituted of a plurality of optical fibers and includes a first inspecting device configured to be provided on one end side of the optical transmission path, a second inspecting device configured to be provided on another end side of the optical transmission path, and a loss calculating unit configured to calculate a loss of each of the plurality of optical fibers. The first and second inspecting devices have a light source unit, a plurality of light input/output ports, an optical switch, a first light detecting unit, a second light detecting unit, a third light detecting unit, and an internal loss recording unit. The light source unit outputs test light. The plurality of light input/output ports each is detachably connected to each of the plurality of optical fibers. The optical switch selectively couples the light source unit with each of the light input/output ports. The first light detecting unit detects a first intensity of the test light input from the inspecting device on a counterpart side and passing through the optical switch. The second light detecting unit detects a second intensity of the test light directed from the light source unit toward the optical switch. The third light detecting unit is optically coupled to another end of a test optical fiber having one end connected to each of the plurality of light input/output ports in place of each of the plurality of optical fibers, and detects a third intensity of the test light received from the light source unit via the test optical fiber. The internal loss recording unit records a loss of an optical path inside the device obtained on a basis of a difference between the third intensity and the second intensity. The loss calculating unit calculates a loss of each of the plurality of optical fibers on a basis of a value obtained by subtracting the first intensity of the second inspecting device, a loss recorded in the internal loss recording unit of the first inspecting device, and a loss recorded in the internal loss recording unit of the second inspecting device from the second intensity of the first inspecting device. 
     An optical transmission path inspecting device of the present disclosure is an inspecting device capable of inspecting an optical transmission path constituted of a plurality of optical fibers and includes a light source unit, a plurality of light input/output ports, an optical switch, a first light detecting unit, a second light detecting unit, a third light detecting unit, and an internal loss recording unit. The light source unit outputs test light. The plurality of light input/output ports each allows the plurality of optical fibers to be detachably connected thereto. The optical switch selectively couples the light source unit with each of the light input/output ports. The first light detecting unit detects a first intensity of the test light input from a different inspecting device and passing through the optical switch. The second light detecting unit detects a second intensity of the test light directed from the light source unit toward the optical switch. The third light detecting unit is optically coupled to another end of a test optical fiber having one end connected to each of the plurality of light input/output ports in place of each of the plurality of optical fibers, and detects a third intensity of the test light received from the light source unit via the test optical fiber. The internal loss recording unit records a loss of an optical path inside the device obtained on a basis of a difference between the third intensity and the second intensity. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view schematically illustrating a constitution of an inspecting system according to an embodiment. 
         FIG. 2  is an enlarged view illustrating a light input/output unit. 
         FIG. 3  is a view illustrating a connection form of the inspecting device in a stage prior to inspection. 
         FIG. 4  is a block diagram illustrating an example of a hardware constitution of a loss calculating unit. 
         FIG. 5  is a view illustrating an example of a polarity of a multi-fiber cable with connectors. 
         FIG. 6  is a view illustrating another example of a polarity of a multi-fiber cable with connectors. 
         FIG. 7  is a view illustrating another example of a polarity of a multi-fiber cable with connectors. 
         FIG. 8  is a view schematically illustrating a constitution of an inspecting device according to a modification example. 
         FIG. 9  is a view schematically illustrating a constitution of an inspecting system as a comparative example. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Problem to be Solved by Present Disclosure 
     In the related art, when an amount of an optical loss of an optical transmission path such as an optical fiber cable in an optical transmission system is measured, in a case in which the optical transmission path includes a plurality of optical fibers, an optical switch is used for consecutively switching between optical fibers which are measurement targets. An optical switch of an inspecting device disposed on one end side of an optical transmission path causes test light output from a light source to be selectively incident on a measurement target optical fiber. In addition, another optical switch of an inspecting device disposed on another end side of the optical transmission path selects the measurement target optical fiber, causes test light to be emitted from the measurement target optical fiber, and detects an optical intensity thereof. The amount of an optical loss of the measurement target optical fiber can be measured on a basis of a difference between an intensity of the test light output from the inspecting device on one end side and the intensity of the test light detected by the inspecting device on the another end side. 
     However, an optical loss occurs not only in the optical fibers but also inside the inspecting devices, particularly in the optical switches. Therefore, in order to accurately measure an optical loss of optical fibers, there is a need to perform adjustment, in which an amount corresponding to an optical loss inside each of the inspecting devices disposed on both sides of an optical transmission path is subtracted in advance, that is, zero calibration. For this reason, in the related art, for example, two inspecting devices disposed on both sides of an optical transmission path are brought together at one place before measuring. Further, these inspecting devices are connected to each other using a short optical transmission path for testing, and zero calibration is performed by means of combination of these inspecting devices. However, normally, an optical transmission path which is a measurement target is long, and one end and another end thereof are far away from each other. Therefore, there is a problem that it takes time and effort to transport the inspecting devices. 
     Hence, an object of the present disclosure is to provide an inspecting system and an inspecting device for an optical transmission path, in which an optical loss of an optical transmission path can be accurately measured without bringing two inspecting devices disposed on both sides of the optical transmission path together at one place. 
     Effects of Present Disclosure 
     According to the present disclosure, it is possible to provide an inspecting system and an inspecting device for an optical transmission path, in which an optical loss of an optical transmission path can be accurately measured without bringing two inspecting devices disposed on both sides of an optical transmission path together at one place. 
     Description of Embodiment of Present Disclosure 
     First, an embodiment of the present disclosure will be enumerated and described. An optical transmission path inspecting system according to the embodiment is a system for inspecting an optical transmission path constituted of a plurality of optical fibers and includes a first inspecting device configured to be provided on one end side of the optical transmission path, a second inspecting device configured to be provided on another end side of the optical transmission path, and a loss calculating unit configured to calculate a loss of each of the plurality of optical fibers. The first and second inspecting devices have a light source unit, a plurality of light input/output ports, an optical switch, a first light detecting unit, a second light detecting unit, a third light detecting unit, and an internal loss recording unit. The light source unit outputs test light. The plurality of light input/output ports each is detachably connected to each of the plurality of optical fibers. The optical switch selectively couples the light source unit with each of the light input/output ports. The first light detecting unit detects a first intensity of the test light input from the inspecting device on a counterpart side and passing through the optical switch. The second light detecting unit detects a second intensity of the test light directed from the light source unit toward the optical switch. The third light detecting unit is optically coupled to another end of a test optical fiber having one end connected to each of the plurality of light input/output ports in place of the plurality of optical fibers, and detects a third intensity of the test light received from the light source unit via the test optical fiber. The internal loss recording unit records a loss of an optical path inside the device obtained on a basis of a difference between the third intensity and the second intensity. The loss calculating unit calculates a loss of each of the plurality of optical fibers on a basis of a value obtained by subtracting the first intensity of the second inspecting device, a loss recorded in the internal loss recording unit of the first inspecting device, and a loss recorded in the internal loss recording unit of the second inspecting device from the second intensity of the first inspecting device. 
     An optical transmission path inspecting device according to the embodiment is an inspecting device capable of inspecting an optical transmission path constituted of a plurality of optical fibers and includes a light source unit, a plurality of light input/output ports, an optical switch, a first light detecting unit, a second light detecting unit, a third light detecting unit, and an internal loss recording unit. The light source unit outputs test light. The plurality of light input/output ports each allows the plurality of optical fibers to be detachably connected thereto. The optical switch selectively couples the light source unit with each of the light input/output ports. The first light detecting unit detects a first intensity of the test light input from a different inspecting device and passing through the optical switch. The second light detecting unit detects a second intensity of the test light directed from the light source unit toward the optical switch. The third light detecting unit is optically coupled to another end of a test optical fiber having one end connected to each of the plurality of light input/output ports in place of the plurality of optical fibers, and detects a third intensity of the test light received from the light source unit via the test optical fiber. The internal loss recording unit records a loss of an optical path inside the device obtained on a basis of a difference between the third intensity and the second intensity. 
     Methods for using the inspecting system and the inspecting device are as follows. First, before inspecting of an optical transmission path using the inspecting system or the inspecting device, for example, during manufacture of each inspecting device, an end of a test optical fiber is connected to each of the light input/output ports of each inspecting device. Further, test light is output from the light source unit. The test light reaches the third light detecting unit through the optical switch, each light input/output port, and the test optical fiber, and the third intensity of the test light is detected by the third light detecting unit. At the same time, the second intensity of the test light is detected by the second light detecting unit. Since the test optical fiber can be made sufficiently short compared to a plurality of optical fibers which are measurement targets, a loss of the optical path inside the device is obtained on a basis of a difference between the third intensity and the second intensity. This loss is calculated by the loss calculating unit, for example, and is recorded in the internal loss recording unit. The test optical fiber is detached from each light input/output port. 
     When an optical transmission path is inspected using the inspecting system or the inspecting device, the plurality of light input/output ports of the first inspecting device are connected to one ends of the plurality of optical fibers, and the plurality of light input/output ports of the second inspecting device are connected to another ends thereof. Next, test light is output from the light source unit of the first inspecting device. This test light is consecutively input to the plurality of optical fibers by the optical switch of the first inspecting device. Test light which has passed through each of the optical fibers reaches the optical switch of the second inspecting device. The optical switch of the second inspecting device selects the optical fiber which has been selected by the optical switch of the first inspecting device and inputs test light thereto. This test light reaches the first light detecting unit of the second inspecting device, and the first intensity of the test light is detected by the second light detecting unit. At the same time, the second intensity of the test light is detected by the second light detecting unit of the first inspecting device. The loss calculating unit calculates a loss of each of the plurality of optical fibers on a basis of a value obtained by subtracting the first intensity of the second inspecting device, a loss recorded in the internal loss recording unit of the first inspecting device, and a loss recorded in the internal loss recording unit of the second inspecting device from the second intensity of the first inspecting device. 
     According to the inspecting system and the inspecting device described above, since a loss of the optical path inside the device is recorded in the internal loss recording unit of each of the inspecting devices, an optical loss of an optical transmission path can be accurately measured without bringing two inspecting devices disposed on both sides of an optical transmission path together at one place. In addition, since each of the inspecting devices has the third light detecting unit, a loss of the optical path inside the device can be easily measured using the functions of each of the inspecting devices. 
     In the foregoing inspecting system, each of the first and second inspecting devices may further have a communication unit for communicating independently from the optical transmission path between the first inspecting device and the second inspecting device. Similarly, the foregoing inspecting device may further include a communication unit for communicating independently from the optical transmission path with a different inspecting device. In this case, the first intensity detected by one inspecting device, and data related to an internal loss recorded in the internal loss recording unit of one inspecting device can be easily transmitted to the other inspecting device. Therefore, the loss calculating unit installed on one end side of the optical transmission path can easily calculate a loss of each of the plurality of optical fibers. In addition, due to a communication path independent from the optical transmission path, even when a connection failure or the like exists in the optical transmission path, a loss of each of the optical fibers can be calculated, and thus an optical fiber in which a connection failure has occurred can be identified. 
     In the foregoing inspecting system, the communication unit of the second inspecting device may transmit the first intensity detected by the second inspecting device, and data related to a loss recorded in the internal loss recording unit of the second inspecting device to the communication unit of the first inspecting device. 
     In the inspecting system and the inspecting device described above, the light source unit may have two or more light sources having output wavelengths different from each other. Since the magnitude of a bending loss of the optical fibers varies depending on the wavelength, it is possible to determine whether or not a loss of the optical fibers is caused due to bending by using two or more light sources having output wavelengths different from each other. 
     In the inspecting system and the inspecting device described above, the optical transmission path is constituted of N (N is an integer of 2 or larger) multi-fiber cables individually including two or more optical fibers, and a number of light input/output ports included in each of the inspecting devices may be equal to or more than the total number of the optical fibers included in the N multi-fiber cables. In this case, even when different multi-fiber cables are erroneously connected, it is possible to easily ascertain the multi-fiber cable which has been erroneously connected by checking the other multi-fiber cables. That is, even when a multi-fiber cable different from a certain multi-fiber cable is erroneously connected to a light input/output unit corresponding to the certain multi-fiber cable, it is possible to easily ascertain the connected multi-fiber cable. 
     In the foregoing inspecting system, the loss calculating unit may be provided inside the first inspecting device or may be provided outside the first inspecting device. 
     Details of Embodiment of Present Disclosure 
     Specific examples of an inspecting system and an inspecting device for an optical transmission path of the present disclosure will be described below with reference to the drawings. The present invention is not limited to these examples. The present invention is indicated by the claims, and it is intended to include all changes within the meaning and the scope equivalent to the claims. In the following description, the same elements are denoted by the same reference numerals in the description of drawings, and repeated description thereof will be omitted. 
       FIG. 1  is a view schematically illustrating a constitution of an inspecting system according to an embodiment of the present disclosure. As illustrated in  FIG. 1 , this inspecting system  1 A is a system for inspecting an optical transmission path  20  constituted of a plurality of optical fibers and includes an inspecting device  10 A that is a first inspecting device provided on one end side of the optical transmission path  20 , and an inspecting device  10 B that is a second inspecting device provided on another end side of the optical transmission path  20 . Since the inspecting devices  10 A and  10 B have the same internal constitution as each other, it is advantageous that the inspecting devices can be shared compared to a case in which the internal constitutions thereof are different from each other. Moreover, the inspecting system  1 A includes a loss calculating unit  30  calculating a loss of each of the plurality of optical fibers. For example, the optical transmission path  20  may be an optical cable constituted of N (N is an integer of 2 or larger, for example, N=10) multi-fiber cables  21  individually including two or more optical fibers. 
     The inspecting devices  10 A and  10 B have a light source unit  11 , an optical switch  12 , light detecting units  13 ,  14 , and  15 , an internal loss recording unit  16 , a control unit  17 , a communication unit  18 , and a light input/output unit  19 . The light source unit  11  is constituted to include a semiconductor light emitting element such as a laser diode, for example, and a circuit for driving this semiconductor light emitting element. The light source unit  11  outputs test light TL used for inspection of the optical transmission path  20 . For example, the wavelength of the test light TL is a wavelength applied to an optical communication system using the optical transmission path  20 . As an example, the wavelength of the test light TL is included in any of a range of 1,260 nm to 1,360 nm (O-band), a range of 1,360 nm to 1,460 nm (E-band), a range of 1,460 nm to 1,530 nm (S-band), a range of 1,530 nm to 1,565 nm (C-band), a range of 1,565 nm to 1,625 nm (L-band), and a range of 1,625 nm to 1,675 nm (U-band). The test light TL may be continuous light or, depending on a test item, may be high-frequency modulated light in which the intensity thereof varies in a cycle. 
       FIG. 2  is an enlarged view illustrating a structure in the vicinity of the light input/output unit  19 . As illustrated in  FIG. 2 , the light input/output unit  19  is constituted to include J (J is an integer of 2 or larger) light input/output ports  19   a . The number J for light input/output ports  19   a  is equal to or more than the total number of optical fibers  24  included in the N multi-fiber cables  21 . When each of the multi-fiber cables  21  has M (M is an integer of 2 or larger, for example, M=12) optical fibers  24 , there are M×N optical fibers  24  in total. The light input/output unit  19  is an optical receptacle, for example, and is detachably connected to an optical connector  22  which is attached to end portions of the multi-fiber cables  21 . Accordingly, each of the light input/output ports  19   a  is detachably connected to the corresponding optical fiber  24 . 
     Refer to  FIG. 1  again. The optical switch  12  is a 1&gt;&lt;J optical switch. That is, a single optical input/output end is provided on one side of the optical switch  12 , and J optical input/output ends are provided on the other side of the optical switch  12 . The optical switch  12  of the inspecting device  10 A functions as an optical switch for one input and J outputs, and the optical switch  12  of the inspecting device  10 B functions as an optical switch for J inputs and one output. The single optical input/output end on one side is optically coupled to the light source unit  11  via an optical path  42  such as an optical fiber for example. The optical input/output ends on the other side are optically coupled to the corresponding light input/output ports  19   a  via optical paths  43  such as optical fibers for example, respectively. Therefore, the optical switch  12  selectively couples the light source unit  11  with each of the light input/output ports  19   a . The optical switch  12  may be an optical switch adopting a method of micro-electro mechanical systems (MEMS) or may be a mechanical optical switch. 
     In the middle of the optical path  42 , a 2&gt;&lt;2 optical coupler  44  is provided. One of two optical input/output ends on one side of the optical coupler  44  is optically coupled to the light source unit  11 , and another one is optically coupled to the light detecting unit  13 . One of two optical input/output ends on the other side of the optical coupler  44  is optically coupled to the optical switch  12 , and another one is optically coupled to the light detecting unit  14 . 
     In the present embodiment, the light detecting unit  13  serves as a first light detecting unit. In the inspecting device  10 B, a part of the test light TL input from the optical switch  12  branches by the optical coupler  44  and reaches the light detecting unit  13 . Accordingly, the light detecting unit  13  of the inspecting device  10 B detects the optical intensity of the test light TL which has been input from the inspecting device  10 A on the counterpart side and has passed through the optical switch  12 . In the present embodiment, this optical intensity is a first intensity. In the present embodiment, the light detecting unit  14  serves as a second light detecting unit. In the inspecting device  10 A, a part of the test light TL output from the light source unit  11  branches by the optical coupler  44  and reaches the light detecting unit  14 . Accordingly, the light detecting unit  14  of the inspecting device  10 A detects the optical intensity of the test light TL directed from the light source unit  11  toward the optical switch  12 . In the present embodiment, this optical intensity is a second intensity. The light detecting units  13  and  14  are constituted to include a semiconductor light receiving element such as a photodiode for example and a circuit which converts a photocurrent output from the semiconductor light receiving element into a voltage signal. 
     Here, constituent elements used in a stage before inspecting the optical transmission path  20  will be described.  FIG. 3  is a view illustrating a connection form of the inspecting devices  10 A and  10 B in a stage prior to inspection. As illustrated in  FIG. 3 , the optical transmission path  20  is detached from the light input/output unit  19  in a stage prior to inspection, and one end of a test optical fiber  41  is connected to the light input/output unit  19 . The test optical fiber  41  may be a single optical fiber or may be an optical cable including a plurality of optical fibers. Another end of the test optical fiber  41  is connected to a light input/output port  51 . The light input/output port  51  and the test optical fiber  41  may be able to be attached to and detached from each other or may be semi-permanently fixed to each other. 
     In the present embodiment, the light detecting unit  15  serves as a third light detecting unit. The light detecting unit  15  is optically coupled to the another end of the test optical fiber  41  via the light input/output port  51 . The light detecting unit  15  detects the optical intensity of the test light TL received from the light source unit  11  via the test optical fiber  41 . In the present embodiment, this optical intensity is a third intensity. Similar to the light detecting units  13  and  14 , the light detecting unit  15  is also constituted to include a semiconductor light receiving element such as a photodiode for example and a circuit which converts a photocurrent output from this semiconductor light receiving element into a voltage signal. 
     The internal loss recording unit  16  is a portion recording data related to a loss of the optical path inside the inspecting device  10 A or  10 B including the internal loss recording unit  16 . The internal loss recording unit  16  can be constituted of a nonvolatile storage device such as a ROM or a hard disk for example. A loss of the optical path inside the device is calculated on the basis of the difference between the optical intensity (third intensity) of the test light TL which is detected by the light detecting unit  15  and the optical intensity (second intensity) of the test light TL which is detected by the light detecting unit  14  and directed from the light source unit  11  toward the optical switch  12 . This calculation may be performed by the control unit  17  or may be performed by the loss calculating unit  30 . 
     The communication unit  18  is a portion for communicating independently from the optical transmission path  20  between the inspecting device  10 A and the inspecting device  10 B. The communication unit  18  is constituted to include a media converter for example. The communication unit  18  converts an electrical signal into an optical signal, transmits the converted optical signal to the inspecting device on the counterpart side, and converts an optical signal received from the inspecting device on the counterpart side into an electrical signal. For this reason, the inspecting devices  10 A and  10 B may be connected to each other through an optical transmission path  52  other than the optical transmission path  20 . The optical transmission path  52  is a single-fiber optical cable for example. The communication unit  18  is connected to the loss calculating unit  30  by cable or radio. The communication unit  18  transmits data related to the optical intensity detected by the light detecting units  13 ,  14 , and  15  to the loss calculating unit  30 . Moreover, the communication unit  18  is used for operating the optical switch  12  of the inspecting device  10 A and the optical switch  12  of the inspecting device  10 B in association with each other. The communication unit  18  is not limited to a media converter. For example, it may be an electrical communication means such as a LAN. 
     The control unit  17  can be constituted of a computer having a CPU, a RAM, and a ROM mounted on a wiring board, for example. The control unit  17  is electrically connected to the light source unit  11  and the optical switch  12  and controls operation of these. The control unit  17  is electrically connected to the light detecting units  13 ,  14 , and  15  and the communication unit  18 . The control unit  17  provides the communication unit  18  with data related to the optical intensity detected by the light detecting units  13 ,  14 , and  15 . The control unit  17  is electrically connected to the internal loss recording unit  16  and causes the internal loss recording unit  16  to record data related to a loss of the optical path inside the device computed by itself or received from the loss calculating unit  30  through the communication unit  18 . 
     The loss calculating unit  30  may be provided inside the inspecting device  10 A as a part of the inspecting device  10 A or may be provided outside the inspecting device  10 A separately from the inspecting device  10 A.  FIG. 1  illustrates a case in which the loss calculating unit  30  is provided outside the inspecting device  10 A. In this case, the loss calculating unit  30  is connected to the inspecting device  10 A by cable or radio and communicates with the communication unit  18  of the inspecting device  10 A. The loss calculating unit  30  calculates a loss of each of the plurality of optical fibers  24  included in the optical transmission path  20 . Specifically, the loss calculating unit  30  calculates a loss of each of the plurality of optical fibers  24  on the basis of the following mathematical expression. 
       [Loss of optical fiber]= Pa−Pb−La−Lb   (1)
 
     In the above mathematical expression (1), Pa is the optical intensity (second intensity) of the test light TL directed from the light source unit  11  toward the optical switch  12  in the inspecting device  10 A. Pa is detected by the light detecting unit  14  of the inspecting device  10 A. Pb is the optical intensity (first intensity) of the test light TL input from the optical switch  12  of the inspecting device  10 B. Pb is detected by the light detecting unit  13  of the inspecting device  10 B in a state in which the optical transmission path  20  is connected to the inspecting devices  10 A and  10 B. La is a loss of the optical path inside the inspecting device  10 A. La is detected by the light detecting unit  15  of the inspecting device  10 A in a state in which the test optical fiber  41  connects each light input/output port  19   a  of the inspecting device  10 A with the light input/output port  51 . La is recorded in the internal loss recording unit  16  of the inspecting device  10 A. Lb is a loss of the optical path inside the inspecting device  10 B. Lb is detected by the light detecting unit  15  of the inspecting device  10 B in a state in which the test optical fiber  41  connects each light input/output port  19   a  of the inspecting device  10 B with the light input/output port  51 . Lb is recorded in the internal loss recording unit  16  of the inspecting device  10 B. In this manner, the loss calculating unit  30  in the present embodiment calculates a loss of each of the plurality of optical fibers  24  on the basis of a value obtained by subtracting Pb, La, and Lb from Pa. 
       FIG. 4  is a block diagram illustrating an example of a hardware constitution of the loss calculating unit  30 . As illustrated in  FIG. 4 , the loss calculating unit  30  is constituted to include a computer including hardware such as a CPU  31 , a RAM  32 , a ROM  33 , an input device  34 , a communication module  35 , an auxiliary storage device  36 , and an output device  37 . The loss calculating unit  30  realizes the functions described above when these constituent elements are operated by a program or the like. 
     Here, a using method and operation of the inspecting system  1 A will be described. First, before inspecting of the optical transmission path  20  using this inspecting system  1 A, for example, during manufacture of the inspecting devices  10 A and  10 B, an end of the test optical fiber  41  is connected to each of the light input/output ports  19   a  of each of the inspecting devices  10 A and  10 B. Further, the test light TL is output from the light source unit  11 . The test light TL reaches the light detecting unit  15  through the optical switch  12 , each light input/output port  19   a , and the test optical fiber  41 , and the optical intensity of the test light TL is detected by the light detecting unit  15 . At the same time, the optical intensity of the test light TL is detected by the light detecting unit  14 . Since the test optical fiber  41  can be made sufficiently short compared to the optical transmission path  20  which is a measurement target, the losses La and Lb of the optical path inside the device can be obtained on the basis of a difference between the optical intensities thereof. The losses La and Lb are calculated by the loss calculating unit  30  or the control unit  17  for example, and are recorded in the internal loss recording unit  16 . Thereafter, the test optical fiber  41  is detached from each light input/output port  19   a.    
     When the optical transmission path  20  is inspected using the inspecting system  1 A, a plurality of light input/output ports  19   a  of the inspecting device  10 A are connected to one ends of the plurality of optical fibers  24  constituting the optical transmission path  20 , and a plurality of light input/output ports  19   a  of the inspecting device  10 B are connected to another ends thereof. Next, the test light TL is output from the light source unit  11  of the inspecting device  10 A. This test light TL is input to one optical fiber  24  via the optical switch  12  of the inspecting device  10 A. The test light TL which has passed through this optical fiber  24  reaches the optical switch  12  of the inspecting device  10 B. The optical switch  12  of the inspecting device  10 B selects the optical fiber  24  which has been selected by the optical switch  12  of the inspecting device  10 A and emits the test light TL from the optical fiber  24 . This test light TL reaches the light detecting unit  13  of the inspecting device  10 B, and the optical intensity Pb of this test light TL is detected by the light detecting unit  13 . At the same time, the optical intensity Pa of the test light TL is detected by the light detecting unit  14  of the inspecting device  10 A. Thereafter, the optical switch  12  of the inspecting devices  10 A and  10 B selects another optical fiber  24 , and similar operation is performed. Thereafter, similar operation is performed for all of the plurality of optical fibers  24  constituting the optical transmission path  20 . The loss calculating unit  30  calculates a loss of each of the plurality of optical fibers  24  on the basis of the foregoing mathematical expression (1). 
     Effects achieved by the inspecting system  1 A and the inspecting devices  10 A and  10 B described above will be described.  FIG. 9  is a view schematically illustrating a constitution of an inspecting system  100  as a comparative example. As illustrated in  FIG. 9 , this inspecting system  100  is a system for inspecting the optical transmission path  20  and includes an inspecting device  110 A provided on one end side of the optical transmission path  20  and an inspecting device  110 B provided on another end side of the optical transmission path  20 . Moreover, the inspecting system  100  includes a loss calculating unit (not illustrated) calculating a loss of each of the plurality of optical fibers  24 . Each of the inspecting devices  110 A and  110 B has the light source unit  11 , the optical switch  12 , the light detecting units  13  and  14 , the control unit  17 , and the light input/output unit  19 . Details of these elements are similar to those of the inspecting devices  10 A and  10 B in the present embodiment. 
     The loss calculating unit of this inspecting system  100  calculates a loss of each of the plurality of optical fibers  24  on the basis of the following mathematical expression. 
       [Loss of optical fiber]= Pa−Pb   (2)
 
     Pa is the optical intensity of the test light TL directed from the light source unit  11  of the inspecting device  110 A toward the optical switch  12  and is detected by the light detecting unit  14  of the inspecting device  110 A. Pb is the optical intensity of the test light TL input from the optical switch  12  of the inspecting device  110 B and is detected by the light detecting unit  13  of the inspecting device  110 B. 
     However, an optical loss occurs not only in the optical fibers  24  but also inside the inspecting devices  110 A and  110 B, particularly in the optical switches  12 . Therefore, in order to accurately measure an optical loss of the optical fibers  24 , there is a need to perform adjustment, in which an amount corresponding to an optical loss inside each of the inspecting devices  110 A and  110 B disposed on both sides of the optical transmission path  20  is subtracted in advance, that is, zero calibration. For this reason, in the related art, for example, two inspecting devices  110 A and  110 B disposed on both sides of the optical transmission path  20  are brought together at one place before measuring. Further, these inspecting devices  110 A and  110 B are connected to each other using a short optical transmission path for testing, and zero calibration is performed by means of combination of these inspecting devices  110 A and  110 B. However, normally, the optical transmission path  20  which is a measurement target is long, and one end and another end thereof are far away from each other. Therefore, there is a problem that it takes time and effort to transport the inspecting devices  110 A and  110 B. In addition, there is also a problem that combination of the inspecting devices  110 A and  110 B is limited to combination which has been subjected to zero calibration and a different inspecting device cannot be used. 
     Regarding these problems, according to the inspecting system  1 A in the present embodiment, since a loss of the optical path inside the device is recorded in the internal loss recording unit  16  of each of the inspecting devices  10 A and  10 B, even if two inspecting devices  10 A and  10 B disposed on both sides of the optical transmission path  20  are not brought together at one place, an optical loss of the optical transmission path  20  can be accurately measured. In addition, since each of the inspecting devices  10 A and  10 B has the light detecting unit  15 , a loss of the optical path inside the device can be easily measured using the functions of each of the inspecting devices  10 A and  10 B. Moreover, since each of the inspecting devices retains a loss of itself, combination of the inspecting devices is not limited, and thus convenience can be improved. For example, when an inspecting device breaks down, it can be replaced with another inspecting device. In addition, it is possible to save the labor of managing combinations of the inspecting devices. 
     In addition, generally, polarities are present in the multi-fiber cables  21  with connectors.  FIGS. 5, 6, and 7  respectively illustrate examples of polarities of the multi-fiber cables  21  with connectors. In these diagrams, the numbers 1 to 12 are assigned to twelve terminals of optical connectors attached to both ends of the multi-fiber cable  21 .  FIG. 5  illustrates a polarity of Type-A (straight) of TIA568.3.  FIG. 6  illustrates a polarity of Type-B (reversed) of TIA568.3.  FIG. 7  illustrates a polarity of Type-C (pairs flipped) of TIA568.3. In the case of the inspecting system  100  according to the comparative example described above, the polarity of a multi-fiber cable for testing are selected in accordance with the polarity of the multi-fiber cable  21  with connectors which is inspection targets, and zero calibration is performed. Thus, there is a problem that the multi-fiber cable  21  having a different polarity cannot be inspected and this lacks versatility. In contrast, in the present embodiment, the internal losses La and Lb are independently recorded for each of the optical fibers  24  in the respective inspecting devices  10 A and  10 B. Therefore, while arbitrary inspecting devices are combined as the inspecting devices  10 A and  10 B, the multi-fiber cables  21  having various polarities can be easily inspected by selecting combination of the internal losses La and Lb in accordance with the polarities of the multi-fiber cables  21 . 
     As in the present embodiment, the inspecting devices  10 A and  10 B may further have the communication unit  18  for communicating independently from the optical transmission path  20  between the inspecting device  10 A and the inspecting device  10 B. Accordingly, information related to the optical intensity Pb detected by the light detecting unit  13  of the inspecting device  10 B and data related to the internal loss Lb recorded in the internal loss recording unit  16  of the inspecting device  10 B can be easily transmitted to the inspecting device  10 A. Therefore, the loss calculating unit  30  installed on one end side of the optical transmission path  20  can easily calculate a loss of each of the plurality of optical fibers  24 . In addition, since a communication path is independent from the optical transmission path  20 , when a connection failure or the like exists in the optical transmission path  20 , or when combination of the optical fibers  24  to be connected is incorrect due to erroneous determination of the polarity, for example, even when a place to be treated with the type A is treated with the type B or the like, a loss of other optical fibers  24  can be calculated and the optical fiber  24  in which a connection failure has occurred can also be identified. Moreover, the control unit  17  controls the optical switch  12  and searches for a path through which light passes so that it is possible to easily determine which optical fibers  24  are connected in combination, and thus analysis of a connection error can be easily performed. 
     As in the present embodiment, the optical transmission path  20  is constituted of the N multi-fiber cables  21  individually including two or more optical fibers  24 , and the number of light input/output ports  19   a  included in each of the inspecting devices  10 A and  10 B may be equal to or more than the total number of the optical fibers  24  included in the N multi-fiber cables  21 . In this case, even when different multi-fiber cables  21  are erroneously connected, it is possible to easily ascertain the multi-fiber cable  21  which has been erroneously connected by checking other multi-fiber cables. That is, even when a multi-fiber cable  21  different from a certain multi-fiber cable  21  is erroneously connected to the light input/output unit  19  corresponding to the certain multi-fiber cable  21 , it is possible to easily ascertain the connected multi-fiber cable  21 . 
     In this case, connection pattern information indicating how each of the ports of the optical switch  12  in the inspecting devices  10 A and  10 B and each of the multi-fiber cable  21  are connected to each other (for example, correspondence information between port numbers and fiber numbers) and grouping information of the multi-fiber cables  21  are recorded and retained. Further, estimated values of an optical loss of each of the multi-fiber cables  21  in each of the connection patterns are compared to measurement results. Therefore, for example, even when the multi-fiber cable  21  is erroneously input and connected to a light input/output unit  19  corresponding a different multi-fiber cable  21 , it is possible to detect such an erroneous circumstance. The foregoing “estimated values” and “measurement results” include not only specific loss values but also the presence or absence of light passing therethrough. 
     Moreover, these pieces of information are retained for each of the patterns to be connected so that even when the polarities of at least two multi-fiber cables  21  differ from each other or when the numbers of fibers of at least two multi-fiber cables  21  differ from each other, abnormal connection or erroneous connection can be easily detected. 
     Modification Example 
       FIG. 8  is a view schematically illustrating a constitution of an inspecting device  10 C according to a modification example of the foregoing embodiment. The inspecting device  10 C has a light source unit  61  in place of the light source unit  11  of the foregoing embodiment. The light source unit  61  has two or more (three in the illustrated example) light sources  61   a ,  61   b , and  61   c  having output wavelengths different from each other. The light sources  61   a ,  61   b , and  61   c  are coupled to the optical path  42  via an optical coupler  46  for three inputs and one output. In addition, the light sources  61   a ,  61   b , and  61   c  are electrically connected to the control unit  17  and consecutively output test light TLa, test light TLb, and test light TLc in accordance with an instruction from the control unit  17 . 
     As in the present modification example, the light source unit  61  may have two or more light sources  61   a ,  61   b , and  61   c  having output wavelengths different from each other. For example, in a single mode optical fiber of ITU-T G625, in a case in which the bending radius is 15 mm, the bending loss is approximately 2.33&gt;&lt;10 −2  dB/m when the wavelength is 1.31 μm, the bending loss is approximately 1.45 dB/m when the wavelength is 1.55 μm, and the bending loss is approximately 4.77 dB/m when the wavelength is 1.65 μm. In this manner, since the magnitude of a bending loss of the optical fibers  24  varies depending on the wavelength, the bending radius can be inferred from the difference between losses in the wavelengths. Thus, it is possible to determine whether or not a loss of the optical fibers  24  is caused due to bending by using two or more light sources  61   a ,  61   b , and  61   c  having output wavelengths different from each other. 
     The optical transmission path inspecting system according to the present invention is not limited to the embodiment described above, and various other modifications can be performed. For example, in the foregoing embodiment, the optical transmission path  20  constituted of a plurality of multi-fiber cables  21  has been described as an example of an inspection target. The optical transmission path  20  may be constituted of a single multi-fiber ribbon or a plurality of multi-fiber ribbons, or a plurality of single optical fibers  24  may constitute the multi-fiber cable  21 . Alternatively, these may be combined and constitute the multi-fiber cable  21 . In addition, in the foregoing embodiment, an example of a case in which each of the inspecting devices  10 A and  10 B has the communication unit  18  has been described. However, the communication unit  18  can also be made unnecessary when information related to the optical intensity detected by the light detecting unit  13  of the inspecting device  10 B and data recorded in the internal loss recording unit  16  can be provided to the loss calculating unit  30  by a different means, for example, optical communication using the optical fibers  24 . 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 A Inspecting system 
               10 A,  10 B,  10 C Inspecting device 
               11  Light source unit 
               12  Optical switch 
               13 ,  14 ,  15  Light detecting unit 
               16  Internal loss recording unit 
               17  Control unit 
               18  Communication unit 
               19  Light input/output unit 
               19   a  Light input/output port 
               20  Optical transmission path 
               21  multi-fiber cable 
               22  Optical connector 
               24  Optical fiber 
               30  Loss calculating unit 
               34  Input device 
               35  Communication module 
               36  Auxiliary storage device 
               37  Output device 
               41  Test optical fibers 
               42 ,  43  Optical path 
               44 ,  46  Optical coupler 
               51  Light input/output port 
               61  Light source unit 
               61   a ,  61   b ,  61   c  Light source 
             TL, TLa, TLb, TLc Test light