Abstract:
A method includes taking backscattering light of a first wavelength in a first direction of a first transmission line and coupling the same so that the light may travel in a second direction of a second transmission line; taking a second wavelength in the first direction of the first transmission line and coupling the same so that the second wavelength may travel in the first direction of the second transmission line; taking backscattering light of the first wavelength in the second direction of the second transmission line and coupling the same so that the backscattering light may travel in the first direction of the first transmission line; taking the second wavelength in the second direction of the second transmission line and coupling the same so that the second wavelength may travel in the second direction of the first transmission line.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a U.S. continuation application filed under 35 USC 111(a) claiming benefit under 35 USC 120 and 3.65(c) of PCT application JP03/01262, filed Feb. 6, 2003, which is hereby incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical amplifying and repeating method and an optical amplifying and repeating system, and, in particular, to an optical amplifying and repeating method and an optical amplifying and repeating system suitable to monitor a loss distribution in an optical transmission line in a longitudinal direction by means of OTDR (Optical Time Domain Reflectometry; optical pulse test). 
   2. Description of the Related Art 
   As a method of monitoring a loss distribution in a longitudinal direction of an optical fiber transmission line, OTDR including coherent OTDR is common (for example, see patent document 1 or non-patent document 1). 
   In a system including an optical amplifying and repeating unit in an optical transmission line, an optical isolator is required in an optical amplifier in terms of avoiding multipath reflection. However, since backscattering light is blocked by the optical isolator in the optical amplifier, it is not possible to carry out OTDR in a condition in which the optical amplifier having the optical isolator built therein is inserted. 
     FIG. 1  shows one example of a configuration of the optical amplifying and repeating unit system in the prior art. In the figure, a wavelength λa, transmitted from an A station, and traveling in an A direction (a direction in which light travels from the A station to a B station) from the A station in a transmission line A (a transmission line in which light travels from the A station to the B station), passes through an optical coupler  10 , an optical amplifier  11  and an optical coupler  12 , travels in the A direction in the transmission line A, and then, is transmitted to the station B. A wavelength λb, transmitted from the B station and traveling in the B direction (a direction in which light travels from the B station to the A station) from the B station in a transmission line B (a transmission line in which light travels from the B station to the A station), passes through an optical coupler  13 , an optical amplifier  14  and an optical coupler  15 , travels in the B direction in the transmission line B, and then, is transmitted to the station A. The above-mentioned optical coupler  10 , optical amplifier  11 , optical coupler  12 , optical coupler  13 , optical amplifier  14  and optical coupler  15  are disposed on an optical amplifying and repeating unit. 
   Backscattering light of the wavelength λa is provided to the light transmission line B from the coupler  12  and via the coupler  13 , then travels in the B direction there, and is monitored by an OTDR measurement unit  17  provided in the A station. Backscattering light of the wavelength λb is provided to the light transmission line A from the coupler  15  via the coupler  10 , then travels in the A direction there, and is monitored by an OTDR measurement unit, not shown, provided in the B station. 
   In OTDR known from patent documents 2, 3, 4, 5 or such, as illustrated with reference to  FIG. 1 , only a loss distribution in a longitudinal direction of the optical transmission line A can be monitored from the A station, and, it is not possible to monitor a loss distribution in a longitudinal direction of the optical transmission line B (opposite line) from the A station. Similarly, only the loss distribution in the longitudinal direction of the optical transmission line B can be monitored from the B station and, it is not possible to monitor the loss distribution in the longitudinal direction of the optical transmission line A (opposite line) from the B station. 
   Patent document 1: Japanese Laid-open Patent Application No. 9-236513; 
   Patent document 2: Japanese Laid-open Patent Application No. 10-229366; 
   Patent document 3: Japanese Laid-open Patent Application No. 9-116502; 
   Patent document 4: Japanese Laid-open Patent Application No. 2001-505312; 
   patent document 5: Japanese Laid-open Patent Application No. 2001-502422; and 
   Non-patent document 1: Submarine Cable Network Systems, written by Shigeyuki Akiba and Naruhito Nishi together, issued by NTT Quality Co., Ltd. 
   If the loss distributions in both A and B directions can be monitored from the A station (or the B station), it is possible to reduce facilities required for a maintenance of the optical transmission lines and a manpower concerning the maintenance. Also, for a system having a long repeating span of optical amplifying and repeating unit, a high dynamic range is required for the OTDR measurement. If the loss distributions in both directions of the transmission lines A and B can be monitored from both A and B stations, it is possible to achieve the dynamic range double that of the prior art by monitoring the half span of the transmission line A and the transmission line B on the side of A from the A station while monitoring the half span of the transmission line A and the transmission line B on the side of B from the B station. However, in the prior art, the above-mentioned improvement of the maintenance or improvement of the dynamic range cannot be achieved. 
   SUMMARY OF THE INVENTION 
   A general object of the present invention is to provide an optical amplifying and repeating method and an optical amplifying and repeating system by which it is possible to monitor loss distributions of optical transmission lines in both directions, as well as it is possible to improve a maintenance and improve a dynamic range. 
   In order to achieve the object, the present invention is configured, in an optical amplifying and repeating method for connecting between a first station and a second station with a first optical transmission line and a second optical transmission line, wherein the first optical transmission line transmits an optical signal in a first direction from the first station toward the second station and includes at least one first optical amplifier on the way; and the second optical transmission line transmits an optical signal in a second direction from the second station toward the first station and includes at least one second optical amplifier on the way, to take backscattering light of a first wavelength traveling in the first direction of the first optical transmission line, immediately after the first optical amplifier, and couple the same in such a manner that the backscattering light may travel in the second direction of the second optical transmission line; to take a second wavelength traveling in the first direction of the first optical transmission line and couple the same in such a manner that the second wavelength may travel in the first direction of the second optical transmission line; to take backscattering light of the first wavelength traveling in the second direction of the second optical transmission line, immediately after the second optical amplifier, and couple the same in such a manner that said backscattering light may travel in the first direction of the first optical transmission line; and to take the second wavelength traveling in the second direction of the second optical transmission line and couple the same in such a manner that the second wavelength may travel in the second direction of the first optical transmission line. 
   By this optical amplifying and repeating method, it is possible to monitor loss distributions of optical transmission lines in both directions, and it is possible to improve maintenance and improve a dynamic range. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a configuration diagram of one example of an optical amplifying and repeating system in the prior art; 
       FIG. 2  is a configuration diagram of a first embodiment of an optical amplifying and repeating system according to the present invention; 
       FIG. 3  is a configuration diagram of a second embodiment of an optical amplifying and repeating system according to the present invention; 
       FIG. 4  is a configuration diagram of a third embodiment of an optical amplifying and repeating system according to the present invention; 
       FIG. 5  is a configuration diagram of a fourth embodiment of an optical amplifying and repeating system according to the present invention; 
       FIG. 6  is a configuration diagram of a fifth embodiment of an optical amplifying and repeating system according to the present invention; 
       FIG. 7  is a configuration diagram of a sixth embodiment of an optical amplifying and repeating system according to the present invention; and 
       FIG. 8  is a configuration diagram of a seventh embodiment of an optical amplifying and repeating system according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention are described now based on figures. 
     FIG. 2  shows a configuration of a first embodiment of an optical amplifying and repeating system according to the present invention. In the figure, wavelengths λa and λb traveling in an A direction (a direction in which light travels from an A station to a B station) in a transmission line A (a transmission line in which light travels from the A station to the B station) after being transmitted from the A station passes through an optical coupler  20 , an optical amplifier  22  and an optical coupler  24 , further travels in the A direction in the transmission line A, and is transmitted to the B station. Further, wavelengths λa and λb traveling in a B direction (a direction in which light travels from the B station to the A station) in a transmission line B (a transmission line in which light travels from the B station to the A station) after being transmitted from the B station passes through an optical coupler  26 , an optical amplifier  28  and an optical coupler  30 , further travels in the B direction in the transmission line B, and is transmitted to the A station. 
   Backscattering light of the wavelength λa transmitted from the A station in the transmission line A is taken from the transmission line A by means of the optical coupler  24  immediately after the optical amplifier  22 , and is coupled in the optical transmission line B (opposite line) immediately before the optical amplifier  28  through an optical filter  32  which passes therethrough only the wavelength λa (or blocks the wavelength λb), in such a manner as to travel in the direction B of the transmission line B by means of the optical coupler  26 . 
   The wavelength λb transmitted from the A station to the transmission line A is taken from the transmission line A by means of the optical coupler  24 , and is coupled in the optical transmission line B (opposite line) immediately before the optical amplifier  28  through an optical filter  34  which passes therethrough only the wavelength λb (or blocks the wavelength λa) in such a manner that the wavelength λb travels in the direction A of the transmission line B by means of the optical coupler  26 . Backscattering light of the wavelength λb coupled in the A direction of the transmission line B, travels in the transmission line B in the B direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of the OTDR measurement unit  40  by transmitting the wavelengths λa and λb in the direction A of the transmission line A from the OTDR measurement unit  40  of the A station. 
   Similarly, backscattering light of the wavelength λa transmitted from the B station in the transmission line B is taken from the transmission line B by means of the optical coupler  30  immediately after the optical amplifier  28 , and is coupled in the optical transmission line A (opposite line) immediately before the optical amplifier  22  through an optical filter  36  which passes therethrough only the wavelength λa (or blocks the wavelength λb) in such a manner as to travel in the direction A of the transmission line A by means of the optical coupler  20 . 
   The wavelength λb transmitted from the B station to the transmission line B is taken from the transmission line B by means of the optical coupler  30 , and is coupled in the optical transmission line A (opposite line) immediately before the optical amplifier  22  through an optical filter  38  which passes therethrough only the wavelength λb (or blocks the wavelength λa) in such a manner that the wavelength λb travels in the direction B of the transmission line A by means of the optical coupler  20 . Backscattering light of the wavelength λb coupled in the B direction of the transmission line A travels in the transmission line A in the A direction. The above-mentioned optical couplers  20 ,  24 ,  26 ,  30 , optical amplifiers  22 ,  28 , optical filters  32  through  38  are disposed in an optical amplifying and repeating unit. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of an OTDR measurement unit of the B station, not shown, by transmitting the wavelengths λa and λb in the direction B of the transmission line B from the OTDR measurement unit of the B station. 
   In this optical amplifying and repeating system, the optical coupler in each of the input and output sides may be configured may be of two stages, monitoring PDs may be added, and thereby, it is possible to monitor input/output power of the optical amplifiers. 
     FIG. 3  shows a configuration of a second embodiment of an optical amplifying and repeating system according to the present invention. In the figure, the same reference numerals are given to the same parts/components as those of  FIG. 2 . In  FIG. 3 , wavelengths λa and λb traveling in an A direction in a transmission line A after being transmitted from the station A passes through an optical coupler  20 , an optical amplifier  22  and an optical coupler  24 , further travels in the A direction in the transmission line A, and then is transmitted to the B station. Further, wavelengths λa and λb traveling in a B direction in a transmission line B after being transmitted from the station B passes through an optical coupler  26 , an optical amplifier  28  and an optical coupler  30 , further travels in the B direction in the transmission line B, and then is transmitted to the A station. 
   Backscattering light of the wavelength λa transmitted from the A station in the transmission line A is taken from the transmission line A by means of the optical coupler  24  immediately after the optical amplifier  22 , and is coupled in the optical transmission line B (opposite line) immediately after the optical amplifier  28  through an optical filter  42  which passes therethrough only the wavelength λa (or blocks the wavelength λb) in such a manner as to travel in the direction B of the transmission line B by means of the optical coupler  30 . 
   The wavelength λb transmitted from the A station to the transmission line A is taken from the transmission line A by means of the optical coupler  24 , and is coupled in the optical transmission line B (opposite line) immediately before the optical amplifier  28  through an optical filter  34  which passes therethrough only the wavelength λb (or blocks the wavelength λa) in such a manner that the wavelength λb travels in the direction A of the transmission line B by means of the optical coupler  26 . Backscattering light of the wavelength λb coupled in the A direction of the transmission line B travels in the transmission line B in the B direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of the OTDR measurement unit  40  of the A station by transmitting the wavelengths λa and λb in the direction A of the transmission line A from the OTDR measurement unit  40 . 
   Backscattering light of the wavelength λa transmitted from the B station in the transmission line B is taken from the transmission line B by means of the optical coupler  30  immediately after the optical amplifier  28 , and is coupled in the optical transmission line A (opposite line) immediately after the optical amplifier  22  through the optical filter  42  which passes therethrough only the wavelength λa (or blocks the wavelength λb) in such a manner as to travel in the direction A of the transmission line A by means of the optical coupler  24 . 
   The wavelength λb transmitted from the B station to the transmission line B is taken from the transmission line B by means of the optical coupler  30 , and is coupled in the optical transmission line A (opposite line) immediately before the optical amplifier  22  through an optical filter  38  which passes therethrough only the wavelength λb (or blocks the wavelength λa) in such a manner that the wavelength λb travels in the direction B of the transmission line A by means of the optical coupler  20 . Backscattering light of the wavelength λb coupled in the B direction of the transmission line A travels in the transmission line A in the A direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of an OTDR measurement unit of the B station, not shown, by transmitting the wavelengths λa and λb in the direction B of the transmission line B from the OTDR measurement unit of the B station. 
   In this embodiment, in comparison to the first embodiment, it is possible to eliminate one filter. However, the backscattering light of the wavelength λa is not amplified by the optical amplifier  22  or  28 . Further, by configuring the optical filters  34  and  38  so that they may have characteristics to pass therethrough the wavelength λb with reflecting wavelengths other than the wavelength λb, it is possible to add photodiodes (PD)  44  and  46  to remaining ports of the optical couplers  20  and  26 , it is possible to detect input power of the respective ones of the optical amplifiers  22  and  28  by means of the photodiodes  44  and  46 , and the detection outputs can be utilized for gain control of the optical amplifiers  22  and  28 . 
     FIG. 4  shows a configuration of a third embodiment of an optical amplifying and repeating system according to the present invention. In this embodiment, contrary to the second embodiment, backscattering light of the wavelength λb is not amplified. In the figure, the same reference numerals are given to the same parts/components as those of  FIG. 2 . In  FIG. 4 , wavelengths λa and λb traveling in an A direction in a transmission line A after being transmitted from the station A passes through an optical coupler  20 , an optical amplifier  22  and an optical coupler  24 , further travels in the A direction in the transmission line A, and then is transmitted to the B station. Further, wavelengths λa and λb traveling in a B direction in a transmission line B after being transmitted from the station B passes through an optical coupler  26 , an optical amplifier  28  and an optical coupler  30 , further travels in the B direction in the transmission line B, and then is transmitted to the A station. 
   Backscattering light of the wavelength λa transmitted from the A station in the transmission line A is taken from the transmission line A by means of the optical coupler  24  immediately after the optical amplifier  22 , and is coupled in the optical transmission line B (opposite line) immediately before the optical amplifier  28  through an optical filter  32  which passes therethrough only the wavelength λa (or blocks the wavelength λb) in such a manner as to travel in the direction B of the transmission line B by means of the optical coupler  26 . 
   The wavelength λb transmitted from the A station to the transmission line A is taken from the transmission line A by means of the optical coupler  20 , and is coupled in the optical transmission line B (opposite line) immediately before the optical amplifier  28  through an optical filter  43  which passes therethrough only the wavelength λb (or blocks the wavelength λa) in such a manner that the wavelength λb travels in the direction A of the transmission line B by means of the optical coupler  26 . Backscattering light of the wavelength λb coupled in the A direction of the transmission line B travels in the transmission line B in the B direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of the OTDR measurement unit  40  of the A station by transmitting the wavelengths λa and λb in the direction A of the transmission line A from the OTDR measurement unit  40 . 
   Backscattering light of the wavelength λa transmitted from the B station in the transmission line B is taken from the transmission line B by means of the optical coupler  30  immediately after the optical amplifier  28 , and is coupled in the optical transmission line A (opposite line) immediately before the optical amplifier  22  through an optical filter  36  which passes therethrough only the wavelength λa (or blocks the wavelength λb) in such a manner as to travel in the direction A of the transmission line A by means of the optical coupler  20 . 
   The wavelength λb transmitted from the B station to the transmission line B is taken from the transmission line B by means of the optical coupler  26 , and is coupled in the optical transmission line A (opposite line) immediately before the optical amplifier  22  through the optical filter  43  which passes therethrough only the wavelength λb (or blocks the wavelength λa) in such a manner that the wavelength λb travels in the direction B of the transmission line A by means of the optical coupler  20 . Backscattering light of the wavelength λb coupled in the B direction of the transmission line A travels in the transmission line A in the A direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of an OTDR measurement unit of the B station, not shown, by transmitting the wavelengths λa and λb in the direction B of the transmission line B from the OTDR measurement unit of the B station. 
   In this embodiment, in comparison to the first embodiment, it is possible to eliminate one filter. However, the backscattering light of the wavelength λb is not amplified by the optical amplifier  22  or  28 . Further, by configuring the optical filters  32  and  36  so that they may have characteristics to pass therethrough the wavelength λa with reflecting wavelengths other than the wavelength λa, it is possible to add photodiodes (PD)  45  and  47  in remaining ports of the optical couplers  24  and  30 , it is possible to detect output power of the respective ones of the optical amplifiers  22  and  28  by means of the photodiodes  45  and  47 , and the detection outputs can be utilized for gain control of the optical amplifiers  22  and  28 . 
     FIG. 5  shows a configuration of a fourth embodiment of an optical amplifying and repeating system according to the present invention. In the figure, the same reference numerals are given to the same parts/components as those of  FIG. 3  or  4 . In  FIG. 5 , wavelengths λa and λb traveling in an A direction in a transmission line A after being transmitted from the station A passes through an optical coupler  20 , an optical amplifier  22  and an optical coupler  24 , further travels in the A direction in the transmission line A, and then is transmitted to the B station. Further, wavelengths λa and λb traveling in a B direction in a transmission line B after being transmitted from the station B passes through an optical coupler  26 , an optical amplifier  28  and an optical coupler  30 , further travels in the B direction in the transmission line B, and then is transmitted to the A station. 
   Backscattering light of the wavelength λa transmitted from the A station in the transmission line A is taken from the transmission line A by means of the optical coupler  24  immediately after the optical amplifier  22 , and is coupled in the optical transmission line B (opposite line) immediately after the optical amplifier  28  through the optical filter  42  which passes therethrough only the wavelength λa (or blocks the wavelength λb) in such a manner as to travel in the direction B of the transmission line B by means of the optical coupler  30 . 
   The wavelength λb transmitted from the A station to the transmission line A is taken from the transmission line A by means of the optical coupler  20 , and is coupled in the optical transmission line B (opposite line) immediately before the optical amplifier  28  through the optical filter  43  which passes therethrough only the wavelength λb (or blocks the wavelength λa) in such a manner that the wavelength λb travels in the direction A of the transmission line B by means of the optical coupler  26 . Backscattering light of the wavelength λb coupled in the A direction of the transmission line B travels in the transmission line B in the B direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of the OTDR measurement unit  40  of the A station by transmitting the wavelengths λa and λb in the direction A of the transmission line A from the OTDR measurement unit  40 . 
   Backscattering light of the wavelength λa transmitted from the B station in the transmission line B is taken from the transmission line B by means of the optical coupler  30  immediately after the optical amplifier  28 , and is coupled in the optical transmission line A (opposite line) immediately after the optical amplifier  22  through the optical filter  42  which passes therethrough only the wavelength λa (or blocks the wavelength λb) in such a manner as to travel in the direction A of the transmission line A by means of the optical coupler  24 . 
   The wavelength λb transmitted from the B station to the transmission line B is taken from the transmission line B by means of the optical coupler  26 , and is coupled in the optical transmission line A (opposite line) immediately before the optical amplifier  22  through the optical filter  43  which passes therethrough only the wavelength λb (or blocks the wavelength λa) in such a manner that the wavelength λb travels in the direction B of the transmission line A by means of the optical coupler  20 . Backscattering light of the wavelength λb coupled in the B direction of the transmission line A travels in the transmission line A in the A direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of an OTDR measurement unit of the B station, not shown, by transmitting the wavelengths λa and λb in the direction B of the transmission line B from the OTDR measurement unit of the B station. 
   In this embodiment, in comparison to the first embodiment, it is possible to eliminate two filters. However, the backscattering light of the wavelengths λa and λb is not amplified by the optical amplifier  22  or  28 . Further, it is possible to add photodiodes (PD)  44  through  47  in remaining ports of the optical couplers  20 ,  24 ,  26  and  30 , it is possible to detect input/output power of the respective ones of the optical amplifiers  22  and  28  by means of the photodiodes  44  through  47 , and the detection outputs can be utilized for gain control of the optical amplifiers  22  and  28 . 
     FIG. 6  shows a configuration of a fifth embodiment of an optical amplifying and repeating system according to the present invention. In the figure, the same reference numerals are given to the same parts as those of  FIGS. 2 through 5 . In  FIG. 6 , wavelengths λa and λb traveling in an A direction (a direction in which light travels from an A station to a B station) in a transmission line A (a transmission line in which light travels from the A station to the B station) after being transmitted from the station A passes through an optical coupler  20 , an optical amplifier  22  and an optical coupler  24 , passes through an optical filter  50  and travels in the A direction in the transmission line A. The optical filter  50  has characteristics to reflect only the wavelength λb with passing therethrough wavelengths other than the wavelength λb. 
   Further, wavelengths λa and λb traveling in a B direction (a direction in which light travels from the B station to the A station) in a transmission line B (a transmission line in which light travels from the B station to the A station) after being transmitted from the station B passes through an optical coupler  26 , an optical amplifier  28  and an optical coupler  30 , passes through an optical filter  52 , and travels in the B direction in the transmission line B. The optical filter  52  has characteristics to reflect only the wavelength λa with passing therethrough wavelengths other than the wavelength λa. 
   Backscattering light of the wavelength λa transmitted from the A station in the transmission line A is taken from the transmission line A by means of the optical coupler  24  immediately after the optical amplifier  22 , and is coupled in the optical transmission line B (opposite line) immediately before the optical amplifier  28  through an optical isolator  54  which passes therethrough only light directed to the optical coupler  26  from the optical coupler  24  in such a manner as to travel in the direction B of the transmission line B by means of the optical coupler  26 . 
   The wavelength λb transmitted from the A station to the transmission line A is reflected by the optical filter  50 , is taken from the transmission line A by means of the optical coupler  24 , and travels to a photodiode  59  through an optical filter  58  after passing through the optical isolator  54  and the optical coupler  26 . The optical filter  58  has characteristics such as to reflect only the wavelength λb, with passing therethrough wavelengths other than the wavelength λb. The wavelength λb reflected by the optical filter  58  is coupled in the transmission line B (opposite line) immediately before the optical amplifier  28  in such a manner as to travel in the A direction of the transmission line B by means of the optical coupler  26 . Backscattering light of the wavelength λb coupled in the A direction of the transmission line B travels in the transmission line B in the B direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of the OTDR measurement unit  40  of the A station by transmitting the wavelengths λa and λb in the direction A of the transmission line A from the OTDR measurement unit  40 . 
   Similarly, backscattering light of the wavelength λa transmitted from the B station in the transmission line B is taken from the transmission line B by means of the optical coupler  30  immediately after the optical amplifier  28 , and is coupled in the optical transmission line A (opposite line) immediately before the optical amplifier  22  through an optical isolator  56  which passes therethrough only light directed to the optical coupler  20  from the optical coupler  30  in such a manner as to travel in the direction A of the transmission line A by means of the optical coupler  20 . 
   The wavelength λb transmitted from the B station to the transmission line B is reflected by the optical filter  52 , is taken from the transmission line B by means of the optical coupler  30 , and travels to a photodiode  61  through an optical filter  60  after passing through the optical isolator  56  and the optical coupler  20 . The optical filter  60  has characteristics such as to reflect only the wavelength λb with passing therethrough wavelengths other than the wavelength λb. The wavelength λb reflected by the optical filter  60  is coupled in the transmission line A (opposite line) immediately before the optical amplifier  22  in such a manner as to travel in the B direction of the transmission line A by means of the optical coupler  20 . Backscattering light of the wavelength λb coupled in the B direction of the transmission line A travels in the transmission line A in the A direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of an OTDR measurement unit of the B station, not shown, by transmitting the wavelengths λa and λb in the direction B of the transmission line B from the OTDR measurement unit of the B station. The optical isolators  54  and  56  should not necessarily be provided. 
   In this embodiment, it is possible to add photodiodes  45  and  47  in remaining ports of the optical couplers  24  and  30 , it is possible to add photodiodes  59  and  61  in the rear of the optical filters  58  and  60 , it is possible to detect input/output power of the respective ones of the optical amplifiers  22  and  28 , and the detection outputs can be utilized for gain control of the optical amplifiers  22  and  28 . 
     FIG. 7  shows a configuration of a sixth embodiment of an optical amplifying and repeating system according to the present invention. In the figure, the same reference numerals are given to the same parts as those of  FIGS. 2 through 6 . In  FIG. 7 , wavelengths λa and λb traveling in an A direction (a direction in which light travels from an A station to a B station) in a transmission line A (a transmission line in which light travels from the A station to the B station) after being transmitted from the station A passes through an optical coupler  20 , an optical amplifier  22  and an optical coupler  24 , and travels in the A direction in the transmission line A, and then is transmitted to the B station. 
   Further, wavelengths λa and λb traveling in a B direction (a direction in which light travels from the B station to the A station) in a transmission line B (a transmission line in which light travels from the B station to the A station) after being transmitted from the station B passes through an optical coupler  26 , an optical amplifier  28  and an optical coupler  30 , and travels in the B direction in the transmission line B, and then is transmitted to the A station. 
   Backscattering light of the wavelength λa transmitted from the A station in the transmission line A is taken from the transmission line A by means of the optical coupler  24  immediately after the optical amplifier  22 , and travels to an optical filter  64  through the optical coupler  26  after passing through an optical isolator  54  which passes therethrough only light directed to the optical coupler  26  from the optical coupler  24  immediately before the optical amplifier  28  of the optical transmission line B (opposite line). The optical filter  64  has characteristics such as to reflect only the wavelength λa, and passes therethrough wavelengths other than the wavelength λa. The above-mentioned backscattering light of the wavelength λa reflected by the optical filter  64  is coupled in such a manner as to travel in the B direction of the transmission line B by means of the optical coupler  26 . 
   The wavelength λb transmitted from the A station to the transmission line A is taken by the optical coupler  24  from the transmission line A, and travels to a photodiode  45  through an optical filter  62  from the optical coupler  24 . The optical filter  62  has characteristics such as to reflect only the wavelength λb with passing therethrough wavelengths other than the wavelength λb. The above-mentioned wavelength λb reflected by the optical filter  62  is coupled in such a manner as to travel in the A direction of the transmission line B by means of the optical coupler  26  immediately before the optical amplifier  28  of the optical transmission line B (opposite line) through the optical coupler  24  and the optical isolator  54 . Backscattering light of the wavelength λb coupled in the A direction of the transmission line B travels in the B direction of the transmission line B. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of the OTDR measurement unit  40  of the A station by transmitting the wavelengths λa and λb in the direction A of the transmission line A from the OTDR measurement unit  40 . 
   Similarly, backscattering light of the wavelength λa transmitted from the B station in the transmission line B is taken from the transmission line B by means of the optical coupler  30  immediately after the optical amplifier  28 , and travels to an optical filter  68  through the optical coupler  20  after passing through an optical isolator  56  which passes therethrough only light directed to the optical coupler  20  from the optical coupler  30  immediately before the optical amplifier  22  of the optical transmission line A (opposite line). The optical filter  68  has characteristics such as to reflect only the wavelength λa, and pass therethrough wavelengths other than the wavelength λa. The above-mentioned backscattering light of the wavelength λa reflected by the optical filter  68  is coupled in such a manner as to travel in the A direction of the transmission line A by means of the optical coupler  20 . 
   The wavelength λb transmitted from the B station to the transmission line B is taken by the optical coupler  30  from the transmission line B, and travels to a photodiode  47  through an optical filter  66  from the optical coupler  30 . The optical filter  66  has characteristics such as to reflect only the wavelength λb with passing therethrough wavelengths other than the wavelength λb. The above-mentioned wavelength λb reflected by the optical filter  66  is coupled in such a manner as to travel in the B direction of the transmission line A by means of the optical coupler  20  immediately before the optical amplifier  22  of the optical transmission line A (opposite line) through the optical coupler  30  and the optical isolator  56 . Backscattering light of the wavelength λb coupled in the B direction of the transmission line A travels in the A direction of the transmission line A. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of an OTDR measurement unit of the B station, not shown, by transmitting the wavelengths λa and λb in the direction B of the transmission line B from the OTDR measurement unit of the B station. 
   In this embodiment, it is possible to add photodiodes  45  and  47  in the rear of the optical filters  62  and  66 , it is possible to detect output power of the respective ones of the optical amplifiers  22  and  28 , and the detection outputs can be utilized for gain control of the optical amplifiers  22  and  28 . 
   By applying optical circulators instead of the optical isolators  54  and  56 , and adding photodiodes in ports to receive light applied in the opposite direction, it is possible to detect input power of the optical amplifiers. Further, in the present optical amplifying and repeating system, by configuring the optical couplers in the input sides of the optical amplifiers in two stages and adding monitoring photodiodes, it is possible to monitor input power of the optical amplifiers. 
     FIG. 8  shows a configuration of a seventh embodiment of an optical amplifying and repeating system according to the present invention. In the figure, the same reference numerals are given to the same parts as those of  FIGS. 2 through 7 . In  FIG. 8 , wavelengths λa and λb traveling in an A direction (a direction in which light travels from an A station to a B station) in a transmission line A (a transmission line in which light travels from the A station to the B station) after being transmitted from the station A passes through an optical coupler  20 , an optical amplifier  22  and an optical coupler  24 , and travels in the A direction in the transmission line A, and then is transmitted to the B station. 
   Further, wavelengths λa and λb traveling in a B direction (a direction in which light travels from the B station to the A station) in a transmission line B (a transmission line in which light travels from the B station to the A station) after being transmitted from the station B passes through an optical coupler  26 , an optical amplifier  28  and an optical coupler  30 , and travels in the B direction in the transmission line B, and then is transmitted to the A station. 
   Backscattering light of the wavelength λa transmitted from the A station in the transmission line A is taken from the transmission line A by means of the optical coupler  24  immediately after the optical amplifier  22 , and is coupled in the optical transmission line B (opposite line) immediately after the optical amplifier  28  through an optical filter  42  passing therethrough only wavelength λa (or blocking the wavelength λb) in such a manner as to travel in the direction B of the transmission line B by means of the optical coupler  30 . 
   The wavelength λb transmitted from the A station to the transmission line A is taken from the transmission line A by means of the optical coupler  20 , and travels to a photodiode  61  from an optical filter  60 . The optical filter  60  has characteristics such as to reflect only the wavelength λb, with passing therethrough wavelengths other than the wavelength λb. The wavelength λb reflected by the optical filter  60  is attenuated by an optical attenuator (ATT)  70 , and travels to a photodiode  59  from an optical filter  58  after passing through the optical coupler  26 . The attenuator  70  is provided for the purpose of avoiding multipath reflection. The optical filter  58  has characteristics to reflect only the wavelength λb with passing therethrough wavelengths other than the wavelength λb. The wavelength λb reflected by the optical filter  58  is coupled in the transmission line B (opposite line) immediately before the optical amplifier  28  in such a manner as to travel in the A direction of the transmission line B by means of the optical coupler  26 . Backscattering light of the wavelength λb coupled in the A direction of the transmission line B travels in the transmission line B in the B direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of the OTDR measurement unit  40  of the A station by transmitting the wavelengths λa and λb in the direction A of the transmission line A from the OTDR measurement unit  40 . 
   Similarly, backscattering light of the wavelength λa transmitted from the B station in the transmission line B is taken from the transmission line B by means of the optical coupler  30  immediately after the optical amplifier  28 , and is coupled in the optical transmission line A (opposite line) immediately after the optical amplifier  22  through an optical filter  42  passing therethrough only wavelength λa (or block the wavelength λb) in such a manner as to travel in the direction A of the transmission line A by means of the optical coupler  24 . 
   Further, the wavelength λb transmitted from the B station to the transmission line B is taken from the transmission line B by means of the optical coupler  26 , and travels to a photodiode  59  from an optical filter  58 . The optical filter  58  has characteristics such as to reflect only the wavelength λb, with passing therethrough wavelengths other than the wavelength λb. The wavelength λb reflected by the optical filter  58  is attenuated by an optical attenuator (ATT)  70 , and travels to a photodiode  61  from an optical filter  60  after passing through the optical coupler  20 . The optical filter  60  has characteristics to reflect only the wavelength λb with passing therethrough wavelengths other than the wavelength λb. The wavelength λb reflected by the optical filter  60  is coupled in the transmission line A (opposite line) immediately before the optical amplifier  22  in such a manner as to travel in the B direction of the transmission line A by means of the optical coupler  20 . Backscattering light of the wavelength λb coupled in the B direction of the transmission line A travels in the transmission line A in the A direction. 
   Thereby, it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines A and B by means of the OTDR measurement unit  40  of the B station, not shown, by transmitting the wavelengths λa and λb in the direction B of the transmission line B from the OTDR measurement unit  40  of the B station. 
   In this embodiment, backscattering light of the wavelength λa is not amplified by the optical amplifier  22  or  28 . Further, it is possible to add photodiodes  45  and  47  in remaining ports of the optical couplers  24  and  30 , it is possible to add photodiodes  59  and  61  in the rear of the optical filters  58  and  60 , it is possible to detect input/output power of the respective ones of the optical amplifiers  22  and  28 , and the detection outputs can be utilized for gain control of the optical amplifiers  22  and  28 . 
   The A station corresponds to a first station of the claims; the B station corresponds to a second station; the transmission line A corresponds to a first optical transmission line; the transmission line B corresponds to a second optical transmission line; the A direction corresponds to a first direction; the B direction corresponds to a second direction; the optical amplifier  22  corresponds to a first optical amplifier; and the optical amplifier  28  corresponds to a second optical amplifier.