Patent claim: Optical amplification stage 1 for OTDR monitoring the stage comprising a first 2a and a second optical signal path 2b having respectively first and second direction of propagation of a respective first and second optical signal the first and second direction of propagation being mutually opposite a first 3a and a second optical amplifier 3b located along the first and second optical signal path respectively and having an optical amplification band each of the first and second optical signal being a WDM signal occupying a WDM band comprised within the optical amplification band a first optical coupler 4a placed along the first optical signal path downstream the first optical amplifier with respect to the first direction of propagation a second optical coupler 4b placed along the second optical signal path downstream the second optical amplifier with respect to the second direction of propagation and an optical bypass path 5 optically connecting the first and the second optical coupler each coupler having a first 6 and a second signal port 7 belonging to the respective optical signal path the first port facing the respective optical amplifier and a first 8 and second bypass port 9 the first bypass port belonging to the optical bypass path wherein the first bypass port is the crossport with respect to the second signal port the optical amplification stage comprising a first 11a and a second optical reflector 11b optically connected to the second bypass port of respectively the first and second optical coupler the first and second reflector being structured for reflecting respectively in a first 30 and second reflection band 31 comprising respectively a first and a second OTDR wavelength comprised within the amplification band the first and second optical reflectors being structured so that the optical power ratio of the optical radiation reflected by each optical reflector to the radiation incident thereto throughout the whole WDM band is less than 25 dB and so that the optical power ratio of the optical radiation reflected by each optical reflector to the radiation incident thereto at the respective first and second OTDR wavelength is greater than or equal to10 dB wherein a first total loss is defined as the loss undertaken by an optical radiation propagating from the second signal port of the first respectively the second optical coupler to the second signal port of the second respectively the first optical coupler through the bypass path and wherein a second total loss is defined as the loss undertaken by an optical radiation propagating from the first signal port of the first respectively the second optical coupler to the first respectively the second optical reflector and then to the second signal port of the second respectively the first optical coupler through the bypass path andwherein the difference of the second total loss to the first total loss at the first and second OTDR wavelengths is less than 10 dB.

Related passage: To the output sections of the optical amplifierrepeaters 22 and 26 are connected 22 optical fiber couplers 23 and 27 respectively to which parts of the output lights of the optical amplifierrepeater are branched.To one each of the branch ports of the optical fiber couplers 23 and 27 are connected one or the other of wavelengthselective reflecting devices 24 and 28.The reflective center wavelength of the wavelengthselective reflecting device 24 here is identical with the wavelength of the probe light source 12 and the reflective center wavelength of the wavelengthselective reflecting device 28 is identical with the wavelength of the probe light source 32.Further the other branch port of the optical fiber coupler 23 to which no wavelengthselective reflecting device is connected and the other branch port of the optical fiber coupler 27 to which no wavelengthselective reflecting device is connected are connected to each other.