Optical repeater, transmission path fiber monitoring method, and optical transmission system

An optical repeater, which is a C+L-band repeater inserted between a first transmission path fiber and a second transmission path fiber, the optical repeater comprising:

• a first optical fiber amplification unit inserted in a first line, and configured to amplify a C-band signal;
• a second optical fiber amplification unit inserted in a second line, and configured to amplify an L-band signal;
• a third optical fiber amplification unit inserted in a third line, and configured to amplify a C-band signal;
• a fourth optical fiber amplification unit inserted in a fourth line, and configured to amplify an L-band signal;
• a first loopback unit provided between an input to the first optical fiber amplification unit or an output from the first optical fiber amplification unit and an input to the third optical fiber amplification unit or an output from the third optical fiber amplification unit;
• and
• a second loopback unit provided between an input to the second optical fiber amplification unit or an output from the second optical fiber amplification unit and an input to the fourth optical fiber amplification unit or an output from the fourth optical fiber amplification unit.