Patent ID: 8135274

Claim:
An optical communication system comprising: line monitoring equipment configured to provide a test signal; a first optical fiber path configured to receive said test signal and carry said test signal in a first direction; a second optical fiber path configured to carry signals in a second direction opposite from said first direction; and a plurality of repeaters coupled to said first and second optical fiber paths, each of said repeaters comprising, a first optical amplifier configured to amplify optical signals on said first optical fiber path, a second optical amplifier configured to amplify optical signals on said second optical fiber path, and a loop back path coupling an output of said first optical amplifier to an input of said second optical amplifier, said loop back path being configured to couple at least a portion of said test signal to said second optical fiber path as a returned test signal, said line monitoring equipment being configured to calculate differential loop gain data associated with each of said loop back paths from said returned test signals of said plurality of repeaters and provide an output identifying a fault in the optical communication system in response to said differential loop gain data, said differential loop gain data comprising, for each one of said loop back paths, a loop gain imparted to said test signal through said each one of said loop back paths, minus a previous loop back path loop gain imparted to said test signal through one of said loop back paths preceding said each one of said loop back paths.