Receiver-based fiber-optic link monitor

An apparatus comprising:

• a digital processor (DP) being configured to obtain a temporal sequence of digital measurements of a first optical signal received by a coherent optical receiver (COR) from an optical fiber link and being configured to estimate a cross-correlation between the temporal sequence of digital measurements and a temporal sequence of powers of a power-modulated second optical signal for a plurality of relative time shifts between the sequences, the first and second optical signals having been transmitted to the optical fiber link in different frequency channels, each of the digital measurements representing a phase distortion of the received first optical signal at a corresponding time;
• and
• wherein the DP is configured to identify a location along the optical fiber link as having a physical change in response to a magnitude of a difference between the estimated cross correlation and reference cross-correlation being greater than a fixed value for one of the relative time shifts;
• and
• wherein the DP is configured to estimate the location of the physical change from the value of the one of the relative time shifts.