Patent ID: 7333681

Claim:
A method for detecting movement of a multimode optical fiber comprising: providing a multimode optical fiber having a first end and a second end; launching pulsed light signals into one end of the optical fiber from a source of light so as to establish a non-uniform mode field power distribution in the optical fiber; for each pulse, sequentially detecting at said one end of the fiber a series of reflected time divided light signals which have been transmitted along the fiber; comparing at least some of the received light signals relative to data obtained from light signals received in response to previous pulses to detect changes of modal power distribution of the received light signals relative to the previously received light signals by separating from the signals separate portions of the light containing the higher order signal modes; analyzing the changes in modal power distribution to determine any changes which are indicative of manipulation of the optical fiber causing movement of a portion thereof along the length thereof; and generating an alarm in response to the detection of any such changes which are indicative of manipulation of the optical fiber causing movement of a portion thereof along the length thereof.