Patent ID: 7974182

Claim:
A method of evaluating the position of a time-varying disturbance on a transmission link, the method comprising: copying, at least in part, an output signal from an optical pulse source, such that there is a pair of signal copies by channeling light from the optical source onto first and second paths such that the power of the optical source traveling along each path is shared in a predetermined manner between the signal copies and one signal copy travels along the first path in an outbound direction with the other signal copy traveling along the second path in the outbound direction; delaying one of the pulse copies relative to the other pulse copy in the outbound direction; combining light traveling in the outbound direction; transmitting the combined light of the differentially delayed pair of signal copies onto the transmission link in the outbound direction; receiving in a return direction from the transmission link return signals comprising backscattered components comprising at least partially returned copies of said signal copies previously transmitted on said transmission link, wherein at least one of said backscattered components has suffered a phase change caused by said time-varying disturbance; combining the received returned signal copies of a transmitted pair so as to produce a combination signal; and using a temporal characteristic in the combination signal of received returned signal copies of a transmitted pair which has traveled along the transmission link to beyond the location of the time-varying disturbance to evaluate the position of the time-varying disturbance on the transmission link, wherein the position of the disturbance is determined from the time of return of said phase-modulated backscattered components of said returned signal copies, the phase-modulated backscattered components including backscattered components of a given pair of pulse copies that are phase-modulated by the disturbance on return.