Distributed optical sensing systems and methods

A distributed optical sensing system comprising:

• a sensing medium;
• an optical source for generating an optical output (OO);
• a means for separating the OO into at least two OO portions;
• a first optical delay means adapted to delay a first OO portion, named Delayed Optical Output Signal (DelayOOS), by a delay time, τ1, with respect to a further OO portion, named Direct Optical Output Signal (DirectOOS);
• a means for directing the DelayOOS and the DirectOOS to the sensing medium;
• a means for receiving distributed backscatter signals from the sensing medium generated by the DelayOOS and the DirectOOS;
• a means for separating the received backscatter signals into at least two backscattered signal portions;
• a second optical delay means adapted to delay a first backscattered signal portion, named Delayed Backscatter Signal (DelayBS), by a delay time, τ2, with respect to a further backscattered signal portion, named Direct Backscatter Signal (DirectBS), wherein:
• one portion of the distributed backscatter generated by DirectOOS, having been received from the sensing medium, provides a portion of the DelayBS, named Direct-DelayBS;
• and
• one portion of the distributed backscatter generated by DelayOOS, having been received from the sensing medium, provides a portion of one or more DirectBS, named Delay-DirectBS;
• an optical receiver configured for measurement of the phase difference between Direct-DelayBS and one or more Delay-DirectBS with full phase quadrature determination (without ambiguity in a range of 2*π radians) to determine optical path length changes along the sensing medium;
• and
• an analysis processor configured to receive measured signals from the optical receiver and analysis of the measured signals thereby to infer physical changes in the sensing medium from the sensed optical path length changes.