Time-Expanded in Distributed Optical Fiber Sensing

Phase-sensitive optical time-domain reflectometry (ϕ OTDR) is a well-known distributed optical fiber sensing technique widely employed in applications that require thousands of measuring points, such as structural health monitoring, seismology or perimeter surveillance. Time-domain interrogators typically offer longer sensing ranges and faster dynamics but significantly worse spatial resolution than their optical frequency domain counterparts. Recently, a new ϕ OTDR scheme exploits the dual-frequency comb technology to cover the performance gap between that typically offered by conventional ϕ OTDR and OFDR interrogators. In this novel sensing technique, called time-expanded (TE-)ϕ OTDR, one optical frequency comb (OFC) probes the fiber to produce a series of backscatter traces. A second comb with almost identical specifications than the previous one but slightly different line spacing serves as a reference. The beating between both OFCs leads to a frequency down conversion of the backscatter traces upon photodetection, allowing obtaining spatial resolution in the cm scale with MHz bandwidth detectors. Thanks to the use of a pair of OFCs under quasi-integer ratio mode and an atomic rubidium clock, we achieve the dynamic interrogation of up to 100,000 sensing points in a fiber with centimeter spatial resolution and in real time.