Time Expansion in Distributed Optical Fiber Sensing

Distributed optical fiber sensing (DOFS) technology has recently experienced an impressive growth in various fields including security, structural monitoring and seismology, among others. This expansion has been accompanied by a speedy development of the technology in the last couple of decades, reaching remarkable performance in terms of sensitivity, range, number of independent sensing points and affordable cost per monitored point as compared with competing technologies such as electrical or point optical sensors. Phase-sensitive Optical Time-Domain Reflectometry (fOTDR) is a particularly interesting DOFS technique, since it enables real-time monitoring of dynamic variations of physical parameters over a large number of sensing points. Compared to their frequency-domain counterparts (OFDR), fOTDR sensors typically provide higher dynamics and longer ranges but significantly worse spatial resolutions. Very recently, a novel fOTDR approach has been introduced, which covers an existing gap between the long range and fast response of fOTDR and the high spatial resolution of OFDR. This technique, termed time-expanded (TE) fOTDR, exploits an interferometric scheme that employs two mutually coherent optical frequency combs. In TE-fOTDR, a probe comb is launched into the fiber under test. The beating of the backscattered light and a suitable LO comb produces a multi-heterodyne detection process that compresses the spectrum of the probe comb, in turn expanding the detected optical traces in the time-domain. This approach has allowed sensing using fOTDR technology with very high resolution (in the cm scale), while requiring outstandingly low detection and acquisition bandwidths (sub-MHz). In this work, we review the fundamentals of TE-fOTDR technology and describe the recent developments, focusing on the attainable sensing performance, the existing trade-offs and open working lines of this novel sensing approach.

Automated summary

Distributed optical fiber sensing (DOFS) technology has witnessed remarkable growth in various fields. A particularly interesting technique, phase-sensitive Optical Time-Domain Reflectometry (fOTDR), enables real-time monitoring over a large number of sensing points. Recently, a novel approach called time-expanded (TE) fOTDR has been introduced, which combines the advantages of long range and fast response of fOTDR with high spatial resolution. This approach utilizes an interferometric scheme and achieves high-resolution sensing with low detection and acquisition bandwidth.