Phase-sensitive optical time-domain reflectometry (ΦOTDR) is a well-established technique that provides spatio-temporal measurements of an environmental variable in real time. This unique capability is being leveraged in an ever-increasing number of applications, from energy transportation or civil security to seismology. To date, a wide number of different approaches have been implemented, providing a plethora of options in terms of performance (resolution, acquisition bandwidth, sensitivity or range). However, to achieve high spatial resolutions, detection bandwidths in the GHz range are typically required, substantially increasing the system cost and complexity. Here, we present a novel ΦOTDR approach that allows a customized time expansion of the received optical traces. Hence, the presented technique reaches cm-scale spatial resolutions over 1 km while requiring a remarkably low detection bandwidth in the MHz regime. This approach relies on the use of dual-comb spectrometry to interrogate the fibre and sample the backscattered light. Random phase-spectral coding is applied to the employed combs to maximize the signal-to-noise ratio of the sensing scheme. A comparison of the proposed method with alternative approaches aimed at similar operation features is provided, along with a thorough analysis of the new trade-offs. Our results demonstrate a radically novel high-resolution ΦOTDR scheme, which could promote new applications in metrology, borehole monitoring or aerospace.

Dual frequency comb (DFC) spectroscopy using electro-optic comb generators stands out for its flexibility, easy implementation, and low cost. Typically, two combs with different line spacing are generated from a common laser using independent electro-optic comb generators. This approach minimizes the impact of laser phase noise; however, the distinct paths followed by the two combs ultimately limit the attainable signal-to-noise ratio and long-term stability of the system. In this work, a common-path DFC is generated using a single modulator driven by an arbitrary waveform generator, thus enabling a remarkable increase of the system stability (up to 0.8 s of integration time) while maintaining high flexibility. The proposed technique is experimentally validated by implementing a dual frequency comb with 3000 lines, covering an optical bandwidth of 4.5 GHz, and demonstrating an optical-to-radiofrequency compression factor of 7500. Our system is able to measure extremely narrowband optical features (in the MHz range) with an accuracy only limited by the master laser stability.

Time-expanded phase-sensitive optical time-domain reflectometry ( T E − Φ O T D R ) is a dual-comb-based distributed optical fiber sensing technique capable of providing centimeter scale resolution while maintaining a remarkably low (MHz) detection bandwidth. Random spectral phase coding of the dual combs involved in the fiber interrogation process has been proposed as a means of increasing the signal-to-noise ratio (SNR) of the sensor. In this Letter, we present a specific spectral phase coding methodology capable of further enlarging the SNR of T E − Φ O T D R . This approach is based on the use of a quadratic spectral phase to precisely control the peak power of the comb signals. As a result, an SNR improvement of up to 8 dB has been experimentally attained with respect to that based on the random phase coding previously reported.

In recent years, the use of highly flexible wings in aerial vehicles (e.g., aircraft or drones) has been attracting increasing interest, as they are lightweight, which can improve fuel-efficiency and distinct flight performances. Continuous wing monitoring can provide valuable information to prevent fatal failures and optimize aircraft control. In this paper, we demonstrate the capabilities of a distributed optical fiber sensor based on time-expanded phase-sensitive optical time-domain reflectometry (TE-ΦOTDR) technology for structural health monitoring of highly flexible wings, including static (i.e., bend and torsion), and dynamic (e.g., vibration) structural deformation. This distributed sensing technology provides a remarkable spatial resolution of 2 cm, with detection and processing bandwidths well under the MHz, arising as a novel, highly efficient monitoring methodology for this kind of structure. Conventional optical fibers were embedded in two highly flexible specimens that represented an aircraft wing, and different bending and twisting movements were detected and quantified with high sensitivity and minimal intrusiveness.

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 (ϕOTDR) 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), ϕOTDR sensors typically provide higher dynamics and longer ranges but significantly worse spatial resolutions. Very recently, a novel ϕOTDR approach has been introduced, which covers an existing gap between the long range and fast response of ϕOTDR and the high spatial resolution of OFDR. This technique, termed time-expanded (TE) ϕOTDR, exploits an interferometric scheme that employs two mutually coherent optical frequency combs. In TE-ϕOTDR, 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 ϕOTDR 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-ϕOTDR 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.

We present a dual-comb scheme based on a single intensity modulator driven by inexpensive board-level pseudo-random bit sequence generators. The result is a simplified architecture that exhibits a long mutual coherence time (up to 50 s) with no need of stabilization feedback loops or self-correction algorithms. Unlike approaches that employ ultrafast arbitrary waveform generators, our scheme makes it possible to produce long interferograms in the time domain, reducing the difference in the line spacing of the combs even below the hertz level. In order to check the system accuracy, we report two spectroscopic measurements with a frequency sampling of 140 MHz. All these results are analyzed and discussed to evaluate the potential of our scheme to implement a field-deployable dual-comb generator.