Time expansion in distributed optical fiber sensing

The work of MRFR and HFM was supported by the MCIN/AEI/10.13039/501100011033 and European Union NextGenerationEU»/PRTR under grants RYC2021-032167-I and RYC2021- 035009-I. The work of MSA and VD was supported by MCIN/AEI/10.13039/ 501100011033 and the FSE invierte en tu futuro under grants PRE-2019- 087444 and RYC-2017-23668, respectively.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.Comunidad de MadridMinisterio de Ciencia e InnovaciónAgencia Estatal de InvestigaciónGeneralitat ValencianaUniversitat Jaume IEuropean Commissio

Time-expanded phase-sensitive optical time-domain reflectometry

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

Time-expanded FOTDR based on Orthogonal Polarization Frequency Comb generation

Phase-sensitive Optical Time-Domain Reflectometry (ΦOTDR) has emerged as an effective and high-performance solution within the realm of Distributed Optical Fiber Sensing (DOFS) technologies, which has promoted its use in an ever-growing number of fields. The challenges arisen by new operation fields demand surpassing the historical trade-offs in this technology, specially aiming for higher resolution without jeopardizing the system simplicity and cost-effectiveness. In this context, time-expanded (TE-)ΦOTDR has been recently proposed as a DOFS solution delivering cm-range resolution with sub-MHz detection and acquisition bandwidths. It is based on the use of an interferometric scheme that employs a dual frequency comb (DFC), consisting of two mutually coherent optical frequency combs with dissimilar repetition rates. In this paper, we present a novel DFC generation scheme for TE-ΦOTDR that exploits the polarization orthogonality. In particular, our approach considerably increases the common path followed by the two frequency combs, thus reducing instability and noise as compared to the conventional generation scheme. Additionally, we employ an IQ modulation scheme with two PRBS generators that increases the scalability of the interrogator while severely reducing its cost and complexity. Results show a reduction in the noise amplitude spectral density especially at low frequency values, which corroborates the stability enhancement of this proposed architecture as compared to the conventional scheme

Common-Path Dual-Comb Spectroscopy Using a Single Electro-Optic Modulator

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

Definition of a FPGA-based SoC architecture for PRBS transmission in optical spectroscopy

Optical spectroscopy is a well-known tool typically employed for characterizing the properties of materials by analyzing their iteration with light. One of the most spread techniques is the dual comb spectroscopy, since it accomplishes ultra-high resolution, and high sensitivity measurements with a relatively simple platform including a single, relatively narrowband photodetector. The employed optical dual comb can be implemented through electro-optical (EO) modulation driven by pseudo-ransom binary sequences (PRBS) at high data rates, commonly in the range of tens of Gbps. For that purpose, the runtime generation and transmission of adaptive PRBS is still an open challenge, often involving expensive and not flexible high-speed digital systems, with a few commercially available solutions that sometimes do not match the application requirements efficiently. In this context, this work describes the definition and implementation of a System-on-Chip (SoC) architecture, based on a FPGA device, capable of generating and transmitting two PRBS for a dual comb, at a data rate up to 5 Gbps. The architecture can be configured and its operation modified in run time, thanks to the general-purpose processor involved, in charge of managing an Ethernet link to receive new PRBS to be transmitted or set up certain parameters. The proposed design has been validated experimentally on a dual comb spectroscopy measurement, where the absorption of a hydrogen cyanide (HCN) gas cell has been successfully characterized.Agencia Estatal de InvestigaciónMinisterio de Ciencia e Innovació

Frequency stability requirements in quasi-integer-ratio time-expanded phase-sensitive OTDR

Time-expanded phase sensitive (TE-φ)OTDR is a recently reported technique for distributed fiber sensing that relies on the use of an electro-optic dual frequency comb (DFC) scheme. A distinctive feature of this approach is its ability to provide high spatial resolution (on the centimeter scale) with detection bandwidths orders of magnitude lower than those of conventional φOTDR systems. The stringent trade-off between resolution, range and sensing bandwidth that exists in TE-φ OTDR has demonstrated to be substantially relaxed by implementing two frequency combs with quasi-integer-ratio repetition rates. However, employing very dissimilar line separations (with a ratio between them > 100) is challenging due to the need of keeping the coherence over long sequences of interferograms, which eventually limits the attainable range. In this paper, we formulate the requirements for the frequency stability of the reference clock used in a quasi-integer-ratio DFC scheme. This analysis allows us to stablish the limits on the number of comb lines (i.e., on the number of available independent sensing points) for a particular reference clock. By using a rubidium atomic clock (with a relative frequency stability of ~10-13), we demonstrate up to 105 sensing points along 2 km of fiber with tens of Hz sensing bandwidth.Comunidad de MadridMinisterio de Ciencia e InnovaciónAgencia Estatal de InvestigaciónGeneralitat ValencianaUniversitat Jaume

Láser ultracompacto anclado en modos basado en InN con pulsos <100 fs

XII Reunión Española de Optoelectrónica, OPTOEL’21, 30 Jun.-02 Jul. 2021, Virtual.Tipo de participación: Participativo - Póster, Entidad organizadora: Comité de Optoelectrónica de la Sociedad Española de Óptica (SEDOPTICA), Forma de contribución: Artículo científicoEn este trabajo se presenta un láser pasivo anclado en modos (mode-locked) basado en Nitruro de Indio (InN) como absorbente saturable (SA) a una longitud de onda de 1.55 μm. Utilizando una lente GRIN como acoplamiento en fibra, se obtiene un sistema compacto sin necesidad de ajustes posteriores una vez fijado el acoplo fibra-lente-SA, así como la obtención de pulsos láser altamente estables en el rango de los femtosegundos.Comunidad de MadridAgencia Estatal de InvestigaciónMinisterio de Economía y Competitivida

Monitoring of a highly flexible aircraft model wing using time-expanded phase-sensitive OTDR

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

Analysis of disturbance-induced "virtual" perturbations in chirped pulse φ-OTDR

When a disturbance acts on a fiber it induces a change in the local refractive index that influences the fiber backscattering trace. If a chirped pulse φ-OTDR setup is used to interrogate the fiber, this refractive index change appears as a local shift of the received trace, linear to the acting perturbation. However, the refractive index change influences the round trip time of all the backscattering components generated by further fiber sections as well. Due to the high sensitivity of chirped pulse φ-OTDR, the change in the round trip time of the backscattering components, which is usually negligible, may appear as a virtual perturbation in certain conditions. In this letter we derive a mathematical model for the virtual perturbation induced by a disturbance acting on the fiber, when the measurement is performed by a chirped pulse φ-OTDR. We experimentally validate the model by inducing a temperatura change on a known span of fiber while monitoring its effects in a further fiber section kept at rest. The experimental results are then analyzed and compared with the theoretical ones.European CommissionMinisterio de Economía y CompetitividadMinisterio de Ciencia, Innovación y UniversidadesComunidad de Madri

Preliminary numerical and experimental tests for the study of vibration signals in dry granular flows

Debris flows are one of the most important hazards in mountainous areas because of their paroxysmal nature, the high velocities, and energy carried by the transported material. The monitoring of these phenomena plays a relevant role in the prevention of the effects of these events. Among different possibilities, fiber optical sensors appear well-suited for this purpose thanks to their fair cheapness (with the exception of the interrogator), the robustness to electromagnetic interferences, the adaptability in extreme harsh conditions (no power supply is required), the possibility of locating the interrogator many kilometers far away from the monitored site, and the unique feature to provide very-dense multipoint distributed measurements along long distances. In this work, the vibro-acoustics signal produced by these phenomena has been focused as a possible source of information for the prediction of incipient movement, and the tracking of their path, velocity and thickness. Few literature works investigate these aspects, and for this reason, a preliminary laboratory and numerical campaign have been carried out with dry granular flume tests on an inclined chute. The discrete element method has been used to simulate the tests and to synthesize the signal measured on an instrumented mat along the channel. The grain shapes of the granular material used in simulations have been obtained by a photogrammetric tridimensional reconstruction. The force-time signal has been also analyzed in time-frequency domain in order to infer the features of the flow. The numerical activity has been preparatory for the experiments carried out by instrumenting the flume with an optical fiber distributed vibration sensing system