Miguel Soriano-Amat

Functional coatings on optical fibers enable selective detection of environmental and chemical parameters, but their use is typically limited to point or quasi-distributed sensing due to localized deposition techniques. In this work, we demonstrate a possible transition towards full-length functional coatings on optical fibers using a draw tower process, enabling their potential use in distributed sensor technology. An optical fiber with Pt:WO₃ nanocomposite polymer functional coating is employed as a proof of concept. The results demonstrate the successful application of this functional coating along hundreds of meters of fibers using a draw tower. When integrated into a distributed sensing configuration, the Pt:WO₃ fiber exhibited a clear change in response with varying hydrogen concentrations from 1% to 4% H₂, with a temperature increase of 2.5 °C at 4 vol.% indicating a promising performance for distributed hydrogen leak detection. This approach opens new opportunities for applying other functional coatings over extended fiber lengths using draw towers, which could be exploited for novel distributed sensing applications.

In symmetric dual-comb spectroscopy, two optical frequency combs with slightly different repetition rates are transmitted through a sample under test. Traditionally, this configuration has been regarded as suitable for measuring only the magnitude of the spectral response of the sample, with the phase information typically sacrificed in favor of a more robust setup against instabilities in the measurement paths, as compared to an asymmetric configuration. In this work, we formulate and demonstrate an algorithm capable of extracting the spectral phase information of a sample from the analysis of high-order Nyquist zones in a symmetric dual-comb spectroscopy configuration. The phase reconstruction strategy is generalized for both standard and quasi-integer ratio (QIR) dual-comb schemes. The capability of recovering both the magnitude and phase spectral profiles by using the generalized phase-reconstruction algorithm in a symmetric configuration is validated by means of two samples, a lossy fiber-based ring resonator and a traceable hydrogen cyanide (HCN) gas cell.

A recent advancement in distributed sensing known as the time-expanded phase-sensitive optical time-domain reflectometry (TE Φ-OTDR) addresses the trade-off between spatial resolution and detection bandwidth, enabling centimeter-scale resolution alongside RF detection bandwidth in the order of MHz. To date, TE Φ-OTDR approaches extract the fiber response from the first Nyquist zone (NZ). In this Letter, we propose a post-processing strategy to enhance the SNR by spectrally averaging different NZs. By evaluating 200 NZs, we have improved the average SNR of the traces by 23.56 dB compared to the standard demodulation scheme without degrading the acoustic sampling and the spatial resolution.

Shape sensing based on optical fiber sensing brings interesting advantages such as high sensitivity, lightweight, long-term reliability, and minimal intrusiveness, among others. However, the fiber interrogation methods employed to date, namely fiber Bragg gratings, OFDR or phase-sensitive (Φ)OTDR are either not entirely distributed, have poor performance in spatial resolution or dynamic sensing or are only valid for short (i.e., few meter-long) fibers. Here, we present a fully distributed optical fiber sensing system able to perform dynamic curvature and torsion sensing over a range of 125 m, with 10 cm resolution and at a sampling rate of 1 kHz. This performance is attained by interrogating three cores of a multi-core fiber (MCF) with time-expanded (TE-) ΦOTDR in its quasi-integer ratio (QIR) mode. With respect to the previously presented TE-ΦOTDR-based curvature sensing proof-of-concept, the use of QIR mode opens the possibility of effortlessly expanding range and/or sampling rate without a significant cost increase. Additionally, in this work the detection of fiber torsion is also addressed, offering for the first time to our knowledge the complete information towards fully distributed shape sensing at acoustic sampling rate with cm-scale resolutions over a record fiber length of >100 m. The unique sensing performance offered by the proposed system may become a valuable tool for a broad range of applications within civil and mechanical engineering, industrial manufacturing or seismology, among others.

We demonstrate spatially resolved sensing by a novel approach that combines an infrared camera and a simplified dual-comb illumination arrangement. Specifically, our scheme employs a continuous-wave laser and only one electro-optic modulator to simultaneously create a pair of mutually coherent optical frequency combs, each one with a slightly different line spacing. The system operates by measuring this dual-comb spectrum from a sequence of acquired images, in order to recover the spectral response of every spatial point of a sample. Thanks to its excellent stability, our approach ensures integration times well in excess of 10 s. The result is an utterly simple multi-spectral imager, capable of resolving up to 127 independent comb lines (spectral channels) across 16 k individual spatial positions, with a digitization sampling rate close to 1 kHz. As a proof of concept, we measure spatial variations of the refractive index of a low-reflectivity etalon undergoing a heating process. This sensing unit has a great potential to work as a field-deployable system for the determination of the spatial distribution of external perturbations such as temperature or strain. We comprehensively discuss the current advantages and limitations of our sensing approach, as well as its further developments.

Time-expanded phase-sensitive (TE-)ΦOTDR has emerged as a novel distributed acoustic sensing (DAS) technology capable of filling the performance gap between typical ΦOTDR and OFDR. It relies on the use of two optical frequency combs whose features are properly engineered to attain the desired DAS performance. Thus, centimeter resolution over kilometer range with kHz sampling rate is readily available, while demanding an extremely reduced detection bandwidth (in the MHz regime). This particular performance makes use of a specialty dual comb that boosts the system tunability, where the reference comb has a repetition rate that is a quasi-integer multiple of the probe's (i.e., the so-called quasi-integer ratio (QIR) TE-ΦOTDR). In this work, we perform, for the first time to our knowledge, a comprehensive analysis of the limits of measurable perturbation (comprising the dynamic range) of TE-ΦOTDR, paying special attention to the QIR scheme. We show that the dynamic range of regular TE-ΦOTDR is similar to that of typical phase demodulation ΦOTDR techniques. However, the particularities of the QIR scheme intrinsically reduce the dynamic range. We set the conditions to determine the maximum detectable perturbation based on the Carson's rule. In turn, the minimum measurable perturbation in the QIR scheme is related to the processing noise induced by the process of sample reordering in a time-varying fiber. Finally, we present a novel post-processing algorithm that mitigates such processing noise, hence improving the sensitivity (and therefore the dynamic range) of the QIR scheme. With the here proposed processing method, we experimentally demonstrate a fourfold improvement in performance of the QIR system with respect to the state of the art, delivering an impressive sensing rate of 100 million interrogated points per second with a sensitivity of -44 dB ref. 1μϵ ² /Hz and a detection bandwidth of only 100 MHz.

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.

Time-expanded phase-sensitive optical time-domain reflectometry is a distributed optical fiber sensing technology based on dual-frequency combs that allows for dynamic and high spatial resolution measurements while maintaining reduced detection requirements. Since the formalization of the technique, different experimental schemes have been satisfactorily tested, with a general performance of cm-scale spatial resolution over hundreds of meters. In this article, we present an optimized scheme with enhanced energy and spectral efficiencies that allows reaching 5 mm spatial resolution. As compared to previous experimental approaches, the presented architecture is based on a free-running dual comb setup generated through pure electro-optical phase modulation. Besides, the introduction of an optical hybrid in the detection stage allows for doubling the spatial resolution while keeping the refresh rate and the sensing range unchanged.

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 narrow-band photodetector. The employed optical dual comb can be implemented through electro-optical (EO) modulation driven by pseudo-random binary sequences (PRBS) at high data rates, commonly in the range of tens of Gbps. For that purpose, the run-time 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.