Spatially resolved dual-comb sensing with a single electro-optic modulator.

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.

Brillouin expanded time-domain analysis based on dual optical frequency combs.

Brillouin Optical Time-Domain Analysis (BOTDA) is a widely-used distributed optical fiber sensing technology employing pulse-modulated pump waves for local information retrieval of the Brillouin gain or loss spectra. The spatial resolution of BOTDA systems is intrinsically linked to pulse duration, so high-resolution measurements demand high electronic bandwidths inversely proportional to the resolution. This paper introduces Brillouin Expanded Time-Domain Analysis (BETDA) as a modified BOTDA system, simultaneously achieving high spatial resolution and low detection bandwidth.

Noise analysis in direct detection and coherent detection phase-sensitive optical time-domain reflectometry systems.

This study compares noise and signal-to-noise ratio (SNR) in direct detection and coherent detection fiber-based distributed acoustic sensing (DAS) systems. Both detection schemes employ the dynamic analysis of Rayleigh-backscattered light in phase-sensitive optical time-domain reflectometry (ΦOTDR) systems. Through theoretical and experimental analysis, it is determined that for photodetection filters with a sufficiently narrow bandwidth, the SNR performance of both detection schemes is comparable.

Dynamic curvature sensing using time expanded ΦOTDR.

Shape sensing can be accomplished using optical fiber sensors through different interrogation principles such as fiber Bragg gratings, optical frequency-domain reflectometry (OFDR), or optical time-domain reflectometry (OTDR). These techniques are either not entirely distributed, have poor performance in dynamic sensing, or are only valid for few-meter-long fibers. Here, we present a system able to perform distributed curvature sensing with a range of 125 m, 10-cm resolution, and a sampling rate of 50 Hz.

Time-expanded φOTDR using low-frequency electronics.

Time expanded phase-sensitive optical time-domain reflectometry (TE-φOTDR) is a recently reported technique for distributed optical fiber sensing based on the interference of two mutually coherent optical frequency combs. This approach enables distributed acoustic sensing with centimeter resolution while keeping the detection bandwidth in the megahertz range. In this paper, we demonstrate that TE-φOTDR can be realized with low-frequency electronics for both signal generation and detection.

Dual electro-optic comb spectroscopy using a single pseudo-randomly driven modulator.

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.

Cancellation of reference update-induced 1/f noise in a chirped-pulse DAS.

Distributed acoustic sensors (DAS) perform distributed and dynamic strain or temperature change measurements by comparing a measured time-domain trace with a previous fiber reference state. Large strain or temperature fluctuations or laser frequency noise impose the need to update such a reference, making it necessary to integrate the short-term variation measurements if absolute strain or temperature variations are to be obtained. This has the drawback of introducing a 1/f noise component, as noise is integrated with each cumulative variation measurement, which is detrimental to the determination of very slow processes (i.

Virtual transparency in ϕ-OTDR using second order Raman amplification and pump modulation.

In distributed optical fibre sensors, distributed amplification schemes have been investigated in order to increase the measurement range while avoiding the limitation imposed by the fibre attenuation and the nonlinear effects. Recently, the use of Raman amplification with an engineered intensity modulation has been demonstrated as an efficient way to produce a virtually lossless trace employing a single-end configuration. In this paper, we propose the combination of this technique with a simultaneous second order Raman pumping scheme for increasing the measurement range.

Quadratic phase coding for SNR improvement in time-expanded phase-sensitive OTDR.

Time-expanded phase-sensitive optical time-domain reflectometry (-) 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 -.

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.

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).

Towards Distributed Measurements of Electric Fields Using Optical Fibers: Proposal and Proof-Of-Concept Experiment.

Nowadays there is an increasing demand for the cost-effective monitoring of potential threats to the integrity of high-voltage networks and electric power infrastructures. Optical fiber sensors are a particularly interesting solution for applications in these environments, due to their low cost and positive intrinsic features, including small size and weight, dielectric properties, and invulnerability to electromagnetic interference (EMI). However, due precisely to their intrinsic EMI-immune nature, the development of a distributed optical fiber sensing solution for the detection of partial discharges and external electrical fields is in principle very challenging.

Long-Range Distributed Solar Irradiance Sensing Using Optical Fibers.

Until recently, the amount of solar irradiance reaching the Earth surface was considered to be a steady value over the years. However, there is increasing observational evidence showing that this quantity undergoes substantial variations over time, which need to be addressed in different scenarios ranging from climate change to solar energy applications. With the growing interest in developing solar energy technology with enhanced efficiency and optimized management, the monitoring of solar irradiance at the ground level is now considered to be a fundamental input in the pursuit of that goal.

Fast and direct measurement of the linear birefringence profile in standard single-mode optical fibers.

Phase birefringence in optical fibers typically fluctuates over their length due to geometrical imperfections induced from the drawing process or during installation. Currently commercially available fibers exhibit remarkably low birefringence, prompting a high standard for characterization methods. In this work, we detail a method that uses chirped-pulse phase-sensitive optical time-domain reflectometry to directly measure position-resolved linear birefringence of single-mode optical fibers.

Distributed sensing of microseisms and teleseisms with submarine dark fibers.

Sparse seismic instrumentation in the oceans limits our understanding of deep Earth dynamics and submarine earthquakes. Distributed acoustic sensing (DAS), an emerging technology that converts optical fiber to seismic sensors, allows us to leverage pre-existing submarine telecommunication cables for seismic monitoring. Here we report observations of microseism, local surface gravity waves, and a teleseismic earthquake along a 4192-sensor ocean-bottom DAS array offshore Belgium.

Fiber-based distributed bolometry.

Optical fibers are inherently designed to allow no interaction between the guided light and the surrounding optical radiation. Thus, very few optical fiber-based technologies exist in the field of optical radiation sensing. Accomplishing fully-distributed optical radiation sensing appears then as even more challenging since, on top of the lack of sensitivity explained above, we should add the need of addressing thousands of measurement points in a single, continuous optical cable.

Selective Monitoring of Oxyanion Mixtures by a Flow System with Raman Detection.

Raman spectroscopy is a selective detection system scarcely applied for the flow analysis of solutions with the aim of detecting several compounds at once without a previous separation step. This work explores the potential of a portable Raman system in a flow system for the selective detection of a mixture of seven oxyanions (carbonate, sulphate, nitrate, phosphate, chlorate, perchlorate, and thiosulphate). The specific bands of these compounds (symmetric stretching Raman active vibrations of carbonate at 1068 cm, nitrate at 1049 cm, thiosulphate at 998 cm, phosphate at 989 cm, sulphate at 982 cm, perchlorate at 935 cm, and chlorate at 932 cm) enabled their simultaneous detection in mixtures.

Sidelobe apodization in optical pulse compression reflectometry for fiber optic distributed acoustic sensing.

We demonstrate a technique to reduce the sidelobes in optical pulse compression reflectometry for distributed acoustic sensing. The technique is based on using a Gaussian probe pulse with linear frequency modulation. This is shown to improve the sidelobe suppression by 13 dB compared to the use of square pulses without any significant penalty in terms of spatial resolution.

Distributed photothermal spectroscopy in microstructured optical fibers: towards high-resolution mapping of gas presence over long distances.

Chemical sensing using optical fibers is often challenging, as it is generally difficult to achieve strong interaction between the guided light and the analyte at the wavelength of interest for performing the detection. Despite this difficulty, many schemes exist (and can be found in the literature) for point chemical fiber sensors. However, the challenge increases even further when it comes to performing fully distributed chemical sensing.

Long-range distributed optical fiber hot-wire anemometer based on chirped-pulse ΦOTDR.

We demonstrate a technique allowing to develop a fully distributed optical fiber hot-wire anemometer capable of reaching a wind speed uncertainty of ≈ ±0.15m/s (±0.54km/h) at only 60 mW/m of dissipated power in the sensing fiber, and within only four minutes of measurement time.

All-optical flip-flops based on dynamic Brillouin gratings in fibers.

A method to generate an all-optical flip-flop is proposed and experimentally demonstrated based on dynamic Brillouin gratings (DBGs) in polarization maintaining fibers. In a fiber with sufficiently uniform birefringence, this flip-flop can provide extremely long storage times and ultra-wide bandwidth. The experimental results demonstrate an all-optical flip-flop operation using phase-modulated pulses of 300 ps and a 1 m long DBG.

Phase-measuring time-gated BOCDA.

We demonstrate a simple scheme that allows performing distributed Brillouin phase spectrum (BPS) measurements with very high spatial resolution (∼7  cm) over long (∼4.7  km) optical fibers. This is achieved by inserting a Sagnac interferometer (SI) in a Brillouin optical correlation domain analysis (BOCDA) configuration.

Resolving 1 million sensing points in an optimized differential time-domain Brillouin sensor.

A differential pulse-width pair (DPP) Brillouin distributed fiber sensor is implemented to achieve centimetric spatial resolution over distances of several kilometers. The presented scheme uses a scanning method in which the spectral separation between the two probe sidebands is kept constant, while the optical frequency of the pump is swept to scan the Brillouin spectral response. Experimental results show that this method avoids detrimental temporal distortions of the pump pulses, which in a standard implementation prevent the DPP method from operating over mid-to-long distances.

SNR enhancement in high-resolution phase-sensitive OTDR systems using chirped pulse amplification concepts.

Phase-sensitive optical time-domain reflectometry (φOTDR) is widely used for the distributed detection of mechanical or environmental variations with resolutions of typically a few meters. The spatial resolution of these distributed sensors is related to the temporal width of the input probe pulses. However, the input pulse width cannot be arbitrarily reduced (to improve the resolution), since a minimum pulse energy is required to achieve a good level of signal-to-noise ratio (SNR), and the pulse peak power is limited by the advent of nonlinear effects.

A Novel Fiber Optic Based Surveillance System for Prevention of Pipeline Integrity Threats.

This paper presents a novel surveillance system aimed at the detection and classification of threats in the vicinity of a long gas pipeline. The sensing system is based on phase-sensitive optical time domain reflectometry (-OTDR) technology for signal acquisition and pattern recognition strategies for threat identification. The proposal incorporates contextual information at the feature level and applies a system combination strategy for pattern classification.