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Standard multimode optical fibers normally support transmission over some 100 modes. Large differences in the propagation constant and the spatial distribution of distinct modes degrade the performance of phase-sensitive optical time-domain reflectometry measurements. In this work, we present a new realization of a coherent time-domain interrogation technique using single-mode operation in multimode fibers. We demonstrate effectively distributed strain sensing on three different multimode optical fibers. Up to 4 km of multimode fiber has been correctly interrogated, featuring a spatial resolution of 20 cm.
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... Ces différences entre les constantes de propagation est par exemple problématique pour interroger les fibres multimodes à l'aide de méthodes OTDR (Optical Time-Domain Reflectometry), où la connaissance de la vitesse de propagation du signal est cruciale pour remonter à la position d'un défaut. Néanmoins, éliminer les modes d'ordre élevé à l'aide d'un filtre adapté  permet de réaliser de telles mesures. Dans un modèle à rayons, le diamètre de coeur des fibres multimodes implique également des variations de l'angle d'incidence sur le dioptre à l'extrémité de la fibre. ...
URL : http://www.theses.fr/2021NANT4049 - FR : La mesure de l'indice optique d’un milieu est un moyen d'obtenir des informations comme les éléments chimiques qui le composent, sa température ou sa pression. Le capteur dit de Fresnel, basé sur la mesure de la puissance réfléchie à l’extrémité d’une fibre optique monomode, est un moyen simple de réaliser des mesures d’indice optique, et est adapté à des mesures in-situ de réticulation ou de teneur en eau dans des structures composites. Néanmoins, ce capteur ponctuel ne permet de faire des mesures que sur une petite zone à l'extrémité de la fibre, le rendant très sensible aux perturbations locales. Cette thèse étudie la possibilité d’utiliser des structures de fibres spéciales en tant que capteur de Fresnel, d’un point de vue théorique et expérimental, afin d’élargir le champ de mesures possibles. Les fibres optiques biréfringentes sont d’abord considérées, avec l'objectif de réaliser des mesures d'anisotropie optique. Les fibres multimodes sont ensuite étudiées afin d'augmenter sensiblement le volume de mesure grâce à leur diamètre de coeur important, et de valider l’emploi de fibres plastiques à bas coût en tant que capteur de Fresnel. Enfin, l'utilisation de la spectroscopie sur un capteur de Fresnel est envisagée, avec l'objectif d'utiliser les possibles bandes d'absorption dans un milieu pour obtenir des informations supplémentaires sur ce dernier, ou bien de prendre en compte la diffusion de la lumière par des particules situées à l'extrémité de la fibre.
... The derived model is valid for any system for which the peak value of a resonance is evaluated through quadratic least-square fitting. In the case of coherent Rayleigh-based DOFS, for instance in direct-detection frequency-scanned φ-OTDR systems, the most widely and commonly-used method to estimate the relative value of the FS between reference and measurement signals is cross-correlation . Cross-correlation is a standard method utilised for delay estimation in sonar and radar systems , and is also adopted in other coherent Rayleigh-based DOFS . The presence of unavoidable additive noise in the traces being correlated fundamentally limits the performance of the cross-correlation estimator and leads to uncertainty in the estimated FS. Besides, other experimental parameters, such as spatial resolution, can also influence the accuracy of estimation. ...
... Recently, multimode fiber has been reported to be used in φ-OTDR systems . A typical multimode fiber can usually support hundreds of spatial modes, which cannot be easily and separately demodulated with standard photodetectors in conventional φ-OTDR systems. ...
We propose and experimentally demonstrate a novel interference fading suppression method for phase-sensitive optical time domain reflectometry (Phi-OTDR) using space-division multiplexed (SDM) pulse probes in few-mode fiber. The SDM probes consist of multiple different modes, and three spatial modes (LP01, LP11a and LP11b) are used in this work for proof of concept. Firstly, the Rayleigh backscattering light of different modes is experimentally characterized, and it turns out that the waveforms of Phi-OTDR traces of distinct modes are all different from each other. Thanks to the spatial difference of fading positions of distinct modes, multiple probes from spatially multiplexed modes can be used to suppress the interference fading in Phi-OTDR. Then, the performances of the Phi-OTDR systems using single probe and multiple probes are evaluated and compared. Specifically, statistical analysis shows that both fading probabilities over fiber length and time are reduced significantly by using multiple SDM probes, which verifies the significant performance improvement on fading suppression. The proposed novel interference fading suppression method does not require complicated frequency or phase modulation, which has the advantages of simplicity, good effectiveness and high reliability.
We propose and experimentally demonstrate a novel interference fading suppression method for phase-sensitive optical time domain reflectometry (φ-OTDR) using space-division multiplexed (SDM) pulse probes in a few-mode fiber. The SDM probes consist of multiple different modes, and three spatial modes (LP01, LP11a, and LP11b) are used in this work for the proof of concept. Firstly, the Rayleigh backscattering light of different modes is experimentally characterized, and it turns out that the waveforms of the φ-OTDR traces for distinct modes are all different and independent. Thanks to the spatial difference of the fading positions for distinct modes, multiple probes from spatially multiplexed modes can be used to suppress the interference fading in φ-OTDR. Then, the performances of the φ-OTDR systems using a single probe and multiple probes are evaluated and compared. Specifically, the statistical analysis shows that the fading probabilities over both the fiber length and the time scale are reduced significantly by using multiple SDM probes, which verifies the significant performance improvement on fading suppression. By introducing the concept of SDM to φ-OTDR, the proposed novel interference fading suppression method avoids the complicated frequency or phase modulation, which has the advantages of simplicity, good effectiveness and high reliability.
While the vast majority of Φ-OTDR sensors use single mode fiber, multimode fiber is also widely deployed by the telecom industry. From a sensor design perspective, multimode fiber also offers advantages compared with single mode fiber, such as higher nonlinear thresholds and immunity to interference fading. However, most attempts to perform distributed strain sensing in a multimode fiber rely on interrogation systems designed for single mode fiber. As a result, these systems discard most of the backscattered light by coupling the multimode fiber under test to a single mode fiber based receiver. In this work, we present a technique that combines a high-speed camera with a time-gated local oscillator to construct a distributed multimode fiber sensor capable of using the entire backscattered speckle field. We demonstrate quantitative, fully distributed strain sensing across a 2 km multimode fiber with a spatial resolution of 20 m, a bandwidth of 400 Hz, and a noise floor of −61 dB re rad2/Hz (4.9 pε/√Hz). The same system can be electronically reconfigured to probe any single sensor position with a bandwidth of up to 20 kHz and a noise floor of −86 dB re rad2/Hz (0.27 pε/√Hz).
Commonly, the frequency shift of back-reflection spectra is the key parameter to measure quantitatively local temperature or strain changes in frequency-scanned Rayleigh-based distributed fiber sensors. Cross-correlation is the most common method to estimate the frequency shift; however, large errors may take place, particularly when the frequency shift introduced by the temperature or strain change applied to the fiber is beyond the spectral width of the main correlation peak. This fact substantially limits the reliability of the system, and therefore requires careful analysis and possible solutions. In this paper, an analytical model is proposed to thoroughly describe the probability of large errors. This model shows that the cross-correlation intrinsically and inevitably leads to large errors when the sampled signal distribution is finite, even under perfect signal-to-noise ratio. As an alternative solution to overcome such a problem, least mean squares is employed to estimate the frequency shift. In addition to reducing the probability of large errors, the proposed method only requires to measure a narrow spectrum, significantly reducing the measurement time compared to state-of-the-art implementations. Both the model and the solution are experimentally verified using a frequency-scanned phase-sensitive optical time-domain reflectometry (
-OTDR) system, achieving a spatial resolution of 5 cm, with a sensing range of 860 m and an acquisition time below 15 s, over a measurable temperature range of more than 100 K with a repeatability of 20 mK, corresponding to a temperature dynamic range of 5000 resolved points.
The extremely low loss of silica fibers has enabled the telecommunication revolution, but single-mode fiber-optic communication systems have been driven to their capacity limits. As a means to overcome this capacity crunch, space-division multiplexing (SDM) using few-mode fibers (FMF) has been proposed and demonstrated. In single-mode optical fibers, Rayleigh scattering serves as the dominant mechanism for optical loss. However, to date, the role of Rayleigh scattering in FMFs remains elusive. Here we establish and experimentally validate a general model for Rayleigh scattering in FMFs. Rayleigh backscattering not only sets the intrinsic loss limit for FMFs but also provides the theoretical foundation for few-mode optical time-domain reflectometry, which can be used to probe perturbation-induced mode-coupling dynamics in FMFs. We also show that forward inter-modal Rayleigh scattering ultimately sets a fundamental limit on inter-modal-crosstalk for FMFs. Therefore, this work not only has implications specifically for SDM systems but also broadly for few-mode fiber optics and its applications in amplifiers, lasers, and sensors in which inter-modal crosstalk imposes a fundamental performance limitation.
An optical time domain reflectometry apparatus for sensing a parameter in a region of interest is characterized in that the optical fiber includes a first section into which optical radiation at the probe wavelength is launched and a second section deployed in the region of interest. The first section has a higher intensity threshold for the onset of non-linear effects than the second section. The source launches the optical radiation into the first section at an intensity lower than the non-linear effects intensity threshold of the first section but higher than the non-linear effects intensity threshold of the second section. The attenuation characteristics of the first section are chosen such that the intensity of the optical radiation at the probe wavelength that reaches the second section is below the threshold for the onset of non-linear effects of the second section.
In an optical time domain reflectometry method and apparatus, in which optical radiation at a first wavelength
(l0) is launched into a fibre, deployed through a region of interest, and back-scattered optical radiation in first and second spectral bands, centred respectively on said first wavelength (l0) and a second wavelength (l-1) equal to the wavelength of an anti-Stokes spectral band which results from inelastic scattering in the fibre of optical radiation at said first wavelength (l0), is used to produce respective first and second sets of output signals, non-simultaneously with optical radiation at said first wavelength (l0), optical radiation substantially at
said second wavelength (l-1') is launched into the fibre and back-scattered optical radiation in a third spectral
band, centred on said second wavelength (l-1), is used to produce a third set of output signals, and a final set
of output signals, dependent upon the values being sensed, is produced by normalising the first set of output
signals to the geometric mean of the second and third sets of output signals.
An overview of fibre-optic interferometry based sensing is given, particularly as it applies to high-performance sensing applications. The operation of a fibre-optic interferometer as a sensor is reviewed. The sensitivity limitations of a fibre-optic sensor are derived, and the system impact of multiplexing many sensors together is explored. A review of the development of the fibre-optic acoustic transducer is presented, as well as system applications and future trends in fibre-optic interferometric sensing.
Raman Distributed Temperature Sensors (RDTS) are attractive for the monitoring of large structures in nuclear power plants such as containment structures and coolant loop systems. We demonstrate the high radiation tolerance of a Raman distributed fiber optic temperature sensor, up to total gamma doses in excess of 300 kGy, using a double-ended configuration and commercially-available optical fibers.
The diversity of spatial modes present within a multimode fiber has been exploited for a wide variety of imaging and sensing applications. Here, we show that this diversity of modes can also be used to perform quantitative strain sensing by measuring the amplitude of the Rayleigh backscattered speckle pattern in a multimode fiber. While most Rayleigh based fiber sensors use single mode fiber, multimode fiber has the potential to provide lower noise due to the higher capture fraction of Rayleigh scattered light, higher non-linear thresholds, and the ability to avoid signal fading by measuring many spatial modes simultaneously. Moreover, while amplitude measuring single mode fiber based Rayleigh sensors cannot provide quantitative strain information, the backscattered speckle pattern formed in a multimode fiber contains enough information to extract a linear strain response. Here, we show that by tracking the evolution of the backscattered speckle pattern, the sensor provides a linear strain response and is immune to signal fading. The sensor has a noise floor of 2.9 pɛ/√Hz, a dynamic range of 74 dB at 1 kHz, and bandwidth of 20 kHz. This work paves the way for a new class of fiber optic sensors with a simplified design and enhanced performance.
Considering middle-term predictions of the need for commercial 10-Tb/s optical interfaces working in 1-P b/s optical transport systems by 2024 and recalling the introduction of the optical bypass when entering the wavelength-abundant era in the early 2000s, we re-evaluate the values of hierarchical optical network architecture in light of the forthcoming massive spatial division multiplexing (SDM) era. We introduce a spatial channel (SCh) network (SCN) architecture, where the SDM layer is explicitly defined as a new networking layer that supports the new multiplexing technology of SDM. In an SCN, optical channels (OChs) accommodated in an express SCh bypass the overlying wavelength cross-connects (WXCs) using spatial cross-connects (SXCs) on the route. As one challenge that SCNs will present, we point out that an excessively large SXC insertion loss reduces the optical reach for spectrally groomed OChs. We show that the optical reach of spectrally groomed OChs can be maintained at almost the same level as that for an OCh transported through a conventional single-layer WDM network thanks to the low-loss features of commercially available spatial switches and foreseeable low-loss SDM multiplexers and demultiplexers. As another challenge, we discuss how to achieve growable, reliable, and cost-effective SXCs. We propose two SXC architectures based on sub-matrix switches and core-selective switches. Simple cost assessment shows that the proposed architectures are more cost-effective than the full-size matrix switch-based architecture with 1+1 equipment protection and conventional stacked WXC architecture.
Celebrating the 20th anniversary of Optics Express, this paper reviews the evolution of optical fiber communication systems, and through a look at the previous 20 years attempts to extrapolate fiber-optic technology needs and potential solution paths over the coming 20 years. Well aware that 20-year extrapolations are inherently associated with great uncertainties, we still hope that taking a significantly longer-term view than most texts in this field will provide the reader with a broader perspective and will encourage the much needed out-of-the-box thinking to solve the very significant technology scaling problems ahead of us. Focusing on the optical transport and switching layer, we cover aspects of large-scale spatial multiplexing, massive opto-electronic arrays and holistic optics-electronics-DSP integration, as well as optical node architectures for switching and multiplexing of spatial and spectral superchannels.
We propose and demonstrate a method to perform quantitative phase-sensitive optical time domain reflectometry (Φ-OTDR) using multimode fiber. While most Φ-OTDR sensors use single-mode fiber, multimode fiber exhibits higher thresholds for non-linear effects, a larger capture fraction of Rayleigh backscattered light, and the potential to avoid signal fading by detecting many spatial modes in parallel. Previous multimode fiber based OTDR sensors discarded most of the backscattered light and thus failed to take advantage of these noise-reducing factors. Here, we show that by performing off-axis holography with a high-speed camera, we can record the entire Rayleigh backscattered field, maximizing the detected light level and making the sensor immune to fading. The sensor exhibits a high degree of linearity, a minimum phase noise of -80 dB [rel. rad2/Hz], and 20 kHz bandwidth.
A method based on coherent Rayleigh scattering distinctly evaluating temperature and strain is proposed and experimentally demonstrated for distributed optical fiber sensing. Combining conventional phase-sensitive optical time-domain domain reflectometry (φOTDR) and φOTDR-based birefringence measurements, independent distributed temperature and strain profiles are obtained along a polarization-maintaining fiber. A theoretical analysis, supported by experimental data, indicates that the proposed system for temperature-strain discrimination is intrinsically better conditioned than an equivalent existing approach that combines classical Brillouin sensing with Brillouin dynamic gratings. This is due to the higher sensitivity of coherent Rayleigh scatting compared to Brillouin scattering, thus offering better performance and lower temperature-strain uncertainties in the discrimination. Compared to the Brillouin-based approach, the φOTDR-based system here proposed requires access to only one fiber-end, and a much simpler experimental layout. Experimental results validate the full discrimination of temperature and strain along a 100 m-long elliptical-core polarization-maintaining fiber with measurement uncertainties of ~40 mK and ~0.5 με, respectively. These values agree very well with the theoretically expected measurand resolutions.
We propose and evaluate performance of the higher order mode filter for 850 nm multimode fiber transmission. The developed passive component reduces impact of mode dispersion on the systems performance. Excellent operation in the 850 nm transmission experiments is shown.
We investigate the influence of air holes on phase sensitivity in microstructured optical fibers to longitudinal strain. According to the numerical simulations performed, large air holes in close proximity to a fiber core introduce significant compression stress to the core, which results in an increase in the effective refractive index sensitivity to longitudinal strain. The theoretical investigation is verified by an experiment performed on four fibers drawn from the same preform and differentiated by air hole diameter. We show that introducing properly designed air holes can lead to a considerable increase in normalized effective refractive index sensitivity to axial strain from -0.21 e-1 (for traditional single mode fiber) to -0.14 e-1.
Space division multiplexing (SDM) is mainly seen as a means to increase data throughput and handle exponential traffic growth in future optical networks. But its role is certainly more diverse. Research on SDM encourages device integration, brings newfunctionality to network elements, and helps optical networks to evolve. As a result, the number of individual components in future networks will decrease, which in turn will improve overall network reliability and reduce power consumption as well as operational expenditure. After reviewing the state-of-the-art in SDMfiber research and development with a particular focus on weakly coupled single-mode multi-core fibers, we take a look beyond the capabilities of SDM as a means of boosting transmission capacity and discuss ideas and concepts on howto exploit the spatial dimension for improved efficiency and resource sharing in optical networks.
In the present paper we propose a novel type of a coherent phase-sensitive optical time-domain reflectometer (OTDR) that utilizes a multimode optical fiber as a sensitive element and is capable of considerable reduction of signal fading. Elimination of OTDR signal fading consequently removes randomly occurring insensitivity of the fiber regions to an external phase action. The backscattered light field at the input of OTDR sensitive multimode optical fiber is represented by a speckle-like pattern, due to a so called modal noise phenomenon. This speckle pattern randomly changes when an optical probe pulse propagates in the fiber line. The backscattered field intensity in every single speckle changes in time statistically independently from the intensity change in every other speckle remote enough from the first one. Thus, on the output of a multimode sensitive fiber, there exist several statistically independent reflectograms, and every single reflectogram contains the same information about external action. The joint independent analysis of these reflectograms can result in reduced or complete fading elimination.
The spectrum of the temporal traces obtained from a phase-sensitive optical time-domain reflectometer is theoretically and experimentally analysed, demonstrating its dependence on the incident optical pulse shape. Numerical simulations and theoretical results are validated experimentally, showing a good matching for rectangular optical pulses. The influence of the photodetector bandwidth on the temporal trace quality is also investigated by simulation and experiment. Results show that the photodetector bandwidth needs to be ~ 40 % wider than the pulse spectrum to acquire time-domain traces of the Rayleigh backscattered light with direct detection.
Extensive research on Brillouin- and Raman-based distributed optical fibre sensors over the past two decades has resulted in the commercialization of distributed sensors capable of measuring static and quasi-static phenomena such as temperature and strain. Recently, the focus has been shifted towards developing distributed sensors for measurement of dynamic phenomena such as dynamic strain and sound waves. This article reviews the current state of the art distributed optical fibre sensors capable of quantifying dynamic vibrations. The most important aspect of Rayleigh and Brillouin scattering processes which have been used for distributed dynamic measurement are studied. The principle of the sensing techniques used to measure dynamic perturbations are analyzed followed by a case study of the most recent advances in this field. It is shown that the Rayleigh-based sensors have longer sensing range and higher frequency range, but their spatial resolution is limited to 1 m. On the other hand, the Brillouin-based sensors have shown a higher spatial resolution, but relatively lower frequency and sensing ranges.
Structural Health Monitoring (SHM) can be understood as the integration of
sensing and intelligence to enable the structure loading and
damage-provoking conditions to be recorded, analyzed, localized, and
predicted in such a way that nondestructive testing becomes an integral part
of them. In addition, SHM systems can include actuation devices to take
proper reaction or correction actions. SHM sensing requirements are very
well suited for the application of optical fiber sensors (OFS), in
particular, to provide integrated, quasi-distributed or fully distributed
technologies. In this tutorial, after a brief introduction of the basic SHM
concepts, the main fiber optic techniques available for this application are
reviewed, emphasizing the four most successful ones. Then, several examples
of the use of OFS in real structures are also addressed, including those
from the renewable energy, transportation, civil engineering and the oil and
gas industry sectors. Finally, the most relevant current technical
challenges and the key sector markets are identified. This paper provides a
tutorial introduction, a comprehensive background on this subject and also a
forecast of the future of OFS for SHM. In addition, some of the challenges
to be faced in the near future are addressed.
A distributed sensor system for detecting and locating intruders based on the phase-sensitive optical-time-domain reflectometer (φ-OTDR) is described. The sensing element is a cabled single-mode telecommunications fiber buried along the monitored perimeter. Light pulses from a continuous-wave Er:fiber Fabry-Pe´rot laser with a narrow (≈3 kHz) instantaneous linewidth and low (few kilohertz per second) frequency drift are injected into one end of the fiber, and the backscattered light is monitored with a photodetector. The effect of phase changes resulting from the pressure of the intruder on the ground immediately above the buried fiber are sensed by subtracting a φ-OTDR trace from an earlier stored trace. In laboratory tests with fiber on reels, the effects of localized phase perturbations induced by a piezoelectric fiber stretcher on φ-OTDR traces were observed. In field tests, people walking on the ground above a buried fiber cable induced phase shifts of several-π radians.
Distributed vibration sensing system using multimode fiber