Few-mode fiber based distributed curvature sensor through quasi-single-mode Brillouin frequency shift.

Distributed fiber-optic sensors employ the conventional fiber as the sensing medium and allow all locations along the optical fiber to be measured simultaneously. Thus the sensing scheme provides an efficient way of simultaneously achieving multi-points sensing with a convenient and low-cost configuration. One of the most popular technologies that enables distributed sensing is based on Brillouin scattering. Characteristics of Brillouin scattering are determined mostly by the material properties of the optical fiber and the Brillouin Frequency Shift (BFS) has linear dependence on both the temperature and strain of the fiber under test (FUT). Based on this principle, intensive research efforts have been made in distributed Brillouin fiber sensors during the past two decades. Among them, Brillouin Optical Time-Domain Analysis (BOTDA) has attracted intensive research interest due to its promising properties for temperature and strain monitoring. In BOTDA the local Brillouin Gain Spectrum (BGS) is reconstructed by scanning the frequency offset of a continuous wave and an optical pulse around BFS when they are counter-propagating inside FUT, and the local BFS and hence the temperature and/ or strain information are retrieved accordingly. Usually curve fitting techniques are employed in determining the BFSs of the measured BGSs along the FUT, in which the measured BGS is fitted using an ideal curve and the BFS is found to be the frequency with the peak gain on the fitted curve. Fitting techniques using different lineshape for ideal curves, e.g. Lorentzian curve fitting (LCF), Gaussian curve fitting (GCF), pseudo-Voigt curve fitting (pVCF), and quadratic curve fitting (QCF) have been used to find the BFS distribution along FUT. However, the curve fitting techniques require careful initialization of model parameters and as many data points as possible collected on the measured BGS to ensure the fitting accuracy, otherwise the accuracy in BFS determination will degrade significantly. In addition, long processing time is needed for the iteration of fitting algorithms to find the BFS, especially when the sensing distance is long with a large number of BGSs collected and processed. Recently we have utilized the nonlinear mapping capability of Artificial Neural Network (ANN) in a BOTDA system and has successfully extracted temperature distribution from the measured BGSs along the FUT. Before temperature extraction ANN is trained to acquire the knowledge about the BGS patterns under different temperatures, thus it allows better accuracy even if the data points on the collected BGS become fewer when large frequency scanning step is adopted during the acquisition of BGSs. Moreover, the processing speed of ANN for temperature extraction is much faster than that of curving fitting techniques. In this presentation, we will review our work on using ANN for temperature extraction in a BOTDA system. Different idea spectrum profiles are used for training ANN, and the corresponding performances are compared with those using conventional curve fitting techniques. Some potential applications using ANN in BOTDA are also discussed. We believe that ANN can be an attractive tool for direct temperature or strain extraction in BOTDA system at high accuracy and fast speed.

Distributed sensor based on stimulated Brillouin scattering, especially Brillouin optical time-domain analysis (BOTDA) technique, attracts much interest recently due to its good performance of long sensing distance and high precision. In BOTDA system, the frequency fluctuation of the light sources is a serious problem, which will cause severe error of the measured Brillouin frequency shift (BFS), considering the narrow bandwidth of Brillouin spectrum. In this paper, a simple and low-cost method to compensate the error induced by frequency fluctuations of laser sources in a BOTDA system is proposed, by use of a local reference fiber which is loose and temperature monitored. The reference fiber, with several ten-meters long, can be part of the sensing fiber. Referring to the measured BFS of the reference fiber, one can obtain the errors induced by frequency fluctuations of light sources, and thus eliminate those errors. The proposed method is demonstrated by preliminary experiment. (3 pages)

Brillouin optical time-domain analysis (BOTDA) enables high-precision distributed sensing by extracting the Brillouin frequency shift (BFS). However, the BFS extraction based on traditional Lorentz curve fitting (LCF) suffers from limited accuracy and low efficiency. This work proposes a spatial-temporal feature fusion method integrating pseudo-video denoising (PVD) network and deep learning to improve the BFS extraction in BOTDA. In our proposal, a PVD network was introduced to improve the signal-to-noise ratio of temporal data by simulating video processing, utilizing multi-frame time redundancy to suppress noise while preserving Lorentz line-shape features. Furthermore, a spatial-temporal feature fusion network combining convolutional neural network (CNN), long short-term memory (LSTM), and multi-head attention (MHA) was constructed. After extracting local spectral features through CNN, LSTM was used to capture the long-term temporal dependence of Brillouin gain spectrum (BGS) signals along the fiber axis, and MHA was introduced to dynamically weight critical frequency points to suppress noise interference. Experiments on a BOTDA system with a sensing fiber of 1.02 km single-mode fibers (SMFs) demonstrated that the proposed method achieved a root mean square error (RMSE) of 0.333 MHz for the BFS extraction, benefiting a higher accuracy by 3.5% as well as 18 times faster processing speed with respect to that with the LCF method. This work provides a high-precision and time-efficient BFS extraction solution for distributed Brillouin sensing, especially for applications under complex noise conditions, exhibiting potentials in structural health monitoring for critical infrastructure such as energy pipeline monitoring, high voltage cable monitoring, and so on.

Few-mode fibers contain a limited number of orthogonal modes, and the Brillouin frequency shift of different modes has different sensitivities to temperature and strain, which provides the possibility of realizing multi-parameter sensing. This paper proposes a novel single-ended Brillouin optical time domain analysis (BOTDA) sensing system that uses Rayleigh backscattered light in the fiber as the probe light, and uses a four-mode graded-index few-mode fiber (4GI-FMF) and a photonic lantern to realize two-parameter sensing. The mechanism of single-ended BOTDA based on FMF is investigated, the modes of 4GI-FMF and the principle of two-parameter measurement are analyzed, and an experimental setup utilizing single-ended BOTDA system with 4GI-FMF is constructed. The performance of the sensing system is analyzed by characterizing and evaluating the Brillouin scattering spectral of different modes and the sensitivity of Brillouin frequency shift to temperature and strain. The proposed sensing system realized reliable temperature and strain discrimination on a 1 km long 4GI-FMF, with temperature and strain accuracies of 1.9 °C and 43.7 με, respectively.

The Brillouin optical time-domain analysis system (BOTDA) is a distributed optical fiber sensing system based on the measurement of the effective Brillouin gain, in which high power pumping pulse is demanded to fulfill optical time domain reflectometry (OTDR) spatial orientation. As for strict rectangular pulse, BGS can maintain Lorentz line profile along the sensor fiber, while the actual rectangular pulse has power transients like the rising edge and failing edge, making BGS broadened or even distorted by the effect of self-phase modulation (SPM), which will induce the decline of the measuring accuracy. A model concerning the effects of pumping pulse power transients on the BGS by means of SPM is established based on the nonlinear Schrodinger equation (NLSE) in regular single mode fiber (SMF).

In this paper, we have characterised Brillouin frequency shift (BFS) in single mode silica fibre. In particular, BFS was characterised in conventional Brillouin Optical Time Domain Analysis (BOTDA) at operating wavelength of 1550 nm by a pump-probe technique. In addition, we also presented Brillouin gain and Brillouin loss mechanism in BOTDA sensing system. The simulation results shows, BFS have strong linear relationship with acoustic velocity and applied strain along the sensing fibre. A frequency shift of 60 MHz was obtained for each 0.1% strain (1000 μ-strain) on fibre. From the measurement results, it was evident that the each μ-strain on the fibre the frequency shift was found as 0.06 MHz. Therefore, BOTDA sensing system based on BFS is a promoting technique for structural health monitoring.

The dependence of Brillouin linewidth and peak frequency on lightwave state of polarization (SOP) due to fiber inhomogeneity in single mode fiber (SMF) is investigated by using Brillouin optical time domain analysis (BOTDA) system. Theoretical analysis shows fiber inhomogeneity leads to fiber birefringence and sound velocity variation, both of which can cause the broadening and asymmetry of the Brillouin gain spectrum (BGS) and thus contribute to the variation of Brillouin linewidth and peak frequency with lightwave SOP. Due to fiber inhomogeneity both in lateral profile and longitudinal direction, the measured BGS is the superposition of several spectrum components with different peak frequencies within the interaction length. When pump or probe SOP changes, both the peak Brillouin gain and the overlapping area of the optical and acoustic mode profile that determine the peak efficiency of each spectrum component vary within the interaction length, which further changes the linewidth and peak frequency of the superimposed BGS. The SOP dependence of Brillouin linewidth and peak frequency was experimentally demonstrated and quantified by measuring the spectrum asymmetric factor and fitting obtained effective peak frequency respectively via BOTDA system on standard step-index SMF-28 fiber. Experimental results show that on this fiber the Brillouin spectrum asymmetric factor and effective peak frequency vary in the range of 2% and 0.06MHz respectively over distance with orthogonal probe input SOPs. Experimental results also show that in distributed fiber Brillouin sensing, polarization scrambler (PS) can be used to reduce the SOP dependence of Brillouin linewidth and peak frequency caused by fiber inhomogeneity in lateral profile, however it maintains the effects caused by fiber inhomogeneity in longitudinal direction. In the case of non-ideal polarization scrambling using practical PS, the fluctuation of effective Brillouin peak frequency caused by fiber inhomogeneity provides another limit of sensing frequency resolution of distributed fiber Brillouin sensor.

A self-heterodyne detection Rayleigh Brillouin optical time domain analysis system

The few-mode fiber (FMF) based Brillouin sensing operated in quasi-single mode (QSM) has been reported to achieve the distributed curvature measurement by monitoring the bend-induced strain variation. However, its practicality is limited by the inherent temperature-strain cross-sensitivity of Brillouin sensors. Here we proposed and experimentally demonstrated an approach for simultaneously distributed curvature and temperature sensing, which exploits a hybrid QSM operated Raman-Brillouin system in FMFs. Thanks to the larger spot size of the fundamental mode in the FMF, the Brillouin frequency shift change of the FMF is used for curvature estimation while the temperature variation is alleviated through Raman signals with the enhanced signal-to-noise ratio (SNR). Within 2 minutes measuring time, a 1.5 m spatial resolution is achieved along a 2 km FMF. The worst resolution of the square of fiber curvature is 0.333 cm-2 while the temperature resolution is 1.301 °C at the end of fiber.

In weakly coupled few mode fibers (FMFs), used in optical communication systems to increase data capacity, the difference between the effective refractive indices of successive spatial modes is a critical parameter. In this paper, we demonstrate that nanostructured FMFs composed of two different types of subwavelength glass rods can be used to effectively control the propagation properties of individual spatial modes. However, the design and optimization of such fibers proves challenging due to the large number of parameters associated with their free-form refractive index distribution. To address this problem, we use an optimization method based on generative inverse design networks (GIDNs) modified to account for the optical properties of weakly coupled FMFs. Our results show that a nanostructured FMF with a core containing multiple angularly modulated concentric rings can achieve differences between the effective refractive indices of 10 spatial modes exceeding 1.2 × 10−3, surpassing other forms of weakly coupled FMFs. Importantly, the parameters of the structure ensure the feasibility of fabricating the designed fibers. Compared to traditional FMF optimization using a deep neural network with a large, randomly generated data set, our method achieves comparable accuracy in predicting refractive index differences, while achieving better spatial mode separation with a six times smaller data set. By using our proposed approach, which could potentially be applied to the design of other types of optical fibers characterized by a large number of parameters, we can significantly reduce the number of training examples required and the time needed to achieve high-quality results.

Brillouin frequency shift (BFS) in a single-mode optical fiber has been measured as a function of both radial and axial strain via the Brillouin optical time domain analysis technique. The effects of the two kinds of strain on the BFS are decoupled by making fiber pretensioned and relaxed while applying pressure. Linear relations have been found between the BFS and both kinds of strain. The radial strain coefficient C(vε(r)) is found to be 0.029 MHz/με, and the axial strain coefficient C(vε(a)) is 0.053 MHz/με. The result may give impetus to some potential applications of the optical fibers, such as a distributed pressure sensor based on Brillouin scattering.

Increasing demand for high data transmission rates and bandwidth availability driven by the rapid development of the broadband services becomes challenging in the context of constraints imposed by nowadays exploited telecommunication optical fibers. Based on single-mode fibers and multiplexing in the wavelength domain (wavelength division multiplexing, WDM), the current technology seems to have reached its fundamental limits. At present, two promising technologies are a subject of intensive research. One of them is the increase of the data transmission through multiplying the number of fiber cores, thus implementing the concept of Multi-Core Fibers (MCF), where each core is used as a separate data transmission channel. The second widely investigated technique is based on the idea of mode division multiplexing (MDM), where different transverse modes of a Few-Mode Fiber (FMF) can be used as different carriers for data channels. In this work, we demonstrate the results of R&D works and the comprehensive tests of the few-mode fibers developed within the project NMKM+. Developed few-mode fibers (both passive and active) and their commercially available counterparts have been tested with respect of transmission parameters and applicability to the real telecom systems. In particular, guided mode profiles, numerical apertures, OTDR, and dispersion characteristics for the passive fibers have been recorded and compared. Also, methods of selective excitation and detection of singular modes have been discussed and partially verified. The results of data transmission experiments have been performed and discussed with respect to the quality of transmitted signals. These have been complemented by the amplification experiments with the use of erbium-doped few-mode fibers in “classical” and microstructural geometry. The results were studied and analyzed, showing the potential for future optimization. Acknowledgement: This work has received support from the National Centre for Research and Development through project NMKM+ (TECHMATSTRATEG1/348438/16/NCBR/2018).

A high sensitivity strain sensor using a sandwich structure of "single mode fiber (SMF)-few mode fiber (FMF)-single mode fiber (SMF)" was proposed and experimentally validated. The designed sensor is achieved by splicing a segment of FMF between two segments of SMFs, and then using a fiber optic fusion tapering machine to double the length of FMF. Introducing tapered optical fibers into the structure to excite more evanescent waves improves the sensitivity of the sensor to the surrounding environment. In addition, due to the fact that the FMF is tapered into a very fine shape, the tensile stress applied to the FMF will increase. Therefore, conical FMF has excellent stress concentration ability, which is easily deformed under stress, thus achieving a high strain sensitivity of -23.9 pm/με. Finally, a cascaded FBG was used to compensate for the temperature cross-sensitivity of the sensor. This strain sensor with an extremely simple structure and high sensitivity has wide application value in the industry.

We propose a unique few-mode fiber (FMF)-based multi-parameter optical-fiber sensor for distributed measurement of temperature and strain. We launch a pump and a probe signal through specific linearly polarized modes into a FMF, and monitor the Brillouin frequency shift (BFS) in each mode. By analyzing the temperature and strain coefficients of the BFS of the two lowest order (LP(01), LP(11)) modes, discrimination of temperature and strain are successfully demonstrated, with an accuracy of 1.2°C and 21.9 με.

A novel model for compensating the nonlocal effects is proposed in BOTDA. A basic experimental configuration is only required. Experimental results show that a hotspot at 39.1 km can be accurately measured under probe power from -14 dBm to +2 dBm, and a 13.5 MHz Brillouin frequency shift error is corrected.