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In practical engineering applications, the real-time monitoring of structural physical field is very important, which requires overall performance improvements on the Brillouin optical fiber sensing, including fast measurement, quick data demodulation and small amount of data storage. A novel Brillouin optical time-domain analysis (BOTDA) scheme has been proposed based on compressed sensing and pattern recognition algorithms, which paves the way for the real time Brillouin optical fiber sensing. In order to test the sensing performance, the Brillouin gain spectrum is measured by the traditional fast BOTDA, where the frequency step and span are 4 MHz and 500 MHz, respectively. By employing the proposed algorithms, the strain information is directly obtained with only 35% of the full data, verifying the feasibility of real-time measurements.

The radiation recovery effect on sensing fibre in Brillouin optical time-domain reflectometer (BOTDR) has been experimentally investigated. In the experiment, three kinds of commercial single-mode G.652.D fibres: Yangtze Optical Fibre and Cable, Corning and Fujikura have been used as sensing fibre in BOTDR system. After 200 Gy dose gamma-ray radiation absorbed by sensing fibre, the performance of BOTDR at different time intervals has been tested. Moreover, by analysing Brillouin gain spectra along with fibre sample, the recovery in radiation-induced attenuation, the variation of Brillouin frequency shift and the variation of stress coefficient are characterised. The results will be helpful for the error correction in the simulation experiment and are a reliable basis for the introduction of BOTDR into the space station.

A thorough study of elastic waves in waveguides, taking into account the full tensorial nature of the stiffness tensor, is presented in the context of stimulated Brillouin scattering. Various approximations of the elastic wave equation used in the stimulated Brillouin scattering literature are implemented and their validity and applicability are discussed. The developed elastic wave mode-solver is also coupled with an electromagnetic counterpart to study the influence of elastic anisotropies on Brillouin gain.

Ultra-narrow spectral features are desirable for a broad range of applications, and they are conventionally realized using ultrahigh Q resonant structures. These structures typically require precision fabrication processes, and moreover, since they are passive, they suffer from signal loss. Here, we demonstrate a novel way to achieve sub-MHz tunable spectral dip in the Brillouin gain spectrum of a spun birefringent fiber (SBF) without loss, and without using a resonator. We show that this dip is unique to SBF, where its polarization eigenmodes are elliptical and frequency-dependent, and the dip only occurs when these orthogonal polarization eigenmodes of the SBF (at the respective pump and signal frequencies) are launched in counter-propagating directions. We experimentally demonstrate a 0.72 MHz spectral dip in the Brillouin gain spectrum of a commercial SBF which is to our knowledge, the narrowest SBS spectral feature ever reported. Furthermore, the linewidth, depth, and spectral location of this dip are tunable on demand by controlling the pump frequency, pump power, and the input polarization of the signal. Its simplicity in implementation, its ultra-narrow linewidth, and its tunability can have a wide range of potential applications, from slow-light to microwave photonics.

Using a stimulated Brillouin gain spectroscopy technique, we have measured the equation of state of bulk liquid helium-4 at $\ensuremath{\sim}1$ K between 0 and 10 bars. Our results are in very good agreement with previous measurements and with theoretical equations of state. However, our specific method allows one to determine it on space/timescales as narrow as 35 $\ensuremath{\mu}\mathrm{m}$/190 ns. This technique is of particular interest for solving a remaining debate about the value of the homogeneous cavitation pressure of liquid $^{4}\mathrm{He}$ at $T\ensuremath{\sim}1$ K.

In this paper, we introduce the basic principle of the Brillouin optical time domain analysis (BOTDA) system and simulate the gain-type BOTDA system. In order to extract the Brillouin frequency shift more accurately, the filtering method is vitally important. In this letter, Gaussian filtering, median filtering and hybrid filtering are used to remove the noise respectively. It is observed that hybrid filtering has the best performance among the three methods, which can improve the signal-to-noise ratio by 9.18 dB. Besides, a peak location method is proposed to quickly extract the center frequency of the filtered Brillouin gain spectrum. This method reduces processing complexity by omitting Lorentz fitting and it possesses the advantages of fast processing rate and high accuracy. Therefore, the peak location method we proposed can be considered as an attractive method for extracting temperature distributions.

We present a microwave photonic bandpass filter with 52dB out-of-band re- jection exploiting interference between orthogonally polarized Brillouin gain resonances. Signal recovery in the presence of an interferer at 500MHz from signal is demonstrated.

Structural damage generally initiates in the form of structural cracks. Thus, developing efficient crack detection techniques is of great importance for the structural health monitoring. In this paper, a new crack identification method is proposed, which is based on the differential pulse-width pair Brillouin optical time domain analysis (DPP-BOTDA) technology and the irregular features of Brillouin gain spectrum (BGS) in the fiber due to structural cracks. The proposed method provides a new way to detect and quantify structural cracks without knowing the strain in the structure. First, the working mechanism of DPP-BOTDA is introduced to illustrate the reason that the DPP-BOTDA, compared to traditional BOTDA technique, can significantly improve the spatial resolution of distributed strain sensing, which is critical for structural crack detection. Then, the BGSs in the fiber with the presence of structural cracks, measured by the DPP-BOTDA, are numerically simulated, from which the crack-induced irregular features of the BGS are summarized. Based these irregular features, new structural crack detection and quantification methods are proposed, which are found to be independent of structural stain. Finally, an experiment is conducted on a simple supported reinforced concrete (RC) beam. The results demonstrate that by using the BGS measured by the DPP-BOTDA, the proposed structural crack identification method successfully detects the occurrence of structural cracks and relatively accurately predicts the crack widths.

We propose a novel method to extract Brillouin frequency shift (BFS) from Brillouin Gain Spectrum (BGS) in Brillouin distributed fiber sensors. The method is based on machine learning of nearest neighbors. In order to find the BFS from the BGS, we design two datasets, one for storing all possible BGS, and the other for storing the corresponding BFS. By comparing the given BGS with the dataset of BGS, we get the minimal kth BFS. The BFS of the given BGS is determined by voting of the kth BFS. By simulations, we compare the performance of both neighbor-based machine learning and curve-fitting. The results show that the method of neighbor-based machine learning is more robust under a wide range of signal-to-noise ratios, pump pulse widths, and frequency scanning steps. The extracting method of neighbor-based machine learning is highly competitive for future Brillouin distributed fiber sensors.

Demonstrated is a unique method for sensing performance enhancement in Brillouin optical time-domain analyzers (BOTDA). A Brillouin gain spectrum (BGS) is superimposed with two symmetric Brillouin loss spectra (BLS). This leads to a complex engineered spectrum shape that is more resistant to the sensing system noise. Instead of only one pump and probe interaction as in the conventional BOTDA setup, three optical probe waves are exploited, with one probe located in the BGS and the other two symmetrically in the BLS. Due to the resistance and insensitivity of the engineered spectrum shape to the noise, the sensing performance is enhanced by 60% and the measurand resolution is doubled.

Brillouin spectrum engineering for the enhancement of slope-assisted Brillouin dynamic sensing is investigated by simulation. By superimposing the Brillouin gain with Brillouin losses, the tradeoff between the dynamic range and the frequency-to-amplitude sensitivity becomes more flexible. Compared to a conventional dynamic Brillouin sensor, a simultaneous enhancement of the dynamic range by 60.27% and the sensitivity by 51.21% has been achieved with the proper parameters. Due to these enhancements, the proposed sensor is 33.87% more robust to system noise and provides a more than 2.2 times of accuracy improvement for strain signal recovery.

The enhancement of stimulated Brillouin scattering in nano-scale waveguides has a great contribution in the improvement of the photonic devices technology. The key factors in Brillouin gain are the electrostriction force and radiation pressure generated by optical waves in the waveguide. In this article, we have proposed a new scheme of nano-scale waveguide in which the Brillouin gain is considerably improved compared to the previously-reported schemes. The role of radiation pressure in the Brillouin gain was much higher than the role of the electrostriction force. The Brillouin gain strongly depends on the structural parameters of the waveguide and the maximum value of 12127 W −1 m −1 is obtained for the Brillouin gain.

The dependence of Brillouin gain spectrum (BGS) characteristics, including the Brillouin frequency shift (BFS) and the BGS bandwidth, on germanium concentration in large-mode-area Ge-doped passive fibers is investigated theoretically and experimentally. The simulation results show that the BFS is inversely proportional to GeO2 concentration, and the BGS bandwidth initially increases with the augment of GeO2 concentration, and then decreases. The BGSs of four fibers with core diameters of 10μm and 20μm for different GeO2 concentrations are compared experimentally. Experimental results demonstrate that with the same core diameter, the variations of BFS and BGS bandwidths with GeO2 concentration accord with the simulation results. Additionally, the BGS characteristics of three large-mode-area passive fibers with diameters of 10μm, 25μm, and 30μm are measured, which confirm that the increasing of the fiber diameters will cause the BGS bandwidth to broaden. We believe that these results can provide valuable references for modulating the high-power narrow-linewidth fiber lasers and Brillouin fiber amplifiers.

The robustness of a recently proposed Brillouin optical time-domain analysis sensor based on dual-tone probe wave with fixed frequency separation is deeply studied. It is discovered that when the distribution of Brillouin frequency shift (BFS) along the sensing fiber is not entirely uniform, non-local effect might be imposed at the middle of sensing fiber, which gives rise to systematic frequency error on the local estimated BFS. Aiming at solving this problem, we propose an approach that acquires both the upper and lower probe sidebands simultaneously using two photodiodes, and the average between the Brillouin gain and loss spectral is calculated to eliminate the detrimental impact of the non-local effect. Moreover, the impact of probe power on the distortions of both pump pulse and Brillouin gain/loss spectrum is experimentally investigated, thus the maximum allowable power of the injection probe wave is determined.

We study stimulated Brillouin scattering (SBS) in a few-mode graded-index fiber in both the intermodal and intramodal configurations. A multi-peak Brillouin gain spectrum (BGS) is observed in both types of SBS interactions owing to the presence of higher-order acoustic modes, whose contribution to the Brillouin gain is found to depend on the modal composition of the pump beam. The numerically simulated results match well with the experimentally observed multi-peak BGS in both the intramodal and intermodal cases. The linewidth of the individual BGS peaks are extracted from a multi-Lorentzian fit to the data and are found to be significantly different for different modal compositions of the pump beam.

We report on a simple and efficient method for enhancing Brillouin scattering signal, i.e., adaptive phase matching (APM) probe-injection technique. In this technique, a low-polarization broad-spectrum probe wave is injected opposite to the pump, which can enhance any stokes signal in its APM range instantly by selective stimulated Brillouin amplification. With advantages of simple scheme, real-time multi-signal enhancement and sweep-free measurement, this technique has a great potential for improving the signal-to-noise ratio of Brillouin gain spectrum in the Brillouin scattering application systems.

Silicon nitride (Si3N4) has emerged as a promising material for integrated nonlinear photonics and has been used for broadband soliton microcombs and low-pulse-energy supercontinuum generation. Therefore understanding all nonlinear optical properties of Si3N4 is important. So far, only stimulated Brillouin scattering (SBS) has not been reported. Here we observe, for the first time, backward SBS in fully cladded Si3N4 waveguides. The Brillouin gain spectrum exhibits an unusual multi-peak structure resulting from hybridization with with high-overtone bulk acoustic resonances (HBARs) of the silica cladding. The reported intrinsic Si3N4 Brillouin gain at 25 GHz is estimated as 7x10^-13 m/W. Moreover, the magnitude of the Si3N4 photoelastic constant is estimated as |p12| = 0.047 +/- 0.004. Since SBS imposes an optical power limitation for waveguides, our results explain the capability of Si3N4 to handle high optical power, central for integrated nonlinear photonics., 7 pages, 3 figures

Nonlinear fiber lasers with short single-mode fibers (≤100m) are investigated. The effects of the length of the Brillouin gain fiber, i.e. the single-mode fiber, and the optical filter are studied. The experimental results indicate that the fiber laser can operate well with 50m single-mode fiber if the optical filter is adopted. But it cannot work when the single-mode fiber is shortened to 25 m. The results show that the nonlinear fiber laser based on single mode fibers as the Brillouin gain medium is hard to realize ultra-short (

Power scaling in high energy fiber-based laser systems is limited by optical nonlinearities. As opposed to the complex micro-structured large mode area (LMA) fibers typically developed to mitigate these parasitic nonlinear effects, the present work instead advocates another approach, a material approach, in which nonlinearities are mitigated through a judicious choice of dopants in the fiber core. To demonstrate the effectiveness of this approach, multicomponent core – silica glass cladding optical fibers are fabricated using the molten core method and the role played by the glass dopants in the mitigation of nonlinearities discussed. More specifically, highlights are made on multicomponent alkaline-earth (Sr, Ca) doped-silica fibers exhibiting concomitant reduction of > 6 dB in the Brillouin gain coefficient, 1 – 2 dB in the Raman gain coefficient, and 2 – 3 dB in the thermo-optic coefficient relative to conventional silica fibers.

International audience; We report a technique based upon the cavity ringdown method that enables to characterize the Brillouin gain coefficient directly in a laser cavity. Material gain, optical cavity parameters and lasing properties can be extracted from measurements whithin a single experiment.

When studying the spatial resolution and Brillouin frequency shift (BFS) uncertainty in Brillouin optical time domain analysis (BOTDA) system with a hotspot, people usually focuses on the point with best performance in the hotspot and neglect the huge error around it. The error is caused by the Brillouin gain spectrum (BGS) contributed by both the hotspot and the ambient segment, which is distorted from standard Lorentzian curve. Due to the distortion of BGS, the estimated BFS near the hotspot is shifted from the actual value and results in BFS error along the fiber. The distorted BGS can be double-peak or single-peak curve that is dependent on both the length of the hotspot that contributes to BGS and the BFS difference between the hotspot and the ambient segment. A fitting technique considering the combined contributions of hotspot and ambient segment is proposed to recover the distorted BGS near the hotspot and evaluate BFS. It is demonstrated that BFS error around hotspot is greatly reduced compare to the conventional Lorentzian fitting and dual Lorentzian fitting schemes.

Detection and quantification of cracks for various civil infrastructures on a large scale are difficult both technologically and economically through current sensing methodologies. This study presents a novel fiber-optic sensor named short-gauged Brillouin fiber optic sensor, which enables basic Brillouin-based analyzers to achieve early crack detection and accurate crack width measurement. The concept and design of the proposed sensor are firstly introduced, followed by respective instrumentation procedures. On this basis, theoretical deduction and numerical simulations of the crack-induced Brillouin gain spectrum (BGS) response using the proposed sensor are carried out, verified subsequently by controlled laboratory tests. The measured BGS responses are then leveraged for crack detection and quantification. A peak-fitting-based methodology was adopted to analyze the BGS data to achieve accurate crack width measurement. The proposed methodology may facilitate economical long-distance distributed crack sensing and quantification for various infrastructures as a generic technique.

We report in this paper the simulation analysis on the simultaneous deployment of Brillouin gain and Brillouin loss in the Golay coded phase-shift pulse Brillouin optical time domain analysis (PSP-BOTDA) fiber optic sensor for improving the sensing distance and at the same time obtaining high spatial resolution measurement. In this technique, the Golay coded pump pulse is alternately frequency modulated for the purpose of generating Stokes and Anti-Stokes scattering. The technique also modulates the coded pump with the return-to-zero (RZ) formats. Simulation analysis has revealed that the simultaneous use of Brillouin gain- and loss and the pulse coding technique in the PSP-BOTDA has improved the dynamic sensing range and the signal-to-noise improvement ratio (SNIR) of the sensor. By performing simulation over 500 m of fiber, we have also successfully demonstrated 10-cm of high spatial resolution measurement with the use of 1ns of coded pulse duration.

The vector model on the polarization properties of stimulated Brillouin scattering is analyzed theoretically with the Stratonovich generator. The quantitative characteristics of average Brillouin gain and alignment of pump and signal waves are numerically investigated. Simulation results show a more general conclusion about the state of polarization that corresponding to the maximum Brillouin gain, when a wider range of birefringence value is considered. In addition, the fluctuation range of signal output is presented. Finally, the statistics of signal output is studied and found to be Gaussian distributed in log-scale. The detailed quantitative study contributes to further understanding of the effect of polarization and birefringence on stimulated Brillouin scattering and statistics of Brillouin gain.

We demonstrate a temperature fiber sensor with enhanced sensitivity based on the second order Brillouin slow light. In this process, the modulated probe wave coincides with the second order Brillouin gain that was generated by circulating the first order Brillouin Stokes signal. Optical delay was induced on the modulated probe wave as it interacts with the second order Brillouin Stokes signal. As the temperature changes, the measured optical delay at the output also changes accordingly. The first order setup has a temperature slope coefficient of 1.126 ns/°C. Compared to the first order setup, the second order setup achieved a coefficient of 1.917 ns/°C, which is almost double the value obtained from the first order experiment.