Distributed Brillouin Fiber Sensor Research Articles

Extraction of Brillouin frequency shift from Brillouin gain spectrum in Brillouin distributed fiber sensors using K nearest neighbor algorithm

The extraction of Brillouin frequency shifts from a Brillouin gain spectrum is a common problem in the engineering applications of Brillouin distributed fiber sensors. In this paper, extraction of Brillouin frequency shifts from Brillouin gain spectrum in Brillouin distributed fiber sensors using K nearest neighbor algorithm is proposed and experimentally demonstrated. The Brillouin frequency shift extraction is treated as a model-free supervised classification problem. The influence of Signal-to-noise ratio and frequency scanning step is studied. The performance of Brillouin frequency shift extraction degrades with the increase of frequency scanning step, and with the decrease of signal-to-noise ratio. The result shows that the algorithm of K nearest neighbor algorithm is a better way to extract the Brillouin frequency shift from Brillouin gain spectrum, comparing with curve-fitting methods, the uncertainty can be 0.44 with frequency scanning step of 1 MHz and SNR of 11 dB.

Extraction of Brillouin Frequency Shift from Brillouin Gain Spectrum in Brillouin Distributed Fiber Sensors Using K Nearest Neighbor Algorithm

Extraction of Brillouin Frequency Shift from Brillouin Gain Spectrum in Brillouin Distributed Fiber Sensors Using K Nearest Neighbor Algorithm

Benefits of Spectral Property Engineering in Distributed Brillouin Fiber Sensing.

As one of the most consolidated distributed fiber sensors based on stimulated Brillouin scattering, the Brillouin optical time-domain analyzer (BOTDA) has been developed for decades. Despite the commercial availability and outstanding progresses which has been achieved, the intrinsic Lorentzian gain spectrum restricts the sensing performance from possible further enhancements and hence limits the field of validity of the technique. In this paper, the novel method of engineering the gain spectral properties of the Brillouin scattering and its application on static and dynamic BOTDA sensors will be reviewed. Such a spectral property engineering has not only provided improvements to BOTDA, but also might open a new way to enhance the performance of all kinds of distributed Brillouin fiber sensors.

Gain Spectrum Engineering in Distributed Brillouin Fiber Sensors

In this paper, a novel method for Brillouin optical time-domain analyzers exploiting the gain spectrum engineering will be proposed for the first time, to the best of our knowledge. By superimposing the Brillouin gain with two symmetric loss spectra, new degrees of freedom will be provided to engineer the spectrum of the Brillouin interaction. This leads to a Brillouin gain shape which is much more robust to noise influences than the conventional one. Simulations and experiment are in very good agreement to each other and demonstrate a more than 3-dB measurement accuracy enhancement. Since for the proposed method the frequency error is less susceptible to the fiber loss, the sensing range can be extended by up to 60%. This paper might open a new way to enhance and adapt the distributed Brillouin sensing performances by an engineering of the spectrum shape.

Fast information acquisition using spectra subtraction for Brillouin distributed fiber sensors.

The traditional methods of extracting sensing information of a Brillouin distributed sensor is by curve fitting the Brillouin gain profiles along the fiber, we propose two concise and time-saving methods to process signals from only the frequency shifted section(s) of the fiber by subtracting the original spectrum from the sensing Brillouin spectrum. Experimental results validate that our methods can provide up to over 9 times faster information acquisition for a 10 km sensing fiber with 800 m frequency shifted section comparing with the traditional way. The proposed methods could be potentially attractive in getting information for the Brillouin distributed sensors as well as the Raman and Rayleigh distributed sensors especially when the processing speed is concerned.

Performance analysis of frequency shift estimation techniques in Brillouin distributed fiber sensors.

The performance of post-processing techniques carried out on the Brillouin gain spectrum to estimate the Brillouin frequency shift (BFS) in standard Brillouin distributed sensors is evaluated. Curve fitting methods with standard functions such as polynomial and Lorentzian, as well as correlation techniques such as Lorentzian Cross-correlation and Cross Reference Plot Analysis (CRPA), are considered for the analysis. The fitting procedures and key parameters for each technique are optimized, and the performance in terms of BFS uncertainty, BFS offset error and processing time is compared by numerical simulations and through controlled experiments. Such a quantitative comparison is performed in varying conditions including signal-to-noise ratio (SNR), frequency measurement step, and BGS truncation. It is demonstrated that the Lorentzian cross-correlation technique results in the largest BFS offset error due to truncation, while exhibiting the smallest BFS uncertainty and the shortest processing time. A novel approach is proposed to compensate such a BFS offset error, which enables the Lorentzian cross-correlation technique to completely outperform other fitting methods.

Brillouin Frequency Shift of Fiber Distributed Sensors Extracted from Noisy Signals by Quadratic Fitting.

It is a basic task in Brillouin distributed fiber sensors to extract the peak frequency of the scattering spectrum, since the peak frequency shift gives information on the fiber temperature and strain changes. Because of high-level noise, quadratic fitting is often used in the data processing. Formulas of the dependence of the minimum detectable Brillouin frequency shift (BFS) on the signal-to-noise ratio (SNR) and frequency step have been presented in publications, but in different expressions. A detailed deduction of new formulas of BFS variance and its average is given in this paper, showing especially their dependences on the data range used in fitting, including its length and its center respective to the real spectral peak. The theoretical analyses are experimentally verified. It is shown that the center of the data range has a direct impact on the accuracy of the extracted BFS. We propose and demonstrate an iterative fitting method to mitigate such effects and improve the accuracy of BFS measurement. The different expressions of BFS variances presented in previous papers are explained and discussed.

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. Such a novel scanning procedure allows the resolving, for the first time in pure time-domain Brillouin sensors, of 1,000,000 sensing points, i.e., 1cm spatial resolution over 10km in a conventional acquisition time.

High-resolution Brillouin optical correlation domain analysis with no spectral scanning.

Distributed Brillouin fiber sensors typically rely on the reconstruction of the steady-state Brillouin gain spectrum (BGS), through spectral scanning of the frequency offset between the pump and signal waves. In this work, we propose and demonstrate an alternative approach, in which the local Brillouin frequency shift (BFS) is extracted from temporal transient analysis of the step response of the amplified signal wave. Measurements are taken at only two arbitrary frequency offsets between pump and signal. No spectral scanning and no prior knowledge of a reference BGS are necessary. The principle is supported by analytic and numeric solutions of the differential equations of stimulated Brillouin scattering. The BFS of a 2 meters-long fiber under test was measured with 1 MHz accuracy and a dynamic range of 200 MHz. Transient measurements were also performed in a Brillouin optical correlation domain analysis (B-OCDA) experiment with 4 cm resolution, standard deviation of 2.4 MHz and 100 MHz dynamic range. A 4 cm-wide hot-spot was properly identified in the measurements. Multiple correlation peaks could be addressed in a single flight of a pump pulse. The results represent the first B-OCDA that is free of spectral scanning. This new measurement concept may be applicable to random-access distributed and dynamic monitoring of sound and vibration.

Novel scanning method for distortion-free BOTDA measurements.

Systematic errors induced by distortions in the pump pulse of conventional Brillouin distributed fiber sensors are thoroughly investigated. Experimental results, supported by a theoretical analysis, demonstrate that the two probe sidebands in standard Brillouin optical time-domain analyzers provide a non-zero net gain on the pump pulse, inducing severe distortions of the pump when scanning the pump-probe frequency offset, especially at high probe power levels. Compared to the impact of non-local effects reported in the state-of-the-art, measurements here indicate that for probe powers in the mW range (below the onset of amplified spontaneous Brillouin scattering), the obtained gain and loss spectra show two strong side-lobes that lead to significant strain/temperature errors. This phenomenon is not related to the well-known spectral hole burning resulting from pump depletion, but it is strictly related to the temporal and spectral distortions that the pump pulse experiences when scanning the Brillouin gain/loss spectrum. As a solution to this problem, a novel scanning scheme for Brillouin sensing is proposed. The method relies on a fixed frequency separation between the two probe sidebands, so that a flat zero net gain is achieved on the pump pulse when scanning the pump-probe frequency offset. The proposed technique is experimentally validated, demonstrating its ability to completely cancel out non-local effects up to a probe power ultimately limited by the onset of amplified spontaneous Brillouin scattering. The method allows for one order of magnitude improvement in the figure-of-merit of optimized long-range Brillouin distributed fiber sensors, enabling measurements along a 100 km-long sensing fiber with 2 m spatial resolution and with no need of added features for performance enhancement.

Going beyond 1000000 resolved points in a Brillouin distributed fiber sensor: theoretical analysis and experimental demonstration.

Distributed fiber sensing possesses the unique ability to measure the distributed profile of an environmental quantity along many tens of kilometers with spatial resolutions in the meter or even centimeter scale. This feature enables distributed sensors to provide a large number of resolved points using a single optical fiber. However, in current systems, this number has remained constrained to a few hundreds of thousands due to the finite signal-to-noise ratio (SNR) of the measurements, which imposes significant challenges in the development of more performing sensors. Here, we propose and experimentally demonstrate an ultimately optimized distributed fiber sensor capable of resolving 2100000 independent points, which corresponds to a one-order-of-magnitude improvement compared to the state-of-the-art. Using a Brillouin distributed fiber sensor based on phase-modulation correlation-domain analysis combined with temporal gating of the pump and time-domain acquisition, a spatial resolution of 8.3 mm is demonstrated over a distance of 17.5 km. The sensor design addresses the most relevant factors impacting the SNR and the performance of medium-to-long range sensors as well as of sub-meter spatial resolution schemes. This step record in the number of resolved points could be reached due to two theoretical models proposed and experimentally validated in this study: one model describes the spatial resolution of the system and its relation with the sampling interval, and the other describes the amplitude response of the sensor, providing an accurate estimation of the SNR of the measurements.

Increasing robustness of bipolar pulse coding in Brillouin distributed fiber sensors.

The robustness of bipolar pulse coding against pump depletion issues in Brillouin distributed fiber sensors is theoretically and experimentally investigated. The presented analysis points out that the effectiveness of bipolar coding in Brillouin sensing can be highly affected by the power unbalance between -1's and + 1's elements resulting from depletion and amplification of coded pump pulses. In order to increase robustness against those detrimental effects and to alleviate the probe power limitation imposed by pump depletion, a technique using a three-tone probe is proposed. Experimental results demonstrate that this method allows increasing the probe power by more than 12.5 dB when compared to the existing single-probe tone implementation. This huge power increment, together with the 13.5 dB signal-to-noise enhancement provided by 512-bit bipolar Golay codes, has led to low-uncertainty measurements (< 0.9 MHz) of the local Brillouin peak gain frequency over a real remoteness of 100 km, using a 200 km-long fiber-loop and 2 m spatial resolution. The method is evaluated with a record figure-of-merit of 380'000.

Time and frequency pump-probe multiplexing to enhance the signal response of Brillouin optical time-domain analyzers.

A technique to enhance the response and performance of Brillouin distributed fiber sensors is proposed and experimentally validated. The method consists in creating a multi-frequency pump pulse interacting with a matching multi-frequency continuous-wave probe. To avoid nonlinear cross-interaction between spectral lines, the method requires that the distinct pump pulse components and temporal traces reaching the photo-detector are subject to wavelength-selective delaying. This way the total pump and probe powers launched into the fiber can be incrementally boosted beyond the thresholds imposed by nonlinear effects. As a consequence of the multiplied pump-probe Brillouin interactions occurring along the fiber, the sensor response can be enhanced in exact proportion to the number of spectral components. The method is experimentally validated in a 50 km-long distributed optical fiber sensor augmented to 3 pump-probe spectral pairs, demonstrating a signal-to-noise ratio enhancement of 4.8 dB.

Stokes and anti-Stokes differential pulse pair based distributed Brillouin fiber sensor with double-sideband probe wave

We propose and demonstrate a distributed Brillouin fiber sensor using Stokes and anti-Stokes differential pulse pair based on double- sideband probe wave, in which the two sidebands of probe wave are used to balance the power of two pump pulses. The spatial resolution is determined by the slightly width difference of the two balanced pulses, without Brillouin gain spectrum broadening. The pulses perform gain-loss process in optical field before the probe signal being detected, without any post-processing or extra measurement time. The proposed technique can achieve high spatial resolution, natural Brillouin gain spectrum linewidth, normal measurement time and long sensing range simultaneously.

Low-noise, single-frequency, single-polarization Brillouin/erbium fiber laser

We demonstrate a low-noise Brillouin/erbium fiber laser (BEFL), which uses only 1.5 m polarization-maintaining erbium-doped fiber as both the Brillouin and erbium gain media. This BEFL presents a phase noise of -125 dB/Hz(1/2) at 1 kHz frequency, at 2 mW Brillouin pump (~3 MHz linewidth) power and 200 mW 980 nm pump power. The polarization extinction ratio of the laser output light is 31 dB. Stable (~2 h) single-mode operation is observed. This BEFL presents potential applications in distributed Brillouin fiber sensors, inteferometric fiber sensors, and optical communications.

Low-Threshold, Single-Mode, Compact Brillouin/Erbium Fiber Ring Laser

In this paper, we derive a numerical model to analyze the compact Brillouin/erbium fiber laser with a short commercialized normal erbium-doped fiber (EDF) as both the Brillouin and linear gain media. The effects of the 980 nm pump power, the Brillouin pump power, the EDF length, and the output coupling ratio on the BEFL output power characteristics are simulated. The theoretical results predict that a short commercialized normal EDF is capable for a high-power low-threshold compact BEFL. Based on the theoretical analyses, a BEFL using only 4 m normal EDF as the gain medium is designed and constructed. The measured results demonstrate that this BEFL presents low pump threshold (~20 mW), high output power (> 10 mW), single-mode operation and low relative intensity noise (-120 dB/Hz). This BEFL shows potential applications in distributed Brillouin fiber sensors, interferometric fiber sensors, and coherent optical communications, etc.

Distributed Brillouin Fiber Sensor With Enhanced Sensitivity Based on Anti-Stokes Single-Sideband Suppressed-Carrier Modulation

We demonstrate an alternative technique to reduce pump depletion and to improve sensitivity in the long-range Brillouin optical time-domain analysis (BOTDA) sensor. Our BOTDA system uses an anti-Stokes single-sideband suppressed-carrier modulation technique that balances the pump depletion due to fiber loss. With this technique, we show both numerically and experimentally a great improvement compared to the dual sideband standard technique.

Theoretical and experimental investigation of an 82-km-long distributed Brillouin fiber sensor based on double sideband modulated probe wave

An 82-km sensing range Brillouin optical time-domain analysis distributed fiber sensor without systematic measurement error is proposed and first experimentally validated, in which the probe wave with two sidebands generates a dual gain-loss Brillouin interaction, giving rise to the remarkable suppressed pump depletion in long sensing range and a relatively narrow Brillouin gain spectrum. Both theoretical analysis and experimental results demonstrate that this technique is capable of accurate long-distance sensing. To the best of our knowledge, it is the simplest technique and does not require extra time for long-distance sensing (>50 km).

Brillouin Distributed Fiber Sensors: An Overview and Applications

A review focused on real world applications of Brillouin distributed fiber sensors is presented in this paper. After a brief overview of the theoretical principles, some works to face the two main technical challenges (large dynamic range and higher spatial resolution) are commented. Then an overview of some real and on-field applications is done.

Long-range distributed Brillouin fiber sensors by use of an unbalanced double sideband probe

We propose and demonstrate a long-range Brillouin Optical Time-Domain Analysis (BOTDA) distributed sensing system making use of an unbalanced double sideband probe formed by a Stokes and an anti-Stokes line. In particular, we show that for each measuring condition an optimal Stokes /anti-Stokes input power ratio exists, allowing a larger suppression of nonlocal effects induced by pump depletion. Experiments on a 50 km single-mode sensing fiber with 5 meters spatial resolution are reported.