Differential Pulse-width Pair Research Articles

Improving BOTDA Performance Based on Differential Pulsewidth Pair and FFDNet

Improving BOTDA Performance Based on Differential Pulsewidth Pair and FFDNet

Enabling Variable High Spatial Resolution Retrieval From a Long Pulse BOTDA Sensor

Spatial resolution (SR) is one of the most important parameters of Brillouin optical time-domain analysis (BOTDA) sensors, which determines the minimum length that a perturbation event can be distinguished. In the field of Internet of Things (IoT), there is an urgent need for sensors with large-scale high-precision sensing capability for scenarios, such as intelligent monitoring of production lines and urban infrastructure. Conventionally, the SR is normally restricted to be longer than 1 m due to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 10$ </tex-math></inline-formula> -ns acoustic lifetime limitation in silica optical fibers. For long-distance smart monitoring systems, the SR is generally on the order of several meters or even worse. However, it does not meet the needs of many applications. Therefore, there is an urgent need to achieve SR in the submeter magnitude. In this work, for the first time to the best of our knowledge, we propose a convolutional neural network (CNN) to process the data of conventional BOTDA sensors, which achieves unprecedented performance improvement that allows to directly retrieve submeter SR from the sensing system that use long pump pulses. By using the simulated Brillouin gain spectrums (BGSs) as the CNN input and the corresponding high SR Brillouin frequency shift (BFS) as the output target, the trained CNN is able to obtain an SR higher than the theoretical value determined by the pump pulse width. In the experiment, the CNN accurately retrieves 0.5-m hotspots from the measured BGS with pump pulses from 20 to 50 ns, and the acquired BFS is in great agreement with 45/40 ns differential pulse-width pair (DPP) measurement results. Compared with the DPP technique, the proposed CNN demonstrates a twofold improvement in BFS uncertainty with only half the measurement time. In addition, by changing the training data sets, the proposed CNN can obtain tunable high SR retrieval based on conventional BOTDA sensors that use long pulses without any requirement of hardware modifications. It is worth mentioning that the proposed method is also applicable to larger pulse widths to retrieve a submeter SR. The proposed data post-processing approach paves the way to enable novel high SR BOTDA sensors, which brings substantial improvement over the state-of-the-art techniques in terms of system complexity, measurement time, reliability, etc.

High-fidelity denoising for differential pulse-width pair brillouin optical time domain analyzer based on block-matching and 3D filtering

In the field of structural deformation and strain measurement for civil structural health monitoring, Brillouin optical time-domain analyzer (BOTDA) system with sub-meter high spatial resolution is required. Differential pulse-width pair Brillouin optical time-domain analyzer (DPP-BOTDA) is an excellent method to achieve the goal without the need to modify the system structure. But the signal-to-noise ratio (SNR) of DPP-BOTDA is poor due to the differential operation, which limits its practical applications. Here, we propose to use Block-Matching and 3D filtering (BM3D) algorithm to improve the SNR of DPP-BOTDA with less degradation of measurement accuracy and spatial resolution. The influence of each BM3D parameter on the denoising performance is fully analyzed in detail for the first time which helps the optimization of the algorithm. Three pulse pairs of 50/47 ns, 50/45 ns and 50/43 ns are used for the demonstration with a sensing range of 9.9 km. With the optimized BM3D, the experimental spatial resolution only degrades by 0.08 m, 0.05 m and 0.04 m after denoising when the SNR improvement is 9 dB, respectively. And the temperature uncertainty for the three cases is improved to be 0.79 °C, 0.36 °C and 0.24 °C, respectively. We believe that this research will improve the efficiency of the parameter optimization when using the BM3D method for noise reduction and contribute to the practical application of the DPP-BOTDA system.

Distributed Airflow Sensing Based on High-Spatial-Resolution BOTDA and a Self-Heated High-Attenuation Fiber.

An all-fiber distributed airflow sensing method based on a differential pulse-width pair Brillouin optical time domain analysis (DPP-BOTDA) and a self-heated high-attenuation fiber (HAF) is proposed and demonstrated. The HAF heated the sensing fiber, producing a gradient temperature distribution in it through physical contact, where the temperature distribution was obtained by DPP-BOTDA with a spatial resolution of 5 cm. The heat loss caused by the airflow was reflected in the decrease in the Brillouin frequency shift and spatially resolved by DPP-BOTDA. Distributed airflow sensing was experimentally demonstrated for measurements of airflow movement, multiple airflow sources and the deflection angle of the airflow. The positioning error of the airflow was no larger than ~2.2 cm; for the deflection angle measurements of the airflow, the maximum demodulation error was 2.5° within the angle range of 0–30°.

Bending-loss-resistant distributed Brillouin curvature sensor based on an erbium-doped few-mode fiber.

We developed a bending-loss-resistant distributed Brillouin curvature sensor based on an erbium-doped few-mode fiber (Er-FMF) and differential pulse-width pair Brillouin optical time-domain analysis. With Er ion amplification compensating for bending-induced optical loss, radii in the ∼0.3 to 2.02 cm range could be monitored correctly. The corresponding Brillouin frequency shift variations were in the range of 91.7 to 9 MHz, which agrees well with theoretical calculations. The sensitivity of the Er-FMF device increased inversely with the bending radius, with the optimal sensitivity of 292.7 MHz/cm obtained at a radius of 0.3 cm. To the best of our knowledge, this is the smallest radius of curvature detected and highest sensitivity reported to date, indicating potential applications in distributed sharp-bend sensing, such as intelligent robotics and structural health monitoring.

Improving the Spatial Resolution of a BOTDA Sensor Using Deconvolution Algorithm

Spatial resolution improvement from an acquired measurement using long pulse is developed for Brillouin optical time domain analysis (BOTDA) systems based on the total variation deconvolution algorithm. The frequency dependency of Brillouin gain temporal envelope is investigated by simulation, and its impact on the recovered results of deconvolution algorithm is thoroughly analyzed. To implement a reliable deconvolution process, differential pulse-width pair (DPP) technique is utilized to effectively eliminate the systematic BFS distortion stemming from the frequency dependency of temporal envelope. The width of the pulse pairs should be larger than 40 ns as is analyzed theoretically and verified experimentally. It has been demonstrated that the proposed method can realize flexible adjustment of spatial resolution with enhanced signal-to-noise ratio (SNR) from an established measurement with long pump pulse. In the experiment, the spatial resolution is increased to 0.5 m and 1 m with high measurement accuracy by using the deconvolution algorithm from the measurement of 60/40 ns DPP signals. Compared with the raw DPP results with the same spatial resolution, 9.2 dB and 8.4 dB SNR improvements are obtained for 0.5 m and 1 m spatial resolution respectively, thanks to the denoising capability of the total variation deconvolution algorithm. The impact of sampling rate on the recovery results is also studied. The proposed sensing system allows for distortion-free Brillouin distributed sensing with higher spatial resolution and enhanced SNR from the conventional DPP setup with long pulse pairs.

Structural Crack Detection Using DPP-BOTDA and Crack-Induced Features of the Brillouin Gain Spectrum

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.

Liquid-level sensing method based on differential pulse-width pair Brillouin optical time-domain analysis and a self-heated high attenuation fiber.

A liquid-level sensing method based on differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA) combining with a self-heated high attenuation fiber (HAF) is proposed, where the principle is to locate the temperature abruption position at the interface between liquid and air caused by their different thermal diffusion rates. A Panda polarization-maintaining fiber is used as the sensing optical fiber, which is tightly glued alongside a laser-powered HAF. Heated by a high-power laser, the temperature of the HAF can increase and exhibits approximately exponential attenuation with the light propagation direction, which induces a similar temperature distribution over the sensing fiber. By using a 5-cm spatial resolution DPP-BOTDA and a 1.4-W heating light, temperature distributions of the sensing fiber are measured for different water levels, and the results indicate that distributed liquid-level sensing with a range of 20 cm and a resolution of 1 cm is realized using our method.

Online distributed strain measurement of fiber Michelson hydrophones based on DPP-BOTDA with a pulsed-probe wave.

We propose and demonstrate a novel differential pulse-width pair Brillouin optical time domain analysis (DPP-BOTDA) system with a pulsed-probe wave for online distributed strain measurement of fiber Michelson hydrophones (FMHs). Different from the conventional DPP-BOTDA using a continuous probe wave, a pulsed-probe wave is used in our scheme to avoid the interferences between the reflected lights from the sensor arm and reference arm of the FMH, where the probe pulse width should be adjusting precisely equal to the time delay between the two arms. The Brillouin frequency shift (BFS) containing the strain and/or temperature information is measured by sweeping the frequency difference of the probe pulse and the pump pulse. In the experiment, an optimized 8/8.5 ns pump pulse pair is applied to realize a 5-cm spatial resolution, and the probe pulses of 624.5 ns and 1269 ns are applied to measure the strain distribution of the sensor arms of two FMHs. We have successfully measured the temperature-induced strain of a FMH with an arm length difference of 62.45 m as temperature increases from -40°C to 80°C and the distributed strain variation of the other FMH with the arm length difference of 126.91 m as the hydrostatic pressure increases from 0.1 MPa to 10 MPa. The results indicate that the proposed scheme can provide a desirable solution for online distributed strain measurement of the FMHs.

High-performance optical chirp chain BOTDA by using a pattern recognition algorithm and the differential pulse-width pair technique

Optical chirp chain Brillouin optical time-domain analysis (OCC-BOTDA) has the capabilities of fast measurement, high Brillouin threshold, and freedom from the nonlocal effect; at the same time, however, it also has problems introduced by transient stimulated Brillouin scattering. The influence of the transient interaction is reflected as the broadened asymmetric Brillouin spectrum, the ghost peak, and the frequency shift of the main peak. This introduces difficulty in computing the fiber Brillouin frequency shift with good measurement accuracy. Besides, the OCC modulation causes additional noise due to the uneven amplitude response for different frequency components. In this work, we propose a high-performance OCC-BOTDA using the principal component analysis (PCA) based pattern recognition algorithm and differential pulse-width pair (DPP) technique. After building the Brillouin spectrum database (i.e., all patterns), the fiber intrinsic Brillouin frequency shift can be recognized by the PCA algorithm from a nonstandard Brillouin spectrum profile, resulting in good measurement accuracy. Meanwhile, the DPP technique, subtracting between two Brillouin signals generated by two wide-width pump pulses, is utilized to reduce the OCC modulation noise and avoid the pulse self-phase modulation effect in long-range BOTDA sensing. In the experiment, a temperature measurement with 1.3 MHz measurement precision, 4 m spatial resolution, and 5 s measurement time is achieved over a 100 km single-mode fiber.

Support Vector Machine based Differential Pulse-width Pair Brillouin Optical Time Domain Analyzer

Support vector machine (SVM) based differential pulse-width pair Brillouin optical time domain analyzer (DPP-BOTDA) has been proposed and experimentally demonstrated. With only one SVM model, temperature distribution along 5 km fiber under test has been successfully extracted from differential Brillouin gain spectrum (BGS) measured under different spatial resolution in DPP-BOTDA. The temperature accuracy by SVM is better than that by Lorentzian curve fitting (LCF), especially when the pump pulse width difference and the number of trace averaging used in the measurement are small, indicating larger tolerance of SVM to high spatial resolution and low signal-to-noise ratio. SVM is also more robust to a wide range of frequency scanning steps and has less accuracy degradation under large frequency scanning step. To extract temperature from 50 000 differential BGSs, 133.17 and 1.12 s are consumed by SVM-0.1 and SVM-1 °C, respectively, both of which are much shorter than that by LCF. The data processing time of SVM is further shortened with the help of principle component analysis for data dimension reduction. SVM for measurand extraction would be especially helpful in the scenario of DPP-BOTDA where high data sampling rate is required to resolve plenty of submeter scale sensing points.

Fast distributed Brillouin optical fiber sensing based on pump frequency modulation

We present a fast distributed Brillouin optical fiber sensor based on pump frequency modulation. The frequency shift of the probe is fixed, while the pump is fast scanned using an arbitrary waveform generator (AWG). Thus, the bandwidth requirement of the AWG is reduced to several hundred megahertz without any expense to the acquiring speed. Combining with differential pulse width pair (DPP) technology, a vibration measurement is implemented in a 230-m polarization-maintaining fiber with a sampling rate of 2.6 kHz and 50 cm spatial resolution. A vibration signal up to 31 Hz is identified successfully with a strain resolution of 15 µε.

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.

Full scale strain monitoring of a suspension bridge using high performance distributed fiber optic sensors

This paper investigated field monitoring of a 1108 m suspension bridge during an assessment load test, using integrated distributed fibre-optic sensors (DFOSs). In addition to the conventional Brillouin time domain analysis system, a high spatial resolution Brillouin system using the differential pulse-width pair (DPP) technique was adopted. Temperature compensation was achieved using a Raman distributed temperature sensing system. This is the first full scale field application of DFOSs using the Brillouin time domain analysis technique in a thousand-meter-scale suspension bridge. Measured strain distributions along the whole length of the bridge were presented. The interaction between the main cables and the steel-box-girder was highlighted. The Brillouin fibre-optic monitoring systems exhibited great facility for the purposes of long distance distributed strain monitoring, with up to 0.05 m spatial resolution, and 0.01 m/point sampling interval. The performance of the Brillouin system using DPP technique was discussed. The measured data was also employed for assessing bridge design and for the assessment of structural condition. The results show that the symmetrical design assumptions were consistent with the actual bridge, and that the strain values along the whole bridge were within the safety range. This trial field study serves as an example, demonstrating the feasibility of highly dense strain and temperature measurement for large scale civil infrastructures using integrated DFOSs.

Long-range BOTDA sensor over 50 km distance employing pre-pumped Simplex coding

We propose and demonstrate a long-range Brillouin optical time-domain analysis (BOTDA) sensor based on the pre-pumped Simplex coding technique. This approach combines Simplex coding with the pulse pre-pump technique, which takes full advantage of the signal-to-noise ratio enhancement provided by optical pulse coding and achieves meter-scale spatial resolution with an unbroadened Brillouin gain spectrum by the use of pre-pumped short pump pulse. Compared to the widely used differential pulse-width pair technique, a comparable performance can be realized while the measurement time is reduced by half. The theoretical analysis of pre-pumped Simplex coding applied to BOTDA systems is presented and a proof-of-concept experiment is carried out along a ∼51 km single-mode fiber composed of two fiber segments with slightly different Brillouin frequency shift values. With the proposed technique, Brillouin frequency shifts of the two different fiber sections at the end of the sensing fiber are clearly distinguished, and the obtained spatial resolution and temperature/strain accuracy along the sensing fiber are respectively 1 m and . According to the experimental results, we believe that the BOTDA employing pre-pumped Simplex coding is able to realize extra-long distance sensing without Raman amplification, while keeping a high spatial resolution and measurement accuracy.

Coherent BOTDA Sensor With Single-Sideband Modulated Probe Light

In order to optimize the coherent Brillouin optical time domain analysis (BOTDA) sensor for more effective chromatic dispersion (CD) suppression, single-sideband intensity-modulated probe (IMP) light is utilized. Obvious Brillouin gain spectrum distortion due to CD observed in the double-sideband IMP case is successfully cancelled. Meanwhile, frequency preshift for local light generation in other coherent BOTDA sensors that have been reported recently can be avoided in our scheme, resulting in a reduction in system complexity. On the other hand, a logarithmic detector, instead of an inphase/quadrature demodulator, is applied to reduce half of the measurement time. A ∼2-m spatial resolution and 0.97 °C temperature accuracy at the end of a 40-km sensing fiber are realized by using a 100-ns/80-ns differential pulsewidth pair.

Brillouin optical time-domain analysis sensor assisted by Brillouin distributed amplification of pump pulses.

We demonstrate the extension of the measurement range of Brillouin optical time-domain analysis (BOTDA) sensors using a distributed Brillouin amplifier (DBA). The technique is based on injecting a DBA pump wave in the fiber to generate an additional Brillouin interaction that amplifies the BOTDA pump pulses and compensates optical fiber attenuation. This amplification does not introduce any significant noise to the BOTDA's probe wave due to the inherent directionality of the Brillouin gain. Additionally, we deploy a differential pulse-width pair measurement method to avoid measurement errors due to the interplay between the self-phase modulation effect and the changes in the temporal shape of the pulses induced by the transient behavior of Brillouin gain. Experimental proof-of-concept results in a 50-km fiber link demonstrate full compensation of the fiber's attenuation with no penalty on the signal-to-noise ratio of the detected signal.

Differential Brillouin fiber sensor based on phase difference on double-sideband pump wave

A configuration based on phase difference on a double-sideband pump wave is proposed to detect the differential variation of temperature or strain in single-mode optical fibers. In our configuration, a probe wave only experiences a differential Brillouin gain contributed by the perturbation of temperature or strain in the sensing fiber. As a result, the power limitation of the probe wave can be alleviated and the photodetector in our configuration does not easily become saturated in the case of a longer sensing range. The spatial resolution is determined by the duration of the phase difference on the two sidebands and the signal-to-noise of our system is nearly twice as high as that of a differential pulse-width pair Brillouin optical time domain analysis sensor since a π -phase shift on the pump wave is employed. The properties and performances of our method are also theoretically derived and experimentally validated.

Phasorial differential pulse-width pair technique for long-range Brillouin optical time-domain analysis sensors.

We introduce a novel phasorial differential pulse-width pair (PDPP) method for Brillouin optical time-domain analysis (BOTDA) sensors that combines spatial resolution enhancement with increased tolerance to non-local effects. It is based on the subtraction of the complex time-domain traces supplied by a sensor configuration that uses a phase-modulated probe wave and RF demodulation. The fundamentals of the technique are first described theoretically and using numerical simulation of the propagating waves. Then, proof-of-concept experiments demonstrate the measurement of the Brillouin frequency shift distribution over 50-km. The system is shown to withstand large variations of the pump power generated by its interaction with a powerful probe wave along the fiber; hence, highlighting the potential of the PDPP technique to increase the detected signal-to-noise ratio in long-range BOTDA. Moreover, the PDPP is also shown to increase the measurement contrast by allowing the use of relatively long duration pulses while retaining 1-m spatial resolution.

Study of Raman amplification in DPP-BOTDA sensing employing Simplex coding for sub-meter scale spatial resolution over long fiber distances

The impact of Raman amplification and Simplex coding is studied in combination with differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA) to achieve sub-meter spatial resolution over very long sensing distances. An optimization of the power levels for the Raman pumps, Brillouin pump and signal has been carried out through numerical simulations, maximizing the signal levels and avoiding at the same time nonlinear effects and pump depletion. A reduction of acoustic-wave-induced distortions in the Brillouin gain spectrum down to negligible levels has also been achieved by numerical optimization of the pulse width and duty cycle of return-to-zero Simplex coding, providing significant signal-to-noise ratio enhancement. Strain–temperature sensing over 93 km standard SMF is achieved with a strain/temperature accuracy of 34µε/1.7 °C, and 50 cm spatial resolution throughout the fiber length.