Brillouin Frequency Shift Change Research Articles

Segmented Noise Reduction Based on Brillouin-Spectrum-Partition in Brillouin Optical Time Domain Sensors

In this paper, the necessity of segmented image denoising in the signal post-processing of Brillouin Optical Time Domain sensors (BOTDS) is investigated firstly, then a novel cross-correlation based Brillouin-spectrum-partition (BSP) method is proposed to denoise the noisy Brillouin gain or loss spectrum in BOTDS. In the BSP filter, the non-correlation factors of adjacent Brillouin scattering spectrum are firstly computed, then the Brillouin spectrum along the fiber is divided into transition sub-images with large changes in Brillouin frequency shift (BFS) and non-transition sub-images with small changes in BFS, and finally the transition and non-transition sub-images are filtered with linear and non-linear filters, respectively. By using an 800m-length G657 sensing fiber with five temperature events in a proof-of-concept experiment, a BFS measurement accuracy improvement of 22% in non-transition section, and a spatial resolution (SR) enhancement of 84% in transition Section can be achieved by using the proposed BSP filter, compared to the results obtained by using non-local means (NLM) filter. Besides, a BFS measurement accuracy improvement of 54% in non-transition section, and a SR enhancement of 71% in transition section, can be achieved compared with the results obtained by using Gaussian filter. Moreover, to deal with the noisy Brillouin spectrum of the 800m-length fiber by using the proposed BSP filter, we obtain a processing time reduction of 37% compared with that by using NLM filter.

Observation of anti-parity-time-symmetry, phase transitions and exceptional points in an optical fibre

The exotic physics emerging in non-Hermitian systems with balanced distributions of gain and loss has recently drawn a great deal of attention. These systems exhibit phase transitions and exceptional point singularities in their spectra, at which eigen-values and eigen-modes coalesce and the overall dimensionality is reduced. So far, these principles have been implemented at the expense of precise fabrication and tuning requirements, involving tailored nano-structured devices with controlled optical gain and loss. In this work, anti-parity-time symmetric phase transitions and exceptional point singularities are demonstrated in a single strand of single-mode telecommunication fibre, using a setup consisting of off-the-shelf components. Two propagating signals are amplified and coupled through stimulated Brillouin scattering, enabling exquisite control over the interaction-governing non-Hermitian parameters. Singular response to small-scale variations and topological features arising around the exceptional point are experimentally demonstrated with large precision, enabling robustly enhanced response to changes in Brillouin frequency shift.

Fast Acquirable Brillouin Optical Time-Domain Reflectometry Based on Bipolar-Chirped Pulse Pair

In this article, fast measurement of Brillouin optical time-domain reflectometry (BOTDR) based on a linearly frequency modulated (LFM) pulse and a digital matched filter is studied theoretically and demonstrated experimentally. The Brillouin frequency shift (BFS) change is detected by measuring the time shift of the Brillouin gain spectrum (BGS) along the time axis instead of the frequency shift, as in traditional BOTDR. In the experiment, we propose to use a positive- and negative-swept pulse, named bipolar-chirped pulse pair (BPP), to determine the BFS change with high accuracy. A BGS acquisition of up to 320 Hz is obtained for a fiber length of 2 km with a strain measurement accuracy of ±8 ${\rm{\mu \varepsilon }}$ . Strain measurements in a range of up to 10000 ${\rm{\mu \varepsilon }}$ are conducted, and the resulting strain coefficient is equal to 0.048 MHz/ ${\rm{MHz}}/{\rm{\mu \varepsilon }}$ , which matches that of standard single-mode fibers. The BPP-BOTDR technique is an intrinsic one-end-injection truly distributed system that exhibits high measurement speed and high accuracy, which may pave the way for various BOTDR applications.

Distributed Salinity Sensor With a Polyimide-Coated Photonic Crystal Fiber Based on Brillouin Dynamic Grating

The distributed measurement of salinity is experimentally demonstrated for the first time, to the best of our knowledge, by Brillouin dynamic grating (BDG) with a polyimide-coated (PI-coated) polarization-maintaining photonic crystal fiber as the active sensing component. The position-related salinity change can be measured by mapping the birefringence change through the BDG, where the salinity altering can induce swelling or shrinking actions of the PI coating through absorbing or releasing water. To reveal the systematic characterizations of the proposed sensor, four PI-coated sensors with different coating thicknesses of 5 μm, 8 μm, 15 μm and 20 μm were fabricated as demos, achieving salinity sensitivities of 30.1 MHz/(mol/L) for 5 μm, 75.4 MHz/(mol/L) for 8 μm, 119.2 MHz/(mol/L) for 15 μm, and 139.6 MHz/(mol/L) for 20 μm, respectively. The proposed sensor shows a linear and reproducible response to the salinity change. And its measurement uncertainty is less than 10 MHz, yielding a salinity accuracy of 0.072 mol/L. The temperature crosstalk can be compensated through measuring the temperature-induced Brillouin frequency shift changes using Brillouin optical time-domain analysis.

Distributed Dynamic Strain Measurement Based on Dual-Slope-Assisted Brillouin Optical Correlation Domain Analysis

In this paper, we propose and demonstrate a high-speed Brillouin optical correlation domain analysis (BOCDA) system by exploiting the slope of the Brillouin gain spectrum (BGS). In the slope-assisted (SA) scheme, the Brillouin frequency shift change is obtained by recording the Brillouin power variation. In this case, the time-consuming frequency sweeping process is avoided, leading to an ultrahigh measurement speed for Brillouin sensors. Here, this SA concept is adopted into the BOCDA system to improve its measurement speed. Moreover, in order to achieve pump-power-independent and fiber-loss-insensitive measurements, a dual-slope-assisted (DSA) scheme is implemented by measuring the ratio between readings taken from both slopes of the BGS. By employing this new technique, a repetition rate of 625 Hz as well as a centimeter-level spatial resolution is achieved, which is verified by measuring dynamic strains with frequencies up to 100 Hz. Meanwhile, by using the proposed system, distributed and dynamic tracking of mechanical wave propagating along the fiber under test is realized. Moreover, the limitations of DSA-BOCDA on measurement accuracy, spatial resolution, and measurement speed are also discussed in detail.

Narrowing Spectral Linewidth in Brillouin Optical Time Domain Analyzers With Adjustable Spatial Resolution

In order to both adjust the spatial resolution (SR) and narrow the Brillouin gain spectrum (BGS) linewidth in Brillouin optical time domain analysis system, a novel scheme based on differential pulse pair technique is proposed. By designing the phase and amplitude of the pulse properly, the higher peak gain and narrower linewidth of BGS are achieved compared to the conventional single pulse scheme with same SR. The pulse parameters for different SR are analytically solved and the performances of the sensor are theoretically and experimentally studied. Experimental results show that the linewidth of BGS is narrowed to 19 and 22 MHz in the case of 2 m SR and 1 m SR, which indicates 2.3 times and 4.3 times frequency accuracy improvements are achieved, respectively. Furthermore, the advantages of our proposal for small Brillouin frequency shift change are also investigated.

Refractive index sensing by Brillouin scattering in side-polished optical fibers.

In this Letter, we demonstrate the possibility to measure the refractive index of a liquid, using the stimulating Brillouin scattering in a 3-cm-long side-polished optical fiber. In addition, we show that by depositing a high-refractive index layer on the polished surface the sensitivity of the Brillouin frequency shift (BFS) can be increased due to a higher penetration of the evanescent field in the outer medium. Experiments show a maximum BFS change of about 11MHz when varying the refractive index of the external medium from 1 (air) to 1.402, and a BFS sensitivity to refractive index of about 293MHz/RIU around 1.40.

Second-Order Nonlocal Effects Mitigation in Brillouin Optical Time-Domain Analysis Sensors by Tracking the Brillouin Frequency Shift Profile of the Fiber

We report on an additional limitation that has been found in Brillouin optical time-domain analysis (BOTDA) sensors due to the so-called second-order nonlocal effects (NLE). Second-order NLE appear in BOTDA setups that deploy a double probe waves to compensate the transfer of energy between the pump pulse and the probe wave, and are related to a spectral distortion of the pump pulse that leads to measurement errors and an effective limit on the maximum probe power that can be deployed in the sensor. We theoretically and experimentally demonstrate that the techniques that have been presented so far in the literature to compensate second-order NLE are only effective in the case that the Brillouin frequency shift (BFS) along the sensing fiber is uniform. However, this requirement for uniformity is not realistic in real world scenarios in which a variety of fibers with different BFS and subjected to different environmental conditions are typically deployed. Therefore, we demonstrate a new method to mitigate the effects of BFS variation in the BOTDA setups that compensate second-order NLE. This method is based on introducing an additional wavelength modulation to the probe wave so as to track the mean BFS changes along the sensing fiber link. With this method, we demonstrate a BOTDA setup that, without coding, distributed amplification, or any other form of performance enhancement, achieves a sensing length of 120 km with 3-m spatial resolution and 2-MHz measurement precision. Moreover, the setup demonstrates, to our knowledge, the largest probe power ever injected in a BOTDA sensing link.

Few-mode optical fiber based simultaneously distributed curvature and temperature sensing.

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.

Principle of Brillouin dynamic grating and its applications in optical fiber sensing

Brillouin dynamic grating (BDG) has been widely studied since it was proposed for the first time to achieve optical storage in 2007. In general, when two beams of pump light (their frequency difference equal to Brillouin frequency shift of the optical fiber) with the same polarization state are injected into the fiber, the coherent acoustic wave can be excited by the stimulated Brillouin scattering effect, forming a BDG. The BDG in an optical fiber has been widely used in optical fiber sensing, characterization of optical fibers, optical storage, all-optical signal processing, microwave photonics and high-precision spectral analysis due to the advantages of all-optical generation and flexible parameter control. In this paper, we analyze the principle of BDG generation and detection, and its applications in optical fiber sensing. The simultaneous measuring of strain and temperature is achieved within a spatial resolution of 20 cm through measuring Brillouin frequency shift and birefringence-induced frequency shift in a polarization-maintaining fiber. A high-sensitivity distributed transverse load sensor based on BDG with a measurement accuracy as high as 0.810-3 N/mm is proposed and demonstrated, whose principle is to measure the transverse-load-induced birefringence change through exciting and probing a BDG in an elliptical-core polarization maintaining fiber. On the basis of the above research, a distributed measurement of hydrostatic pressure is demonstrated by using a 4-m photonics crystal fiber with a measurement error less than 0.03 MPa at a 20-cm spatial resolution, while the temperature cross-talk to the hydrostatic pressure sensing can be compensated for through measuring the temperature-induced Brillouin frequency shift changes by using Brillouin optical time-domain analysis. A system based on BDG in polarization maintaining fibers is reported to achieve a spatial resolution below one centimeter, while preserving the full accuracy on the determination of temperature and strain through measuring Brillouin frequency shift. Taking advantage of creating a long BDG in an optical fiber, an ultra-narrow bandwidth optical filter is realized by operating a BDG in a single-mode fiber, and the optical spectrometry is performed by sweeping the center wavelength of the BDG-based filter through a swept-tuned laser, where a 4 fm (0.5 MHz) spectral resolution is achieved by operating a BDG in a 400 m single-mode fiber.

Dependence of Brillouin frequency shift on water absorption ratio in polymer optical fibers

We studied the dependence of the Brillouin frequency shift (BFS) on the water-absorption ratio in poly(methyl methacrylate)-based polymer optical fibers (POFs) to clarify the effect of the humidity on POF-based Brillouin sensors. The BFS, deduced indirectly using an ultrasonic pulse-echo technique, decreased monotonically as the water absorption ratio increased, mainly because of the decrease in the Young's modulus. For the same water absorption ratio, the BFS change was larger at a higher temperature. The maximal BFS changes (absolute values) at 40, 60, and 80 °C were 158, 285, and 510 MHz, respectively (corresponding to the temperature changes of ∼9 °C, ∼16 °C, and ∼30 °C). Thus, some countermeasure against the humidity is indispensable in implementing strain/temperature sensors based on Brillouin scattering in POFs, especially at a higher temperature. On the other hand, Brillouin-based distributed humidity sensors might be developed by exploiting the BFS dependence on water absorption in POFs.

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

We proposed and demonstrated a few-mode fiber (FMF) based optical-fiber sensor for distributed curvature measurement through quasi-single-mode Brillouin frequency shift (BFS). By central-alignment splicing FMF and single-mode fiber (SMF) with a fusion taper, a SMF-components-compatible distributed curvature sensor based on FMF is realized using the conventional Brillouin optical time-domain analysis system. The distributed BFS change induced by bending in FMF has been theoretically and experimentally investigated. The precise BFS response to the curvature along the fiber link has been calibrated. A proof-of-concept experiment is implemented to validate its effectiveness in distributed curvature measurement.

Experimental evaluation of a distributed Brillouin sensing system for measuring extensional and shear deformation in rock

Distributed Brillouin sensing systems (DBSs) have growing applications in engineering and are attracting attention in the field of underground structures, including mining. The capability for continuous measurements of strain over large distances makes DBSs a promising monitoring approach for understanding deformation field evolution within a rock mass, particularly when the sensor is installed away from excavation damaged zone (EDZ). A purpose-built fiber optic sensing cable, a vital component of DBSs, was assessed in laboratory conditions.A test program was performed to observe DBSs response to various perturbations including strain and joint movements, including opening and shearing of joints. These tests included assessment of the strain-free cable response and the application of extensional and lateral displacement to various sensing cable lengths (strained lengths), from 1m down to 1cm. Furthermore, tests were done to evaluate the time-dependent behavior of the cable and to observe the effect of strain transfer using a soft host material (e.g. a soft grout) under lateral displacement.The noise level of the DBSs range was ±77με, determined through repeated measurements on an unstrained cable. Stretching test results showed a clear linear correlation between applied strain and Brillouin frequency shift change for all strained lengths above half the spatial resolution of the DBSs. However, for strained lengths shorter than half the spatial resolution, no strain response was measurable and this is due to the applied internal signal processing of the DBSs to detect peak Brillouin gain spectrum and noise level. The stability with time of the measurements was excellent for test periods up to 15h.Lateral displacement test results showed a less consistent response compared to tension tests for a given applied displacement. Although the Brillouin frequency shift change is correlated linearly with the applied displacement in tension, it shows a parabolic variation with lateral displacement. Moreover, the registered frequency response (correlated with strain) of the system decreased significantly when the sensing cable was embedded in a sand-filled tube compared with direct cable displacement.

Determining the Change of Brillouin Frequency Shift by Using the Similarity Matching Method

We have proposed a similarity matching method (SMM) to obtain the change of Brillouin frequency shift (BFS), in which the change of BFS can be determined from the frequency difference between detecting spectrum and selected reference spectrum by comparing their similarity. We have also compared three similarity measures in the simulation, which has shown that the correlation coefficient is more accurate to determine the change of BFS. Compared with the other methods of determining the change of BFS, the SMM is more suitable for complex Brillouin spectrum profiles. More precise result and much faster processing speed have been verified in our simulation and experiments. The experimental results have shown that the measurement uncertainty of the BFS has been improved to 0.72 MHz by using the SMM, which is almost one-third of that by using the curve fitting method, and the speed of deriving the BFS change by the SMM is 120 times faster than that by the curve fitting method.

Performance of strain-free stimulated Brillouin scattering suppression fiber

We have investigated experimentally the performance of an SBS suppression fiber with a nonuniform dopant concentration along its length fabricated by the VAD method. We have achieved an SBS threshold of 15 dBm against a continuous wave at 1.55 /spl mu/m. This is 7 dB larger than that of conventional fiber. We have confirmed experimentally that the increase in the SBS threshold can be estimated from the ratio of the Brillouin frequency shift change over the effective interaction length to the intrinsic Brillouin gain bandwidth. Moreover, we have confirmed that there is no BER degradation after a transmission of 138 km through a 2.5 Gb/s intensity modulation direct detection system when the SBS suppression fiber is used with an input power of 16 dBm.

Thermal effects on the Brillouin frequency shift in jacketed optical silica fibers

Brillouin frequency shift in two kinds of jacketed optical single-mode silica fiber, with an ultraviolet curable resin coat and a nylon coat, has been measured at temperatures ranging from -30 to +60 degrees C. It has been found that there are two reasons for the Brillouin frequency shift change in jacketed optical fibers against temperature change. One is the Brillouin frequency shift change for bare fibers. The other is the thermal stress due to the differences in thermal expansion coefficients in bare fiber and its coating material.