Dynamic distributed optical fiber sensing based on simultaneous measurement of Brillouin gain and loss spectra with frequency-agile technique.

To simplify the experimental equipment and improve the signal-to-noise ratio (SNR) of the traditional Brillouin optical time-domain analysis (BOTDA) system, we propose a scheme using the frequency-agile technique to measure Brillouin gain and loss spectra simultaneously. The pump wave is modulated into the double-sideband frequency-agile pump pulse train (DSFA-PPT), and the continuous probe wave is up-shifted by a fixed frequency value. With the frequency-scanning of DSFA-PPT, pump pulses at the -1st-order sideband and the +1st-order sideband interact with the continuous probe wave via stimulated Brillouin scattering, respectively. Therefore, the Brillouin loss and gain spectra are generated simultaneously in one frequency-agile cycle. Their difference relates to a synthetic Brillouin spectrum with a 3.65-dB SNR improvement for a 20-ns pump pulse. This work simplifies the experimental device, and no optical filter is needed. Static and dynamic measurements are performed in the experiment.

Ring-core few-mode fiber and DPP-BOTDA-based distributed large-curvature sensing eligible for shape reconstruction.

This study proposes a distributed large-curvature sensor based on ring-core few-mode fiber (RC-FMF) and differential pulse-pair Brillouin optical time-domain analysis (DPP-BOTDA). The RC-FMF is adhered to a thin steel substrate and an asymmetric hump shape is reconstructed using the Frenet-Serret algorithm. The proposed curvature sensor demonstrates a larger curvature-sensing range, excellent tolerance to bending-induced optical loss, and increased Brillouin gain coefficient. The proposed sensor also demonstrates longer sensing distance and continuous absolute measurement compared to other sensors. The proposed model can be applied to the end tracking of soft robotics and structural health monitoring of civil infrastructures.

Brillouin dynamic grating erasure technique for fast all-optical signal processing.

Brillouin dynamic grating (BDG) is an attractive storage unit for all-optical signal storage and processing. However, the processing speed of the traditional "write-read" scheme is severely limited by the inter-process interference (IPI) due to the residual BDG. Here, we propose an all-optical "write-read-erase" scheme to avoid the IPI effect, which can effectively eliminate the residual BDG through an erase pulse. In a numerical simulation, for multi-processes to store a 7 × 7-bits Simplex code, each time, the residual BDGs from the former process are erased for the proposed scheme, and the power fluctuation of the retrieved waveform is suppressed within ±10%. In a preliminary experiment, residual BDG erase efficiencies up to 88.5% can be achieved by introducing erase pulses to neglect the IPI effect on the retrieved waveform. Without the IPI effect, all-optical signal processing will availably be speeded up, especially for short on-chip integrated circuits.

High-sensitivity dynamic distributed pressure sensing with frequency-scanning φ-OTDR.

We propose a high-sensitivity dynamic distributed pressure sensor using frequency-scanning phase-sensitive optical time-domain reflectometry (φ-OTDR) in a single-mode fiber (SMF), where an injection locking laser working as both filter and amplifier is used to generate the multi-frequency signals under a double-sideband modulation. The pressure sensitivity of the SMF is measured to be 702.5 MHz/MPa, which is approximately 1000 times larger than that of the Brillouin optical time-domain analysis technique. Subsequently, a dynamic pressure experiment is carried out in the case of rapid pressure relief from 2 to 0 MPa so that a maximum sampling rate of 33.3 kHz for a 25-m SMF is achieved, and the measurement uncertainty of 0.61 kPa for the proposed scheme is demonstrated.

High-resolution and large-strain distributed dynamic sensor based on Brillouin and Rayleigh scattering.

A wide-dynamic-range and high-resolution optical fiber sensor based on Brillouin and Rayleigh scattering is proposed, which merges frequency-scanning phase-sensitive optical time-domain reflectometry (Φ-OTDR) and Brillouin optical time domain analysis (BOTDA) via an adaptive signal corrector (ASC). The ASC suppresses the accumulated error of Φ-OTDR with the reference of BOTDA, which breaks through the measurement range limitation of Φ-OTDR so that the proposed sensor can perform a high-resolution measurement in a wide dynamic range. Its measurement range is determined by BOTDA, and can reach the limitation of optical fiber, while the resolution is limited by Φ-OTDR. In proof-of-concept experiments, a maximum strain variation of up to 302.9 µÉ› is measured with a resolution of 5.5 nɛ. Furthermore, with an ordinary single-mode fiber, high-resolution dynamic pressure monitoring in the range from 20 MP to 0.29 MPa is also demonstrated with 0.14-kPa resolution. This research represents the first time, to the best of our knowledge, that a solution for merging data from a Brillouin sensor and a Rayleigh sensor which achieves the advantages of the two sensors at the same time has been realized.

Quasi-acoustic impedance matching distributed opto-mechanical sensor with aluminized coating optical fibers.

The uncoated single-mode fiber has been extensively researched as an opto-mechanical sensor since it can achieve substance identification of the surrounding media by exciting and detecting transverse acoustic waves via forward stimulated Brillouin scattering (FSBS), but it has the danger of being easily broken. Although polyimide-coated fibers are reported to allow transverse acoustic waves transmission through the coating to reach the ambient while maintaining the mechanical properties of the fiber, it still suffers from the problems of hygroscopic property and spectral instability. Here, we propose a distributed FSBS-based opto-mechanical sensor using an aluminized coating optical fiber. Benefiting from the quasi-acoustic impedance matching condition of the aluminized coating and silica core cladding, aluminized coating optical fibers not only have stronger mechanical properties and higher transverse acoustic wave transmission efficiency but also have a higher signal-to-noise ratio, compared with the polyimide coating fibers. The distributed measurement ability is verified by identifying air and water around the aluminized coating optical fiber with a spatial resolution of 2 m. In addition, the proposed sensor is immune to external relative humidity changes, which is beneficial for liquid acoustic impedance measurements.

Polarization separation assisted optomechanical time-domain analysis with submeter resolution.

Optomechanical time-domain analysis (OMTDA) is a novel approach to measure distributed acoustic impedance of surrounding media with a high spatial resolution based on coherent forward stimulated Brillouin scattering probing. However, the spatial resolution is still limited by the polarization noise and influence of activation pulse. In this Letter, we propose a polarization separation based OMTDA to further improve the resolution. By isolating the acoustic activation and probing process in orthogonal polarization states, the backward Brillouin scattering of an activation pulse is effectively suppressed. Accompanied with the reduced polarization fluctuation brought by polarization-maintaining fiber, a spatial resolution of 0.8 m is experimentally demonstrated over a 34-m-long fiber and the precise distinction between air and alcohol is realized.

Temperature-compensated distributed refractive index sensor based on an etched multi-core fiber in optical frequency domain reflectometry.

We proposed a temperature-compensated distributed refractive index (RI) sensor using an etched multi-core fiber (MCF) in optical frequency domain reflectometry. The MCF contains inner and outer cores and is etched until the outer core is exposed. Therefore, the outer core can be used for distributed RI sensing, and the inner core can be used for temperature compensation. The sensing length of 19 cm and the spatial resolution of 5.3 mm are achieved in the experiment. The RI sensing range is as wide as 1.33-1.44 refractive index units (RIU), and the maximum sensitivity of 47 nm/RIU is obtained around 1.44 RIU. Additionally, the temperature sensitivity is 9.8 pm/°C. Using this sensor, we successfully detected the glycerol diffusion process in water.

Millimeter-level recognition capability of BOTDA based on a transient pump pulse and algorithm enhancement.

Brillouin optical time-domain analysis requires a pulsed pump to obtain a distributed Brillouin gain spectrum (BGS) containing environmental information, whose width corresponds to spatial resolution (SR). We propose a rising edge demodulation (RED) algorithm acting on Brillouin information generated by a transient pump pulse (

Temperature-compensated multi-point refractive index sensing based on a cascaded Fabry-Perot cavity and FMCW interferometry.

We proposed a novel temperature-compensated multi-point refractive index (RI) sensing system by the combination of the cascaded Fabry-Perot (FP) sensors and the frequency modulated continuous wave (FMCW) interferometry. The former is used for simultaneous sensing of RI and temperature, and the latter is used for multiplexing a series of the cascaded FP sensors to realize multi-point sensing. By means of Fourier transform-based algorithms, the interference spectra of each sub-FP sensors can be divided and demodulated independently. Experimentally, three cascaded FP sensors are multiplexed to verify multi-point RI and temperature sensing ability. RI sensitivity up to ∼1200 nm/RIU is obtained within RI range from 1.3330 to 1.3410, and temperature sensitivity up to ∼0.17 nm/°C is obtained within temperature range from 20 °C to 80 °C. The RI precision is as high as 10-5 RIU and the temperature precision is as high as 0.05 °C. In addition, the prospective multiplexing number could reach about 4000 estimated by the minimum detectable light power. The proposed sensing system has potential advantages in the practical applications that require a large number sensing points.

Fast Brillouin optical time-domain reflectometry using the optical chirp chain reference wave.

A new technique for the fast implementation of Brillouin optical time-domain reflectometry has been proposed and demonstrated with the optical chirp chain (OCC) reference wave. By using the fixed bandpass filter and envelope detection, the spontaneous Brillouin spectrum can be online demodulated in the time domain for truly distributed, one-end access and fast measurement. The measurement time is only limited by the pulse repetition rate and averaging times. For a 400 m single-mode fiber, a 31.58Hz strain vibration on a 2 m fiber segment is measured for a wide dynamic range (∼3200µÎµ) with an equivalent sampling rate of 200Hz when 200 times of averaging is performed. Furthermore, the performance on the measurement accuracy is investigated with different OCC frequency spans and durations.

Fast Brillouin optical time-domain analysis using frequency-agile and compressed sensing.

A fast Brillouin optical time-domain analysis (BOTDA) sensor has been proposed and experimentally demonstrated based on the frequency-agile and compressed-sensing technique. The proposed scheme employs a data-adaptive sparse base obtained by the principle component analysis algorithm, enabling the sparse representation of Brillouin spectrum. Then, it can be reconstructed successfully with random frequency sampling and orthogonal matching-pursuit algorithms. In the experiment, the Brillouin gain spectrum (BGS) is mapped by the conventional fast BOTDA, where the frequency step and span are 4 MHz and 500 MHz, respectively. By using compressed-sensing technology, the BGS is successfully recovered with 37 random frequency samples, the number of which is only 30% of the full data. With fewer sampling frequencies, the compressed-sensing technology is able to improve the sensing performance of the conventional fast BOTDA, including a 3.3-time increase in sampling rate and 70% reduction in data storage.

Phase-coded Brillouin optical correlation domain analysis with 2-mm resolution based on phase-shift keying.

We report a phase-coded Brillouin optical correlation-domain analysis (BOCDA) based on phase-shift keying (PSK), in which the pseudo-random binary sequence (PRBS) phase coding is realized using a Mach-Zehnder modulator (MZM). Unlike the conventional phase-coded BOCDA using a phase modulator, which suffers from the non-rectangular transition in the encoded phase, the PSK can realize perfect phase switches between 0 and π with zero-width edges. It is not sensitive to the bandwidth of the modulator and the power of the radio-frequency modulation signal. Numerical simulations and experimental results prove that it can effectively suppress the Brillouin amplification in the off-peak positions. In experiment, a 2-mm spatial resolution sensing is realized using only a 20-GHz bandwidth MZM.

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.

Imaging enhancement based on stimulated Brillouin amplification in optical fiber.

The detection of weak optical signals embedded in strong background illumination has broad application prospects. We propose an imaging enhancement method based on stimulated Brillouin scattering (SBS) in a single-mode fiber, which is capable of amplifying the weak optical signal while neglecting the broadband background noise because of its narrow gain bandwidth. In experiment, a high gain of 60 dB was achieved. An imaging enhancement experiment was carried out, where a target which cannot be seen because of transmission loss could be clearly captured with the amplification of SBS in the fiber. Because of the employment of continuous pump rather than a pulsed pump, this system has wide application in the monitoring of non-cooperative targets.

Low-noise and high-gain of stimulated Brillouin amplification via orbital angular momentum mode division filtering.

We demonstrate a Brillouin amplifier scheme by using orbital angular momentum mode division filtering, which is able to amplify the weak optical signals with low noise and high gain. The system retains the advantages of a conventional collinear Brillouin amplifier structure, and employs a liquid-crystal spatial light modulator to generate distinguishable degree-of-freedom of beams. As we all know, the noise mainly derives from the unwanted coupling in stimulated Brillouin scattering (SBS) processes, which severely limits the amplifier's performances. The efficient SBS noise-filtering method is discussed, which could improve the output beam quality effectively. The experimental results show the proposed amplifier scheme can overcome this obstacle, providing the magnification (or called signal gain) of 71 dB for an input signal of 4.7×10-12 J. In addition, the amplified signal is recognized from the amplifier spontaneous-emission (ASE)-like noise with a signal-to-noise ratio of 6.7 dB.

Distributed Brillouin optical fiber temperature and strain sensing at a high temperature up to 1000 °C by using an annealed gold-coated fiber.

In this study, the distributed temperature and strain sensing with an annealed single mode gold-coated optical fiber over a wide temperature range up to 1000 °C is demonstrated by using the differential pulse pair (DPP) Brillouin optical time domain analysis (BOTDA). Owing to the protection provided by the gold coating, the fiber can withstand high temperature environments and maintain a high strength, which enables the gold-coated fiber acting as a repeatable high-temperature sensor. After annealing twice to remove the internal stress, the temperature coefficient of the gold-coated fiber is stable and consistent with a nonlinear function. Owing to the residual stress accumulated during the cooling process of coating and the low yield strength of gold, a pre-pulling test is essential to measure the strain of a gold-coated fiber. An equal axial force model is used to recalculate the strain distribution induced by the large temperature difference within the furnace. The high-temperature strain coefficient of an annealed gold-coated fiber decreases with temperature, i.e. from ~0.046 MHz/µÎµ at 100 °C to ~0.022 MHz/µÎµ at 1000 °C, mainly due to the increase in Young's modulus of silica with temperature. To the best of our knowledge, this is the first time that an annealed gold-coated fiber has been applied for distributed high-temperature strain sensing, which demonstrates the potential applications for strain monitoring in complex, high-temperature devices such as jet engines or turbines.

150 km fast BOTDA based on the optical chirp chain probe wave and Brillouin loss scheme.

Distributed long-range Brillouin optical time domain analysis (BOTDA) is an extremely time-consuming sensing scheme, which requires frequency mapping of the Brillouin spectrum and a large number of average times. Here, we propose a fast long-range BOTDA based on the optical chirp chain (OCC) probe wave and Brillouin loss scheme. The OCC-modulated probe wave is enabled by cascading fast-frequency-changing microwave chirp segments head-to-tail, which covers a large frequency range around the anti-Stokes frequency relative to the pump wave. The combination of the OCC technique and Brillouin loss scheme provides several advantages, i.e., fast measurement, a high Brillouin threshold, no additional amplification scheme, and freedom from the nonlocal effect. In the experiment, 6 m spatial resolution, 3.2 s measurement time, and 3 MHz measurement precision were achieved over a 150 km single-mode fiber.

Single-shot BOTDA based on an optical chirp chain probe wave for distributed ultrafast measurement.

Brillouin optical time-domain analysis (BOTDA) requires frequency mapping of the Brillouin spectrum to obtain environmental information (e.g., temperature or strain) over the length of the sensing fiber, with the finite frequency-sweeping time-limiting applications to only static or slowly varying strain or temperature environments. To solve this problem, we propose the use of an optical chirp chain probe wave to remove the requirement of frequency sweeping for the Brillouin spectrum, which enables distributed ultrafast strain measurement with a single pump pulse. The optical chirp chain is generated using a frequency-agile technique via a fast-frequency-changing microwave, which covers a larger frequency range around the Stokes frequency relative to the pump wave, so that a distributed Brillouin gain spectrum along the fiber is realized. Dynamic strain measurements for periodic mechanical vibration, mechanical shock, and a switch event are demonstrated at sampling rates of 25 kHz, 2.5 MHz and 6.25 MHz, respectively. To the best of our knowledge, this is the first demonstration of distributed Brillouin strain sensing with a wide-dynamic range at a sampling rate of up to the MHz level.

Detecting cm-scale hot spot over 24-km-long single-mode fiber by using differential pulse pair BOTDA based on double-peak spectrum.

In distributed Brillouin optical fiber sensor when the length of the perturbation to be detected is much smaller than the spatial resolution that is defined by the pulse width, the measured Brillouin gain spectrum (BGS) experiences two or multiple peaks. In this work, we propose and demonstrate a technique using differential pulse pair Brillouin optical time-domain analysis (DPP-BOTDA) based on double-peak BGS to enhance small-scale events detection capability, where two types of single mode fiber (main fiber and secondary fiber) with 116 MHz Brillouin frequency shift (BFS) difference have been used. We have realized detection of a 5-cm hot spot at the far end of 24-km single mode fiber by employing a 50-cm spatial resolution DPP-BOTDA with only 1GS/s sampling rate (corresponding to 10 cm/point). The BFS at the far end of 24-km sensing fiber has been measured with 0.54 MHz standard deviation which corresponds to a 0.5°C temperature accuracy. This technique is simple and cost effective because it is implemented using the similar experimental setup of the standard BOTDA, however, it should be noted that the consecutive small-scale events have to be separated by a minimum length corresponding to the spatial resolution defined by the pulse width difference.