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A distributed pressure sensor is proposed and designed for dynamic pressure measurement by using frequency-agile fast Brillouin optical time-domain analysis (BOTDA). A double coated single-mode fiber (SMF) is used as the sensing fiber because of its 1500- $\mu \text{m}$ thickness outer coating that can significantly improve the dependence of the Brillouin frequency shift on pressure. Then, the frequency-agile technique is used to generate frequency-agile pump pulse in BOTDA system so that a fast Brillouin gain spectrum distribution measurement can be achieved. An injection locked scheme is used as both optical filter and optical amplifier to improve the measurement accuracy. As a result, the static pressure sensitivity of the double coated fiber is −3.46 MHz/MPa with the pressure ranging from 0 to 30 MPa. Subsequently, a large dynamic range pressure measurement is performed with a sampling rate of 33.3 kHz and maximum sensing distance of 25 m. Furthermore, it is proved that the uncertainty of dynamic pressure measurement system is 0.03 MPa.

In the past few years, Brillouin-based optical fiber sensors have gained significant ground for distributed temperature and strain measurements in the real world. Among these sensors, the optical chirp chain (OCC) based Brillouin optical fiber sensor is a good candidate to realize ultrafast distributed sensing, which is of great importance to distinguish quick-changing events in practical applications. In this paper, the concept, principle and sensing scheme of OCC are introduced, including the OCC Brillouin optical time-domain analysis and OCC Brillouin optical time-domain reflectometry. In OCC Brillouin optical time-domain analysis, the influence of transient interaction is reflected as three types of spectral distortions, i.e., the back end distortion, the frequency lag of main peak and the frequency saltation distortion, which are verified in the simulation and experiment and attributed to the rapid frequency sweeping. In addition, this paper reviews the OCC based cutting-edge schemes and technologies aspects to upgrade Brillouin sensors with ultra-fast and high-performance sensing capability. In the future, there are still many challenges in OCC based distributed sensing, such as high spatial resolution, real-time demodulation and low-cost scheme.

A high-sensitivity distributed temperature sensor based on Brillouin scattering is theoretically proposed and experimentally demonstrated. Double-coated single-mode fibers (SMF) are used as sensing fibers to improve the temperature sensitivity of Brillouin frequency shift (BFS). The thermomechanical model of double-coated fiber based on Lame formula is established, and the reasons for improving the BFS temperature sensitivity are analyzed. In the simulation, the BFS temperature sensitivity is improved with increasing of outer coating thermal expansion coefficient, Young’s modulus, and radius, but the effect of Poisson’s ratio can be ignored. In the experiment, four kinds of optical fibers are designed to verify the theoretical results, including a single-coated fiber and three double-coated fibers. The outer coating radii of double-coated fibers are $450~\mu \text{m}$ , $1000~\mu \text{m}$ and $1500~\mu \text{m}$ , respectively. The BFS temperature sensitivity of these fibers are calculated to be 1.07 MHz/°C, 3.28 MHz/°C, 4.57 MHz/°C and 6.04 MHz/°C within the temperature range of −30 to 40 °C, respectively. With a spatial resolution of 1.5 m, the maximum BFS temperature sensitivity of the double-coated SMFs is measured, which is about 6 times higher than that of single-coated SMF, achieving a maximum measurement error less than 0.05 °C.

The uncoated optical fiber has been extensively researched as an optomechanical sensor since it can achieve substance identification of the surrounding media by exciting and detecting transverse acoustic waves via forward stimulated Brillouin scattering (FSBS), yet within danger of being easily broken. Although polyimide-coating fibers are reported to allow transverse acoustic waves transmission to the coating for reaching the ambient while maintaining the mechanical properties of the fiber, it still suffers from the problems of humidity sensitivity and spectral instability. Here, we propose for the first time a robust distributed FSBS-based optomechanical 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 has a higher signal-to-noise ratio, compared to 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 the external relative humidity changes, which is a preponderance for liquid acoustic impedance measurements.

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.

Brillouin-based optical fiber sensors have been developed over the past few years and played a significant role for distributed temperature and strain measurements, which include many properties such as high measurement accuracy, large measurement range and environmental suitability. Among these sensors, Brillouin optical fiber sensors via optical chirp chain (OCC) become an ideal choice for ultrafast distributed measurement to distinguish quick-changing events in practical applications. This paper begins with the introduction of the concept and the generation schemes of OCC, the spectra distortion characteristic of OCC Brillouin signal are analyzed. The efforts towards such OCC based Brillouin optical fiber sensing for ultrafast measurement are reviewed here as well, with the OCC based Brillouin optical time domain analysis and the OCC based Brillouin optical time domain reflectometry, which give distinguished performance for dynamic measurement, long distance measurement and one end access measurement. Meanwhile, the consequent future challenges of OCC based distributed sensing are discussed and presented, such as as high spatial resolution and high accuracy measurement.

A fast Brillouin optical time-domain reflectometry (BOTDR) for dynamic strain measurement is proposed and experimentally demonstrated based on the frequency-agile technique. Employing the band-pass filter and envelope detection, the spontaneous Brillouin gain spectrum can be online demodulated in the time domain for a truly distributed, one-end-access and dynamic strain measurement. A strain sampling rate of 62.5 Hz is achieved for the proposed sensor over 172 m single-mode fiber with 100 sweeping frequencies and 64 times of averaging. In the experiment, the vibration frequencies of 6.82 Hz and 14.77 Hz have been measured with 2 m spatial resolution and 4000 μϵ dynamic range. The measurement accuracy is calculated to be ±30 μϵ by the standard deviation of the static fiber Brillouin frequency shift. Furthermore, the performance on the strain sampling rate and measurement accuracy is investigated with different averaging times. It should be noted that the upgraded schemes for the frequency-agile based Brillouin optical time-domain analysis can be also introduced into the proposed fast BOTDR.

A distributed birefringence measurement of a polarization-maintaining fiber (PMF) can be realized by using a Brillouin dynamic grating (BDG). However, the pump depletion decreases the intensity of the BDG along the PMF and limits the measurement range to a few hundred meters. We demonstrate a distributed birefringence measurement of the PMF with an extended range to 3030 m based on an enhanced BDG with an 18.5-ns pump-1 pulse, frequency-upshifted continuous-wave pump-2, and 4-ns read pulse (spatial resolution: 40 cm). The enhanced BDG is promising to characterize the design of a fiber coil to improve the fiber optic gyroscope performance.

The phase-sensitive optical time-domain reflectometry (φ-OTDR) is a good candidate for distributed dynamic strain sensing, due to its high sensitivity and fast measurement, which has already been widely used in intrusion monitoring, geophysical exploration, etc. For the frequency scanning based φ-OTDR, the phase change manifests itself as a shift of the intensity distribution. The correlation between the reference and measured spectra is employed for relative strain demodulation, which has imposed the continuous measurement for the absolute strain demodulation. Fortunately, the Brillouin optical time domain analysis (BOTDA) allows for the absolute strain demodulation with only one measurement. In this work, the combination of the φ-OTDR and BOTDA has been proposed and demonstrated by using the same set of frequency-scanning optical pulses, and the frequency-agile technique is also introduced for fast measurements. A 9.9 Hz vibration with a strain range of 500 nε has been measured under two different absolute strains (296.7με and 554.8 με) by integrating the Rayleigh and Brillouin information. The sub-micro strain vibration is demonstrated by the φ-OTDR signal with a high sensitivity of 6.8 nε, while the absolute strain is measured by the BOTDA signal with an accuracy of 5.4 με. The proposed sensor allows for dynamic absolute strain measurements with a high sensitivity, thus opening a door for new possibilities which are yet to be explored.

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.

For several years, Brillouin-based optical fiber sensors plays an important roles in the fields of distributed temperature and strain measurements in the real world. Among these sensors, the optical chirp chain (OCC) based Brillouin optical fiber sensor is a good candidate to realize ultrafast distributed sensing, which is of great importance to distinguish quickchanging events in practical applications. In this paper, the principle of OCC and the OCC based Brillouin optical timedomain analysis (BOTDA) sensing are introduced. In OCC-BOTDA, there are three types of spectral distortions, i.e. the back end distortion, the frequency lag of main peak and the frequency saltation distortion, which are influenced by the transient stimulated Brillouin scattering, are verified in the simulation and experiment in this paper and attributed to the rapid frequency sweeping.

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.

Without adding additional hardware, a post-processing algorithm with nonlinear weight coefficients is proposed to enhance the spatial resolution of BOTDA system meanwhile a unique self-averaging method is conducted to denoise the demodulated Brillouin signals.

Dynamic Brillouin optical fiber sensors based on fast scanning of Brillouin gain spectrum (BGS) are one of the most promising techniques to measure dynamic strains, where an ~11-GHz bandwidth arbitrary waveform generator (AWG) or a vector microwave generator is essential for frequency agility. A dynamic Brillouin optical fiber sensor based on dual-modulation is proposed here, which aims to realize dynamic sensing via a low-bandwidth AWG. In this protocol, the scanning of BGS is implemented by the combination of a single-frequency modulation and a frequency-agility modulation. The frequency of the single-frequency modulation is slightly lower than the Brillouin frequency shift of the fiber under test so that the tuning range of the frequency-agility modulation is required to cover only several-hundred MHz for the scanning of the BGS, which significantly reduces the bandwidth requirement for the AWG. In experiment, an 11.8-Hz strain is measured with a 30-m fiber, where the spatial resolution and the sampling rate are 1 m and 200 Hz, respectively. Furthermore, by tracking the damping vibration of the optical fiber, its resonant frequency is measured with a sampling rate of 100 Hz.

Supplemental Document

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 ( < p h o n o n lifetime) via a nonlinear weight matrix to enhance SR. The distributed BGS generated by using an 8-ns transient pump pulse is processed by the RED algorithm, and its SR is enhanced from 0.8 m to 1 mm (i.e., 800-fold improvement) in the simulation and 8 mm (i.e., 100-fold improvement) in the experiment.

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 31.58 H z strain vibration on a 2 m fiber segment is measured for a wide dynamic range ( ∼ 3200 µ ε ) with an equivalent sampling rate of 200 H z when 200 times of averaging is performed. Furthermore, the performance on the measurement accuracy is investigated with different OCC frequency spans and durations.

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.

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., Sensors: Lighting the way to low-cost, ultrafast sensors Scientists have developed an inexpensive and ultrafast sensor that could be used in a range of applications from monitoring geo-engineering to the structural integrity of large infrastructure. Compared with conventional electrical sensing, optical fiber sensors are much smaller and cheaper, immune to external electromagnetic interference and can offer millions of sensing points simultaneously. However, processing the information provided by optical fibers can be time-consuming, limiting their applications. Now, a team of researchers from China and Canada, led by Yongkang Dong from Harbin Institute of Technology in China, has developed an innovative technique that generates a probe wave comprising short optical chirps that can be quickly demodulated by injecting a single-shot pump pulse into the fiber. The work paves the way for ultrafast sensors for use in applications from the infrastructure to geophysical research.

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.

A dynamic distributed Brillouin optical fiber sensor based on dual-modulation is proposed, in which the scanning of the Brillouin gain spectrum (BGS) is implemented by the combination of a single-frequency modulation and a frequency-agility modulation. The frequency of the single-frequency modulation is slightly lower than the Brillouin frequency shift of the fiber (∼10.8 GHz for silica fibers), while the tuning range of the frequency-agility modulation is required to cover only several-hundred MHz for the scanning of the BGS, which can significantly reduce the bandwidth requirement for the arbitrary waveform generator and ultimately reduce the cost of dynamic Brillouin sensors. With a 30-m fiber, an 11.8-Hz strain is measured with a spatial resolution of 1 m and a sampling rate of 200 Hz.

We propose and demonstrate a novel fast Brillouin optical time-domain analysis system using the coefficient K spectrum which is defined as the ratio of phase-shift and gain of Brillouin amplification, where K features linear response, immune to the variation of pump power and a wide measure range. For a 30ns-square pump pulse, the frequency span of K spectrum can reach up to 200MHz. In dynamic strain experiment, a multi-slope assisted K-BOTDA with the measured strain of 5358.3μe and the vibration frequency of 6.01Hz and 12.05Hz are demonstrated.

A distributed hydrostatic pressure sensor based on Brillouin dynamic gratings (BDGs) was proposed and demonstrated for the first time to the best of our knowledge. Through measuring the pressure-induced birefringence changes through exciting and probing the BDGs, the hydrostatic pressure sensing is realized. The thin-diameter PM-PCF is used as the fiber under test. The temperature can be compensated by measuring the temperature-induced Brillouin frequency shift (BFS) through differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA). A distributed measurement is reported with a 20-cm spatial resolution and measurement accuracy as high as 0.025 MPa.

Guided acoustic wave Brillouin scattering has gained considerable interest in recent years because of its capacity to detect mechanical features of materials surrounding the optical fiber. Nevertheless, distributed measurements using this mechanism are rarely taken because of the impracticality of the method’s forward scattering mechanism. Recently, remarkable work using ingenious schemes has managed to address the difficulty, which opens a brand new way to achieve position-resolved substance identification. However, due to the long acoustic wave lifetime and insufficient signal-to-noise ratio (SNR), current spatial resolution is restricted to 15–50 m, which is far from practical requirements. Here we propose a novel opto-mechanical time-domain analysis based on coherent forward stimulated Brillouin scattering probing to greatly improve the achievable spatial resolution. The coherent transverse acoustic wave is first created by a long activation pulse and then probed by a short two-tone probe pulse. The two-tone probing process involves a coherent stimulated interaction between the probe pulse and the excited transverse acoustic wave. The interaction, which we first propose here, shows a distinct phase-sensitive characteristic. This new coherent stimulated probing process, if it is well controlled, will enhance the forward stimulated Brillouin scattering intensity and thus improve the SNR of the sensing. Moreover, higher SNR backward stimulated Brillouin scattering is used to detect the intensity evolution of the probe pulse. Owing to this new sensing scheme combined with a more robust demodulation algorithm, we demonstrated a 2 m spatial resolution opto-mechanical measurement over a 225 m long fiber in which we were able to distinguish air from alcohol. These advances greatly facilitate the practicability of forward stimulated Brillouin scattering.

Brillouin-based optical fiber sensing has gained considerable attention for the past few years thanks to its ability to offer distributed sensing. The major limitation of a Brillouin-based optical fiber sensor is its complexity because a time-consuming frequency-sweeping process is needed to obtain a local Brillouin gain spectrum (BGS) and to calculate the local Brillouin frequency shift (BFS). Thus, it is only suitable for static or slow-varying measurements. In this article, we propose an approach to achieve truly distributed and ultra-fast fiber-optic sensing based on an active and distributed bandpass microwave photonic filter (MPF) through stimulated Brillouin scattering (SBS). To obtain a truly distributed BFS, a counter-propagating single-shot pump pulse is launched into the fiber link and a microwave multi-tone (MMT) signal with a random initial phase distribution which is phase modulated on an optical carrier is launched into the fiber link from the other end. Due to the SBS effect, the −1st order sideband of the phase-modulated signal will experience Brillouin amplification while the +1st order sideband will experience Brillouin attenuation, and the phase-modulated signal is converted to an intensity-modulated signal. The entire operation is equivalent to a bandpass MPF. By detecting the optical signal at a photodetector (PD), a regenerated MMT signal with its magnitude and phase that are shaped by the MPF is obtained. By evaluating the regenerated MMT signal, the Brillouin information corresponding to the temperature or strain change at a specific location is revealed. The major advantage of the approach is that time-consuming frequency-sweeping process is avoided. Truly distributed strain, temperature, and vibration sensing with a 2 m spatial resolution over 49.5 m distance at a speed up to 83.3 kHz is experimentally demonstrated.

We demonstrate a high-spatial-resolution fast Brillouin optical time-domain analysis scheme based on frequency agility and differential double-pulse for distributed dynamic measurement. The frequency-agility probe wave is obtained from the second-order sideband of modulated light by using frequency-agility microwave signal from a wideband arbitrary waveform generator. The differential double-pulse technique is proposed to improve the spatial resolution while keeping the capability of dynamic measurement. In experiment, a spatial resolution of 20 cm is achieved by using a 52/50 ns differential double-pulse, and the distributed vibration measurement is demonstrated over a 50-m Panda fiber with a maximum vibration frequency of up to 50 Hz. With only five averages, the standard deviation of the strain accuracy is of 14 μV.

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.

We numerically calculate and experimentally investigate the characterization of phase-shifted Brillouin dynamic gratings (PS-BDGs) in a polarization maintaining fiber (PMF). A phase-shifted point is induced into the middle of a conventional BDG through phase-modulating one of the two pump pulse, generating a PS-BDG thanks to the stimulated Brillouin scattering (SBS). When the frequency difference between a high frequency pump1 pulse with 1ns and π-1ns and a low frequency pump2 pulse with 100ps is equal to the Brillouin frequency shift of the PMF, a transient PS-BDG with a 3dBbandwidth of 354MHz of the notch spectrum is simulated based on the coupled-wave equations of BDG. By increasing the repetition rate up to 250MHz, an enhanced PS-BDG with a deep notch depth is obtained since the residual acoustic wave of the former SBS process is enhanced by the optical waves of the latter SBS process. Then a proof-of-concept experiment is built to verify the transient PS-BDG and the results show that the notch feature is consistent with the simulation results and the notch frequency of the PS-BDG can be changed by tuning the phase shift Δϕ . The proposed PS-BDGs have important potential applications in optical fiber sensing, microwave photonics, all-optical signal processing and RoF (radio-over-fiber) networks.

A temperature-compensated distributed hydrostatic pressure sensor based on Brillouin dynamic gratings (BDGs) is proposed and demonstrated experimentally for the first time, to the best of our knowledge. The principle is to measure the hydrostatic pressure induced birefringence changes through exciting and probing the BDGs in a thin-diameter pure silica polarization-maintaining photonic crystal fiber. The temperature cross-talk to the hydrostatic pressure sensing can be compensated through measuring the temperature-induced Brillouin frequency shift (BFS) changes using Brillouin optical time-domain analysis. A distributed measurement of hydrostatic pressure is demonstrated experimentally using a 4-m sensing fiber, which has a high sensitivity, with a maximum measurement error less than 0.03 MPa at a 20-cm spatial resolution.

An up to 1100°C and 1200°C high-temperature distributed Brillouin sensing based on a GeO 2 -doped single mode fiber (SMF) and a pure-silica photonics crystal fiber (PCF) are demonstrated, respectively. We found that the BFS dependence on temperature of SMF and PCF agrees well with an exponential function instead of a linear function, which is mainly attributed by the change of the acoustic velocity in a silica fiber. A Brillouin frequency shift (BFS) hopping is observed in both kinds of fiber between 800°C–900°C in the first annealing process and after that the BFS exhibits the stability and repeatability with a measurement accuracy as high as ±1.42°C for SMF and ±2.6°C for PCF, respectively. The BFS hopping is a highly temperature-dependent behavior, which means that a high-temperature (> 800°C) would accelerate the process of BFS hopping to reach a stable state and after BFS hopping, both SMF and PCF shows good repeatability from 1000°C to 1100°C and 1000°C to 1200°C without annealing. The process of coating burning of silica fiber not only introduces a loss induced by micro-bending but also imposes a compressive stress on the bare fiber, which contributes to an additional BFS variation at the temperature period of coating burning (272°C–564°C).

Brillouin dynamic gratings (BDGs) has been, in recent years, addressed using a polarization-maintaining fiber with many prominent advantages and proved that the BDGs have a profound high sensitivity to fiber birefringence changes. When the two parallel polarized counter-propagating pump waves with a frequency offset of fiber Brillouin frequency shift, and the orthogonally polarized probe pulse wave satisfied with the phase-matching condition, the BDGs would be excited through stimulated Brillouin scattering and read, while the optical frequency difference between the pump and probe waves is determined by the birefringence. The birefringence-induced frequency shift (BireFS) associated with the impact of external environment may be affected by local static pressure. Though the measurement of the BireFS changes along the fiber, one can realize a distributed fiber static pressure sensing. In our presentation, a temperature-insensitive distributed static pressure sensor based on BDGs is proposed and experimentally demonstrated for the first time, to the best of our knowledge. The measurement principle is to interrogate the static-pressure-induced fiber birefringence changes through generating and mapping the BDGs in the fiber under test (FUT). The experimental setup adopted two pump waves to excite a BDG and a short probe pulse to read the Brillouin grating spectrum associated with the birefringence with a spatial resolution of 20 cm. The sensing technique features a distributed measurement, temperature-insensitivity and high sensitivity to the static pressure. The distributed transverse load measurement experiment is conducted in an temperature-insensitive elliptical-core polarization-maintaining fiber with a measurement accuracy as high as 0.8 × 10−3 N/mm; and the distributed hydrostatic pressure measurement experiment is also performed in a thin-diameter pure silica polarization-maintaining photonic crystal fiber with a measurement accuracy as high as 0.025MPa with a character of temperature compensation.

Two novel phase-shifted Brillouin dynamic gratings (PS-BDGs) are proposed using single pump phase-modulation (SPPM) in a polarization maintaining fiber (PMF) for the first time to our knowledge. Firstly, based on the stimulated Brillouin scattering (SBS), a transient PS-BDG with a 3-dB bandwidth of 354MHz is written by a 2-ns pump1 pulse and a 100-ps pump2 pulse, where the phase of pump1 pulse is shifted with π from its middle point through phase modulation. Then, with a high repetition rate of 250MHz for both pump pulses, an enhanced PS-BDG with a deep notch depth is obtained and its notch frequency can be easily tuned by changing the phase shift. We demonstrate a proof-of-concept experiment of the transient PS-BDG and show the notch frequency changing by tuning the phase shift. The proposed PS-BDGs have important potential applications in microwave photonics, all-optical signal processing and RoF (radio-over-fiber) networks.

We propose and demonstrate a dynamic Brillouin optical fiber sensing based on the multi-slope assisted fast Brillouin optical time-domain analysis (F-BOTDA), which enables the measurement of a large strain with real-time data processing. The multi-slope assisted F-BOTDA is realized based on the double-slope demodulation and frequency-agile modulation, which significantly increases the measurement range compared with the single- or double- slope assisted F-BOTDA, while maintaining the advantage of fast data processing and being suitable for real-time on-line monitoring. A maximum strain variation up to 5000μe is measured in a 32-m fiber with a spatial resolution of ~1m and a sampling rate of 1kHz. The frequency of the strain is 12.8Hz, which is limited by the rotation rate of the motor used to load the force on the fiber. Furthermore, the influence of the frequency difference between two adjacent probe tones on the measurement error is studied theoretically and experimentally for optimization. For a Brillouin gain spectrum with a 78-MHz width, the optimum frequency difference is ~40MHz. The measurement error of Brillouin frequency shift is less than 3MHz over the whole measurement range (241MHz).

In this work, a distributed hydrostatic pressure sensor based on Brillouin dynamic gratings (BDGs) was proposed and demonstrated for the first time to the best of our knowledge. The hydrostatic pressure sensing is realized through measuring the pressure-induced birefringence changes through exciting and probing the BDGs; while the temperature can be compensated by measuring the temperature-induced Brillouin frequency shift (BFS) through differential pulsewidth pair Brillouin optical time-domain analysis (DPP-BOTDA). The thin-diameter high-birefringent PM-PCF is used as the fiber under test and its porous-structure cladding is liable to be deformed exhibiting a high sensitivity to pressure. A distributed measurement with a 20-cm spatial resolution and a measurement accuracy as high as 0.025 MPa is reported.

A 100-J-level Nd:glass laser system in nanosecond-scale pulse width has been constructed to perform as a standard source of high-fluence-laser science experiments. The laser system, operating with typical pulse durations of 3–5 ns and beam diameter 60 mm, employs a sequence of successive rod amplifiers to achieve 100-J-level energy at 1053 nm at 3 ns. The frequency conversion can provide energy of 50-J level at 351 nm. In addition to the high stability of the energy output, the most valuable of the laser system is the high spatiotemporal beam quality of the output, which contains the uniform square pulse waveform, the uniform flat-top spatial fluence distribution and the uniform flat-top wavefront.

We demonstrate an up to 1200°C high-temperature distributed Brillouin sensing based on a pure-silica photonics crystal fiber. A Brillouin frequency shift (BFS) hopping is observed between 800°C-900°C for the first annealing process and after that the BFS exhibits the stability and repeatability with a measurement accuracy as high as ±2 °C . The BFS dependence on temperature in the range of room temperature to 1200°C agrees well with an exponential function instead of a linear function, which is mainly attributed by the change of the acoustic velocity in a silica fiber.

We propose and demonstrate a dynamic Brillouin optical fiber sensing based on the multi-slope analysis, which features a large measurement range of strain and the capability of real-time data processing. The multi-slope analysis is realized by using the frequency-agility modulation and it can significantly increase the measurement range compared with the single-slope analysis, while maintaining the advantage of fast data processing time and suitable for real-time monitoring. In experiment, we performed a measurement of strain up to 5000 μe with the frequency of 13 Hz by using a six-slope analysis.

We propose a bend-insensitive distributed Brillouin optical fiber sensing by using a singlemode-multimode-singlemode optical fiber structure for the first time to the best of our knowledge. The sensing fiber is a graded-index multimode fiber (GI-MMF) sandwiched by two standard single-mode fibers (SMFs) with central-alignment splicing at the interface between GI-MMF and SMF to excite the fundamental mode in GI-MMF. The sensing system can resist a minimal bend radius of 1.25mm while maintain the measurement performance, with which the measured coefficients of strain and temperature are 421.6MHz/% and 0.826MHz/°C, respectively. We also demonstrate that the higher-order modes excited in GI-MMF can be easily influenced by bending, so that exciting the fundamental mode is essential for bend-insensitive distributed sensing.

We demonstrate a high-spatial-resolution fast Brillouin optical time-domain analysis scheme based on frequency agility and differential double-pulse for distributed dynamic measurement. The frequency-agility probe wave is obtained from the second-order sideband of modulated light by using frequency-agility microwave signal from a wideband arbitrary waveform generator. The differential double-pulse technique is proposed to improve the spatial resolution while keeping the capability of dynamic measurement. In experiment, a spatial resolution of 20 cm is achieved by using a 52/50 ns differential double-pulse, and the distributed vibration measurement is demonstrated over a 50-m Panda fiber with a maximum vibration frequency of up to 50 Hz. With only five averages, the standard deviation of the strain accuracy is of 14 μV.