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.
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.
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 5358.3με 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.
A dynamic distributed Brillouin optical fiber sensing 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 a little less than the Brillouin frequency shift of the fiber (∼10.8 GHz for silica fiber), while the tuning range of the frequency-agility modulation is required to cover only several-hundred MHz for the scanning of 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, a 11.8-Hz strain is measured. The spatial resolution and the sampling rate are 1 m and 200 Hz, respectively.
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.
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 με with the frequency of 13 Hz by using a six-slope analysis.
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 the 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 14με;.
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