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 high resolution distributed dynamic strain sensing has been proposed and experimentally demonstrated based on the combination of Brillouin and Rayleigh scattering. The proposed scheme employs the same set of frequency-scanning optical pulses modulated through the frequency-agile technique for fast measurements. The Brillouin optical time domain analyzer (BOTDA) technology is used to provide absolute measurement benchmarks, while the phase-sensitive optical time domain reflectometer (φ-OTDR) technology is used to capture relative strain changes in details. Two groups of 100 Hz vibrations with different amplitude (300 nε and 250 nε) have been measured under two different absolute strains (1173.9 με and 525.3 Με), which allows for dynamic absolute strain measurement with a high resolution of 8.4 nε.
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.
In view of the limitations of the traditional Brillouin optical time domain analysis (BOTDA) system such as low sampling rate, large transmission and storage space, a fast BOTDA scheme based on compressed sensing technology has been proposed to realize the random frequency sampling of Brillouin gain spectrum (BGS). The proposed scheme uses a data-adaptive sparse base obtained by the principle component analysis algorithm to realize the sparse representation of Brillouin spectrum. Then, it can be reconstructed successfully with orthogonal matching-pursuit algorithm. Compared with the traditional uniform spectrum sampling with a step size of 4 MHz, the proposed compressed sampling scheme can recover the BGS using 30% of the frequency. With fewer sampling frequencies, compressed sensing technology can improve the sensing performance of traditional fast BOTDA, including increasing the sampling rate by 3.3 times and reducing the amount of data storage by 70%.
In this paper, we analyzed the Brillouin scattering characteristics in micro-nano fibers. According to the elastic mechanics equation and the Helmholtz equation, the mathematical model of the acoustic wave field and the optical field in the cylindrical waveguide with the sub-wavelength scale were established. According to the relationship between intrinsic acoustic mode and dispersion, the mechanism of Brillouin scattering in micro-nano fiber was analyzed. Considering the effects of electrostriction and radiation pressure comprehensively, the influence of two main physical effects, photo-elastic effect and moving-boundary effect on Brillouin gain in the process of acousto-optic coupling was studied. On this basis, the influence of the structure, material, and size of the micro-nano fiber on the Brillouin frequency shift and gain was analyzed and verified experimentally by Brillouin optical correlation domain analysis.
Lidar has been widely used in both military and civil applications. Its received optical signal undergoes considerable loss and disturbance from background noise, which limits its performance especially in bad weather or air conditions. A high-gain and frequency-selective amplifier for weak optical signal based on stimulated Brillouin scattering in single mode fiber is proposed, which is an excellent candidate for the signal enhancement in lidar system. The characteristics of the amplifier were studied numerically and experimentally. In experiment, a 430-nW (peak power) pulsed signal was amplified by 70 dB with a signal-noise ratio of 14 dB, which was in good agreement with theoretical result.
High spatial resolution measurement of the Brillouin frequency shift (BFS) is accomplished using transient analysis. The step response of the output signal wave is recorded for an arbitrary, known frequency offset between pump and signal, and compared against a library of modeled, reference functions. No spectral scanning or prior knowledge of the BFS is necessary. The principle is employed in a Brillouin optical correlation domain analysis (B-OCDA) of a 4 cm-wide fiber segment. A dynamic range of 130 MHz and an accuracy of ±1.9 MHz are achieved. Transient analysis may improve the precision and acquisition rate of B-OCDA.
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 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;.
Characteristics of transmitted temporal profile of active stimulated Brillouin scattering (SBS) optical limiting are studied. Methods of impairing transmitted residual peak power and shortening the delay time of SBS are proposed. Based on coupled wave equations of SBS process, physical models of optical limiting based on active SBS are constructed. Using an implicit finite differencing in time and a downward differencing scheme in space, theoretical calculation models of active SBS limiting for numerical simulations are built up. Three parameters, i.e. residual peak power, delay time and limited power are defined renewably to characterize active SBS optical limiting. The relationship of transmitted temporal profile to seed peak power is obtained by numerical calculations. The influence of the injected seed power on transmitted residual peak power is analyzed in detail. It is shown that the residual peak power is weakened with strengthening of the seed power. By selecting suitable power of seed, residual peak is removed and consequently the ideal flat pulse output is achieved. On the basis of theoretical analysis, active SBS optical limiting temporal characteristics are examined experimentally with a 20 ns Nd:YLF laser and one nonlinear medium (CCl4). The experimental results are compared with analytical and numerical calculations.
A new method to reduce the residual pulse peak in stimulated Brillouin scattering (SBS) optical limiting by an injected seed is presented. The pulse shapes of the transmitted pump beam are studied for various delay times and injected seed powers theoretically. The numerical simulated results show that the height of the residual pulse peak can be controlled by changing the delay time and injected seed intensity. Experimentally, various optical limiting pulse shapes with different heights of the residual peak are observed by changing the delay time and intensity of the seed injected into the SBS optical limiter. The experimental and theoretical results show that the residual pulse peak in SBS optical limiting can be controlled by the delay time and power of an injected seed.
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