Dexin Ba

High-Spatial-Resolution Fast BOTDA for Dynamic Strain Measurement Based on Differential Double-Pulse and Second-Order Sideband of Modulation

We demonstrate a high-spatial-resolution fast Brillouin optical time-domain analysis (BOTDA) for distributed dynamic strain measurement based on differential double-pulse and second-order sideband of modulation. The frequency-agility probe wave is obtained from the second-order sideband of the modulated light by using the microwave signal from a wideband arbitrary waveform generator (AWG), which reduces the bandwidth requirement of the AWG by half to ~ 5.5 GHz. The differential double-pulse scheme 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 polarization-maintaining fiber observing the vibration frequency of up to 50 Hz. With only five averages, the standard deviation of the strain accuracy is measured to be 14 με.

High-quality near-field beam achieved in a high-power laser based on SLM adaptive beam-shaping system

We demonstrate a high-power laser system with a high-quality near-field beam by using a liquid-crystal spatial light modulator (SLM). An efficient spatial beam shaping algorithm is discussed which can improve the output nearfield beam quality effectively. Both small-signal and large-signal amplification situation of the laser are considered in the beam shaping algorithm. The experimental results show that the near field fluence modulation of output is improved from 1.99:1 to 1.26:1 by using the liquid-crystal SLM. Obvious uniform spatial fluence distribution and near-field beam quality improvement are observed.

Distributed measurement of dynamic strain based on multi-slope assisted fast BOTDA.

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 5000με 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).

Slope-assisted BOTDA based on vector SBS and frequency-agile technique for wide-strain-range dynamic measurements

We present a slope-assisted BOTDA system based on the vector stimulated Brillouin scattering (SBS) and frequency-agile technique (FAT) for the wide-strain-range dynamic measurement. A dimensionless coefficient K defined as the ratio of Brillouin phase-shift to gain is employed to demodulate the strain of the fiber, and it is immune to the power fluctuation of pump pulse and has a linear relation of the frequency detuning for the continuous pump and Stokes waves. For a 30ns-square pump pulse, the available frequency span of the K spectrum can reach up to 200MHz, which is larger than fourfold of 48MHz-linewidth of Brillouin gain spectrum. For a single-slope assisted BOTDA, dynamic strain measurement with the maximum strain of 2467.4με and the vibration frequency components of 10.44Hz and 20.94Hz is obtained. For a multi-slope-assisted BOTDA, dynamic measurement with the strain variation up to 5372.9με and the vibration frequency components of 5.58Hz and 11.14Hz is achieved by using FAT to extend the strain range.

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.Sensors: Lighting the way to low-cost, ultrafast sensorsScientists 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.

Dynamic Distributed Brillouin Optical Fiber Sensing Based on Dual-Modulation by Combining Single Frequency Modulation and Frequency-Agility Modulation

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.

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.

High-precision small-scale laser focal spot measurements

In this paper, a method of two-dimensional fine-scanning with charge coupled device has been conducted to precisely measure spatial position and intensity distribution of small-scale focal spot (diameter in microns). The measurement accuracy of the small-scale focal spot position is better than 1 mu m when the fluctuations of the light energy and background noise are relatively small. The theoretical analysis is consistent with the experimental results.

Generation of multi-line spectrum via intensity modulation for the broadening of Brillouin gain bandwidth

It is an effective approach to use pump with multi-line spectrum to broaden the bandwidth of slow light based on stimulated Brillouin scattering (SBS). A novel method of generating multi-line spectrum is proposed and simulated, through which arbitrary numbers of spectral lines with arbitrary intervals can be obtained. The amplitude of each line is able to be tuned independently. These two merits enable to construct the shape of the gain spectrum of SBS as expected. The maximum broadened bandwidth of the pump is only limited by the bandwidth of related hardwares.

Residual pulse peak reduction in stimulated brillouin scattering optical limiting by an injected seed

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.