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

Abstract

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).

Fast Frame Synchronization Design and FPGA Implementation in SF-BOTDA

“…Therefore, in light of Equation (9), it can be seen that the calculation complexity is substantially reduced to (3 × N + 4) + 8 × (M − 1) multiplications and (7 × N − 5) + 16 × (M − 1) summations. For the M = 1152 and N = 128 used in References [14,15], using the standard method requires 446,976 multiplications and 1,026,432 summations, while using the proposed method only required 9596 multiplications and 19,307 summations, reducing the calculation amount by about 98%, which makes the real-time frame synchronization in BOTDA possible.…”

“…To solve the problem of long acquisition times, a variety of scanning-free BOTDA (SF-BOTDA) or single-shot methods was proposed [13][14][15][16][17][18]. For example, slope-assisted BOTDA is realized based on tuning the frequency of probe optical signal to half of BGS which can effectively shorten the measurement time [13].…”

“…For example, slope-assisted BOTDA is realized based on tuning the frequency of probe optical signal to half of BGS which can effectively shorten the measurement time [13]. In Ba et al [14], another kind of scanning-free BOTDA is proposed based on multiple probes and pump pairs that allows for the fast measurement of strain or temperature changes along the fiber. Moreover, Jin et al proposed a digitally generated optical-frequency-comb-based BOTDA to realize scanning-free BOTDA and single measurement BOTDA in 2015 and 2017 respectively, with an acquisition time of 100 µs for a 10 km optical fiber [15,16].…”

Recent Progress in Fast Distributed Brillouin Optical Fiber Sensing

“…It can be clearly seen that the time traces are in good agreement with the reference time trace in Figure 13a-c while several distortion points (the blue ellipse in Figure 13d) and severe waveform distortion (zoom-in view in Figure 13e) are generated for the frequency interval of 180 MHz. Therefore, the maximum frequency interval for this scheme is 160 MHz, which is twice of that of the BGS-based multi-slope-assisted BOTDA scheme [42]. This results in a doubled sampling rate.…”

“…The frequency relationship[42] for (a) a double-slope-assisted scheme and (b) a multi-slopeassisted scheme.…”