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

Abstract

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.

Recent Progress in Fast Distributed Brillouin Optical Fiber Sensing

“…Generally, for a silica SMF, the BFS is~11 GHz [1,5,9] at the communication wavelength of 1550 nm. It is noted that the BFS has a good linear relationship [1,11] with the strain and temperature of the fiber and is given by the equation below.…”

“…Then, the distributed BGS can be obtained through a frequency-sweeping process for the probe wave and is curve-fitted to get the distributed BFS. To measure the distributed BFS, the typical BOTDA scheme is operated by the "Pump-Probe" method [5,11,28,29], which is shown in Figure 1. A pulsed-pump is injected into the FUT at the position z = 0 while a CW-probe wave is injected into the FUT at the position z = L, which is shown in Figure 1a.…”

“…For the probe wave, it experiences not only the Brillouin amplification but the Brillouin phase-shift [11] (i.e., the vector SBS effect), which is shown in Figure 1b. The BGS and BPSS can be depicted by the equations below.…”

“…Instead of the slope of the BGS, the slopes of the BPSS [43,44] and RF phase-shift profile [45] are selected as the sensing curve resulting in the dynamic ranges of 40 MHz and 128 MHz, respectively. Recently, a dimensionless coefficient K [11] is defined as the expression below.…”

“…When their 2 of 17 frequency-detuning is close to the Brillouin frequency shift (BFS) of the FUT, the power will be dramatically transferred from the high-frequency optical wave to the low-frequency optical wave via the stimulated Brillouin scattering (SBS) effect [8,9]. By scanning the frequency-detuning, the distributed Brillouin gain spectrum (BGS) can be obtained and its central frequency (i.e., BFS) computed by curve-fitting has a linear relationship [1,4,5,10,11] with an environmental strain or temperature.…”