Strain variation measurement with short‐time Fourier transform‐based Brillouin optical time‐domain reflectometry sensing system

Abstract

A novel technique to perform dynamic strain measurement with a Brillouin optical time-domain reflectometry (BOTDR) sensing system based on short-time Fourier transform (STFT) is proposed. Unlike traditional frequency scanning methods, the proposed system samples the Brillouin backscattering signal directly after it has been downconverted to the intermediated frequency (IF) region. The Brillouin frequency shift (BFS) of the sensing fibre was reconstructed from the recorded time-domain data continuously without dead-time, which provided a much higher refreshing rate of the strain measurement. Therefore, dynamic strain variation over an optical fibre can be measured with a single-end structure that is more flexible and reliable. A 16.7 Hz strain variation on a 12 m section was successfully detected at the end of a 270 m sensing fibre with 4 m spatial resolution and 45 με uncertainty.Introduction: Distributed optical fibre sensors based on Brillouin scattering have attracted significant interest in the past few decades. By monitoring the Brillouin frequency shift (BFS) of the scattering signal generated by an injected probe pulse, the sensors can detect temperature and strain simultaneously [1, 2] along the entire length of a sensing fibre. Generally, there are two main kinds of technologies, namely, Brillouin optical time-domain reflectometry (BOTDR) and Brillouin optical timedomain analysis (BOTDA). Traditionally, both technologies require a time-consuming frequency scanning of the entire Brillouin gain spectrum (BGS) to obtain the BFS, which restricts the application of both technologies to static measurement only. Moreover, because of the weak spontaneous Brillouin backscattering signal, a great amount of average is needed even if coherent detection is introduced in the BOTDR sensing system [3]. For the BOTDA system, the signal is stronger because of the high stimulated Brillouin gain. Similar signal-to-noise ratio (SNR) can be achieved with much shorter integration time, that provide this technique with the potential for dynamic strain detection [4][5][6][7], especially when it works with the help of the slope-assisted technique. However, the requirement of having a two-end structure and the inability of straintemperature distinction [8] decrease its attraction and also limit its application realm.As a substitution for frequency scanning, Fourier transform was introduced to a BOTDR system. The BGS spectrum can be obtained without frequency scanning. In the literature [9], it takes 1 s to measure the temperature by using the Brillouin fibre laser and Fourier transform technique with 1024 data averages. Apparently, if one records the down-shifted backscattering power continuously with good SNR, the one-end measurement of the dynamic strain variation could be achieved.In this Letter, a dynamic strain-sensing BOTDR system based on short-time Fourier transform (STFT) is proposed and demonstrated. The BFS of the sensing fibre can be reconstructed from the recorded time-domain data continuously without dead-time, which...

Dynamic Strain Measurement Using Small Gain Stimulated Brillouin Scattering in STFT-BOTDR

“…Brillouin based dynamic systems can measure the absolute strain with a large strain range higher than 10 4 με. Brillouin based systems demonstrate a slow sampling rate (tens of Hz to a few hundred Hz) and a limited sensing distance from tens of meters to a few hundred meters [10]- [12].…”

“…However, BOTDR usually needs a large number of averaging due to the weak power of spontaneous Brillouin scattering (SpBS) and the conventional frequency scanning method [13]. Recently, Short-time Fourier transform (STFT) algorithm is replacing the frequency scanning in BOTDR [12], [14]. STFT can reduce the averaging times and realize the dynamic strain measurement [12], [14], whose performance is limited by the signal-to-noise ratio (SNR) that is usually low in BOTDR [13].…”

Detrimental Effect Elimination of Laser Frequency Instability in Brillouin Optical Time Domain Reflectometer by Using Self-Heterodyne Detection

“…Among these studies, optical fiber sensors based on Brillouin scattering have attracted much attenuation due to their capability to measure the distribution of temperature and/or strain, vibration and moisture in large civil structures, power cables, geological disasters, and road embankments [8,9,10,11,12,13]. In general, Brillouin-based distributed sensors can be implemented by using either stimulated Brillouin scattering (SBS) [4,12] or spontaneous Brillouin scattering (SPBS) [8,9,10].…”

“…Among these studies, optical fiber sensors based on Brillouin scattering have attracted much attenuation due to their capability to measure the distribution of temperature and/or strain, vibration and moisture in large civil structures, power cables, geological disasters, and road embankments [8,9,10,11,12,13]. In general, Brillouin-based distributed sensors can be implemented by using either stimulated Brillouin scattering (SBS) [4,12] or spontaneous Brillouin scattering (SPBS) [8,9,10]. Nevertheless, in a Brillouin optical time domain reflectometer (BOTDR) system, the round-trip delay time from launching the pulsed light to receiving the backscattered Brillouin light at the front end of a fiber can be used to determine the scattering location, and the frequency shift and intensity of spontaneous Brillouin light in the fiber can be used to simultaneously measure distributed temperature and strain by accessing only one end of fiber, offering extra advantages over the SBS-based Brillouin optical time domain analysis system.…”

Active Compensation Method for Light Source Frequency Drifting in <inline-formula> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula>-OTDR Sensing System

“…Compared to other vibration detecting DOFS [4]- [6], -OTDR has the advantages of fast response, high sensitivity and multi-point detection capacity. Therefore, it has been applied in various applications such as intruder detection [1] and structure health monitoring [2].…”