Distributed Strain Measurement with High Spatial Resolution Using Fiber Bragg Gratings and Optical Frequency Domain Reflectometry

Abstract

High spatial resolution and sensitivity are required in distributed strain measurements for structural health monitoring. In this paper we show that the distributed strain measurements with the high spatial resolution can be implemented by optical frequency domain reflectometry (OFDR). We could easily map the strain profile along a long gauge FBG from a spectrogram by applying Fourier transform analysis with a sliding window to the signal obtained from the measurement system. The OFDR signal can be simulated by the transfer matrix method based on coupled mode theory. We evaluated the spatial resolution and the sensitivity from the results of simulations and experiments. We confirmed that the spatial resolution of the current system is about 4 mm and it may be changed according to the location of the grating in the sensing fiber. In addition, the strain distribution adjacent to holes of an aluminum tensile specimen was measured. The agreement between this experimental result and the simulated one was good. IntroductionDistributed measurements using fiber optic sensors attract the attention of researchers studying on structural health monitoring (SHM) systems, because they can effectively obtain useful information on the structural integrity all over a structure from those systems [1]. Fiber optic distributed strain sensors can provide the information on harmful deformation or stress concentration due to damage or an unexpected large load. In order to assess the structural integrity properly according to the strain/stress distributions obtained from the distributed strain sensors, it is important that the sensors can detect slight anomaly in the distributions [2]. Therefore, the high spatial resolution and the sufficient sensitivity are required in the measurement system.In recent years various techniques for distributed strain measurements have been proposed. Among them, the distributed sensing techniques based on Brillouin scattering have been developed by many researchers and have drawn attention in SHM fields [3,4]. While fiber Bragg grating (FBG) sensors with some interrogation systems are ordinarily employed for quasi-distributed strain or temperature measurements, they can be also applied to full-distributed sensing. If a strain distribution measured is monotonic, the strain profile along the grating can be determined by analyzing the reflection spectrum [5]. A strain distribution along a chirped grating is measured by the phase-based sensing method or the low-coherence reflectometry [6,7]. However, these distributed sensing methods have a drawback, such as elimination of uniform gratings or short sensing range. By using a tunable laser demodulation, the strain profile along a uniform grating with a long gauge length can be mapped based on Fourier transform analysis [8]. Although this sensing system can measure strain distributions with the high spatial resolution, it seems that signals outputted from several photodetectors in the sensing system have to be acquired individually and analyzed step by step.In t...

Long fiber Bragg grating sensor interrogation using discrete-time microwave photonic filtering techniques

“…In the context of sensing applications, different methods have been proposed and implemented in order to interrogate the Bragg-frequency distribution along a FBG with the aim of implementing distributed temperature/strain sensors. Some of these salient techniques include optical low-coherence reflectometry (OLCR) [6], optical frequency domain reflectometry (OFDR) [7], synthesis of optical coherence function (SOCF) [8], and timefrequency analysis [9].…”

Time-frequency analysis of long fiber Bragg gratings with low reflectivity

“…For these reasons, FBGs have been extensively implemented for many potential applications such as sensors [1], switches [2], temporal integrators [3] and filters [4]. Several methods have been proposed to characterize the Bragg-frequency distribution along FBGs [5][6][7], based on optical low-coherence reflectometry and optical frequency domain reflectometry. However, both techniques show a limited measurement range and a slow response and require very complicated setup.…”

Resin flow monitoring in vacuum-assisted resin transfer molding using optical fiber distributed sensor

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