AS1E.2.pdf
ACP Technical Digest © 2012 OSA
Development of N ano-Strain-Resolution Fiber Optic Static Strain Sensor for Crustal Deformation Monitoring I
Zuyuan He/,2 Qingwen Liu, 1,2 and Tomochika Tokunaga
3
State Key LaboratolY ofAdvanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, China 2 Dept. ofElectrical Engineering and lriformation Systems, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 3 Dept. of Environment Systems, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8563, Japan
[email protected]
Abstract: The fIrst realization of nano-strain-resolution fIber optic static strain sensor is introduced. Theoretical analysis and experimental demonstration are reviewed. With this sensor, crustal deformations induced by oceanic tide and by earthquake were clearly observed. OC]S codes: (060.2370) Fiber optics sensors; (060.3735) Fiber Bragg gratings; (120.0280) Remote sensing and sensors
1. Introduction For geo-science applications, it is required to monitor the earth's deformation continuously at locations as many as possible with a resolution better than 10 nano-strains (nE), which corresponds to the crustal deformation induced by oceanic tide. Traditionally, sensors such as extensometers and laser strain-meters installed underground are used for this purpose. These conventional sensors, however, are difficult to be installed widely due to their size of tens to hundred meters in length. Also due to their length, they can only give integrated strain information over that length. Recently, we started a project to investigate the feasibility to employ fIber optic sensors, specifIcally, fIber Bragg grating (FBG) sensors for geophysics applications. FBG sensors are small in size, and thus low-cost in installation, and can easily be multiplexed, making them attractive for geophysics if they can provide a resolution down to nano strains (nE). They have already been widely adopted in strain measurement for applications in smart materials and structures. For most of these applications, a strain resolution about 1 micro-strain (IlE) is generally satisfactory. Although there are several reports of dynamic FBG strain sensing at kHz region realizing even better than n E sensitivity, the realization o f static strain sensing i s much more difficult. A dynamic sensing can b e self-referenced, but a static strain sensor has to be compared with an extra standard.
In fact the static strain resolution is mainly
limited by environmental temperature disturbance, because FBGs are sensitive to both strain and temperature. For strain measurement with high resolution, a common way is to employ a strain-free reference FBG for temperature compensation. The reference FBG is also useful in eliminating other common drift such as the influence from light source wavelength/intensity variation.
Strain information can be retrieved by evaluating the different wavelength
shift between the sensing FBG and the reference FBG.
Many demodulation algorithms have been proposed to
determine the Bragg wavelength of FBGs, among which the cross-correlation algorithm can determine the Bragg wavelength difference directly and exhibits good ability of suppressing random uncertainty. In this paper, we introduce the realization of a nano-strain-resolution FBG static strain sensor. The theoretical analysis and experimental demonstration are reviewed. With this sensor, a resolution of 2.6 nE without strain applied and a resolution of 17.6 nE with strain applied were demonstrated, respectively, and crustal deformations induced by oceanic tide and by earthquake were clearly observed, for the fIrst time to the best of our knowledge.
2. System configuration and theoretical analyses [1] The confIguration of proposed sensor is shown in Fig. 1 (a). The sensor consists of pairs of FBGs. Each pair has the same nominal Bragg wavelength for sensing and reference, respectively. The sensing FBG is mounted on the measurand, and the reference FBG is strain-free. The two FBGs are places close so that they experience the same thermal fluctuation. The common noise such as wavelength drift of laser and environmental thermal variation has the same influence on both FBGs. As a result, the Bragg wavelength difference between the FBGs is only sensitive to the strain applied to the sensing FBG. During the in-lab experiments, the sensor system consists of one pair of identical FBGs with center wavelength of 1550 nm. For the in situ experiments, the system has two pair of FBGs with center wavelength of 1535 nm and 1555 nm, respectively. We performed a thorough analysis on the static strain resolution of the sensor employing a cross-correlation algorithm for interrogating the strain-induced Bragg wavelength shift. The influences of the wavelength inaccuracy of the laser source, the intensity noise, the shape of FBG's reflective spectrum, etc., on the strain resolution are analyzed.
Fig. l(b) shows the influence of the wavelength inaccuracy of the laser source as an example. The
AS1E.2.pdf
ACP Technical Digest © 2012 OSA
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Fig. 1. (a) Configuration of multiplexed high resolution FBG strain sensors. (b) Example of theoretical analysis results: strain resolution vs. FBG bandwidth under different wavelength inaccuracy of the laser source.
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analyses provide guidelines for the design and the optimization of the FBG sensor to realize ultra-high static strain resolution, and show that nano static strain sensing can be realized with an FBG sensor if it is designed properly. 3.
Experimental verification and field demonstration [2]
The in-lab experiments were carried out with and without strain applied, respectively. At first, both FBGs were put in relaxation and no strain was applied to ascertain the ultimate performance of the sensing system. The measured wavelength difference over 24 hours is shown in Fig. 2(a). According to the deviation of the wavelength difference, a strain resolution as high as 2.6
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strain to the sensign FBG. As shown in Fig. 2(b), the measured wavelength shift followed well the variation of the applied strain, and a strain resolution of 17.6
m;
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After the validation on the performance of the sensor in laboratory, we performed in-situ demonstration at Aburatsubo Bay, Kanagawa, Japan, to observe the crustal deformation induced by oceanic tide. As shown in Fig. 3(a), a vault is built at the coastline with two sections. Currently 38-m long extensometers are set in Section A to monitor the crustal deformation. Recently we set up I-m long FBG sensors in Section B for the same purpose. Fig. 3 (b) and (c) illustrate the oceanic tide level and the strain measured by FBG sensors, showing that the crustal strain induced by oceanic tide is clearly recorded by the FBG sensors. The different amplitudes between the sensor units suggest the feasibility of measuring the distribution of deformation in rock mass with high spatial resolution and low cost. Fig. 3(d) is the measured strain by the extensometer, providing a strain standard for comparison. The strain resolution of FBG sensor is calculated to be 10 ns, which is the first in-situ demodulation of 10 ns order static strain resolution with FBG sensors. During the measurement, strain changes induced by earthquakes are also recorded as shown in Fig. 4, which is the data due to an earthquake of M7. 1 occurred on July 10, 20 1 1 at Sanriku coast (N38°, EI43.5°), about 500-1an far away from Aburatsubo Bay. The Japan Meteorological Agency
(JMA) seismic intensity
scale at Aburatsubo is 2. Due to the sampling period of 1 min, the seismic waveform is not recorded. Optical fiber sensors capable of both static and dynamic strain sensing with ns-order resolution are now under development [3, 4].
AS1E.2.pdf
ACP Technical Digest © 2012 OSA
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References [1] [2] [3] [4]
Q. Liu, T. Tokunaga, and Z. He, "Realization of nano static strain sensing with fiber Bragg gratings interrogated by narrow linewidth tunable lasers," Opt. Express. 19 (21),20214-20223 (20II). Q. Liu, T. Tokunaga, K. Mogi, H. Matsui, H. F. Wang, T. Kato, and Z. He, "Ultra-high resolution multiplexed fiber Bragg grating sensor for crustal strain monitoring," IEEE Photon. Jour.,4 (3),996-1003 (2012). Q. Liu, T. Tokunaga, and Z. He, "Ultra-high-resolution large-dynamic-range optical fiber static strain sensor using Pound-Drever-Hall technique," Opt. Len. 36 (20),4044-4046 (20II). Q. Liu,T. Tokunaga,and Z. He, "Sub-nano resolution static strain fiber sensor using a sideband interrogation technique," Opt. Len. 37 (3), 434-436 (2012).
