{"_buckets": {"deposit": "bc7fe855-b205-4d05-8cf5-799e45f0f712"}, "_deposit": {"id": "5507", "owners": [], "pid": {"revision_id": 0, "type": "depid", "value": "5507"}, "status": "published"}, "_oai": {"id": "oai:repository.dl.itc.u-tokyo.ac.jp:00005507"}, "item_7_alternative_title_1": {"attribute_name": "\u305d\u306e\u4ed6\u306e\u30bf\u30a4\u30c8\u30eb", "attribute_value_mlt": [{"subitem_alternative_title": "\u5730\u7403\u7269\u7406\u7814\u7a76\u7528\u8d85\u9ad8\u6b6a\u5206\u89e3\u80fd\u5149\u30d5\u30a1\u30a4\u30d0\u30bb\u30f3\u30b5"}]}, "item_7_biblio_info_7": {"attribute_name": "\u66f8\u8a8c\u60c5\u5831", "attribute_value_mlt": [{"bibliographicIssueDates": {"bibliographicIssueDate": "2012-03-22", "bibliographicIssueDateType": "Issued"}, "bibliographic_titles": [{}]}]}, "item_7_date_granted_25": {"attribute_name": "\u5b66\u4f4d\u6388\u4e0e\u5e74\u6708\u65e5", "attribute_value_mlt": [{"subitem_dategranted": "2012-03-22"}]}, "item_7_degree_grantor_23": {"attribute_name": "\u5b66\u4f4d\u6388\u4e0e\u6a5f\u95a2", "attribute_value_mlt": [{"subitem_degreegrantor": [{"subitem_degreegrantor_name": "University of Tokyo (\u6771\u4eac\u5927\u5b66)"}]}]}, "item_7_degree_name_20": {"attribute_name": "\u5b66\u4f4d\u540d", "attribute_value_mlt": [{"subitem_degreename": "\u535a\u58eb(\u5de5\u5b66)"}]}, "item_7_description_5": {"attribute_name": "\u6284\u9332", "attribute_value_mlt": [{"subitem_description": "Geophysical research requires monitoring the earth\u0027s deformation continuously at locations as many possible with nano-strain (n\u03b5) order resolution and large dynamic range in the static to low frequency domain. Currently sensors for this purpose include the borehole strain meter, extension and free-space laser interferometers. Those types of sensors provide sufficient strain resolution; however, their large length of tens to hundreds of meters and high cost restrict the wide adoption of those sensors, especially in the deep underground. On the other hand, optical fiber strain sensors have the well-known advantages such as light weight, small size, low cost, ability of remote and multiplexed sensing, etc. They are very attractive for geophysical research if they can provide the required strain resolution. Although strain resolution even better than n\u03b5 have been reported in dynamic strain sensing with optical fiber sensors, the resolution in the quasi-static frequency domain the resolution is limited to the order of the micro-strain (\u03bc\u03b5) for most of sensors. It is generally satisfactory for applications such as smart material and structure health monitoring. But it has to be improved by about 3 orders of magnitude for the sensors to utilized in the geophysical applications. The strain resolution is one of the most important parameters to evaluate the performance of strain sensors. It is defined as the smallest change in strain that produces a distinguished response in the measurement. The resolution of a sensor is limited by the random noise level in the system and by the ambient interference. Two factors relate to high resolution. First, the sensor must have high strain sensitivity, which is defined as the induced variation in the output of sensor for a given change of strain. The other factor is to effectively suppress the random noise level and the effect of ambient interference. This does not only involve choosing high precision instruments which will significantly increase the cost of sensor; more importantly, the mechanism of the sensor has to been studied sufficiently to find the main noise sources and then suppress them by certain methods. At the beginning of this thesis, we theoretically analyzed the performance of a static strain optical fiber sensor interrogated by a tunable laser, and deduced the expression of the strain resolution for this type of sensors. Following on the analysis, a series of optical fiber sensors were developed with ultra-high strain resolution. Further attempt on multiplexed sensing is also presented in the later part of this thesis. Optical fiber strain sensors have already achieved ultra-high resolution in dynamic strain sensing field; however, the strain sensing in the quasi-static domain is not so successful yet. Those facts are a direct consequence of an essential difference between the dynamic and static strain sensing. The dynamic sensing deals with the periodical strain signal, and thus it is self-referenced; but the static strain sensing measures arbitrary (usually slowly varying) signals, and an extra reference is required, which is usually a frequency-stabilized component or an additional sensor head identical to the strain sensor but free of strain. In our research, the proposed quasi-static strain sensors have two identical sensor heads. One is for strain sensing and the other is strain-free working as a reference for compensation of the fluctuation of both the laser source and the sensor heads. The key for ultra-high resolution static strain sensing credits to the precise measurement of the output difference between the two sensor heads. Two types of sensor heads are presented in this thesis, and different technologies are proposed for the interrogation of the sensor heads. The structure of the thesis is stated below. In chapter 2, we designed an ultra-high strain-resolution fiber Bragg grating (FBG) sensor which is interrogated by a narrow linewidth tunable laser. The sensor consists of a pair of FBGs for strain sensing and reference, respectively. The wavelength of the laser source sweeps to obtain the spectra of FBGs. The difference in the Bragg wavelength of the FBGs is calculated utilizing a cross-correlation algorithm. The performance of the sensor is theoretically studied. First, the main noise sources in the sensor are discussed. Then, the expression of the resolution is deduced with a Gaussian-curve model for the FBG\u0027s spectrum. The theoretical prediction agrees well with numerical simulation, and is further verified by experimental results. With the expression, the guidelines to optimize this type of the sensor are revealed in detail, providing a firm base for the construction of practical n\u03b5 resolution FBG sensors. In chapter 3, we built a FBG strain sensor based on the analysis. With the sensor a wavelength resolution of 3.1 fm was obtained in laboratory without strain applied, corresponding to a static strain resolution down to 2.6 n\u03b5. This is the first demonstration that a n\u03b5-order static resolution is achied with a simple sensor configuration. With a variable strain applied by a piezo-stage, a strain resolution of 17.6 n\u03b5 was demonstrated, which is mainly limited by the precision of the testing stage. Later, the sensor is put into field test to measure the crustal deformation induced by oceanic tide at Aburatsubo Bay, Japan, which is currently monitored by 38m-long extension-meters. Wavelength division multiplexing (WDM) technique is used for interrogation of two sets of FBG strain sensors. The deformation induced by oceanic tide is clearly recorded with resolution about 10 n\u03b5, and the strain staggers around earthquakes are also observed. Compared with the extension-meters, our FBG sensor has a comparable resolution with a much smaller size and lower cost, providing a powerful tool for geophysical measurements. In chapter 4, we developed an opticcal fiber static strain sensor by using a pair of fiber Fabry-Perot interferometer (FFPI) sensor heads, to overcome the wavelength repeatability problem which limits further improvement on the resolution of FBG sensors. A frequency modulation (FM) technology is used to dither the laser frequency, and then a digitalized demodulation configuration is employed to extract the detuning information between the laser source and the resonance frequency of the FFPIs. A cross-correlation algorithm is used to calculate the resonance difference from the extracted signals with high precision. An ultra-high static wavelength resolution corresponding to strain resolution down 5.8 n\u03b5 was demonstrated in experiment, with dynamic range large than 100 \u03bc\u03b5. Together with the small laser sweeping range (5 pm) and the short measuring period, this research provides a high resolution, large dynamic, short measuring period and low cost strain sensor for the geophysical applications. Then we invented a novel sideband interrogation technology to interrogate FFPI sensors for even higher strain resolution in chapter 5. This technology avoids the wavelength nonlinearity of the tunable laser during large-range sweeping in the typical FM configuration. A special designed radio frequency signal is used to drive an intensity modulator (IM) to generate a sideband. The sideband is used to interrogate the sensing FFPI, while the laser carrier is used to interrogate the reference FFPI with typical FM configuration. Experiment of static strain sensing is carried out using a tunable laser, and a cross-correlation algorithm is employed to calculate the resonance difference. With a sweeping rang of only 0.1 pm and measuring period of a few seconds, a standard deviation of measured resonance difference of 29 kHz was obtained, corresponding to a strain resolution of 0.3 n\u03b5. This is the first time that a sub-n\u03b5 static strain resolution was demonstrated with optical fiber sensors. Real-time sensing is achieved by locking the laser carrier and sideband to the reference and sensing FFPIs, respectively. Furthermore, real-time sensing is achieved by locking the laser carrier and the sideband to reference and the sensing FFPIs, respectively. With a specially designed radio frequency modulator to drive the IM, a strain resolution down to 0.05 n\u03b5 is realized in real-time srain sensor, and the measuring rate is 7 Hz. With the ultra-high strain resolution and the ability of real-time sensing, the proposed sensor meets the strictest standard for geophysical research, especially, for earthquake measurements. In chapter 6, we designed a multiplexing technology with identical FFPI sensor heads based on a dual-modulation configuration. The modulation method is presented after a thorough analysis on the FM modulation technique, and a modulator for the dual-modulation is designed using a commercially available differential quadrature phase shift keying (DQPSK) modulator. Numerical simulation results prove that, the strain and the position can be measured simultaneously in the multiplexed sensor with two identical FFPIs. The last chapter concludes the research. 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