| Fiber optic interferometric sensor is an important branch of the fiber optic sensor.White-light interferometry is a widely used technique of the optical interferometry. Thewhite-light interferometry, which is applied to fiber optic interferometric sensor, canmeasure the absolute optical path difference (OPD), and possess the abilities to providelarge dynamic measurement range and high measurement resolution. Spectral-domainwhite-light interferometry requires no local receiving interferometer to mechanically scanthe OPD. The measurement setup of the spectral-domain white-light interferometry hasgood stability. The setup is small in size and is convenient for integrating and practicalengineering application. The demodulation algorithm of the white-light interferometryplays a critical role in the spectral-domain fiber optic white-light interferometry technique.The measured parameter detected and transmitted by the white-light interferometricmeasurement setup can be demodulated through the white-light interferometric algorithm.In this thesis, the spectral-domain fiber optic white-light interferometric demodulationalgorithm is investigated. The application of the fiber optic white-light interferometry isalso studied in the thesis. The main research work and innovated achievements are outlinedas following.Firstly, a wavenumber scanning based Fourier transform white-light interferometry(FTWLI) is proposed. The detected original white-light optical spectrum is mapped fromwavelength domain to wavenumber domain by using the wavelength-wavenumber domaintransforms. Then, the white-light optical spectrum in wavenumber domain is calibrated andequally resampled along the wavenumber. For a certain OPD, the period of the white-lightoptical spectrum distributed along the wavenumber is constant. The chirp in the period isentirely removed. The Fourier spectrum of the white-light interferometric spectrum doesnot expand, and the phase error caused by chirp is avoided. The measurement resolution ofthe FTWLI is increased. In the experiment, a fiber optic extrinsic Fabry–Perotinterferometric (EFPI) sensor with an OPD of4.6mm is measured, and the broadening ofthe Fourier spectrum decreases by65%. The measurement resolution of the wavenumber scanning based FTWLI can achieve0.004μm.Secondly, a FTWLI based on nonlinear wavelength sampling is proposed. Thenonlinear wavelength sampling method avoids the wavelength-wavenumber domaintransforms. The white-light optical spectrum can be directly calibrated and resampled in thewavelength domain. The distribution and the chirp characteristics of the white-light opticalspectrum will be retained by using the nonlinear wavelength sampling. However, thebroadening of the Fourier spectrum and phase error caused by chirp will be avoided. Thenonlinear wavelength sampling method satisfies the assumption of the fast Fouriertransform (FFT) algorithm that the input sampling points should be equally spaced. Theexpanding of the Fourier spectrum can decrease by67.9%by using the nonlinearwavelength sampling based FTWLI. The variation range of the measurement resultsdecreases to0.019μm, and the standard deviation of the measurement results can reach0.005μm, when a fiber optic EFPI sensor with an OPD of3mm is measured.Thirdly, a multiple-level cross-correlation based fiber optic white-light interferometry(multiple-level cross-correlation method) with wavelet transform de-noising is proposed.The white-light optical spectrum is decomposed by the discrete wavelet transform. Thenoise existing in the detail components is removed by using the threshold method. Then,the glitches in the white-light optical spectrum, which are caused by the noise and mayinfluence the results of the cross-correlation calculation, are eliminated. For achieving thesame measurement resolution, the computation complexity of the multiple-levelcross-correlation method decreases by three orders of magnitude comparing with that of theconventional single-level cross-correlation method. The efficiency of the measurement iseffectively improved. When measuring the fiber optic interferometric sensor with a shortOPD, the multiple-level cross-correlation method can avoid the uncertainty in determiningthe optical spectrum peaks of the peak-detection based method and the phase distortion ofthe FTWLI method. The measurement resolution for fiber optic interferometric sensor withshort OPD is effectively improved, and the measurement range of the fiber opticwhite-light interferometry is extended. In the experiment, when the OPD of the measuredfiber optic EFPI sensor is176μm, the variation range of the measurement results is3.008nm, and the measurement resolution is1.369nm by using the proposed wavelet transform de-noising and multiple-level cross-correlation based fiber optic white-light interferometry.Fourthly, the transmission spectrum of the high-finesse fiber optic EFPI sensor iscaused by the multiple-beam interference, and contains periodic sharp peaks. The Fourierspectrum of the transmission spectrum consists of multiple frequency components. A noveldemodulation algorithm is proposed to measure the high-finesse fiber optic EFPI sensorwith a short cavity length. In this demodulation method, the FTWLI extracts the high orderfrequency component of the Fourier spectrum to measure the cavity length. In theexperiment, when a high finesse fiber optic EFPI sensor of which the cavity length is120μm is measured, the FTWLI extracts the second order frequency component to interrogatethe cavity length. The standard deviation of the measurement results is9.132nm. When ashort EFPI sensor is measured, the proposed demodulation method effectively avoids thephase distortion and improves the measurement resolution of the FTWLI method. Themeasurement range of the FTWLI method is extended on the short OPD.Finally, a fiber optic EFPI displacement sensor for fault measurement in geomechanicsis proposed and experimentally demonstrated. By designing a packing structure, the EFPIdisplacement sensor can demonstrate high strength and high stability, as the effects of theharsh environment are mitigated. A sensor placement method is also proposed, and theEFPI displacement sensor can be placed easily and accurately in the fault zone. Atemperature-compensated structure is also proposed in the EFPI sensor. Thus, the sensoritself is capable of compensating for temperature. The proposed EFPI displacement sensoris measured by the fiber optic white-light interferometry system in the experiment. Theexperimental results show that the ratio of the cavity length to the temperature decreases by68.3%because of the temperature-compensated structure. The quoted error caused by1°Cchange of the temperature is only0.0015%. When the temperature is continuously changedfrom17to80°C, the measurement results vary over a range of4.954μm and the standarddeviation is only1.196μm. |
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