| Fiber-optic Fabry-Pérot(F-P)interferometric sensor has plenty inherent advantages such as small size,high sensitivity,high temperature resistance,corrosion resistance,anti-electromagnetic interference and intrinsic safety.Its high-speed dynamic measurement plays an important role in the fields of acoustic detection,vibration monitoring and nano precision motion control.Spectral demodulation technology with full spectrum analysis capability can provide accurate and dense sensing information,has the characteristics of high stability and large dynamic range,which is a hot spot of research and application in the field of optical fiber sensing.However,high-speed spectrometers are usually expensive and difficult to achieve large-scale multiplexing of fiber-optic F-P sensors.Limited by the spectrum sampling rate,spectral demodulation methods are mostly used for relatively low-frequency acoustic and vibration measurements.The spectral acquisition scheme based on tunable laser has a non-negligible Doppler-induced error when used in dynamic interferometry,and this kind of error will be tens or even hundreds of times of the actual cavity length variation.Fast demodulation technology with high flexibility that can meet the complex measurement requirements in different scenarios has become one of the key technologies that need to be researched and developed in the current interferometric optical fiber sensing field.This thesis conducts in-depth research on key issues in fast dynamic measurement of fiber-optic F-P sensors by investigating all-optical acoustic sensing,vibration sensing and dynamic displacement measurement techniques.Based on the fast modulation of discrete wavelengths of the modulated grating Y-branch(MG-Y)laser,we proposed a fiber-optic programmable laser interferometry.This technology has the advantages of fast measurement speed,high stability and large dynamic measurement range,and can easily realize multi-parameter detection and array multiplexing.Major works of this thesis are listed as follows:We systematically introduced the measurement principles and system design scheme of fiber-optic programmable laser interferometry,then developed a programmable laser interferometry system by innovatively integrating intensity demodulation,quadrature phase demodulation,and spectral demodulation schemes together.A multi-channel demodulation system was constructed with FPGA(Field programmable gate array)as the core unit.Through flexible switching of different measurement modes,it achieves 100～250 k Hz fast phase demodulation while possessing full spectrum analysis capability.The resolution of absolute cavity length measurement based on white light interferometry is 0.87 nm,and the resolution of relative cavity length measurement based on phase shifting interferometry is 0.72 nm.In addition,a high-speed system has been developed to meet the measurement needs of higher sampling rates,with a maximum phase sampling rate of 10 MHz.Common-path quadrature-phase demodulation of F-P acoustic vibration sensors is realized by fast and programmable modulation of discrete wavelengths.The full-spectrum acquisition capability of the MG-Y laser greatly simplifies the parameter calibration steps,and the flexible wavelength control can be adapted to different F-P cavity lengths.The feedback-stabilization mechanism of quadrature phase relationship further improves the system robustness in harsh environments.In addition,phase shifting interferometry with real-time phase-step calibration is proposed,which solves the multiplexing problem of dynamic fiber-optic F-P sensors.Through the space-division multiplexing of four F-P acoustic sensors,multi-channel simultaneous phase demodulation and sound source localization are realized based on the same laser modulation parameters,and the localization error is less than 2 cm.Combined with time division multiplexing and wavelength division multiplexing technology,we also developed a fast dynamic measurement system with 1 to 16 channels.Using the phase shifting interferometry based on 4-step Carré algorithm,the measurement range of the dynamic fiber-optic F-P sensor is extended greatly.We realized the measurement of large-amplitude vibrations and dynamic displacements with cavity length changes in tens or even hundreds of microns.Combined the tunable laser-based phase shifting interferometry with the equal-step phase shifting algorithm,it avoids the demodulation error caused by changes in the phase-shift step,reflected light intensity and fringe visibility in the process of rapid and large-scale cavity length changes.Large dynamic measurement range is achieved with unknown initial cavity length and phase-shift step.Additionally,for traditional dual-cavity passive quadrature phase demodulation technology,we proposed an improved real-time parameter calibration scheme,which expands its application in the field of dynamic displacement measurement.Both of the above schemes enable dynamic displacement measurements over 1000 μm.To overcome the dependence of multi-parameter detection systems on the combination of multiple discrete systems,and to realize high-precision multi-parameter measurement based on a single demodulation system,we proposed phase-shifting white light interferometry and 4+1programmable sampling demodulation techniques.Through the specific design of the sampling process,fast phase demodulation of the dynamic F-P sensor and the acquisition of full spectrum information are simultaneously accomplished.The phase shifting white light interferometry uses the phase-shifted spectra to analyze the DC cavity length of the sensor,which is used to characterize the quasi-static parameter.The 4+1 programmable sampling demodulation technology conducts the phase demodulation of the F-P vibration sensor and the spectral demodulation of the fiber Bragg grating(FBG)temperature sensor through four fixed optical frequencies and one scanning optical frequency.The phase sampling rate and spectrum sampling rate for the two programmable sampling schemes are 200 k Hz,50 Hz and 100 k Hz,20 Hz,respectively.The fiber-optic programmable laser interferometry realizes flexible switching of different measurement schemes such as intensity demodulation,quadrature phase demodulation,and spectral demodulation,and can meet the practical measurement requirements of fiber-optic F-P sensors such as fast,large dynamic range phase demodulation and array multiplexing.This kind of high-performance miniaturized programmable measurement system based on MG-Y semiconductor lasers has broad application prospects in dynamic interferometry in aerospace and marine exploration fields. |
