Research On High-resolution Dynamic Demodulation Technique For Fiber-optic Fabry-Perot Sensor And Its Application

Fiber-optic Fabry-Perot(F-P)sensor is recognized to have wide industrial applications due to its anti-electromagnetic interference,electric insulation,high sensitivity,intrinsic safety,high resolution,etc.The high-resolution dynamic demodulation technique of the fiber-optic F-P sensor plays an important role in structural tilt monitoring under obvious vibration interference,and dynamic strain monitoring of aero turbine engine,nuclear submarine engine and nuclear reactor.In this paper,the high-resolution dynamic demodulation technique of the fiber-optic F-P sensor and its applications in dynamic tilt and strain sensing are studied.They are of great significance to the high-speed high-resolution robust measurement of fiber-optic F-P sensors.To solve the problem of phase jump in total phase demodulation of fiber-optic F-P cavity,the principle of F-P cavity length demodulation algorithm based on Buneman frequency estimation and total phase is thoroughly studied.The reasons and influences for phase jump are analyzed.The period number and phase of interference fringes are obtained by the direct reading method from the amplitude spectrum and phase spectrum of the interference signal.The pre-estimation compensation technique of initial phase of the interference spectrum is utilized to optimize the demodulation method.The phase jump problem caused by Buneman frequency estimation error and initial phase drift is solved.The highest demodulation rate of 70 kHz and the highest cavity length demodulation resolution of 0.027 nm are achieved.Moreover,a dynamic demodulation method of fiber Bragg grating(FBG)central wavelength based on Buneman frequency estimation formula is proposed,which is especially applicable for FBG central wavelength demodulation under the condition of low spectral resolution.The demodulation resolution of FBG central wavelength of 0.048 pm is achieved at 2-kHz spectral acquisition rate and 0.156-nm spectral resolution.Additionally,an optical fiber F-P tilt sensor based on vertical cantilever structure is proposed.The optimized F-P dynamic demodulation algorithm is used to process and separate the DC and AC signals of the real-time cavity length signal to realize the tilt measurement in the vibration environment.The sensor achieves a static inclination resolution of 0.03" in the inclination range of ±1.048° and a dynamic inclination resolution of 0.66" under vibrations.In order to realize the simultaneous measurement of tilt and vibration acceleration,a fiber-optic F-P sensor based on simple pendulum structure is proposed.Soft carbon fiber ropes with high mechanical strength and temperature insensitivity are used as pendulum strings.With the single vibration characteristics of pendulum mode,the DC signal and AC signal are separated from the real-time absolute cavity length signal.The nonlinear fitting of the dynamic frequency response curve is used to calculate the vibration acceleration amplitude.The simultaneous measurement of tilt angle and vibration acceleration amplitude is achieved of the proposed sensor.Finally,a scheme for dynamic strain measurement using a Rayleigh enhanced intrinsic Fabry-Perot Interferometer(REIFPI)is proposed.The REIFPI is composed of two nano-grating defects written by femtosecond laser pulse.The experimental results show that the method could be used to detect the dynamic strain at 800-? temperature environment and obtain 0.6-?? strain measurement resolution using the dynamic demodulation technique.The problem of vibration measurement in high temperature environment is solved.