| High frequency vibration detection and analysis are widely used in the fields of material nondestructive testing and diagnosis.Traditional electrical sensors are limited by resonant frequency,so it is difficult to realize broadband measurement.At present,the widely used ultrasonic sensing system based on phase-shift grating intensity demodulation has narrow working window and is easy to be affected by ambient temperature.To achieve the goal of large working range and broadband high-frequency vibration detection,a high-frequency vibration sensing system based on ordinary fiber Bragg grating(FBG)reflected light phase detection and demodulation is proposed in this paper.The vibration sensing system is experimentally studied from the aspects of frequency response,working range and variable temperature measurement.Explain and understand the source of modulation signal noise,and a differential sensing system of reflected and transmitted light is proposed to reduce the influence of modulator frequency drift.T The important work of this subject is as follows:1.The effects of refractive index and grating length on the spectral slope of fiber Bragg grating are analyzed by simulating the grating spectral model.A simulation sensing system based on FBG reflected light phase sensing and demodulation is constructed to verify the feasibility of the scheme.The demodulation systems based on reflectivity and phase slope are compared and analyzed,and the demodulation performance of the system under laser frequency noise is simulated.A multi FBG multiplexing simulation sensing system is built,and the demodulation results of pulse reflection beat light when multiple vibration signals act on the grating sensor at the same time are described.2.A detection system based on FBG reflected light phase sensing is established.The system uses heterodyne detection to include the vibration information in the beat signal phase.The optimal frequency estimation algorithm and orthogonal demodulation method are used to successfully restore the vibration signal.Taking the piezoelectric ceramics transducer(PZT)as a reference,the scheme can achieve 83 d B signal-to-noise ratio and 43 d B sensor sensitivity at the frequency point of 400 k Hz and has good linear sensing characteristics.The frequency response of the sensing system with a SNR of more than 70 d B in the wide frequency range of 100 k Hz-2MHz.The laser wavelength scanning test and 20℃ variable temperature sensing test show that the scheme is suitable for FBG with different reflectivity,and can have a large working window of at least 0.2 nm and the ability to resist temperature drift.3.It is analyzed that the signal fluctuation of FBG reflected light sensing system is due to the random drift of modulator frequency.Based on the principle that the slopes of FBG reflected and transmitted light are inversely equal at the same wavelength,an FBG differential sensing system is built,which can improve the signal fluctuation and double the signal amplitude. |
