Research On High-speed Demodulation Of Weak Grating Array Based On Microwave Photonics And Chromatic Dispersion

The identical weak grating array is widely used in many engineering fields because of its excellent characteristics such as low reflection loss,weak signal crosstalk,low writing cost and easy realization of large capacity.However,in the measurement fileds of multipoint and high frequency,for example,in the dynamic monitoring of rotating machinery and the wheeltrack monitoring of high speed train,the increasingly complex measurements put forward higher requirements on the dynamic and static high speed demodulation capability,multiplexing capacity and operation cost of the identical weak grating sensing system.In this paper,based on the microwave photonics technology and the chromatic dispersion effect,the grating wavelength shift information is converted to the frequency domain and the beat frequency signal of the grating can rapidly form by crossing microwave sweep cycle.Combined with the optimized frequency spectrum zooming algorithm,the high-speed and high-precision demodulation of identical weak grating array is realized.The main research contents are as follows:?1?The temperature and vibration sensing mechanism of grating are analyzed.The influences of Rayleigh scattering,multipath reflection and grating reflectivity on the multiplexing capacity of the identical weak grating array are studied theoretically and simulatively.The principles of the microwave photonics technology and the chromatic dispersion effect are also studied,and a mathematical model of wavelength domain to frequency domain conversion is established.?2?The high-speed demodulation system of weak grating array based on the microwave photonics and chromatic dispersion is researched.In the construction of optical path,a continuous frequency-varying microwave signal whose sweep cycle and frequency bandwidth are 40kHz and 500MHz¹.5GHz respectively is used to drive the LiNbO₃ modulator for the broadband light modulation,and the erbium-doped fiber amplifier and dispersion compensation fiber are adopt to provide the optical gain and dispersion coefficient.In the circuit design,the microwave mixer,low-pass filter and other circuits are designed to collect the beat frequency signal.In the principle of demodulation,the wavelength demodulation theory of the system is deduced theoretically,and the crossing cycle mode of sweeping signal is proposed to generate the beat frequency signal quickly,which can avoid the speed limitation of the system for the long length of dispersion fiber.?3?The algorithm of high-speed demodulation system for weak grating array based on the microwave photonics and chromatic dispersion is researched.Through analyzing the influences of fence effect and frequency sidelobe on the system accuracy in detail,an optimized frequency spectrum algorithm based on the Chirp-Z and Hanning window is designed to achieve high-speed demodulation while reducing the spectrum sidelobe crosstalk between multi-grating arrays.?4?The system performance optimization methods are researched and verified.Using reference grating,the influence of temperature disturbance on the dispersion fiber is weakened by solving the frequency difference between the sensing grating and the reference.Introducing the single mode fiber,the demodulation errors when the temperature influence the transmission fiber of sensing array are corrected by parallelling the wavelength and frequency relationship of two-channel.In addition,a dispersion correction model is established to further calibrate the demodulation deviation caused by dispersion approximation.?5?An identical weak grating sensing array of 105 gratings with 1m spacing and0.1%reflectivity is fabricated,and multi-parameter static and dynamic experiments are built to test system performances.Experiment results show that the demodulation accuracy of this system is about 5.8pm,the average linearity of the grating array temperature demodulation is 0.9988,the demodulation rate can reach 40kHz,and the system has good demodulation capability of dynamic and static signal.