Design Of Fiber Loop Ring-Down Cavity Temperature Sensing System Based On FPGA

In the process of freezing and thawing,the internal temperature of ice layer is the key physical parameter of ice engineering research on the ice mechanical properties,material balance and energy change of ice sheet,ice evolution process.The continuous detection method and technology play an important theoretical research value and practical significance for the reflection of climate conditions,hydrologic forecasting,and ice break-up warning.However,the inherent complex characteristics of ice material and the special low temperature environment restrict the continuous detection of temperature variation in the process of ice generation and dissipation.As a new detection technology developed since the 1970 s,optical fiber sensing technology has the advantages of electromagnetic interference immunity,corrosion resistance,waterproof and moisture-proof,real-time online monitoring,which has been widely used in engineering practice.For ice temperature detection problems and the advantages of optical fiber sensing technology,fiber Bragg grating with high sensitivity and dynamic detection as the sensing element is combined with fiber loop down cavity sensing technology which has advantages on structure and demodulation.The model of sensing system is established based on the characteristics of fiber Bragg grating loop ring-down cavity,and the temperature sensing experimental device is built to explore the continuous detection of internal temperature during ice generation and dissipation.The main research of this paper is as follows:1.The sensing theory of fiber Bragg grating loop ring-down cavity.The characteristics of fiber Bragg grating and loop ring-down cavity temperature sensing are studied to establish the theoretical model.The ring down characteristics and the best optimal coupling ratio of couplers composed of the fiber Bragg grating loop ring-down sensing system are obtained by numerical simulation;2.Design of tunable pulsed laser source based on FPGA.As the main control unit,a crossclock synchronization scheme is proposed to solve the asynchronous timing problem of FPGA,and high-speed tunable electric-pulse signal is obtained by frequency multiplying technology,which is modulated into pulsed laser;Bias voltage controller module of electro-optic modulator is designed;The internal state machine from FPGA is designed to adjust the bias voltage,which is used to solve the problem of the best work-point drift to realize the optimization of the pulsed laser source;3.Design the Erbium-doped fiber amplifier system based on FPGA.According to analyze the working principle of Erbium-doped fiber amplifier,the amplification system of initial pulsed laser and the voltage controlled constant current driving circuit of 980 nm semiconductor laser are built.Based on the dynamic control of FPGA,the output power with 1.079?W of the pulsed laser by electro-optic modulator is amplified to 6.7m W;4.Experiment of temperature sensing system.Based on the theoretical analysis,the fiber Bragg grating ring-down cavity temperature sensing system based on FPGA as a control core and test platform are built.The experimental results show: the theoretical analysis is basically consistent with the experimental results;During ice generation and dissipation,the continuous temperature variation law of ice layer in the range of-30??30? is obtained;The method of the pulsed laser source optimization can improve the stability and repeatability of the proposed temperature sensing system,and the temperature detection sensitivity is 0.03337?s/?,and the resolution is 0.03?.