Research On Temperature Demodulationsystem Based On FFP-TF And Dualreference FBG

Fiber Bragg Grating(FBG),with the characteristics of all fiber fabrication and wavelength modulation,occupies an important position in the sensing field.FBG is an indispensable part,especially in the fields of national defense industry,power system,health monitoring on civil engineering project,and so on.Considering the unique characteristic of the sensing parameters to the central wavelength modulation of FBG,the method of demodulating the sensing grating is universal.The Fiber Fabry-Perot Tunable Filter(FFP-TF)as the wavelength demodulation devices.Combined with two reference FBGs to calibrate the dynamic characteristic parameters of FFP-TF,the subject has developed a set of Demodulation system with complete display function of temperature and wavelength.And this Demodulation system can achieve spectral reappearance.In this paper,we reviewed the development of FBG and the research progress at home and abroad,and the mature demodulation technology from the literature research.We also deduced the sensing principle of FBG.In addition,the transmission characteristics of FFP-TF are analyzed in theory.The research focuses on the temperature sensing,which from two aspects of the temperature characteristic calibration method and the wavelength demodulation system design of the sensing grating.The main contents are as follows:A method to calibrate the temperature sensing characteristics of FBG is designed with spectrometer and high precision digital thermometer DS18B20.According to this method,calibrating the temperature sensing characteristics of 1550 nm FBG,the sensitivity of temperature is 9.899pm/?.Compared with the traditional water temperature box calibration method,the range of temperature sensitivity error in 0.619pm/ ?,the proposed calibration method with the characteristics of low cost and high efficiency is effective and reliable.Taking sinusoidal signal as an example,the implementation of Direct Digital Synthesizer(DDS)on hardware is deduced,and the hardware structure of signal generator is explained.And combined with high speed parallel DAC module and voltage amplifying circuit,a triangular wave signal generator that meets the requirements of driving FFP-TF is designed.The high-speed parallel ADC module is selected to sample the photoelectric signal of FBG.At the same time,the digital Gaussfilter is designed and implemented on the hardware.Based on the starting time of driving FFP-TF,a fast peak seeking algorithm based on Field Programmable Gate Array(FPGA)is proposed and implemented.In addition,a data compression method combined with pre-peak seeking operation and interval sampling is innovatively proposed,which not only ensures the peak finding effect,but also improves data transmission efficiency.Finally,the data packet format is defined and the effective data is packed to the upper computer.According to the functional requirements of the system,software with simple peak seeking function and system software with completed emodulation function are written in C#(C Sharp)language.First,the interactive operation of hardware and software is designed,and the characteristic parameters of FFP-TF are calibrated in real-time on the basis of the peak finding of the reference FBG.Then,the peak finding results of the sensing FBG are replaced by the transmission characteristics of FFP-TF to calculate the wavelength,and then the temperature parameters are reversed.Lastly,the waveform data unpack spectral reconstruction in the drawing window.At the same time,the result of processing by the host computer is stored in the specified path in the TXT text format.The software and hardware platform is set up to test the performance of FFP-TF.A triangular wave signal with a frequency of 500 Hz and a peak-peak of 11.5V drives FFP-TF:(1)at constant temperature,it is found that the time domain resolution of FFP-TF(the time required to distinguish the unit wavelengths)remains unchanged.Such as,in 31? under the environment,the time domain resolution is 29.8475ns/pm;(2)for 3 fixed wavelengths and 8 temperature nodes,it is found that when the same wavelength is demodulated,the time domain resolution will be changed.However,due to the hysteresis effect of FFP-TF,which leads to the introduction of the nonlinear problem.Finally,according to the test of FFP-TF,FFP-TF with the same driving triangular wave signal,placed in the environment of 29 ?.It is found that the temperature error of the demodulation system is below 1.36 ?,which was included the temperature error of calibration method 1.52 ?.Therefore,the effectiveness of demodulation function to temperature and wavelength is explained.At the same time the test result pointed out that the resolution of temperature and wavelength were 3.31 pm,0.32 ?.