Development Of High Speed FBG Fiber Grating Demodulation Instrument

The demodulation technique of fiber Bragg grating obtained fast development dueto its unique advantages. With the changing of its applied environment and the increasingof requirements, how to develop a corresponding demodulator according to the differentneeds of the environment will be crucial.In this paper, we settled a system of demodulation based on the linear filter, ofwhich the basic principle is that to obtain the changes of external temperature andpressure with the ratio changes of two beams’ optical power. Through theoretical analysiswe find that the relationship between the ratio and the transmittance of the linear filter isproportional in perspective of the traditional demodulation, and the traditional method offilter demodulation has been improved. By the following aspects, the stability of thedemodulation system, the demodulating speed and the demodulating precision have beenadvanced.First of all, in the experiment we found that when the power of the light sourcedecreased, the ratio changed greatly, and the instability of the system caused directly theimprecision of the demodulation. In order to improve the stability of demodulationsystem, the method of ratio correction has been adopted. Through the analysis on thechange of two optical powers, we found out the background noise is the major cause ofthe two lights’ ratio changes, and the background noise have been processed. Besides, theinstability of linear filter has been solved by introducing the temperature control device.The experimental results show that the ratio is constant and the system has a goodstability when the FBG sensors are at different temperatures and change the light sourcepower has been changed after the background noise have been handled. In addition,when the method of the virtual wavelength is introduced and the virtual wavelength isadopted to demodulate the temperature, we can demodulate the temperature directlywithout calibrating the actual wavelength, which improved the speed of demodulationsystem to great extent. The fitting relationship between actual wavelength and the ratiocan be derived by the relationship between virtual wavelength and the ratio. The experimental data show that the method of virtual wavelength is correct. Thirdly, in theexperiment, the relationship between the wavelength and the ratio were fitted innonlinear, after which I compared the results of wavelength and the demodulatingtemperatures by the demodulator developed by myself. And the results show that, underthe condition of nonlinear fitting, the average value of temperature precision of thedemodulation system measured is better than the situation of the linear fitting that itimproved the accuracy of the demodulation system.Finally, the fitting coefficient hadbeen unable to experimental measurements because of the change of the ratio caused byjoint loosening of the optical fiber. In this paper, joining the method of the coefficientcorrection, to fitting the coefficient once again without establish a new database, save alot of time, the experimental results show that the coefficient had been corrected fordemodulation experiment have a good stability, high precision, achieve the objective ofmeasure a physical parameter in a long time use a group of database data.The demodulation device has such advantages as high stability, fast response speed,high precision and so on. It can be accessed with four road FBG sensor, and realizes themulti-point measurement of temperature.