Technology Of Signal Interrogation Of Long Period Fiber Grating

Abstract：Fiber grating sensing technology represents one of the major researchdirection of fiber-optic sensing. It has many advantages, such as high sensitivity,corrosion resistance, small size, distributed measurement and so on, and it has beenmore widely appreciated in recent years. Fiber grating can be divided into fiberBragg grating (FBG) and long period fiber grating (LPFG), and there are manysimilarities in producting methods and theoretical basis between them, however, withdifferent features and advantages in sensor performance. Currently the FBG sensingtechniques have been developed maturely, but the LPFG sensor technology have yetto continue to be explored and researched.Interrogate technique which is the key of technology application in fiber gratingdetermines the performance of sensor system directly, and this thesis is started forstudy LPFG sensor technology as the background. LPFG sensing mechanism isbased on the principle that measured parameter changes into its response to themovement amount of loss characteristics wavelength. Therefore, the movementamount of this wavelength can reflect the size of test parameters. The core of LPFGsignal interrogation is how to detect tiny movement amount of characteristicswavelength. In an actual signal interrogation of LPFG sensor, cross-sensitivitybetween the measured parameters and temperature affects the effectiveness of signalinterrogation, which is the key issue in the LPFG research that must be resolved, andmust be considered. At present, our laboratory commonly use spectrometer as signalinterrogate method, which is high cost、bulky and is not suitable for practical systemneeds in Fiber grating sensing technology. Therefore, people propose a variety ofways in LPFG signal interrogate technique based on wavelength encoding. Thispaper is based on existing research and study a new type of methods named arraywaveguide grating AWG interrogation systematically.The specific research of this paper includes:1. Firstly, a large number of references were studied and various methods based onLPFG signal interrogation were analyzed and summarized; and then several major techniques as F-P scan-based interferometer method, monochromatic edgefiltering method, edge filter with FBG device were analyzed and reviewed; atlast, on the basis, a new method based on arrayed waveguide grating AWGinterrogation was focused on, analyzed systematically and in-depth andexperimental researched. The strain is used as the measured parameter andtemperature as a cross-sensitivity parameter in the experiment, then the strainsignal interrogate effect is studied based on the condition of temperaturecross-sensitivity environment. This design principle is applied in the paper tocomplete the experiment and obtain data to build cross-sensitivity test modelbetween wavelength shift and strain.2. As the temperature and strain cross-sensitivity problem is existed in the signalinterrogate experiment, therefore, through analysis and compare differentsensitivity solutions, such as reference fiber grating method and so on, we decideto use regression analysis of data fusion algorithm to remove temperaturecross-sensitivity effects in the experiment. Based on this method, we build atheoretical model and design the corresponding matlab program to eliminatetemperature cross-sensitivity in the form of software.3. AWG-based signal interrogate method is optimized to improve the design, andthe cantilever method is chosen to apply strain. Then cantilevers of glass andmetal materials are designed and made, and the experimental relationshipbetween the hanging heavy of free end of the cantilever and LPFG strain signalis studied.4. This paper do the experiment to test LPFG signal interrogate effect under thetwo parameters-temperature and strain, at the same time, design the signalinterrogate experiment based on spectroscopy, obtain data to establish theoreticalmodels.Then use regression analysis method to fit the data, analysis theeliminate effect of temperature cross-sensitivity, and compare the result withAWG-based signal interrogate methods.