The Study On Sensors Based On Long-period Fiber Gratings

Long-period fiber Grating(LPFG) sensor is one of the potential fiber devices that have been developing quickly in recent years. The principle of transmitting light based on LPFG is that forward transmitting mode of core couples to forward transmitting modes of cladding. So LPFG has no backward transmitting light and it need not fix the isolator between LPFG and light source. The resonant wavelengths and the values of wastage are more sensitive to changes of environment because of this characteristics. Therefor it has higher sensitivity on temperature, bend, distortion, transverse load,concentration,refractive index than that of Bragg fiber gratings.Furthermore LPFG sensor is characterized by its high flexibility, wide range of measure, not disturbed by electromagnetism , not easy to be rotten , being light in weight and small in bulk . So, LPFG is widely used for measure in industry and agriculture real time and on-line.it specially fit for the dangerous situation of strong magnetic field intensity , or the condition of a high level of radiation , or the surroundings of being easy to be rotten. Though they are both fiber gratings, LPFG is different from FBG in working mechanism, which determines their different features and application.This paper attempts a detail study on the working mechanism and the sensor application of LPG in five parts as followling:1. Introduce the development and sort of the fiber grating.2. Analyse the working mechanism of LPFG and its application in communication and sensors realm.3. Detailedly analyse the characteristics of LPFG and draw some useful conclusions as following:(1) The effective refractive index of core approximately linear minish along with the wavelength increase; It will increase when the radius of core and the refractive index of cladding increase.(2) The effective refractive index of cladding decrease with the increase of wavelength and the orders of cladding mode; Changes of core radius do not obviously affect on the effective refractive index of cladding,Yet Changes of cladding radius obviously affect on the effective refractive index of cladding. (3) The variety of peak value and position of transmissions result from changes of fiber parameters.(4) The coupling coefficients are sensitive to the change of radius core with decrease of wavelength, Yet the coupling coefficients are sensitive to the change of radius cladding with increase of wavelength.4. Detailedly study the LPFG sensors characteristics of the refractive index, the temperature and the axial strain in detail. Based on the study on the temperature and axial stress and refractive index of LPFG , we put forward and analyse a project of combining the temperature and the strain and coated a thin film on the cladding to improve the temperature sensibility of LPFG , and a project of compensating temperature sensitivity of LPFG in communication realm 5. Briefly introduce the theory of three-cladding-layer LPFG . Equations about the fields of electricity and magnetism in realm of core, cladding, film, environment are obtained by using optical-guided theory .Resolve dispersion equation and obtain the effective refractive index of cladding using method of numerical calculation.