Study On Characteristics Of Long Period Fiber Grating Sensing In Visible Region

The resonant wavelengths of traditional optical fiber grating sensor are usuallylocated in infrared region and the broadband infrared light source is applied as worklight source. The high resolution detection in infrared region and the infrared lightsource with high cost and big in volume, which resisted the popularity of fiber gratingsensor technology. In recent years, the high performance, cheap and high power whitelight LED source technology development rapidly, plus the low cost of highresolution wavelength detection in visible bands, so the optical fiber sensor works invisible region will play a more important role in optical fiber sensing fields.At present the research and application of fiber Bragg grating sensor in infraredregion and testing system have more mature, the research of fiber Bragg grating invisible region has just started. The research report of long period fiber grating invisible region has not been reported yet. Based on the advantages of the long periodfiber grating，the design of long period fiber grating in visible region, the sensingproperties and some new coupling properties of long period fiber grating areinvestigated in this thesis and the main content are as follows：(1) Introductions are given to the types of fiber grating, production technologyand application。(2) Introductions are given to the long period fiber grating coupling theory andthe basic method of analyzing long period fiber grating characteristics. The changingof long period fiber grating core fundamental mode and cladding modes effectiverefractive index, the distribution of mode field, the variation of coupling coefficientand the simulation calculation of transmission spectrum, which are all analyzed.(3) Based on the three-layer fiber model and the coupling theory, the effectiverefractive index of core mode and the cladding modes, the changing relation of thecoupling coefficient of cladding modes at the lowest few ones with varyingwavelength, are numerically calculated and analyzed in Long-period grating (LPG). Itis revealed that the mode number of the maximum coupling coefficient of claddingmodes decrease with wavelength, the coupling coefficient with wavelength isdifferent in variant fiber parameters. These results are valuable for the long periodgrating designing, since the coupling coefficient directly affect the peak loss of thegrating transmission spectrum。 (4) By using numerical calculation method, we get the relationship between theresonance wavelength and period of long period fiber grating in visible region，Inaccordance with the necessary grating transmission spectrum parameters, the designof the ideal long period fiber grating in visible region is given. According totemperature and strain theory model of the long period fiber grating，the numericalsimulation analysis is given to temperature and strain characteristics of the longperiod fiber grating in visible region. The study found that the resonant wavelengthof long period fiber grating in visible region with temperature and strain has verygood linear behavior，loss peak amplitude of the grating is not sensitive for axialstrain and temperature. The temperature and strain sensitivity of long period fibergrating in visible region is lesser than in infrared region, which is much greater thanthe sensitivity of fiber Bragg grating in visible region.