Study On The Characteristics And Sensing Applications Of Long Period Fiber Gratings

Long period fiber grating(LPFG) is a kind of fiber grating with a period of several microns, which has good prospects in optical fiber sensor and optical fiber communication areas with advantages of low insertion loss, low back-reflection, easy fabrication and low cost. In the thesis, a new LPFG fabrication method based on carbon dioxide laser is proposed, and the optical spectrum characteristics and sensor applications of the LPFG is studied. The main research works includes:1. Based on vector Helmholtz equation and electromagnetic field boundary conditions, the implicit dispersion equation of three-layer cylindrical waveguide can be derived. A repaid scan and step-by-step method are adopted to solve the effective refractive index of cladding modes with high resolution and fast speed. Theoretical analysis and numerical simulation of transmission spectral characteristics is a good reference to the fabrication and sensor application of LPFG.2. A new symmetric fabrication method based on three focused high-frequency carbon dioxide laser beams is promoted for the first time. The system with advantages of simple structure, easy to control and adjustment can eliminate the asymmetric refractive index modulation caused by side exposure method. The LPFG fabricated with the system has low insertion loss and narrow bandwidth, also have relative uniform bending character, low polarization dependence loss. The fabrication method is patent protected now.3. LPFGs which couple guide modes to low order cladding modes are fabricated on telecommunication fibers of SMF-28(Corning) with the method as mentioned above. The characteristics of temperature, axial strain, bending and surrounding refractive index of LPFGs are investigated in the thesis. The LPFGs have high temperature sensitivity and high temperature stability in comparison with fiber Bragg gratings(FBG), which are suitable to be used under high temperature. Compared with FBG, the axial strain sensitivity of LPFG is smaller. The bending measurements show that LPFG fabricated with side exposure exists the most bend-sensitive two orientations and the most bend-insensitive two orientations, but LPFG fabricated with symmetric exposure method shows similar bend characteristics. The refractive index experiment shows that the LPFG can be used to measure refractive index and concentration with fast speed and high resolution. The refractive index sensitivity of LPFG can be further improved by etching the cladding radius.