Design And Fabrication Of Long Period Few Mode Fiber Grating

Long-period fiber grating(LPFG),whose grating period is generally around few hundred micrometers,is an optical device widely used in both sensing and communication applications.With the fast technique development,both the new kinds of fiber gratings and corresponding fabrication technologies are emerging.Moreover,the design strategy of LPFG needs the updating,due to the existence of specialty optical fiber.In the thesis,we carry out both numerical simulation and experimental fabrication in order to explore the structure parameters of long period few-mode fiber grating with respect to the mode coupling characteristics.After the study on the light guiding mechanism of specialty fiber grating,mode coupling theory and the laser interaction with transparent material,we archive some research fruits on the propagation characteristics of hollow-core fiber(HOF),mode coupling characteristics of LPFG and femtosecond laser grating inscription.The main outcomes of my thesis are summarized as follows,(1)We introduce the type of fiber gratings and recent progress of fiber grating fabrication technique over the world.Meanwhile,based on various LPFG applications,we explain the advantages and disadvantages of individual fabrication technique.(2)With the help of COMSOL software,we numerically investigate the propagation characteristics of HOF,including the electrical field distribution,mode field diameter,effective refractive index,cut-off wavelength,and dispersion.It is identified that the combination of HOF and LPFG has the potential of innovative sensor applications.(3)We numerically investigate the LPFG inscribed on the conventional two-mode fiber(TMF),and identify the effect of mode coupling characteristics on the LPFG performance.When both the grating length and the refractive index modulation depth increase,the resonance peak first increases and then weakens,meanwhile the side lobes of the resonance peak are enhanced.In addition,as the grating period increases,the center wavelength of the coupled peak is read-shifted.Based on the femtosecond laser fabrication platform,we successfully obtain the TMF-based LPFG with a resonance peak of-13.765 dB.