Fabrication Of Long Period Few-mode Fiber Grating Based On Femtosecond Laser

Long period fiber grating(LPFG)has been commonly-used in fiber communication and fiber sensing.Femtosecond laser inscription is an efficient technique to fabricate all-fiber devices including resonators,micro-channels and grating.Based on the femtosecond laser fabrication platform,my thesis is focused on the development of long period few mode fiber grating.The main research outcomes can be summarized as follows,(1)We introduce the mode coupling theory of few-mode fiber and LPFG in details.The performance of two-mode fiber(TMF)to be inscribed is numerically investigated by COMSOL first.Then,we analyze the effect of LPFG parameters on its transmission spectrum.Moreover,we qualitatively describe the interaction mechanism and fabrication process between femtosecond laser and transparent material.(2)We design and build up a fabrication platform based on femtosecond laser.After explaining the operation principle,we carry out parameter optimization of femtosecond laser processing platform.The optimization of both free-space beam propagation and the precise alignment of few-mode fiber with respect to the femtosecond laser is emphasized.(3)We carry out an experimental characterization of the fabricated LPFG.The effect of fabrication parameters on the transmission spectrum of single mode fiber based LPFG is experimentally investigated for the ease of platform calibration.Then,TMF based LPFG is fabricated to achieve the mode conversion from LP₀₁ to cladding mode with a coupling efficiency of 99%over 8nm wavelength range.