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

Due to characteristics of extremely short pulse-width and ultra-high peak power,femtosecond laser(fs-laser)can induce highly localized refractive index modulation,during its nonlinear interaction process with transparent medium materials.Thus,fs-laser has become a new generation of high-quality light sources for material micro/nano-processing,indicating the advantages of high precision and good flexibility during the process of fabricating all-fiber micro-devices.Meanwhile,in order to solve the capacity crunch of traditional standard singlemode fiber(SSMF),mode division multiplexing(MDM)technique based on few-mode fiber(FMF)has attracted worldwide research interests.Consequently,the mode converter(MC)becomes a vital device for the implementation of MDM transmission.My thesis is focused on the fs-laser direct inscription technique for the fabrication of few-mode long period fiber grating(FM-LPFG),aiming to provide broadband and flexible MSC for the MDM transmission.The main research outcomes can be summarized as follows.(1)The principle of fs-laser micro-machining are summarized,after the interaction with transparent dielectric material is introduced.Three categories of fs-laser induced refractive index modulation are emphasized.Research progress of the FM-LPFG based mode converter and the fs-laser enabled LPFG fabrication are reviewed.In particular,the problem of focus distortion during the fs-laser axially inscription onto the free-standing fiber is analyzed.(2)The mode theory of FMF and the coupled mode theory of LPFG are described.Based on the mode coupling theory of LPFG,the MC characteristics of FM-LPFG are explained,and a modified radial line-by-line inscription method is proposed.(3)The LPFG based MC is firstly designed,then successfully fabricated by the selfdeveloped fs-laser direct inscription platform,and finally completed the performance evaluation.Our fabricated MC covering the C+L band is able to realize an average conversion efficiency of more than 90%,an average insertion loss of less than 5 d B,and a polarization dependent loss of less than 3 d B.