Research On Fabrication And Properties Of Long Period Fiber Grating

Unlike conventional fiber Bragg grating,long period fiber grating?LPFG?is one kind of grating with a period of hundreds of micrometers.It has been widely used in fiber communication,fiber sensors and fiber lasers because of the advantages of low insertion loss,high coupling efficiency,sensitiveness to environment and no back scattering.This article contains the design,fabrication and application of LPFGs,and the main contexts are given as follow:?1?The developments,classifications,fabrications and applications of FBG and LPFG are introduced simply.Also,the present hot area of this research is summarized here.They are the sensing characteristics of helical LPFG,the generation of orbital angular momentum?OAM?by mode converter based on LPFG and research on LPFG's dispersion compensation point.?2?The mode coupling between optical fiber core modes and cladding modes is analyzed using the mode coupling theory,and the calculation of transmission spectrum of FBG and LPFG are also analyzed in the Sec.2.Moreover,the relationship between grating spectrum and parameters of 2?m waveband LPFG and double-cladding fiber LPFG are stimulated,which providing theoretical guidance for the LPFG fabrication.Using theoretical analysis,we also calculate the phase matching condition of two-mode fiber and dew-mode fiber.?3?The self-constructed fabrication system platform of LPFG based on line-focused continue-wave CO₂ laser irradiation,the fabrication process and spectrum measurement are introduced.The spectrum measurements and characteristics of different types LPFG are investigated in this section using the fabrication system.These LPFGs contain 2?m waveband LPFG,LPFG written in two-mode fiber and four-mode fiber and double-cladding fiber LPFG.?4?The sensitiveness to temperature and surrounding refractive index of the 2?m waveband LPFG are tested using self-constructed measurement system.A ultra-broadband mode converter based on LPFG operating at 1?m is achieved and the principle of the ultra-broadband is also analyzed,which providing a method for the controlling of bandwidth in this device.Moreover,LP₀₁-LP₁₁/LP₂₁ mode converters and generation of OAM are obtained by introducing special treatment to LPFG written in four-mode fiber.Based on the transmission spectrum of double-cladding fiber LPFG,the feasibility of experiment of suppressing of stimulated Raman scattering in high power amplifier and oscillator using this device are analyzed in the last section.