Mode-locked Fiber Lasers With Cylindrical Vector Beams Generation Based On Few-mode Fiber Gratings

Cylindrical vector beams（CVBs）belong to spatially non-uniform polarized beams with polarization symmetry.Due to their unique high-numerical-aperture focusing properties,they have potential applications in particle trapping,laser machining,and many other fields.For this reason,it has been widely concerned by researchers in recent years.There are many methods for generating CVBs,among which,the method to excite higher-order modes in all-fiber structure has the advantages of stable structure and high beam quality,and can be perfectly combined with fiber lasers.In recent decades,mode-locked fiber lasers have great potential for development in the fields of optical communication,remote sensing,sensing technology,and laser medicine due to their characteristic of small size,low cost,and superior stability.In this dissertation,few-mode fiber gratings will be used to the generation of CVBs,and it will be used in several all-fiber mode-locked fiber lasers to obtain mode-locked pulsed CVBs with high purity.The main research contents are as follows:1、Ytterbium-doped fiber laser with CVB generation based on few-mode fiber Bragg grating:The working principle of fiber grating is analyzed using the diffraction theory of grating,and the mode coupling process of few-mode fiber gratings is explained with coupling mode theory.Dislocation fusion of single-mode fiber and few-mode fiber Bragg grating（FM-FBG）is performed to excite higher-order modes.When the mixed-mode beam passes through the FM-FBG,it can output CVBs due to its transverse mode separation characteristic.This structure is applied to an ytterbium-doped fiber laser using dual-cladding gain fibers,and the mode-locking is accomplished by using the nonlinear optical loop mirror technology,obtaining CVB mode-locked pulse with high power of 320mW.2、Erbium-doped fiber lasers with CVB generation based on a few-mode fiber grating combination:Utilizing the high-efficiency mode conversion of two-mode long period fiber grating（TM-LPFG）,the mode selectivity and spectral filtering of two-mode fiber Bragg grating（TM-FBG）,higher-order modes with high purity are generated in an all-fiber mode-locked laser by cascaded the TM-LPFG and the TM-FBG.It is worth noting that longitudinal mode locking is achieved using a semiconductor saturated absorption mirror（SESAM）,and the purity of CVB pulse is measured greater than 98%.3、An all-few-mode fiber mode-locked laser based on few-mode fiber gratings:The wavelength division multiplexing-mode selective coupler（WDM-MSC）is studied,and the phase matching conditions that need to satisfy the mode conversion are deduced by the coupled mode theory.On the basis of the results of calculation and simulation,WDM-MSC was fabricated by using single-mode fiber and few-mode fiber using fused taper technology.Combining this device with TM-LPFG and TM-FBG,an all-few-mode fiber laser was constructed with a SESAM used as longitudinal mode locker.As a result,not only can the mixed mode of LP₀₁ mode and LP₁₁mode oscillates in the cavity,but also the CVB pulse with a mode purity of up to 99.4%is output due to the combination of few-mode fiber gratings.