Passively Mode-locked Cylindrical Vector Beam Fiber Lasers Based On Few-mode Fiber Devices

As a common light source in current communication and other systems,fiber laser has been widely studied.However,due to the limitation of single transverse mode in single-mode fiber,it can no longer satisfy the applications for future development.The emergence of mode-division multiplexing(MDM)makes it possible to greatly enhance the transmission capacity of communication syestem.The key technology of MDM is the mode converter.The mode converter can realize the excitation and conversion of different modes.All-fiber mode converters are easy to connect with optical fiber communication systems with low coupling loss,which has attracted extensive attention.This thesis mainly studies the mode coupling characteristics of three kinds of few-mode fiber decives and their applications: Few-mode fiber Bragg grating(FM-FBG),two-mode fiber optical coupler(TMF-OC)and wavelength division multiplexing-mode selective coupler(WDM-MSC).All these three devices were then used in passively mode-locked fiber laser to control the transverse modes of fiber lasers,and cylindrical vector beam(CVB)pulses have been obtained.The specific research contents are as follows:1.Mode coupling characteristics of FM-FBG and its application for CVB pulse generation from passively mode-locked fiber laser.Based on the fiber grating diffraction theory,mode coupling of FM-FBG was analysed,and the relationship between the propagation constant and the wavelength was simulated.Under the condition that the grating period was determined,the self-coupling and cross-coupling wavelengths could be found.The higher-order modes can be excited and selected by means of the dislocation fusion of single-mode fiber(SMF)and FM-FBG.This structure was then used to a linear-cavity passively mode-locked fiber laser with commercial semiconductor saturable absorption mirror(SESAM)inside the cavity for mode locking,and finally CVB pulses have been obtained.2.Symmetric two-mode fiber coupler for direct generation of higher-order modes and its application for CVB pulse generation.Based on coupled mode theory,the working principle of the symmetric TMF-OC was analysed.The finite element method was used to analyze the relationship between the diameter of the two-mode fiber and the mode effective refractive index at a fixed wavelength of 1550 nm.According to the principle of total reflection of the fiber,the size range of the two-mode fiber(TMF)is determined when the fundamental mode is bound to the core without coupling and the higher-order mode starts diffusing to the cladding.The beam propagation method was used to simulate the mode field distribution and power exchange when the size was chosen the right one.Based on the theory and simulation results,TMF-OC was fabricated using the modified flame brushing technique.The TMF-OC is then used in passively mode-locked fiber ring laser with nonlinear polarization rotation for mode locking.By using the TMF-OC as higher-order mode output,the CVB pulses have been obtained.3.Wavelength division multiplexing-mode selective coupler(WDM-MSC)and its application for mode-locked fiber laser.Based on the coupled mode theory,the working principle of WDM-MSC was analysed,and the phase matching conditions for the mode conversion were studied.The combination of higher-order modes at 976/1550 nm was simulated by using the beam propagation method when the proper ratio of radius of single-mode fiber and low-mode fiber was selected.Based on the theory and simulation results,the device was fabricated by pre-tapering technology.The device was then applied to the all few-mode fiber mode-locked figure-8 cavity laser with nonlinear optical loop mirror(NOLM)acting as equivalent saturable absorber for mode locking.The output of the CVB pulses has been realized from a fiber laser with all few-mode fibers.