All-fiber Ultra-fast Cylindrical Vector Beam Mode-locked Laser

In recent years,the research of cylindrical-vector beams(CVBs)has attracted more and more attention.Unlike the uniformly polarized Gauss beams,the electric field direction of the CVBs is centrally symmetrical on the cross section,the intensity distribution is circular,and the intensity at the center is zero.Because of its unique polarization characteristics,it has been used in many fields such as laser processing and particle manipulation.Various methods for generating CVBs have been developed,including active and passive methods in free space or optical fibers.All-fiber mode-locked lasers have always been an ideal source for high-performance femtosecond pulses.After more than several decades of development,the performance of the fiber lasers with respects of pulse width and energy has been greatly improved and the fiber lasers have been effectively used in ultrafast spectroscopy,biophotonics,optical communication,laser processing,imaging and other fields.By incorporating the method of CVB generation in optical fiber into all-fiber mode-locked lasers,all-fiber cylindrical-vector beam mode-locked fiber lasers have been proposed and investigated in this thesis.The purpose is to obtain short pulse and high energy CVBs,so as to make the application of the CVBs more convenient.The main research works are as follows:1?Cylindrical vector beam fiber lasers based on two-mode fiber grating: Pure azimuthal or radial CVBs have been obtained by combining dislocation fusion excitation of CVB with mode-locked fiber laser based on carbon nanotubes.At the first,the theoretical simulations of the dislocation fusion excitation of CVB have been carried out.The results indicated that mode conversion can reach maximal value at the displacement distance of 5 ?m.Then,a polarization-insensitive all-fiber mode-locked laser is constructed with a commercial saturable absorber mirror(SESAM)for mode-locking,as well as a two-mode fiber grating for mode conversion.The experimental results show the laser can output picosecond cylindrical vector beams.2?Multi-wavelength cylindrical-vector beam mode-locked fiber laser with mode-selection coupler: According to the coupled mode theory,when LP01 mode in single-mode fiber(SMF)and LP11 mode in two-mode fiber(TMF)meet the phase matching condition,LP01 mode in SMF will be converted into LP11 mode in TMF.Firstly,the mode-selection coupler(MSC)was fabricated by fiber tapering technique.Then,the birefringence of the MSC induced by non-uniformity makes it possible to achieve wavelength switchability and dual-wavelength cylindrical-vector beam,in combination with all-fiber mode-locked laser based on carbon nanotubes.And the pulse width of the obtained CVB is in several hundreds of femtosecond order.3?CVB mode-locked fiber laser with various mode-locking operation states: Based on nonlinear polarization rotation for mode-locking,mode-locked CVB fiber laser with various operation states has been demonstrated with MSC for mode conversion.By changing the pump power,the single-pulse,three-pulse and bound-state CVBs have been obtained from the same laser.The pulse width of single CVB pulse is about 175 fs.