Investigation On End-pumped Lasers With High-order Laguerre And Bessel Gaussian Beams Output Based On Gain And Loss Control

With the optical applications in various fields, high-order Laguerre Gaussian (LG) beams and high-order Bessel Gaussian (BG) beams have aroused the widespread attention in the last twenty years due to their unique optical properties. High-order LG beams and BG beams are not only applied in optical tweezers, meta-material laser processing, stimulated emission depletion technology, free space optical communications and other classical fields, but also in the field of quantum entanglements and quantum communications. One of the key and fundamental reasearch of high-order LG beams and BG beams is how to obtain the beams with high mode purity. End-pumped solid-state lasers, with high-order LG beams or BG beams output, have the advantages of compact structure, stability, high efficiency, high output power and high mode purity. This kind of lasers has gotten a lasting and in-depth study.In the first part, applications and generation methods of high-order LG beams and BG beams are involved. Research process of end-pumped lasers with the output of high-order LG beams or BG beams are introduced. In this dissertation, end-pumped solid-state lasers with high-order LG and BG beams output based on gain and loss control are proposed and studied. Several achievements and contributions are summarized:Based on gain control in the principle of laser resonator transverse mode selection, an end-pumped solid-state laser with annular pumping is proposed and studied. In experiment, vortex Laguerre Gaussian (vLG) beams are realized with continuous and pulse output. An apparatus and method of generating annular pumping beams are proposed and analyzed from the ray tracing and diffraction integral in detail. Then the annular pumping beams are verified in experiment. Based on the principle of gain control, We directly produce continuous-wave first-order vLG beams from an annular end-pumped Nd:YVO4 laser with a general fiber coupled LD source. The maximum output power of continuous working in this laser is 574mW. The orbital angular momentum of the output beam is measured by Mach-Zehnder interferometer. By inserting passively Q switched crystal Cr4+:YAG, pulsed vLG beams with a repetition rate of 65KHz, and pulse width of 198ns are generated.Thermal effect model of rod laser crystal with annular end-pumping is proposed and investigated. Starting from one-dimensional heat conduction equation, annular beams are equivalent as the radial off axis Gaussian beams model, and theoretical derivation of temperature distribution, thermal induced phase difference distribution, equivalent thermal focal length and thermal induced loss in the rod laser crystal with annular end-pumping are given. Taking 1.3% doped rod Nd:YAG crystal as an example, this thermal model is investigated in detail. The simulation results show that under the same pump power, compared to the solid beams pumping, annular beams pumping could be more suitable for high power working for the reason that the temperature in the center for annular beams pumping is lower than the one for the solid beams pumping. Besides, with respect to the thicknesses of the annular pump beams, the locations of the center of the maximum intensity more influence the thermal distribution in the rod laser crystal.Based on loss control in the principle of plan-concave laser resonator transverse mode selection, an end-pumped solid-state laser inserting amplitude mask with the output of high-order LG beams in the resonator is proposed and investigated. Starting from the spatial intensity distributions of high-order LG beams, addition losses are introduced to increase the losses for the other modes by inserting the amplitude mask in the resonator. This proposed mechanism can be divided into three categories:radial loss, azimuthal loss, and the combination of radial and azimuthal loss, which correspond to excite radial high-order LG modes, azimuthal high-order LG modes, and regular high-order LG modes, respectively. By controlling the locations and thicknesses of opaque rings and lines on the mask, all kinds of high-order LG modes can be obtained. With the application of mode purity, all the generated modes possess high mode purities greater than 93% in simulation. And the distributions of the losses in this end-pumped active resonator with the amplitude mask are established and investigated. Finally, a method of converting high-order LG beams into high-order vLG beams outside the resonator is proposed and discussed.Based on loss control in the principle of axicon laser resonator transverse mode selection, an end-pumped solid-state laser inserting amplitude mask with the output of superposed high-order BG beams in the resonator is proposed and investigated. By investigating the oscillating behaviors of high-order BG beams in the axicon resonator, the oscillating high-order modes in the resonator are superposition states of high-order BG modes with the opposite vortices. The additional loss for the target mode in the resonator is introduced by the amplitude mask. By controlling the locations and thicknesses of opaque lines on the mask, all kinds of superposed high-order BG modes can be obtained. Then, the free space propagation and angular spectrum of the laser beam are studied. The simulation results show that the output beam has the characteristics of non-diffraction, which is the same as high-order BG beams. By analyzing the orbital angular momentum of the output beam, the simulation results show that the output beams from the active axicon resonator are superposition of high-order BG beams with the opposite vortices.