Study On High Power Laser Resonators With Large Gain Zone

Laser resonator is the core component of laser sysytem, and optimal design of laser resonator can improve laser power and beam quality. In this dissertation, in order to get laser beam with high power and high beam quality, theoretical and experimental studies on laser resonators with large gain zone are presented. The main contents are classified as follows.(1) The numerical model of high power laser resonator with big Fresnel Number is founded. The calculation precision and efficiency of different discrete finite element methods are analyzed, and a new discrete finite element method with characteristics of the same space and the same area to improve calculation efficiency is provided. A distinguishing method of dominant mode for transverse mode competition of laser resonator with big Fresnel Number is put forward, in which the mode distinguishing factorσis defined as the product of of absolute value of eigenvalueγof resonator transit matrix and the efficient area coefficientη0.05 of laser spot, the mode of(which have) the biggestσis dominant, the validity of which is tested in several experiments of laser resonators. By means of a optics focus system and Collins formula, model to calculate the beam quality parameter, including focus beam radius, far-field divergence angle, M2 factor etc, is established, which is benefit to evaluating beam quality.(2) Based on two- dimensional diffraction integral equation, the theoretical model of laser resonator with non-circle aperture is founded. The results of laser resonator with a square aperture show that there are some regular distribution mode, rotary symmetry distribution mode, cross distribution mode and diagonal symmetry distribution mode. The rotary symmetry distribution mode, cross distribution mode and diagonal symmetry distribution mode can not be abtained from Separation Variable Method. The experimentals on beam characteristics of laser resonators with square, rectangle and triangle aperture are studied. TEM99 mode square spot and TEM0-11 mode rectangle spot and hexagon spot are achieved respectively. The output beam characteristics of 28mm×28mm square aperture and circle aperture with diameter 28mm are compared, when a square aperture is used, laser output power is higher than circle aperture is used, but the divergence angle is same. The laser phase transformation hardening case on 45# steel caused by the square aperture beam is more uniform than circle aperture beam.(3) A new type of toric concave mirror laser resonator is presented. The effects of different aperture size and curvature radius on output beam property is discussed systematically. For a resonator with length of 3m, curvature radius of 15m, and aperture radius of 16mm, its Fresnel Number is about 8, and annular distribution mode with M2 factor of 1.1613 and far-field divergence angle (full Angle) of 0.75mrad is obtained. The change of curvature radius has no obvious effect on spot size of output beam, far-field divergence angle, focus spot size and M2 factor etc, whereas, when the curvature radius of concave mirror of plane-concave stable resonator is from 10m to 50m, the output beam mode of plane-concave stable resonator is from TEM04 to TEM01.(4) The experiments of toric concave mirror laser resonator, plane-concave stable resonator and parallel plane resonator with the same aperture size are done on HUST2000 high power transverse flow CO2 laser. Laser beam of 1820W with annular distribution, laser beam of 1860W with TEM06 distribution and laser beam of 1000W with approximate TEM03 distribution are got, respectively at discharge current of 5A. Scanning welding test on 316L stainless steel at 1m/imn velocity by annular distribution beam, the welding depth is more than 2.4mm, whereas the output beam of plane-concave stable resonator and parallel plane resonator can not be used for deep penetration laser welding.(5) Theoretical and experimental studies on property of lower-order mode laser beam of folded resonator are done. A dynamic adjustment method for folded resonator with structure of outside bridge and inside cavity is presented, so the problem of resonator malajustment is solved. A N-type three-folded resonator on transverse flow CO2 laser for industry is developed, which is applied to the laser welding system of gas generator body of automobile airbag.