Research On Beam Characteristics And Stability In Large Fresnel Number Off-axis Unstable Resonator

The off-axis unstable resonator with a large Fresnel number has been adopted in high power diffusion-cooled RF-excited slab CO₂ laser for the extraction of high output power.When the equivalent Fresnel number over 500 for unstable resonator,the analysis error of traditional analytical methods,such as virtual source method and eigenvector method would become serious as the approximation conditions for these methods are no longer valid.In this thesis,different resonator structures and beam characteristics for high power diffusion-cooled RF-excited slab CO₂ laser were investigated by numerical simulation and experiments,while the methods to obtain doughnut-like mode were discussed.The main contents of this thesis are as following.?1?The characteristics of symmetric and off-axis unstable resonators were discussed.The power extraction efficiency in misaligned unstable resonators were studied by performing the "Rigord analysis".The results show that the number and the power content in the side lobes of the far field distribution of the output-beam are clearly reduced for off-axis unstable resonators when compared with symmetric ones,the M2 factor of the output beam is only 1/5 of the symmetric unstable resonator.In addition,compared with positive-branch unstable resonators the negative-branch ones are far less sensitive to misalignment.Therefore,the off-axis negative-branch unstable-waveguide hybrid resonator is suitable for high power RF-excited slab CO₂ lasers.?2?A positive-branch unstable resonator whose equivalent Fresnel number is relatively small and a negative-branch unstable resonator with the same size of gain area have been investigated.For positive-branch unstable resonator,there was no significant difference in the beam characteristics between resonator having parabolic mirrors or spherical mirrors,the simulation results are in good agreement with the experimental results.However,for negative-branch unstable resonator,the output beam direction deviates-1.79mrad from the optical axis when resonator with spherical mirrors,while the output beam is is almost a collimated one for parbolic mirror resonator.The power extraction efficiency are nearly same for parabolic mirror resonator and spherical mirror resonator when they were misaligned.These results show that the spherical aberration has little influence on beam characteristics for unstable resonator with a relatively small Fresnel number.?3?The influence of spherical aberration on the resonator of a 2kW RF-excited slab CO₂ laser whose equivalent Fresnel number is 673 was analyzed.Experimental and numerical wave-optics simulations of the beam characteristics for paraholic mirror and spherical mirror resonator were presented.The beam characteristics for resonators with different mirrors radius of curvature and cavity length were also investigated.It turned out that output beam direction deviates-3.5mrad from the optical axis for spherical mirror resonator,while the output beam has a smaller beam spot size and higher power density at the focal plane and propagate along the optical axis for parbolic mirror resonator.Using parabolic mirror resontor the power extraction efficiency is 5.6%higher for mirror tilt angles varied between-6mrad to 6mrad,the simulation results are in good agreement with the experimental results.In addition,the power extraction efficiency of the shorter cavities and the mirrors radius of curvature incressed resonator are 3.3%,3.1%higher than confocal parabolic mirror resonator,respectively.?4?The propagation characteristics of the output beam passing through the beam shaping system for spherical mirror resonator and nonconfocal parabolic mirror resonator were investigated,the beam shaping system for both resonators were redesigned.Two methods had been adopted to obtain doughnut-like mode.Firstly,a mirror could achieve doughnut-like mode output in far field was designed by G-S algorithm,but the shape beam has a large divergence angle which not suitable for remote processing.Then,through a gradual phase change from-??? in a circular fashion across the incoming beam wave front obtained the doughnut-like mode whose M2 factor is 2.29,the diffraction efficiency is 97.1%,the shaped beam have good propagation characteristics.?5?The“loaded resonator" model of off-axis unstable resonator with huge Fresnel number was established,The rear and outcoupling mirrors misalignment characteristics of 2kW RF-excited slab CO₂ laser were studied.It turned out that the output power decreased by 28.4%and 34.4%when the tilt angle were +8mrad and-8mrad,respectively.While,the output power showed little change when the tilt angles were negative for rear mirror,but the output power decreased sharply for positive tilt angles,the output power decreased by 69%when the tilt angle were +8mrad.Moreover,the symmetry of the shaped beam and the M2 factor in unstable direction were also affected when resonator mirrors were misaligned.These results are useful for the design of 3kW RF-excited slab CO₂ laser.