Webster K L, Sung C C
Appl Opt. 1992 Jan 20;31(3):319-28. doi: 10.1364/AO.31.000319.
A high-power CO(2) laser beam is known to deteriorate after a few microseconds because of a mode-medium instability (MMI) that results from an intensity-dependent heating rate that is related to the vibrationalto-translational decay of the upper and lower CO(2) lasing levels. An iterative numerical technique has been developed to model the time evolution of the beam as it is affected by the MMI. The technique is used to study the MMI in an unstable CO(2) resonator with a hard-edge output mirror for different parameters, e.g., the Fresnel number and the gas density. The results show that the mode of the hard-edge unstable resonator deteriorates because of the diffraction ripples in the mode. We use a Gaussian-reflectivity mirror to correct the MMI. This mirror produces a smoother intensity profile, which significantly reduces the effects of the MMI. Quantitative results on the peak density variation and beam quality are presented.
众所周知，由于模式 - 介质不稳定性（MMI），高功率CO₂激光束在几微秒后会劣化，这种不稳定性是由与上下CO₂激光能级的振动 - 平动衰减相关的强度依赖加热速率引起的。已开发出一种迭代数值技术来模拟光束受MMI影响时的时间演化。该技术用于研究具有硬边输出镜的不稳定CO₂谐振器中不同参数（例如菲涅耳数和气体密度）下的MMI。结果表明，硬边不稳定谐振器的模式因模式中的衍射波纹而劣化。我们使用高斯反射率镜来校正MMI。该镜产生更平滑的强度分布，从而显著降低MMI的影响。给出了关于峰值密度变化和光束质量的定量结果。
