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随着离轴抛物面反射聚焦镜在激光加工领域的广泛应用,如何提高其聚焦光斑质量成为一个亟需解决的问题。理论上,抛物面反射聚焦方式可以消除球差的影响,得到高质量的聚焦光斑。但是抛物面反射镜是最难调整的反射镜系统之一,对光轴失准的敏感度极高。细微的失准引发的像散将大大降低焦斑的光强。为了深化对其的认识,通过几何光学和物理光学的方法,对离轴90°抛物面镜应用于激光聚焦光路时,光轴的角度失准对聚焦光斑造成的影响进行分析。结果表明,细微的角度失准将导致焦斑面积大大增加,从而大大降低了焦斑的光强。并通过二阶矩法证明了角度失准将导致抛物面镜焦平面附近产生一对互相垂直的焦线,同时证明了两焦线所处z平面与焦点F的绝对距离相等,并与入射偏角θ呈线性关系。焦线的存在使入射高斯光束转变为椭圆高斯光,降低了焦面处的峰值光强。抛物面镜对角度失准的敏感度非常高,几个mrad的失准角将导致焦面处的峰值光强下降一半。
With the wide application of off-axis parabolic reflector in the field of laser processing, how to improve the quality of focused spot becomes an urgent problem to be solved. Theoretically, the parabolic reflection focusing method can eliminate the influence of spherical aberration and obtain high-quality focused light spot. Parabolic mirrors, however, are among the most difficult to adjust mirror systems and have a high sensitivity to misaligned optical axes. Astigmatism caused by subtle misalignment will greatly reduce the intensity of the focal spot. In order to deepen its understanding, the influence of angular misalignment of the optical axis on the focal spot is analyzed by using the geometrical optics and physical optics when the off-axis 90 ° parabolic mirror is applied to the laser focusing optical path. The results show that subtle angle misalignment will lead to greatly increased focal spot area, thus greatly reducing the intensity of the focal spot. The second-order moment method is used to prove that the misalignment will cause a pair of mutually perpendicular focal lines near the focal plane of the parabolic mirror. At the same time, it is proved that the absolute distance between the z-plane and the focal point F of the two focal lines is equal to the angle of incidence θ A linear relationship. The presence of the focal line transforms the incident Gaussian beam into elliptical Gaussian light, reducing the peak intensity at the focal plane. Parabolic mirrors are highly sensitive to angular inaccuracies and misalignments of a few mrad will cause the peak intensity at the focal plane to be reduced by half.