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 mirror in laser processing, how to improve the quality of focal spot becomes a problem to be solved. In theory, the parabolic reflecting focusing approach can eliminate the influence of spherical aberration, and get high-quality focal spot. But the parabolic mirror is one of the most difficult adjustment reflector systems, and it is very sensitive to the optical axis misalignment. So that, the focal power density will decrease rapidly due to the tiny angular misalignment.In order to deepen understanding on the poor off-axis propertites of the parabolic mirror, by geometrical optics and physical optics, the influence of the misalignment angle on the 90° off-axis parabolic mirror focal spot characteristic is studied. The results indicate that the focal spot size is increased and the intensity is decreased greatly because of the tiny angular misalignment. By second-order moments, it is proved a pair of focal lines is produced near the focal plane due to the angular misalignment. It is also proved that the distances between the z planes at which the two focal lines are located and the focus F are equal, and are proportional to the misalignment angle.Because of the existence of the pair of focal lines, the incident Gaussian beam becomes elliptical Gaussian mode, which decreases the peak intensity at the focal plane. The parabolic mirror is very sensitive to the misalignment, and the misalignment angle of several mrad will make the peak intensity at the focal plane drop to the half.