部分相干光被环形透镜聚焦而产生的焦移

 从理论上研究部分相干光被一环形透镜聚焦，在焦点附近的轴上点的光强分布。研究结果表明，当部分相干光被一环形透镜聚焦时，最大聚焦光强不在几何焦点，而是位于透镜与几何焦点之间，出现焦移现象。并且，焦移量不仅依赖于透镜外半径的菲涅耳数，还依赖于部分相干光的空间相干度和中心拦截比。透镜的菲涅耳数越小，焦移越大；部分相干光的空间相干度越低，焦移越大。当菲涅耳数一定时，环形透镜的中心拦截比越大，焦移越大；当空间相干度很小时，情况就变得比较复杂。

Focal shifts of partially coherent light focused by an annular lens

The on-axis intensity distribution was investigated in the focal region of partially coherent light focused by an annular lens. The results show that, when the partially coherent light is focused by an annular lens, the best focusing point is not located at the geometrical focus. This means that the phenomenon of focal shifts occurs. Moreover, the amount of the focal shifts depends on the Fresnel number of the focusing lens, the spatial coherence of the partially coherent light and the central obstruction ratio of the annular lens etc. It is shown that the smaller the Fresnel number, the larger the focal shift; the lower the spatial coherence, the larger the focal shift;for a fixed Fresnel number, the larger the central obstruction ratio is, the larger the focal shift is. However, when the spatial coherence is very low, the relation among them becomes complicated.