Annular pupils, generally used in focal systems for radially polarized beams, can compress the size of the focus spot to improve the transverse resolution of confocal microscope.However, there is a lack of criterion for the comparison of the performance of pupil with different apertures.To overcome this limitation, we investigated the changes of the performance of the pupil by varying the aperture of the pupil in three different ways:1) increase the aperture of pupil with a fixed angle width;2) increase the aperture of pupil with fixed effective incident light quantity;3) increase light sheltering coefficient with a fixed outer edge aperture of the pupil.Our results indicate that with the varying size of pupil aperture, both the transverse and longitudinal focal fields exhibit obviously different changes under these three different ways.The changes of the focusing performance in such three different ways are almost the same only when the angle aperture of the narrow pupil is larger than 1.1 rad.As the pupil aperture increases, the ratio of longitudinal field to transverse field increases as well, which means that a bigger aperture of the pupil is better for side-lobe suppression.When the angle aperture of the pupil is increased to 1.26 rad, the half angle width of the focus spot reaches a minimum value of 0.4 λ, only about 0.625 times compared with that of the solid light beam with the same angle aperture.%环形光瞳滤波常被用于径向偏振光聚焦系统,以实现焦斑压缩,进而提高共焦显微的横向分辨率.但是在对不同孔径的光瞳进行分析时缺乏比较的标准,无法进行定量的对比.本文对三种不同的光瞳孔径变化方式进行了定量对比研究,它们分别是:固定环角宽度,增大环孔径;固定环有效入射面积,增大环孔径;固定环形瞳外半径,改变遮光比.结果表明,不同模式下,焦平面处纵、横向场分布变化方式明显不同,只有当三者均为大孔径(大于1.1 rad)的窄光环时才基本等同.另外随着孔径的增大,纵、横向场之比也随之增大,说明孔径的增大可以抑制旁瓣光场.三种孔径增大方式均表明当窄环孔径角增大到1.26 rad时焦斑压缩将到达极限值0.4 λ,与同数值孔径标量实心光束相比,窄环对焦斑压缩了1.6倍.
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