Abstract：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 sidelobe 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.
