基于紧聚焦条件下的矢量衍射理论推出了一个解析解形式的环半径公式，利用此公式设计得到的二元相位波带片可以使入射光的紧聚焦区域呈现轴向双焦点分布。并且，公式中的两个焦点的轴向距离和环半径直接相关，可以设计一系列的具有不同轴向距离的二元相位波带片，用来调制高数值孔径物镜，使其产生轴向可调的双焦点。以径向偏振贝塞尔-高斯光束为例，数值模拟了在不同轴向偏移距离的二元相位波带片的调制下的紧聚焦场的空间强度分布。数值模拟结果表明，基于此类二元相位波带片可以成功地实现轴向距离可调的双焦点。此外，利用一些具有特殊轴向距离的二元相位波带片，还可产生“光泡”与“光针”等特殊的紧聚焦场分布。因此，此类二元相位波带片有望应用于微粒子的动态操控与捕获。

An analytic formula of the radius of a zone plate is deduced by the vector diffraction theory under the tight-focusing conditions. Based on the binary phase zone plate designed by the formula, axial two focuses can be created in the focusing area of a high numerical aperture (NA) objective lens. Because the formula of the radius of the zone plate is related to the axial shifting distance of the two focuses, it is possible to design a set of zone plates with different axial shifting distances for generating tunable axial two focuses. For a high NA objective illuminated by a radial polarized Bessel- Gaussian light, the field distributions of the objective modulated by the binary zone plate with different axial shifting distances are simulated. It is shown that axial tunable two focuses can be realized successfully. In addition, using the zone plate with suitable axial distance, some special focusing fields, such as“optical bubble”and“optical needle”can also be realized under the tightfocusing condition. Hence, such binary zone plates could be utilized for optical trapping and manipulation of particles.