基于可调谐复振幅滤波器的超长焦深矢量光场

The focused vectorial fields with ultra-long depth of focus generated by the tunable complex filter

根据矢量光场衍射积分理论和离散复振幅光瞳滤波原理,通过一种由双λ/2波片和离散复振幅滤波器组成的可调谐复振幅滤波器,研究了大数值孔径下超长焦深聚焦矢量光场的构建与调控.给出了一个六环带区的离散复振幅滤波器,对入射光场的偏振态、振幅滤波和相位滤波三者进行同步优化,获得了焦深接近10λ的三维平顶光场;通过调控双λ/2波片夹角来改变聚焦光场的矢量化结构,使之在光针场、平顶光场、光管场及中间结构光场之间交替变化.研究结果揭示了入射光场矢量化结构演化与聚焦光场矢量化结构变换之间的关系,解决了获取动态的、可调控的超长焦深聚焦光场的问题.两种基本的聚焦光场光针场、光管场的独自使用或三维平顶光场的调和使用,将会在光学显微、光学微纳操控以及光学精细加工领域获得重要应用.

According to the diffraction integral theory of vector field and the pupil filtering method with a discrete complex amplitude, we present a tunable pupil filter to achieve and manipulate the focused vector fields with ultra-long depth of focus. The filter consists of a polarization rotator with two λ/2 wave plates and a discrete complex amplitude filter with six zones. Amplitude transmissions of these zones are different and increase along the radial direction. And every two adjacent transmitted zones have the opposed phases 0 and π. With optimized cylindrical polarization （α0 = 52°） and discrete amplitude, the generalized cylindrical vector field can be tightly focused into a three-dimensional （3D） flat-top field with an extended depth of focus （～10λ） by a high numerical aperture lens. For the main outermost zone and the other five inner zones, we analyze the intensity distributions of the three polarized components and the total polarized component in the focal region. We find that the axially and azimuthally polarized components are the major contributors. The outermost zone offers the central field of the focused field while the other five zones affect the side lobe more obviously. Through adjusting the included angle φ between the double wave plates, we can change the polarization states of the incident vector field and alter the structures of the focused fields among the 3D fiat-top focused field, needle-like field, tube-like field and the other fields with intermediate form. Result obtained is superior to that of the past research for the adjustable freedom between these forms, and it reveals the dynamic relation between the evolved vectorial form of incident field and the vector structure of the focused field. Our work paves a way to achieve the controlled dynamical focused field with a long depth of focus. The needle-like field, tube-like field and the well-matched 3D fiat-top focused field will meet the demand of applications in optical microscope, optical micro-manipulating, optical micromachining and so on.