Connolly Peter W R, Ren Ximing, McCarthy Aongus, Mai Hanning, Villa Federica, Waddie Andrew J, Taghizadeh Mohammad R, Tosi Alberto, Zappa Franco, Henderson Robert K, Buller Gerald S
Appl Opt. 2020 May 10;59(14):4488-4498. doi: 10.1364/AO.388993.
Large-format single-photon avalanche diode (SPAD) arrays often suffer from low fill-factors-the ratio of the active area to the overall pixel area. The detection efficiency of these detector arrays can be vastly increased with the integration of microlens arrays designed to concentrate incident light onto the active areas and may be refractive or diffractive in nature. The ability of diffractive optical elements (DOEs) to efficiently cover a square or rectangular pixel, combined with their capability of working as fast lenses (i.e., ∼/3) makes them versatile and practical lens designs for use in sparse photon applications using microscale, large-format detector arrays. Binary-mask-based photolithography was employed to fabricate fast diffractive microlenses for two designs of 32×32 SPAD detector arrays, each design having a different pixel pitch and fill-factor. A spectral characterization of the lenses is performed, as well as analysis of performance under different illumination conditions from wide- to narrow-angle illumination (i.e., /2 to /22 optics). The performance of the microlenses presented exceeds previous designs in terms of both concentration factor (i.e., increase in light collection capability) and lens speed. Concentration factors greater than 33× are achieved for focal lengths in the substrate material as short as 190µ, representing a microlens f-number of 3.8 and providing a focal spot diameter of <4µ. These results were achieved while retaining an extremely high degree of performance uniformity across the 1024 devices in each case, which demonstrates the significant benefits to be gained by the implementation of DOEs as part of an integrated detector system using SPAD arrays with very small active areas.
大幅面单光子雪崩二极管（SPAD）阵列常常存在填充因子较低的问题，填充因子即有效面积与像素总面积之比。通过集成微透镜阵列可大幅提高这些探测器阵列的探测效率，微透镜阵列旨在将入射光聚焦到有效区域上，其性质可以是折射型或衍射型。衍射光学元件（DOE）能够有效覆盖方形或矩形像素，再加上其具备快速透镜（即f/3）的功能，使其成为适用于使用微尺度大幅面探测器阵列的稀疏光子应用的通用且实用的透镜设计。采用基于二元掩模的光刻技术为两种32×32 SPAD探测器阵列设计制造了快速衍射微透镜，每种设计具有不同的像素间距和填充因子。对透镜进行了光谱表征，并分析了在从广角到窄角照明（即f/2到f/22光学系统）的不同照明条件下的性能。所展示的微透镜在聚光因子（即光收集能力的提高）和透镜速度方面的性能均超过了以往的设计。对于衬底材料中短至190µ的焦距，实现了大于33倍的聚光因子，这代表微透镜的f数为3.8，焦斑直径小于4µ。在每种情况下，在1024个器件上都保持了极高的性能均匀性，这证明了将DOE作为使用具有非常小有效面积的SPAD阵列的集成探测器系统的一部分来实施可带来显著益处。
