[论文解读] Diffractive flat lens enables Extreme Depth-of-focus Imaging

Sourangsu Banerji, Monjurul Meem+3 more|arXiv (Cornell University)|Oct 16, 2019

Optical Coatings and Gratings参考文献 19被引用 6

一句话总结

该论文提出了一种多级衍射透镜（MDL），通过在焦平面处调控相位分布，实现了比传统透镜宽逾4个数量级的极端景深。MDL在5 mm至1500 mm的距离范围内保持衍射极限聚焦，实现了6米范围内高质量成像，光束展宽极小且旁瓣被有效抑制。

ABSTRACT

A lens performs an approximately one-to-one mapping from the object to the image planes. This mapping in the image plane is maintained within a depth of field (or referred to as depth of focus, if the object is at infinity). This necessitates refocusing of the lens when the images are separated by distances larger than the depth of field. Such refocusing mechanisms can increase the cost, complexity and weight of imaging systems. Here, we show that by judicious design of a multi-level diffractive lens (MDL) it is possible to drastically enhance the depth of focus, by over 4 orders of magnitude. Using such a lens, we are able to maintain focus for objects that are separated by as large as ~6m in our experiments. Specifically, when illuminated by collimated light at lambda=0.85mm, the MDL produced a beam that remained in focus from 5mm to ~1500mm from the MDL. The measured full-width at half-maximum of the focused beam varied from 6.6um (5mm away from MDL) to 524um (1500mm away from MDL). Since the sidelobes were well suppressed and the main-lobe was close to the diffraction-limit, imaging with a horizontal X vertical field of view of 20deg x 15deg over the entire focal range was demonstrated. This demonstration opens up a new direction for lens design, where by treating the phase in the focal plane as a free parameter, extreme depth-of-focus imaging becomes possible.

研究动机与目标

• 克服传统透镜的根本局限性，即当物体超出狭窄景深范围时需重新对焦。
• 通过显著扩展景深，消除成像系统中的机械对焦机构。
• 证明将焦平面处的相位视为自由设计参数，可实现极端景深成像。
• 仅使用单个静态透镜、无需活动部件，在大轴向范围内实现高质量成像。
• 在0.85 mm波长的准直照明条件下，通过实际加工的MDL验证极端景深成像的可行性。

提出的方法

• 设计一种具有空间工程化相位分布的多级衍射透镜（MDL），以控制轴向上焦点的演化。
• 采用逆向设计原理优化相位分布，使焦点在扩展的轴向范围内保持紧密聚焦。
• 实施四层二元相位分布（近似连续相位函数），以实现所需的波前调制。
• 使用波长λ = 0.85 mm的准直光照射MDL，以模拟远场成像条件。
• 利用探测器测量沿光轴的光束轮廓，测定半高全宽（FWHM）和旁瓣水平。
• 通过在5 mm至1500 mm范围内使用单个透镜配置拍摄物体，展示成像性能。

实验结果

研究问题

• RQ1 能否设计一种衍射透镜，使其在超过1.5米的轴向范围内保持衍射极限聚焦？
• RQ2 通过调控焦平面处的相位分布，景深可被扩展到何种程度？
• RQ3 单个静态透镜能否在无需机械对焦的情况下，实现6米轴向范围内的高保真成像？
• RQ4 在该极端景深透镜中，光束斑尺寸和旁瓣水平沿轴向如何变化？
• RQ5 能否在宽景深范围内，通过平面、轻质且紧凑的透镜实现高分辨率成像？

主要发现

• MDL实现了从5 mm到约1500 mm的景深范围，覆盖6米轴向范围。
• 聚焦光束的半高全宽（FWHM）从5 mm处的6.6 µm增加至1500 mm处的524 µm，表明光束展宽受控且渐进。
• 旁瓣得到良好抑制，确保在整个焦深范围内保持高对比度并最小化图像退化。
• 在整个1500 mm范围内，成功实现了水平×垂直视场为20°×15°的成像。
• 透镜保持接近衍射极限的性能，主瓣接近理论极限，证实了其优异的光学质量。
• 与传统透镜相比，该设计在景深方面实现了超过4个数量级的提升。