結合適應性光學校正系統之流體透鏡特性與像差研究

Adaptive optics-corrected system for performance enhancement and aberration measurement of adaptive fluidic lens

林明欣(Ming-xin Lin)

指導教授 ： 傅尹坤

國立中央大學/工學院/光機電工程研究所/碩士(2012年)

摘要

本論文是關於流體透鏡的相關性質與像差研究，並且結合適應性光學相關應用。本論文研究重點在(1)將適應性光學當作像差產生源，以流體透鏡達到校正效果，(2)利用簡單方法降低微流體透鏡所造成的球形像差，(3)結合適應式光學與流體透鏡，改善影像品質。
(1)利用流體透鏡校正可變形反射鏡所產生的像差
可調式流體透鏡藉由曲率變化產生不同的焦距，並且有機會當作校正像差的裝置，例如活塞、散焦和散光等等的像差。在本文中，流體透鏡被使用來校正所引起的波前像差系統研究，與解釋Zernike多項式和勘探液體透鏡的潛力。此外，像差將以Zernike多項式表示，前六項多項式通常出現在眼科學且被仔細檢查。藉由商品化的MEMS (micro-electrical-mechanical systems) DM (deformable mirrors)包含140個制動器，故意產生三種不同的波前像差，Shack-Hartmann波前感測器被用來測量流體透鏡的光學性質和特點。實驗結果表明，活塞像差可顯著改善，從0.972μm降至0.031 μm在注入至少0.02 mL去離子水(產生曲率半徑為392.8 mm)的情況。類似的改進，可以發現散焦&散光像差皆同時減少，分別從-0.15 μm & -0.48 μm降到0.02 μm & 0.085 μm。當注入0.1毫升體積（曲率半徑78.9毫米），我們可以實驗推斷傾斜&散焦像差，藉由MEMS DM引起的像差值大小0.486 μm & -1.472 μm，流體體透鏡只能輕微改善至0.245 μm & 0.305 μm。
(2)改變流體透鏡的薄膜厚度比，降低球形像差
可調式雙凸微透鏡，直徑1000 μm的微機械加工和壓力驅動產生可變焦距的應用。微透鏡由兩個薄膜建構形狀，流體室和微流道相互連結。焦距調整範圍從35 mm到250 mm，變焦比率達7倍，且沒有任何機械運動部件。像差特性使用了Shack-Hartmann波前感測器測量系統，以澄清焦距調變所產生潛在的不利影響，在這裡特別以Zernike多項式解釋。實驗結果表明，球形像差數值從-0.037 μm到-0.095 μm顯著增加，注入去離子水分別為1.11 μL至4.43 μL。提出一個容易的和具有成本效益的方法來補償球形像差，根據調節彈性薄膜的厚度差。當施加均勻壓力時，改變雙凸微透鏡的薄膜厚度比例，可被用來控制變形程度以及表面輪廓的差異度。且以ZEMAX模擬和實驗來驗證最佳厚度比的設計理念和研究。當注入3.32 μL固定液體量時，以波前感測實驗測量其相對應的球形像差值，厚度比1:1球形像差為-0.053 μm與厚度1:4球形像差-0.023μm相比，獲得56 ％的改善，得到其最佳厚度比為1:4。所提出的微透鏡是實用的，可應用於醫療成像系統中，例如：一個動態的環境和適應性光學系統。

(3)利用適應性光學校正流體透鏡所產生的像差
可調式流體透鏡是利用曲率變化，通過不斷調整注入液量，實現可變焦距的特性。然而，自然曲率變化性質和折射率不匹配會引發內在空間畸變，嚴重導致圖像品質降低的情形。在這裡，我們提出可調式流體透鏡產生畸變的實驗研究和使用適應性光學補償波前像差。適應性光學的基礎計劃，在三種不同注入液體量的情況下，大幅減少造成的波前誤差，其RMS分別從0.42、1.05、1.49降至0.2、0.21、0.23 μm，分別相對應於100-200 mm的可調焦距。

關鍵字

適應性光學 ； 流體透鏡 ； ZEMAX ； 球形像差 ； 可變形反射鏡

並列摘要

This paper mainly research performance enhancement and aberration measurement of adaptive fluidic lens.
(1) Adjustable fluidic lenses for correcting aberrations induced by MEMS deformable mirrors
An adjustable fluidic lens is typically utilizing curvature change and is promising to correct the aberrations such as piston, defocus and astigmatism. In this paper, capability of adjustable fluidic lenses to correct the induced wavefront aberrations is investigated systematically, with particular attention placed on interpretation of Zernike modes and exploration of the potentials of fluidic lenses. In addition, aberrations are expressed in Zernike polynomials and the first six modes commonly encountered in ophthalmology are carefully examined. Three different wavefront aberrations are intentionally induced by a commercial MEMS (micro-electrical-mechanical systems) DM (deformable mirrors) with 140 actuators. The optical properties of fluidic lenses corrections are characterized by Shack-Hartmann measurements. Experimental results show that piston mode (Z1) can be significantly improved from 0.972μm to -0.031μm using fluidic lenses by injecting DI water as little as 0.02ml (resultant meniscus curvature is 392.8mm). Similar improvements can be found in defocus (Z5)/astigmatism (Z6) and aberrations are reduced for both modes from -0.15μm/-0.48μm to 0.02μm/0.085μm, respectively. When injected volume of 0.1ml (resultant meniscus curvature is 78.9mm), we can experimentally deduce that aberrations of Z3/Z5 induced by MEMS DM at a magnitude of 0.486μm/-1.472μm, fluidic lenses can only marginally improved to -0.245μm/0.305μm.

(2) Characterizing aberration of a pressure-actuated tunable biconvex microlens with a simple spherically-corrected design
A tunable biconvex microlens of 1000 μm diameter is micromachined and pressure-actuated for variable-focusing applications. The microlens consists of two thin membranes with reconfigurable shapes, a fluid chamber and the interconnected microchannel. The back focus length tuning range is demonstrated from 35 mm to 250 mm and zoom ratio of up to 7 without any mechanical moving components. Aberration characterization is carried out systematically by Shack-Hartmann measurements to clarify the potential adverse effect associated with focal length tunability, with particular attention placed on interpretation of Zernike modes. Experimental results show that spherical mode (Z13) can be significantly degraded from -0.037 μm to -0.095 μm by injecting DI water of 1.11 to 4.43 μL, respectively. A facile and cost-efficient approach has been proposed to compensate spherical aberration based on differential thickness of elastic membranes. The thickness variation for the biconvex microlenses can be manipulated as the difference in deformation contour as well as the resultant surface profile when subjected to uniform applied pressure. Both ZEMAX simulation and experiment are used to validate the design concept and search for the optimal thickness ratio. By injecting the fixed fluid volume of 3.32 μL, the optimal thickness ratio of 1:4 can be experimentally obtained and the measured spherical aberration is -0.023 μm, or 56% improvement compared with 1:1 thickness ratio of -0.053 μm. The proposed microlens is robust and can be used potentially in medical imaging systems, for example, a dynamic environment and adaptive optics.

(3) Adaptive optics correction of a tunable fluidic lens for ophthalmic applications
Tunable fluidic lenses are utilizing curvature change via continuously adjusting injected liquid volumes to achieve variable-focusing properties. Nevertheless, the nature of curvature change and refractive index mismatch causes inherent spatial aberrations that severely degrade image quality. Here we present the experimental study of the aberrations in tunable fluidic lenses and use of adaptive optics to compensate for the wavefront errors. Adaptive optics based scheme is demonstrated for three injected liquid volumes, resulting in a substantial reduction of the wavefront errors from 0.42, 1.05, 1.49 to 0.2, 0.21, 0.23 μm, respectively, corresponding to the focal length tunability of 100-200mm.

並列關鍵字

Fluidic lens ； Adaptive optics ； Deformable mirrors ； Spherical aberration

參考文獻

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