[技術分享] 液態可變焦透鏡原理 Focus tunable lenses

2020 年 10 月 19 日 Acousto-Optic AOM 技術分享

Introduction

Traditional optics are based on solid glass or plastic lenses, which are moved back and forth to focus or zoom. A very old but successful system, however, is completely different: the eye! It consists of an elastic lens material, which is bent in order to focus. Optotune has developed and patented a series of lenses that basically copy the principle of the eye.

Instead of moving lenses back and forth, the human eye focuses by reshaping its lens

Working principle

Optotune’s focus tunable lenses are shape-changing lenses based on a combination of optical fluids and a polymer membrane. The core element consists of a container, which is filled with an optical liquid and sealed off with a thin, elastic polymer membrane. A circular ring that pushes onto the center of the membrane shapes the tunable lens. The deflection of the membrane and with that the radius of the lens can be changed by pushing the ring towards the membrane or by exerting a pressure to the outer part of the membrane or by pumping liquid into or out of the container.

Working principle of ML lenses

Working principle of Optotune’s manual lens ML-20-35, which achieves a lens shape ranging from concave to flat to convex. The ring that forms the lens is pushed towards the container, thus filling the lens with liquid

Working principle of Optotune’s electrical lens EL-10-30. In this case, the lens-shaper ring remains in place relative to the container. The only movement is a ring that pushes down on the membrane in the outer part of the lens with increasing current, thus pumping the liquid into the center of the lens.

Possible variations of the shape-changing polymer lens

Advantages

A change in lens radius of several micrometers can have the same optical effect as moving the entire lens several centimeters. Optical systems can be designed more compact, oftentimes with less lenses and usually with less or no translational movement. Accordingly, there is no more need for expensive mechanical actuators. Less movement also leads to a more robust design, which can be completely closed so that no dust can enter. Furthermore, the materials employed are lighter than glass, saving overall weight. Less movement and weight also means less power consumption and that the response time of systems with tunable lenses can be very low, in the order of milliseconds. Another advantage becomes obvious during production. The fact that less optical parts are moved combined with the tunability of the radius during operation results in reduced tolerance sensitivity and thus higher yield rates.

To summarize, these are the five main advantages of focus tunable lenses over traditional optics:

Compact design
Less mechanics
Fast response
Low power
Less tolerance sensitivity

技術

工作原理

Optotune的核心技術是基於可變形鏡頭的工作原理。鏡頭是由一個容器組成，容器內裝有光學流體，並用彈性聚合物薄膜進行密封。電磁致動器對容器施壓後進而導致鏡頭彎曲，從而通過致動器線圈內的電流來控制鏡頭的焦距。實際上，電流和屈光度(與焦距成反比)成線性關係。

大多數可調焦液態鏡頭均設計呈正方向彎曲，即致動器朝薄膜方向推進。通常需要增加一個平凹補償鏡片以實現負屈光度。但新一代鏡頭的驅動器與薄膜相連接，可通過負電流將薄膜推離容器，從而形成凹透鏡頭。因此，無需額外添加補償鏡片便可實現負屈光度。

薄膜厚度是影響鏡頭調焦範圍的重要參數。薄膜厚度越小，屈光度調節範圍越大。此外，鏡頭的屈光度還可以通過使用折射率不同的光學流體來進行調整。 Optotune採用折射率為約1.3的光學流體，具有低色散特性(阿貝數約為100)，這使得鏡頭極其適合用於多色應用領域。

反應時間

與質量彈簧系統類似，液態鏡頭需要極短且有限的反應時間作動，其原因在於致動器移動必需克服流體的物理慣性。當採用階躍電流時，鏡頭的上升時間大約為幾毫秒。由於共振激發的作用，鏡頭僅需10-20毫秒即可完全就位，具體所需的時間還與鏡頭尺寸有關關。右圖顯示了鏡頭對階躍電流的光學影響，其結果由四象限光電二極管測得。測量均在室進行。

採用經優化的階躍電流或閉環系統可大幅抑制共振。右圖為EL-10-4 2-0F鏡頭的楷形響應，該款鏡頭帶有用於閉環屈光度控制的整合光回饋系統。結果顯示未出現振盪現象。

波前品質

大致上Optotune的可調焦液態鏡頭呈現球面透鏡的形狀。波前誤差一般約為0.1 (測量條件：8mm通光孔徑,波長為525 nm)，可進行高品質成像和顯微檢查。通常薄膜厚度越薄，調節範圍越大，但同時波前誤差也會變大。如右圖中藍、紅實線所示。

由於薄膜具備彈性，因此鏡頭形狀會受到重力影響。當鏡頭垂直使用時(光軸為水平方向)，會因重力而產生Y軸慧(Y-Coma)。右圖紅線虛線和實線充分證明了這一點。我們還可以從這些虛線和實線中看到薄膜越厚，質地便越硬，因而受到重力的影響越小。因此，為了使鏡頭發揮最佳的性能,建議將光軸置於垂直方向。

透光率

光學流體和薄膜材料均具備極高的透光性，透光波段為400-2500 nm。以下透射光譜儀表示透鏡材料的透光率，如：假設採用了理想的蓋玻片。由於薄膜具有彈性，因此無法使用標準程序鍍膜，所以薄膜反射率為4%，如有需求，我們可以再蓋玻片上針對特需波長進行鍍膜優化處理。 Optotune對於蓋玻片提供兩種在可見光和近紅外光區域的標準寬波段鍍膜。如有需求，我們可對蓋玻片進行特殊鍍膜處理。