A few types of focusable fluidic lenses have been developed over the years; one form such a lens can take is that of a chamber with a transparent membrane that flexes as fluid pressure changes within the chamber (see Laser Focus World, August 2003, p. 11). True focusable liquid lenses—those with a liquid surface that changes shape on its own—eliminate the optical problems associated with flexing membranes, but are difficult to implement. Engineers at Varioptic (Lyon, France) have created a liquid lens focusable by "electrowetting" that contains two dissimilar liquids in a chamber; the shape of the interface between the liquids can be altered by varying an electric field. The company is targeting high-volume consumer-market devices such as miniature digital cameras for mobile phones.
In electrowetting, an applied voltage changes the wettability of a droplet of conducting fluid that contacts a surface. For example, a beaded droplet of water resting on a normally nonwettable surface can flatten out under a voltage. The Varioptic lens actually contains two fluids—one water and the other a nonconductive immiscible fluid having a different refractive index—in contact with each other (see figure). The spherical fluid-to-fluid interface terminates on a conical ring-shaped support in the chamber; as the water becomes more or less wettable, the contact angle of the interface to the support changes, altering the curvature of the interface. The lens chamber is completely enclosed, eliminating problems of contamination and evaporation.
The interface between water and an immiscible fluid in a liquid-lens chamber takes a spherical shape that is changed by varying the voltage across two electrodes (right; dotted and solid lines). The contact angle between the interface and one of the electrodes is a function of voltage; the two liquids have different refractive indices, thus forming a lens. A liquid lens can be made part of a small focusing lens assembly for a digital camera (prototype shown in inset, right). The two photos above were taken 0.03 seconds apart by a 1/4-in. CCD camera outfitted with a fixed glass lens and a liquid focusing lens.
In a typical liquid lens with a 5-mm aperture, the focal length changes from -200 mm to infinity to +50 mm as the voltage is varied from 0 to 50 V (early prototypes required voltages of up to 250 V; engineers are working to further reduce the maximum voltage required). Typical wavefront distortion over a 4.5-mm clear aperture is 0.7-µm peak-to-valley, and the optical axis remains stable in its lateral position to ±20 µm.
"This is our core know-how," says Bruno Berge, president of Varioptic. "Every liquid lens must incorporate a centering mechanism; for a lens to be useful, its optical axis should be stable. In our case, we have investigated several centering methods, and the one that is currently under use relies on the geometry of the supporting piece. We have shown that all cavity surfaces that are less curved (curvature is defined locally) than the sphere spontaneously center the liquid drop. We use a conical surface, but cylindrical or toroidal surfaces could work as well."
The liquid-lens element can be integrated into a multilens assembly as is normally required for aberration correction in cameras. Requiring no mechanical moving parts, such a lens is simpler and potentially lower in cost than standard focusing lenses. Engineers at Varioptic have already developed an autofocus-lens micromodule for miniature digital cameras and are working on a zoom module (which requires two liquid lenses). The technology has attracted the attention of a large Asian manufacturer of electronic parts, with whom Varioptic has signed a large contract for developing prototypes and producing a pilot series.