A liquid lens with electro-optical focus and tilt control

Small-sized camera modules have become key subsystems in mobile phones and camera handsets. In camera phones, the industry trend has been to increase pixel numbers while maintaining small sensor formats for overall size and cost reasons. However, pixel shrinking has led to degraded light sensitivity, thus requiring increased exposure times. In turn, these impact image quality because of handshake blur.¹ Several groups have highlighted this as a major problem in mobile imaging, which can only be resolved by employing optical-image-stabilization systems² (OISs). Among the OIS technologies currently under development,^3,4 a liquid-lens component based on electrowetting^5–7 can generate an electrically controlled optical tilt in two directions. The relevant tilt range and response time are suitable for OIS designs that can be integrated with miniature mobile-phone camera modules.⁸ OIS liquid lenses can also provide focus control and realize small-sized OIS auto-focus (AF) camera modules without any moving mechanical parts.

A liquid lens is composed of two fluids with the same density. One of these is electrically insulating (such as oil), while the other is an electrolyte. They are characterized by a refractive-index difference so that they form an optical interface with optical power depending on the liquid interface's curvature radius. Both liquids rest on a hydrophobic dielectric coating. When voltage is applied to the latter, the wettability of the liquids is modified and the curvature of the interface changes: see Figure 1(A), (B), and (C). This process is highly reversible with low hysteresis. In a liquid lens, the oil droplet is spatially confined to a conical cavity and acts as a lens with a variable focal length, depending on the applied voltage: see Figure 1(D).

Figure 1. (A) Schematic principle of electrowetting involving a drop of oil (o) with contact angle αon an insulating coating (green) of thickness (d), surrounded by a conducting fluid (w). V: Power source. (B) and (C) The same oil drop on a parylene coating at 0 and 60V rms (AC frequency: 1kHz). (D) Schematic view of the component's bottom part.

When the same voltage is applied to the entire surface of the dielectric coating, the shape of the liquid interface remains both spherical and centered on the conical cavity's symmetry axis. When a nonuniform voltage is applied along the dielectric coating, we can obtain a tilted liquid-interface shape. This is the key principle of our OIS and AF liquid-lens component. In practice, a nonuniform voltage along the cone is obtained with a design that includes four electrodes separated by a resistive layer (see Figure 2).

Figure 2. Optical-image-stabilization system (OIS) liquid lens with four electrodes.

Our AF/OIS liquid lens is a modular component that can be simply plugged into an existing camera to transform a fixed-focus module into an AF and OIS device⁹ (see Figure 3). Its driving circuitry is very compact because of the availability of a customized driver chip from Maxim Integrated Products (see Figure 4). Auto-focus adjustment of the camera module can be achieved with a closed-loop command of the optical power of the lens from an estimation of the sharpness of the image by the built-in image processor. The principle of optical-image stabilization includes measurements of the instantaneous handshake tilt of the camera module with a two-axis gyroscope and generation of a reverse tilt with the liquid lens.^2,10

Figure 3. Optical scheme of a 5Mpix OIS/auto-focus camera module with a ¼inch sensor. The liquid lens is attached to the camera-module lens. The design offers a focus range from 5cm to infinity and a ±0.6°tilt-correction range for the OIS function. f: Focal ratio.

Figure 4. Liquid lens and MAX14574EWL+ driver with external components.

Our OIS liquid lens is mainly dedicated to image stabilization for miniature camera modules. The electrical command of both its optical power and the optical tilt, combined with a miniature component, can also be used in a wide range of applications in optics, such as for optical-beam tuners, active spectral filtering, or active laser-beam injection in optical fibers (see Figure 5).

Figure 5. Optimized fiber injection using two OIS liquid lenses and afiber-collimation lens.

In summary, with no moving parts, our new OIS/AF liquid-lens component is silent, robust, and easy to integrate. It can combine OIS and smooth AF for pictures and video with low electrical consumption. The next step in this technology is for our customers to integrate liquid lenses in miniature camera modules which should happen in early 2011.