Electrically tunable optical zoom system using composite liquid crystal lenses

Optical zoom systems that regulate image magnification are a widely used feature of camera and projection devices. Typically, an optical zoom consists of many solid lenses. The objective can be focused, zoomed in, and zoomed out by controlling the relative positions of the lenses. However, conventional systems are bulky and fragile, and thus are not practical for portable device applications such as cell phones, pico projectors, and webcams. A better solution would be an electrically tunable system with no mechanically moving optical parts.

Realizing such a system requires optical elements with electrically tunable focal lengths, such as liquid lenses, deformable mirrors, and liquid crystal (LC) lenses. LC lenses in particular have the advantage of low cost, light weight, and no moving parts. The main mechanism of the electrically tunable focal length of the lenses results from the parabolic distribution of refractive indices due to the orientations of the LC directors (i.e., the average direction of the molecular axes). The wave front of the incident plane wave is then bent into a converging or a diverging spherical wave, which indicates the lensing effect for the incident wave as a positive or a negative lens. In 1992, Tam proposed and carried out a theoretical analysis of electro-optical zoom lenses.¹ Subsequently, experimental scientists^2–4 also demonstrated electro-optical zoom systems.^2–4 However, the proposed designs have drawbacks, including discrete magnification, two optical zooms (2 and 0.5×) for an objective distance of 50cm, large overall size (>50cm), and a small zoom ratio (2 : 1∼4 : 1).

Figure 1. The structure of composite liquid crystal (LC) lenses. The focal length of the polymeric layer is fixed. When voltage V₁>V₂, the composite LC lens acts as a positive lens. When V₁<V₂, the composite LC lens acts as a negative lens. NOA81: Norland Optical Adhesive 81. ITO: Indium tin oxide. PVA: Polyvinyl alcohol.

To achieve an optical system with a continuously tunable optical zoom, we performed an optical analysis and designed a new system for portable devices using composite LC lenses.⁵ The structure and operating principle of the lenses consists of an LC layer and a built-in planar polymeric layer whose focal length is fixed (see Figure 1).⁶ The function of the polymeric lens is to shift the tunable range of the focal length of the composite LC lens. The lenslike distribution of the refractive indices of the polymeric layer serves to fix its focal length, whereas that of the sub-LC layer is voltage-controllable. When V₁>V₂, the sub-LC layer acts as a positive lens to convert the incident plane wave into a converged spherical wave by virtue of the distribution of orientations of the LC directors (see Figure 1). When V₁2, the sub-LC lens acts as a negative lens to convert the incident plane wave into a diverged spherical wave. According to geometric optics, the total lens power—defined as the inverse of focal length—of the composite LC lens equals the sum of the lens power of the sub-LC and polymeric layers. The composite LC lens has a small absolute value of the negative focal length due to the shifting effect of the polymeric layer. This characteristic is crucial in realizing an electrically tunable optical zoom system with a continuously tunable and large optical zoom ratio.

Figure 2. Structure of the electrically tunable optical zoom image system. The structures of the LC objective and eyepiece lenses are shown in Figure 1. f_L: Focal length of the lens.

Figure 2 shows both the zoom with the LC lens pair and a camera system consisting of a solid lens and an image sensor. A polarizer is attached to the LC eyepiece lenses. The mechanism is based on the concept of a confocal telescope. The magnification changes electrically when the focal lengths of the two composite LC lenses change in response to applied voltages. As a result, no matter where the target is, we can obtain a clear (or focused) image by properly controlling the focal lengths of the composite lenses. Moreover, as with a telescope, the magnification can also be modified.

However, the focal length of the composite LC lenses is limited, which in turn limits magnification. The optical zoom ratio of the system is defined as the ratio of maximum magnification to minimum magnification. Consequently, the optical zoom ratio is determined by the composite lenses. In the experiments, we set the distance between LC lenses at 10cm. When the lens power of the composite LC lenses is in a range of −13:5m⁻¹ to 21:8m⁻¹, the optical zoom ratio of the designed system reaches 7:9 : 1 (see Figure 3).

Figure 3. Photographs of the images in the electrically tunable optical zoom system at a fixed object distance of 10cm. The magnification of (a) the zoom-in image is 2.3 and (b) the zoom-out image is 0.29.

In summary, we have reported an electrically tunable optical zoom system using a pair of composite LC lenses. Images can be focused, zoomed in, and zoomed out continuously by changing the applied voltage of the composite lenses. However, the light efficiency in the system is limited by a polarizer and the composite lens apertures. Our next step will be to develop polarizer-free LC lenses with large aperture size to improve the light efficiency of the system and to make it more practical for use in portable devices.