We have developed an optical zoom system based on tunable lens with polymer membrane and multilayered structure. The presented system employed two tunable lenses and two doublet lenses as the main refractive units. Each tunable lens had a delicately designed, solid-liquid mixed structure with multiple internal slim holes, which helped to improve the lens optical properties and stability against gravity. The optical structure and regulation principle of the zoom system is presented, as well as the detailed fabrication process of the tunable lens. Under different displacement loads, the lens deformation properties, and the adjustment ability and imaging characteristics of the zoom system were measured and analyzed. Besides, the spot diagram, field curvature and distortion of the system were simulated using the Zemax software. The magnification of the designed system could be regulated from 0.2X to 4.3X flexibly depending on the focal variation of each tunable lens.

Keywords: zoom system, polymer membrane, tunable lens, integrated structure

Zoom system is often found in many optical applications such as cell phones, microscopes, telescopes, pico projectors and digital cameras.^1-3 With development of the optics technology, the requirements for integration, flexibility and adjustment ability of the zoom system become higher and higher. Therefore, it is of great necessity to develop the zoom system with compact structure, stable optical property and large magnification ratio.
The conventional optical zoom system generally consists of multiple solid lens groups, electrical motors and transmission mechanisms, which usually has limited magnification ratio, bulky package and complicated driving module. Recent years, the tunable lenses with adjustable focus have attracts much attention,^4-6 showing the potential to be used for various flexible zoom systems. According to regulating principle, the focus of tunable lens can be adjusted by changing the lens refractive index or the lens surface shape. The liquid crystal (LC) lens is a typical kind of tunable lens, which controls the refractive index through altering the molecular distribution structure of the LC material.⁷ Utilizing two LC lenses as the main adjustable unit, an electrically tunable-focusing zoom system was presented, which realized a zoom ratio of 7.9:1.⁸ A optical imaging system was demonstrated which used one LC lens to maintain the imaging position and two other LC lenses in charge of zoom function.⁹ Due to the existed inhomogeneity of the LC material, the LC lens system was usually faced with the problems of energy loss and optical distortions.
The liquid tunable lens adjusted the focus by changing the shape of lens refractive surfaces, such as electro-wetting lens, the fluidic tunable lens, and dielectric elastomer lens.^10-12 Various optical imaging systems were developed and analyzed using the liquid tunable lens. For example, a two-liquid-lens zoom system without moving elements was reported which had a zoom factor of 1.8 and a compact structure of 10 mm.¹³ An optofluidic zoom system was presented which utilizes liquid optical path switchers to reduce the structure and increase the zoom range.¹⁴ A three-element zoom system for laser beam expanders based on liquid tunable lens was presented, as well as the design principle and paraxial properties of the system.¹⁵ The electrowetting based liquid tunable lens has compact structure and large zoom range, but this kind of tunable lens and imaging system usually had a small aperture and requires a relatively high voltage. The pressure driven liquid lens mainly consists of elastic membrane, optical liquid and holder, and adjust the focus through changing the liquid pressure to alter the lens surface deformation.^16,17 This kind of tunable lens had good integration, strong adjustability and flexible aperture, showing broad application potentials. As the liquid material is usually susceptible to the external vibration and gravity effect, the stability and optical properties of the pressure driven tunable lens and imaging system remains to be improved.
In this paper, we have developed an optical zoom system based on tunable lens with polymer membrane and multilayered structure. Compared with the previous zoom system based on liquid tunable lens, we used a solid-liquid mixed structure with multiple slim holes to design the porous tunable lens, in order to improve the stability and optical quality of the zoom system. Each tunable lens was made of solid-liquid mixed material with multiple optical layers, which would not only reduce the proportion of the liquid to improve the lens stability, but also offer more optical freedoms for the practical applications. Besides, multiple slim holes were drilled in the front aperture, inner lens and holder of the tunable lens, which can increase the fluidic damping resistance to improve the lens adaptability to gravity and enhance the stability of the optical axis. In the following section, the optical structure and regulation principle of the zoom system are presented, as well as the detailed fabrication process of the tunable lens. Under different displacement loads, the lens deformation property, adjustment ability and imaging characteristics of the zoom system were measured and analyzed. Furthermore, the spot diagram, field curvature and distortion of the presented system are also discussed.