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A varifocal lens with a large focal length range is an indispensable optical part for applications such as biomedical engineering, intelligent robot, industrial image, and virtual-real. A key feature of Alvarez lenses is that the focal length can be tuned using small lateral rather than axial translation. Dielectric elastomer (DE) is a kind of ‘smart’ non-ionic electroactive polymer material. DE can be used as a soft actuator to produce the lateral translation for the Alvarez lens due to its reversibly large strains with a fast response speed, low cost, lightweight, and simple manufacturing process. When a driving voltage is applied to the compliant electrodes of the DE, an electrostatic force named Maxwell stress reduces the distance between the electrodes, thus the DE expands in the lateral directions. The focal length range of the Alvarez lens actuated by DE is not only determined by the parameters of the Alvarez lens but also by the parameters of the DE. To achieve large lateral displacement for achieving a large focal length range, three Alvarez lenses actuated by DE with different pre-stretched ratios and diameters are experimentally studied in this paper. Firstly, we develop and fabricate three Alvarez lenses actuated by DE with different pre-stretched ratios and diameters. Two Alvarez lenses, actuated by DE membranes with the same diameter of 38 mm, but with different pre-stretched ratios, i.e., 200% and 260%, respectively, are fabricated. In addition, an Alvarez lens actuated by DE membrane with a pre-stretched ratio of 260% and diameters of 30 mm is fabricated. Secondly, a driving voltage is applied to the DE membrane to actuate the Alvarez lenses. The driving voltage is generated using a function generator and the amplitude is amplified by a high-voltage converter. Lastly, the focal length ranges of the three Alvarez lenses are measured by the magnification method. A young embryo of the capsella bursa-pastoris section is selected as the imaging object and placed 2.0 mm from the fabricated Alvarez lenses. By measuring the object height in the captured image under different driving voltages using a microscope, the focal lengths of the three Alvarez lenses are obtained. The results show that the Alvarez lens actuated by a dielectric elastomer with a large pre-stretched ratio and diameter has a wider focal length range under the same driving voltage. The study is beneficial to designing and developing an Alvarez lens actuated by DE for optical imaging applications that require wide focal length tuning capacity.
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