In order to improve the adaptability of cylindrical lenses and expand their application fields, a continuous zoom liquid micro-cylindrical lens system was fabricated. Cylindrical lenses are widely used in engineering fields such as beam shaping, scanning equipment, and holographic display due to their irreplaceable ability in beam manipulation. However, the commonly used focal length of cylindrical lenses is always fixed, and most of the research on zoom systems always focuses on the commonly used symmetric circular lenses, while zoom cylindrical lenses are relatively unexplored. Therefore, a series of liquid zoom cylindrical lenses have been designed in our previous study, including compound-type and capillary-type liquid core zoom cylindrical lenses. They change the refractive index of the core by changing the type or concentration of the liquid filled in the hollow area of the lens and then achieve the variable focal length. However, the compound-type lens has a large size and is not easy to integrate. The capillary-type micro cylindrical lens system has a short focal length and small zoom range, which limit its application. To address the above problems, we aim to design and fabricate a new type of liquid zoom cylindrical lens system based on a polydimethylsiloxane (PDMS) substrate, which is characterized by a high zoom ratio, stable structure, small volume, and easy integration. The continuous zoom liquid micro-cylindrical lens system can be used to replace the fixed focus cylindrical lens in beam manipulation, providing a higher degree of freedom and adaptability and applied in the accurate measurement of liquid refractive index and liquid diffusion coefficient.

The design and fabrication of a continuous zoom liquid micro-cylindrical lens system mainly include four processes: establishment and optimization of the initial structure, zoom ability and imaging quality evaluation, tolerance analysis, and processing and fabrication. In this paper, the capillary-type liquid core micro-cylindrical lens based on a PDMS substrate designed in our previous work is selected as the original structure, and its parameters are quadrupled as the initial structure to lengthen the focal length. When the refractive index of the liquid filled in the capillary varies by changing the type or concentration of the liquid, the focal length of the lens system changes, which can be considered as the different zoom states of the system and simulated by using the multiple structures in ZEMAX. We set the curvature radii, thicknesses, and glass material types of the cylindrical lens in the zoom system as variables and establish the evaluation function to optimize the system structure iteratively. We also analyze the optimized system's zoom ability and imaging quality until a high zoom ratio and good imaging quality over the zoom range are obtained. Then, the tolerance analysis, including curvature radius, decentering, as well as thickness of every surface in the molding process, and tilting of each cylindrical lens in the installation process, are performed to evaluate the feasibility of the design. We send the designed cylindrical lenses for processing and embed them into a PDMS substrate to complete the preparation. The setup of the observation system is complete, and then the zoom ability and the imaging quality of the continuous zoom liquid micro-cylindrical lens system based on a PDMS substrate are measured to verify the feasibility of the design scheme.

The zoom lens system we have designed is composed of two symmetrical meniscus lenses and a biconvex cylindrical lens, which are all embedded in a PDMS substrate (Fig. 2). The edges of the two meniscus lenses are glued to form a cavity, and the focal length of the system can be changed continuously by varying the refractive index of the liquid filled in the cavity. The rationally designed biconvex cylindrical lens can control the aberrations of the cylindrical lens system in the whole zoom range. The detailed parameters are listed in Table 1. The dimension of the PDMS substrate is 14.7 mm×6.0 mm×10.0 mm. When the refractive index of the liquid injected into the cylindrical lens system changes from 1.3330 to 1.5530, the back focal length of the system changes from 52.292 mm to 4.972 mm continuously and smoothly (Fig. 3). In the whole zoom range, the radial root mean square radius of the diffuse spot of the cylindrical lens system is always less than 5 μm (Fig. 5), and the MTF curves are close to the diffraction limit in most zoom structures (Fig. 6). The possible tolerance of the cylindrical lens system is analyzed in detail (Figs. 7-10), and permissible tolerance is given. Then, the fabrication of the lens system (Fig. 11), as well as the measurement of the back focal length (Fig. 12) and the MTF curves (Fig. 13) are completed. The measured values and curves are close to the simulation results.

A continuous zoom liquid micro-cylindrical lens system based on a PDMS substrate is designed and fabricated in this paper, and a cavity is used to inject liquid; a biconvex cylindrical lens is used to control the aberrations, and the square PDMS substrate ensures the stability of the lens system. Both the simulation and measured results have confirmed its high zoom ratio and great imaging quality. Compared with the original structure, the zoom range is enlarged by about 5 times while ensuring the imaging quality. The zoom system has the advantages of a high zoom ratio, small size, simple and stable structure, and high imaging quality, which can be used in integrated micro-devices.