Characterizing aberration of a pressure-actuated tunable biconvex microlens with a simple spherically-corrected design

A tunable biconvex microlens of 1000 μm diameter is micromachined and pressure-actuated for variable-focusing applications. The microlens consists of two thin membranes with reconfigurable shapes: a fluid chamber and the interconnected microchannel. The back focus length tuning range is demonstrated from 35 mm to 250 mm and zoom ratio of up to seven without any mechanical moving components. Aberration characterization is carried out systematically by the ShackHartmann measurements to clarify the potential adverse effect associated with focal length tunability, with particular attention placed on interpretation of the Zernike modes. Experimental results show that spherical mode (Z13) can be significantly degraded from -0.037 μm to -0.095 μm by injecting DI water of 1.114.43 μL. A facile and cost-efficient approach has been proposed to compensate spherical aberration based on differential thickness of elastic membranes. The thickness variation for the biconvex microlenses can be manipulated as the difference in deformation contour as well as the resultant surface profile when subjected to uniform applied pressure. Both ZEMAX simulation and experiment are used to validate the design concept and search for the optimal thickness ratio. By injecting the fixed fluid volume of 3.32 μL, the optimal thickness ratio of 1:4 can be experimentally obtained and the measured spherical aberration is -0.023 μm, or 56% improvement compared with 1:1 thickness ratio of -0.053 μm. The proposed microlens is robust and can be used potentially in medical imaging systems, for example, a dynamic environment and adaptive optics.