Presentazioni e Articoli Tecnici

The expression adaptive optics was initially termed for the technology used in the telescopes to deform the mirrors to allow for phase correction of the incoming light. Soon, the adaptive optics was also implemented in microscopes, optical communication systems and in optical imaging systems. In conventional imaging systems, the lenses are mechanically moved to focus an image, whereas, with the adaptive optics lenses the lens’s surface curvatures are changed to focus an image. The focus tunable lens or simply called as the adaptive lens uses micro-actuators to change the radius of curvature of a deformable surface to modify the focus/refractive power of the lens. One such adaptive lens with an integrated piezoelectric actuator was developed in Laboratory for Microactuators, IMTEK- Department of Microsystems Engineering, University of Freiburg, Germany.

The adaptive lens consists of a circular ring-shaped piezoelectric bimorph actuator and a flexible membrane attached on top of a fluid chamber to form a fluid-membrane interface. The piezoelectric actuator, under the applied electric field, applies force on to the fluid to generate pressure in the fluid chamber, thereby deforming the fluid-membrane interface to form an aspherical membrane surface.

Here, we present the adaptive lens modelled with COMSOL Multiphysics®. For the simulation, physics modules such as the piezoelectric devices, the fluid-structure interaction and the heat transfer in solids and fluids are coupled together to study the interaction between the piezoelectric forces and the fluid forces acting on the deformable membrane at different temperatures.

The adaptive lens is modelled as a 2d axisymmetric model, consisting of a piezoelectric actuator, a flexible membrane, and a fluid chamber. The piezoelectric actuator is modelled in ‘solid mechanics’ and ‘electrostatics’, the flexible membrane in ‘solid mechanics’ and the fluid in ‘laminar flow’. Since in COMSOL® Multiphysics, the direct coupling of the piezoelectric effect and the fluid-structure interaction is not possible, a ‘moving mesh’ physics module is used to transfer the piezoelectric forces to fluid forces. Further, the ‘heat transfer in fluids’ and ‘heat transfer in solids’ physics modules are coupled into the model to simulate the influence of fluid thermal expansion on the deformable membrane.

In the simulation, the piezoelectric actuator is set to different electric potentials to determine the actuator deflection, the fluid pressure and the radius of curvature of the membrane at different temperatures. The simulated results are in close agreement with the experimental results. The adaptive lens can vary the refractive power from -16m^‑1 to +17 m^-1 at 25°C and from -15 m^-1 to +28 m^-1 at 75°C.