Shape Memory Alloy for Adaptive Optics

Shape memory alloys (SMAs) are smart materials that, upon deformation, return to their original shape when heated above a temperature threshold. This special behaviour makes SMAs very attractive as effcient strain/force actuators with very large strains (up to 8%). For this reason, SMAs have been widely used in several applications, from surgery to space. In thesis work, we considered the use of SMAs to build a deformable lens, i.e. a lens with tunable focal length. Usually, optical systems are realized by using optical components such as lenses and mirrors that have fixed optical power (focal length) and that sometimes can be moved, as it occurs in zoom systems. In such cases the movements are achieved manually or by using motors that are slow and subjected to mechanical wear. More recently, optical elements with variable focal length, hence called "adaptive" have been developed, allowing for a great simplification in optical design. Among all the available materials used for actuation, SMAs possess a higher strain with respect to the common actuation materials used to build deformable lenses (such as the piezoelectric). Moreover, they don't need high actuation voltages and, by using SMA wires, allow to realize very small actuators, and consequently very compact devices. Herein, we demonstrate that SMA wires can be efficiently employed for developing lenses with variable focal length with small dimensions, easily embeddable in wearable optics.