Femtosecond laser writing of freeform gradient index microlenses in hydrogel-based contact lenses

Femtosecond lasers can be used to write a variety of gradient index refractive devices. Writing devices with an arbitrary optical profile (i.e., freeform) requires knowing the functional dependence of the phase change that the wavefront will experience when passing through a region written under different exposure parameters. We measured this dependence as a function of writing speed and power in hydrogel-based contact lenses. Regions of constant refractive index change were written under different conditions and then the phase change was measured with a Mach-Zehnder interferometer and a phase retrieval algorithm. This functional dependence was tested by writing arbitrary Zernike polynomials with varying magnitudes.