Femtosecond laser writing of optical waveguides using astigmatic-elliptical beams produced by a phase-only spatial light modulator

Beam shaping techniques provide a way to control the shape of the focal volume in laser micromachining applications. This approach has been successfully applied for fs-lasers both in surface and bulk processing of materials [1, 2, 3]. Particularly the use of elliptically shaped beams produced either with cylindrical telescopes [2] or slits [3] has enabled the fabrication of waveguides with nearly circular symmetry by direct fs-laser writing. However, these beam shaping techniques rely on the use of conventional optics and are therefore not flexible enough to be easily adapted to changes of the processing conditions (different processing depths, materials with different refractive indices, or different numerical apertures). Spatial light modulators (SLM's) provide, in turn, a flexible tool for this and other applications like aberration pre-compensation [4].