Femtosecond Laser Axial Intensity Manipulation with a Liquid Crystal on Silicon Spatial Light Modulator for Dielectric Machining

Abstract

In the field of ultrafast laser processing, diffractive optical elements (DOEs) are used to form desired wave fronts that can be used to create tailored microstructures and/or to accelerate the process through parallel machining. Liquid crystal on silicon spatial light modulators (LCoS-SLMs) have significant advantages in applications such as these because they are able to achieve all the light field manipulation effects of fixed optical elements (e.g. lenses, gratings) while being much more flexible, programmable and dynamic. Generally, LCoS-SLMs are applied to laser micromachining across only two dimensions – in which the laser is focused at multiple points on a fixed, single plane that is perpendicular to the axis of laser propagation [1, 2]. There has also been implementation of altered axial beam profiles to improve machining times with such beam shapes as Bessel and Vortex beams [3, 4]. However, while threedimensional intensity patterns have been used for machining [5], their utility in fast dicing has yet to be thoroughly explored. My research project exploits the 3D spatial shaping capabilities of an LCoS-SLM to improve laser micromachining efficiency in a wide range of materials, beginning with dielectrics. This project focuses upon the phase manipulation of Gaussian, femtosecond laser pulses at a wavelength of 800 nm to create multiple focal points in multiple locations along the axis of our laser beam. A quick, effective way to create this multiple focal point intensity distribution is found by using concentric Fresnel lens phase masks. The multiple focal point configuration is then used to machine a substrate of 1 mm thick soda lime glass. It is seen that simultaneous machining of the top and bottom of a transparent dielectric is possible, and that the technique can be adapted for different materials with different refractive indices and thicknesses. We believe that this flexible machining technique will allow us to approach machining of transparent materials with unprecedented speed, and will have application in the micromachining industry for dicing of materials.