Impact of Nanograting Direction on Ultrafast Laser Stress Generation in Fused Silica Glass

Abstract

Ultrafast laser stress figuring (ULSF) is able to deterministically correct low spatial frequencyerrors without imparting mid-spatial frequency errors. This process uses ultrafast laser pulses to create modifications within a transparent substrate. The modifications generate stress within the bulk substrate that creates a bending moment used to alter the shape of the substrate. ULSF can be applied to both flat and curved optics. Polarization plays a large role in the figuring process. Nanograting modifications are generated with moderate pulse energy and low repetition rate of the ultrafast laser and form perpendicular to the laser polarization direction. This thesis discusses a characterization of the relationship between the polarization state of the laser and the stress generated using ULSF. A calibration system is created to determine the output polarization state of the laser. This calibration routine discovered that the previous system had elliptical polarization which was remedied to create linear polarization. The stress generated by nanograting modifications was characterized by rotating the polarization state from perpendicular to the writing direction to parallel to the writing direction. The stress generated tends toward zero as the polarization is rotated away from orthogonal for nanograting modifications. An initial model of two independent stress states, one stationary and one rotating, is presented with this parameter study which was found to not describe the observed data. Additionally, initial results from performing ULSF on curved substrates, the next step in applying this technology, show that ULSF can induce deformation in curved substrates.