Estimation of Ultrafast Laser-Induced Stress in Fused Silica from Evaluation of Stress Birefringence

Abstract

Ultrafast laser stress figuring (ULSF) is capable of deterministically modifying low-spatial frequency height of thin mirrors, without creating higher-spatial frequency errors by generating stress using focused ultrafast laser pulses. The permanent stress field caused by sub-picosecond laser pulses varies in both profile and magnitude depending on pulse parameters and material properties, and results in stress birefringence. Ultrafast laser pulses also generate nanogratings, causing form birefringence in the modification region. The ability to visualize and quantify the birefringence from these stress fields and nanogratings allows for higher precision figuring as well as an understanding of the polarization effects caused by ULSF at high spatial frequencies. This thesis demonstrates the ability to visualize these stress fields through single shot polarization microscopy. Our procedure makes use of division of focal plane (DoFP) imaging to measure fields of birefringence surrounding laser-induced modifications created through ULSF. We do so by propagating near monochromatic circularly polarized light through a modified sample to a DoFP camera and use the subsequent intensity data to output the local stress birefringence at each pixel. We then demonstrate the creation and use of a finite element model to simulate both the form and stress birefringence generated in laser-induced modifications. We then attempt to compare experimental measurements to those generated in the finite element model of the laser-induced modification to infer the stress state in the modification itself. The proposed imaging polarimeter allows for quantification of the extent and magnitude of these stress fields, which will improve the precision of the ULSF process.