Investigation of Correction Capabilities of Ultrafast Laser Stress Figuring for Advanced Optical Fabrication

Abstract

Ultrafast laser stress figuring (ULSF), in which ultrafast laser-generated bending moments permanently deform mirror substrates, is a viable noncontact alternative to traditional optics fabrication techniques. It has been previously demonstrated to flatten 100 mm-diameter mirror substrates by ~5 μm RMS to ~10 nm RMS flatness. For significantly larger magnitude or higher spatial-frequency corrections, however, a substrate cannot be fully corrected due to limited space available in the substrate. A predictive model of the magnitudes and spatial frequencies that ULSF is capable of correcting is needed to implement ULSF for mirror substrate manufacturing. To this end, corrections of randomly generated and representative surface maps were simulated, using linear optimization of the correctable RMS height error, to understand the capabilities of ULSF correction. This thesis describes the mechanics of ULSF, the optimization process to minimize achievable height error, and the ULSF process capabilities gleaned from the simulations. Large-magnitude deformations imparted onto fused silica substrates using an optimized ULSF process are demonstrated. Finally, ULSF system changes are proposed for wider application in large optics fabrication, along with experimentally determined processing parameters when using a 0.2 numerical aperture focusing objective in a ULSF system.