OSC Dissertation Defense: Kevin Laverty

Thin and lightweight mirrors are important in precision optical systems such as spaceborne telescopes and x-ray observatories. Their reduced stiffness, however, makes them more susceptible to deformation, allowing surface errors to develop during the fabrication, coating, and mounting processes. In particular, coating and mounting can introduce errors after the optical surface has already undergone its final figuring step. Since conventional figuring techniques rely on removing material from the optical surface to produce the desired figure, they generally cannot be used to correct these errors without disturbing the finished optical surface or assembly.

 

Stress-based figuring techniques offer an alternative approach, in which applied stresses are used to controllably deform the substrate and produce changes in figure without requiring additional material removal from the optical surface. Ultrafast laser stress figuring (ULSF) is one such technique. ULSF uses focused sub-picosecond laser pulses to produce subsurface modifications within a substrate. The residual stresses associated with these modifications produce controlled bending of the substrate, allowing the surface figure to be altered without removing material. ULSF therefore offers a potential method for correcting mirrors after they have been polished, coated, and potentially mounted.

 

This dissertation investigates the development and application of ULSF across several substrate materials and geometries. First, the ultrafast-laser-induced stress responses of fused silica, ultra-low expansion (ULE) glass, Eagle XG glass, and c-plane sapphire are characterized across a broad range of processing conditions. Fused silica is then investigated in greater detail, revealing systematic changes in the magnitude and composition of the generated stress across the processing landscape. The ULSF process is then developed and demonstrated through the deterministic figure correction of a 100 mm diameter, 0.585 mm thick c-plane sapphire wafer, resulting in a 96% reduction in figure error over three iterations. The limits of first-pass correction accuracy are then investigated by evaluating the influence of several process and substrate errors on the expected first-pass correction of a fused silica wafer. Finally, ULSF is extended from flat plates to curved shells. The process is demonstrated on 100 mm diameter, 2 mm thick spherical fused silica shells with a 500 mm radius of curvature (F/2.5). The low-order surface figure errors of one uncoated shell are corrected, while a second shell with a protected aluminum coating is figured from its initial spherical shape into an off-axis parabola-like figure with a corresponding off-axis distance of 55 mm. These results establish ULSF as a versatile process for deterministic, noncontact figure correction across a range of substrate materials and geometries.

Monday, August 17, 2026, 2:00 pm – 5:00 pm, in Meinel 821. Please email Kevin at ( kevinlaverty@arizona.edu  ) or graduate student advisor Jini Kandyil (jini@optics.arizona.edu) for the Zoom link.