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Stress fields imparted with an ultrafast laser can correct low spatial frequency surface figure error of mirrors through ultrafast laser stress figuring (ULSF): the formation of nanograting structures within the bulk substrate generates localized stress, creating bending moments that equilibrize via wafer deformation. For ULSF to be used as an optical figuring process, the ultrafast laser generated stress must be effectively permanent or risk unwanted figure drift. Two isochronal annealing experiments were performed to measure ultrafast laser-generated stress stability in fused silica and Corning ultra-low expansion (ULE) wafers. The first experiment tracked changes to induced astigmatism up to 1000 °C on 25.4 mm-diameter wafers. Only small changes were measured after each thermal cycle up to 500 °C for both materials, but significant changes were observed at higher temperatures. The second experiment tracked stress changes in fused silica and ULE up to 500 °C but with 4 to 16× higher signal-to-noise ratio. Change in trefoil on 100 mm-diameter wafers was measured, and the induced stress in fused silica and ULE was found to be stable after thermal cycling up to 300 °C and 200 °C, respectively, with larger changes at higher temperatures. 

Although stress is a critical factor in many ultrafast laser-based fabrication techniques, its relationship to different laser operating parameters remains poorly understood. Here, we investigate the stress landscape within fused silica generated by ultrafast laser pulses, with repetition rates of 100???900?kHzand pulse energies of 200???4000?nJ, by measuring all three components of the stress-induced curvature change of fused silica plates. We find that for all repetition rates, there is an inflection in the stress when the average laser power is about 60?mW, and this inflection is not correlated with the morphological transition from nanogratings to melting, as observed from cross-section imaging. The equibiaxial and antibiaxial components of stress exhibit a characteristic average ratio of about ?1.65 up until the visually observed onset of melting within the modifications, which occurs when the average laser power is about 423?mW. We conclude that nanogratings produce a characteristic stress state, with a maximum magnitude that is reached at lower pulse energy than nanograting erasure. Beyond nanograting erasure, the stress state is more variable and distinct from the nanograting-induced stress state. 

Fabricating freeform mirrors relies on accurate optical figuring processes capable of arbitrarily modifying low-spatial frequency height without creating higher-spatial frequency errors. We present a scalable process to accurately figure thin mirrors using stress generated by a focused ultrafast laser. We applied ultrafast laser stress figuring (ULSF) to four thin fused silica mirrors to correct them to 10-20 nm RMS over 28 Zernike terms, in 2-3 iterations, without significantly affecting higher-frequency errors. We measured the mirrors over a month and found that dielectric-coated mirrors were stable but stability of aluminum-coated mirrors was inconclusive. The accuracy and throughput for ULSF is on par with existing deterministic figuring processes, yet ULSF doesn’t significantly affect mid-spatial frequency errors, can be applied after mirror coating, and can scale to higher throughput using mature laser processing technologies. ULSF offers new potential to rapidly and accurately shape freeform mirrors.