Freeform off-axis three-mirror anastigmatic system for deep ultraviolet femtosecond laser processing

Deep ultraviolet (DUV) femtosecond lasers combine high single-photon energy with high peak power, enabling exceptional spatial resolution and minimal thermal damage to surrounding material during processing. In such systems, the focusing optics not only concentrate the laser beam but also facilitate real-time monitoring in the visible spectrum. Reflective optical systems, known for their broad chromatic correction, are particularly advantageous for applications spanning a wide wavelength range. This paper presents the design and implementation of a freeform off-axis three-mirror anastigmatic (TMA) system specifically developed for DUV femtosecond laser processing. Based on nodal aberration theory and structural constraints, an automated procedure was presented to generate the initial TMA configuration. Subsequent optimization with freeform surfaces achieved diffraction-limited imaging performance. This work represents the first implementation, to our knowledge, of a freeform off-axis TMA system in DUV femtosecond laser processing. Experiments demonstrated the system's strong potential for realizing real-time monitoring and nanoscale fabrication with feature sizes below 100 nm. Additionally, ablation craters with atomic-scale depth were successfully achieved.