Towards Freeform Reflective Fused Silica Optics using Ultrafast Laser-Assisted Etching

While conventional methods like diamond turning can achieve the necessary precision for manufacturing image slicers, they are often expensive in cost and time, and restrictive in terms of the materials to which they can be readily applied. Ultrafast laser-assisted etching (ULAE) is an emerging manufacturing technology that could potentially be used to manufacture free form reflective optics using fused silica, as it enables µm-level precision shaping of fused silica over several millimetres scales. Here we demonstrate the potential of ULAE for manufacturing fused silica image slicers by fabricating precision 8 × 1 mm flat fused silica surfaces using ULAE. The waviness meets the required level for this application, staying below 1 µm. Specifically, we measured S_10z = 0.164 µm. The roughness varies with surface inclination; for a flat surface at 0^◦ inclination, we measured S_q = 109 nm, while at a 5^◦ inclination, it increased to S_q = 204 nm. If combined with a suitable polishing technique to remove the high spatial frequency roughness, we believe this work demonstrates that ULAE provides a new route to manufacture freeform reflective monolithic fused silica optics such as image slicers for ground- and space-based applications.