Surface and bulk damage resistance of calcium fluoride optics assessed by x-ray induced color centers

A rhodium-emitted X-ray with a photon energy at 20 KeV was employed to irradiate CaF₂ with different surface finishing. Color centers associated with the irradiation damage were determined by spectral measurements. The depth distribution of the color centers was calculated and confirmed in a wedge spot generated by magnetorheological finishing. The results suggest that the color center formation is directly associated with the damage resistance of the optics. Laser durable grade CaF₂ demonstrates higher damage resistance to the X-ray irradiation when compared to excimer ArF laser grade CaF₂. Subsurface-damage-free CaF₂ surfaces further enhance the damage resistance.