Material Properties and Window Design Constraints

Calcium fluoride has a transmission bandwidth ranging from 180 nm to 8.0 μm, with a refractive index of approximately 1.43286 at 632.8 nm and an extinction coefficient close to zero. The in-crystal transmittance can exceed 99.8%/cm at 248 nm and exceeds 99.6% at 193 nm. Behind these optical advantages lie inherent processing limitations: calcium fluoride has a Mohs hardness of only about 4, with a (111) cleavage plane and low fracture toughness, making it extremely prone to cleavage fractures or subsurface microcracks during grinding and polishing. Research data indicates that the deep, coarse scratches left on the calcium fluoride surface by conventional polishing abrasives are the primary cause of laser-induced damage, directly limiting the service life of the components. Therefore, the process design for flat windows cannot simply replicate glass processing parameters; instead, a balance must be struck between low-stress material removal and surface integrity.

Polishing Reaction Layer: A Hidden Source of Transmittance Loss

For uncoated, double-sided polished calcium fluoride flat plates, transmittance loss due to Fresnel reflection alone is approximately 8%. However, an even more challenging issue lies in the chemical reaction layer introduced during the polishing process. When using traditional cerium dioxide polishing slurries, the slurry reacts with the calcium fluoride surface to form an amorphous residue layer or one with a deviated stoichiometric ratio; this reaction layer exhibits significant absorption in the ultraviolet (UV) spectrum. Academic research has confirmed that cerium oxide soft abrasives tend to adhere to the component surface after reacting with calcium fluoride, causing absorption peaks in the UV spectrum. The core of the process solution to this problem lies in adding a controlled acidic cleaning step after polishing. A brief immersion in dilute nitric acid or dilute acetic acid can effectively strip away the reaction layer and compensate for the loss of surface fluorine; however, the permissible concentration and time window are extremely narrow, and excessive etching can actually worsen surface roughness. In the mass production of calcium fluoride windows, Yutai Optics manages the matching of the polishing solution system and the cleaning formulation as independent process variables, rather than relying on a generic glass cleaning process.