|
The components used with high power lasers are frequently exposed to high thermal loads, which can cause a variety of detrimental effects including thermal deformation, which can lead to aberrations, and contamination-induced heating, which can lead to catastrophic component failure. In this presentation, we discuss several topics in thermal design for high power laser components. The first involves dirt or contamination. When a dirty optic is hit with high power laser light, the contamination absorbs the light very efficiently, sending the surface temperature to thousands of degrees. In order to prevent the surface from creating large numbers of free carriers due to the extreme temperature, the materials near the surface must have a very large bandgap. Indeed, we have shown that different materials can display more than an order of magnitude different CW (continuous wave) laser damage thresholds. Contamination-resistant optics must be designed solely of high bandgap materials. A second topic involves thermal mismatch in optical coatings. Thermal expansion mismatch between layers in a coating or between the coating and its substrate can deform an optical component, but such deformation can be eliminated at the coating design stage by optimizing the mechanical parameters of the coating in tandem with the optical properties. To first order, this is a matter of incorporating layer thickness, elastic modulus, thermal expansion, and intrinsic stress levels into a mechanical multilayer model that ensures flatness across a wide range of temperatures. Additional topics build on the first two. It has been found that there can be a significant reduction in film stress as the density of a thin film is reduced, and there seems to be a threshold for sharp transitions at a certain density level. These characteristics can be used to build low-stress contamination-resistant high reflectivity coatings from a single material with a high bandgap, such as SiO2. Finally, since optical elements are so sensitive to thermal deformation via changes in optical path length (shape change and refractive index), cavities formed by such elements can be used a probe of temperature variation or indeed other processes that change local refractive index, such as turbulence.
|
