Many optical how to align laser mirrors structures require mirrors that push the bounds of manufacturability and supply near-best reflectivity and pondered-wavefront control. Consequently, it’s regularly effective for optical machine designers, or all of us choosing mirrors for steerage lasers on an optical bench, to interact optical coating engineers early withinside the layout process. After all, optimizing machine overall performance calls for know-how the tradeoffs among replicate reflectivity, wavefront, weight, thermomechanical overall performance, laser harm threshold, and cost.
Many laser mirror are Bragg reflectors that take benefit of more than one Fresnel reflections and optical interference to expand reflectivity through a multilayer dielectric stack of alternating excessive- and low-index-of-refraction skinny films. Such mirrors are commonly synthetic the usage of vacuum bodily vapor deposition (PVD) strategies which include evaporation and sputtering. As wavelength decreases, movie losses because of scatter and absorption increase. Understanding how segment affects reflectivity is likewise essential. It is defined through the Fresnel equations that, because the perspective of mild incident on a replicate floor will increase, the reflectivity of s-polarized mild at every skinny-movie boundary will increase and the reflectivity of p-polarized mild decreases as much as Brewster’s perspective.
Achieving the identical theoretical reflectivity at excessive angles calls for extra layers to laser mirror p-polarized mild than s-polarized mild. This has repercussions for each replicate loss and pondered wavefront. At excessive angles, p-polarized mild spends extra time withinside the replicate stack than s-polarized mild, ensuing in more losses because of absorption and scatter and decrease typical reflectivity. For excessive-quality, low-loss IBS films, this distinction in reflectivity is small—commonly measured in elements in keeping with million. For evaporated near-IR mirrors, the distinction in reflectivity among s- and p-polarized mild may be as excessive as numerous tenths of a percentage at excessive angles. For excessive-overall performance mirrors, the size approach is as essential because the deposition approach. A well-calibrated business spectrophotometer is able to measuring the pondered overall performance of a replicate to roughly ±0.2 %R. To gain higher precision, custom laser-primarily based totally metrology answers are used.