An optical coating is one or more thin layers of material deposited on an optical component such as a lens or mirror, which alters the way in which the optic reflects and transmits light. The most common types of coatings are anti-reflection (AR), high-reflective (HR), beamsplitter, and filter. AR coating reduces unwanted reflections from surfaces, and is commonly used on spectacle and photographic lenses. HR coating can be used to produce mirrors which reflect greater than 99.99% of the light which falls on them
Different materials are necessary when operating within certain wavelength ranges of the electromagnetic spectrum such as the ultraviolet (UV) or the visible/near-infrared (VIS-NIR) regions. Different materials are also used depending on whether or not the application calls for high-power continuous or high-power pulse irradiation. For example, continuous-wave (CW) laserscan cause an optical coating to heat up and melt, whereas short-pulse lasers can generate high-intensity electromagnetic fields.
Unfortunately, coating designers are limited by the number of materials suited for high-power applications. For example, high-reflective mirror coatings are created by alternating quarter-wavelength thick layers of high-index and low-index materials. The design of this stack of materials can significantly alter the laser-induced damage threshold (LIDT) of the coating. For instance, simply adding a half-wave thick layer of low-index material can drastically increase the LIDT. When choosing the appropriate low-index and high-index materials, coating technicians favor dielectric metal oxides because of their low absorptivity. Silicon dioxide (SiO2) is the generally accepted and ubiquitous choice for low-index layers; however, choosing a material for the high-index layers is not as straightforward: oxides of titanium, tantalum, zirconium, hafnium, scandium, and niobium are all popular choices.
A clean coating chamber, appropriate choice of thin-film materials, and good control of process parameters are also essential. After deposition, coating technicians must carefully control contamination, which may lead to the formation of failure-inducing absorption sites. For this reason, meticulous cleaning procedures are also required at the assembly stage, typically under strict clean-room operating conditions.
This sensitivity to organic or particulate residue presents a very real challenge to coating technicians and underscores the importance of a thorough cleaning process. Clean-rooms are essential for minimizing the risk of recontamination after cleaning the optical element to be coated. Most manufacturers use lint-free wipes without silicone constituents when manually cleaning in their final clean process. Additionally, they use solvents of extremely high purity - typically, methanol, isopropanol, or acetone. Ultrasonic cleaning can be another useful tool as it is more effective (and less error-prone) at dislodging residual polishing compounds than cleaning by hand.
A typical multi-stage manual process includes a surfactant wash and several wipes with an ammonia solution followed by a drag-wipe technique. This drag-wipe stage produces very high shear forces, resulting in the removal of any remaining contaminants from the material’s surface.
Contaminants from the tools and walls of the coating chamber itself can also contribute to contamination of the substrate to be coated. For example, backstreaming can occur with an inefficient diffusion pump, resulting in organic contamination. Meanwhile, if excess material on the chamber walls is not removed prior to deposition, it can begin to flake off and loose particulates can be transferred onto the optical element. Preventing this is as easy as lining the walls with foil and periodically replacing it when it begins to exhibit excess material buildup.