Precision optical components such as lenses or mirrors are almost always provided with an optical coating. The applied layers have different effects, such as the enhancement of transmission of certain wavelengths or the reduction of reflection. However, an optical coating is not only used in optics and technology sectors, but also in everyday objects such as glasses, displays or on window panes to reduce energy losses.
In our two-part article series on this subject, we present various optical coatings with their fields of application as well as modern coating technologies. Learn more about selected coating options below.
What is an optical coating?
Coating is the process by which a material is applied as a thin, firmly adhering layer to a base material. In optical industry, dielectric and metallic materials are used for this purpose. An optical coating can control different properties of light by optical interference. This includes influencing the degree of reflection, transmission and polarization as well as the change in phase of the light. The behavior of an optical coating depends on the number and thickness as well as the difference in refractive index between the single layers.
Thin films with great effect – coating possibilities
The most common optical coatings are:
• Metal optical coating (mirrors)
• Dielectric mirrors
• Beam splitter coating
• Optical filters and
• Anti-reflective coatings (AR)
Reflective coatings or metallic mirrors refer to the mirroring of surfaces based on metals (e. g. Al, Ag, Au). The mirrors have a very wide wavelength range with an almost constant reflectivity and are, therefore, a cost-optimized process for the production of reflectors. Due to the low mechanical and chemical resistance of the metal layers, it is beneficial to protect them with dielectric layers, which often increases the reflection too. These simple mirrors are commonly used for beam guidance in optical systems.
Using dielectric coating materials, absorption-minimized mirrors can be produced, which are used, for example, as deflecting mirrors or pump laser mirrors in laser resonators. For certain wavelength ranges, these layers achieve reflectances higher than 99.9%.
Coatings for beam splitters divide incoming light into a reflected and transmitted part, which is necessary e. g. for Michelson interferometers. Based on thermally very stable dielectric layers, customer-specific partial ratios can be realized. Typically, these are 50%/50% (R/T) or 30%/70% (R/T). A wavelength-specific coating of beam splitters is also possible (e. g. red reflected and green transmitted).
Filter coatings are used in many different industrial applications, e. g. in fluorescence microscopy. This type of optical coating is used to transmit, reflect, absorb or attenuate light of certain wavelengths. The use of dielectric layers makes it possible to separate different wavelengths as short or long pass filters. Short pass filters transmit short wavelengths while long wavelengths are reflected. Long pass filters allow long wavelengths to pass through and reflect the short wavelengths. Filter coatings are used, among other things, to separate the pump radiation from the laser radiation. For example, a crystal with a laser diode emitting 940 nm is optically pumped in the laser resonator. This light passes through two filters in the resonator, reflecting the radiation at 940 nm from one to the other. The crystal generates laser radiation at 1064 nm. This radiation has to be decoupled by filters, which reflect at 940 nm. This means that these filters have to have a high transmission at 1064 nm.
Anti-reflective coatings are essential for maximum optical transmission in many applications. A distinction can be made between single layer, broadband optical coating or anti-reflective coating especially for laser applications. Depending on the glass used, a residual reflection of < 0.1% can be achieved. The main field of application for anti-reflection coating is to minimize reflection losses in the beam guidance of optical systems by the coating of all optical surfaces. When light passes through an interface, part of the light is reflected, depending on the substrates. This reflected light is no longer contained in the transmitted light. The losses add up with the number of interfaces in the optical system. To keep these losses as low as possible, antireflective coating has to be applied.
Depending on the properties of the materials and the requirements of the optical systems different processes are used to apply the presented optical coatings. We will soon be posting a separate blog article on coating technologies.