Optical Coating 101

by | Jul 15, 2022 | Optics explained

Introduction to optical coating

Optical coatings are used to enhance the transmission, reflection or polarisation properties, and are composed of thin layers of oxides, rare earth materials, metals, and other metallic and dielectric materials. The performance of the coating is dependent on the thickness of the coat, number of layers and their refractive index.

Coatings have been prominent within the optics industry since 1935. Invented and patented by Alexander Smakula, the first procedure was a single-layer antireflection coat. Traditionally, Smakula created a process which includes vacuum technology to apply a fluoride compound to a glass lens, resulting in adverse reflection. Since then, more production techniques have been created and the methods of coating optical components have been improved and modified to not only be more effective, but cost less.

Coating processes

Some of the types of vacuum coating technologies that Addoptics offers are; evaporation deposition, ion-beam sputtering, and advanced plasma reactive sputtering.

Evaporation deposition was the first method created for optical coating. This is a thermal process in which source materials are changed into a vapour state by either resistance heating or electron-beam bombardment inside of vacuum chambers. Holders are then rotated for coating uniformity, so that the evaporated materials can be condensed and adhered to substrates.

Ion – beam sputtering is a technique which is used for high-precision theoretical designs as it has impressive accuracy. The process includes an ion gun to produce energetic ions, these are then accelerated up to tens of eV by an electric field. There is then a transfer of kinetic energy to create a stream of collisions that sputters material to the targeted surface. The sputtered particles can either fly from the target to energetically impact the substrate resulting in deposition or, the particles can collide with the gas atoms in the chamber to thermally condense on the substrate, if the particles are at a higher gas pressure.

Similar to ion-beam sputtering, advanced plasma reactive sputtering uses mid – frequency dual – magnetrons to achieve consistent and reliable depositions with thin-film optical characteristics. There is a reactive gas introduced into the sputtering chamber, which causes a chemical reaction that the particles undergo which enables the production of oxide and nitride films.

These processes create coatings to serve a purpose on the optics. There are hard coatings to protect the optic, anti-reflective and reflective coatings available. 

Types of coatings

Hard coatings

These coatings can also be called protective coatings, as that is their main purpose. They are usually applied to polymer optics, which are not machined, moulded or diamond-turned as these optics can be more prone to scratched, abrasions, and chemical attacks. However, by applying this type of coating these issues can be solved without interfering with the integrity of the optic. Hard coatings can pass ASTM D1044 test Taber Abrasion (Δ % Haze) with levels of <7 after 500 revolutions. They will also hold up against chemical attacks from sodium hydroxide, 1% solutions of hydrochloric acid, and acetone, to name a few.

Reflective coatings

Reflective coatings are used to minimise the loss while reflecting light sources, and lasers. Once coated, during reflection, absorption and scattering can cause decreased throughput or even potential laser-induced damage. The coatings help in many applications where light reflection is needed such as within laser optic applications such as laser beam path manipulation and laser cavity end mirrors.

Addoptics commonly uses aluminium (AI) combined with Sio2 (serving as a protective layer). Silver and gold coatings are also a possibility. Metallic surfaces can reflect light due to loosely attached electrons which freely oscillate with incident light waves, without much hindrance. However, all metals will absorb some amount of incident light. As such, metallic mirror coatings are susceptible to damage when using high power lasers.

Anti- Reflective coatings

This type of coating creates an optimal lens performance by immensely improving the efficiency of the optic by enhancing contrast, increasing transmission and eliminating ghost images. Anti-reflection coatings are resistant to physical and environmental coatings which highly improves the quality and durability of the optic. As such, AR coatings are an optimal choice for transmissive optics.

Coating your custom optics

Addoptics offers smart manufacturing of optics as a service. We can provide custom optics within a couple of days. Without high investment costs, expensive tooling, or a minimum order quantity. For optical engineers, our service enables you to experience design freedom, while remaining within budget. Explore new possibilities with our custom optics in combination with optical coatings.

Contact us here, we are always happy to help with any challenges or questions you have. 

Sources

https://www.nacl.com/industrial-optical/dip-applied-coating/

https://www.edmundoptics.com/globalassets/knowledge-center/articles/optical-coating-technology-and-applications-past-and-present-to-future-en.pdf

https://www.edmundoptics.com/knowledge-center/application-notes/lasers/an-introduction-to-optical-coatings