Optical coating techniques have a wide range of important applications; for example, the development of eyeglasses for people with light-sensitive eyes and the improvement of camera lenses’ imaging capabilities.
This article explores the fundamental principles of optical coatings, looking at what they are and how they work, as well as outlining the different technologies involved in their manufacture.
Image Credit: Avantier Inc.
What is an Optical Coating?
An optical coating is a layer of extremely thin film applied to an optical component’s surface in order to enhance transmission, modify its reflecting qualities, or adjust the polarization of light emitted by the component in question.
Typical optical coatings may be as basic as a sheet of aluminum or another metal or as complex as a dielectric coating comprised of several thin layers of material, possessing precisely regulated parameters governing the coating’s thickness, composition, and number of layers.
Since 1935, coatings have played a significant role in advancing the optical industry. The original technique, patented by Alexander Smakula, involved applying a single layer of anti-reflection coating. From the outset, Smakula utilized vacuum technology to infuse a fluoride component into glass lenses, a process that, while innovative, also led to unwanted reflections.
Since then, new production techniques have been established, and the coating operations for optical components have been refined and altered to become more productive and affordable.
Types of Optical Coatings
Different types of optical coatings exist, and these each function differently to produce specific results based on use.
Filter Coatings
Optical filter coatings can improve or degrade an image’s appearance by applying one or more thin layers to a substrate. These coatings are also able to reflect specific wavelengths during image transmission.
Beam Splitter Coatings
A beam splitter coating is able to divide a single light beam into two separate beams. This division typically exhibits a transmission-to-reflectance ratio of equal or varying value, and in some scenarios, this coating can be used to manage multiple beam pathways, recombine routes, and control the combined beams’ polarization.
High-Reflective Coating
A high-reflective (HR) coating can be placed on a surface to reflect either all or part of the light that reaches it.
For example, an uncoated glass optic would have a reflection of 4 %, while metal coatings will fundamentally change the features of the optic. An aluminum coating applied to the same optic will result in a reflection of approximately 88-92 % of visible light.
Used in conjunction with a suitable dielectric coating, silver coatings have the potential to increase reflectivity in the far infrared band to 99.9 %. It is important to note that in this case, reflection will be lower in the UV and some visible spectrum regions.
Dielectric mirrors frequently utilize coatings comprised of two distinct repeating layers - one layer with a high index (such as TiO2 or ZnS) and one with a low index. These levels are generally divided by an intermediate layer (such as MgF or SiO2).
Dielectric coatings have the potential to offer a reflectance that is exceedingly high over the band stop (a comparatively narrow range of wavelengths).
HR coatings are central to the operation of laser equipment because these coatings reduce the amount of laser power required in many widely used optical applications.
Anti-Reflective Coating
Anti-reflection coatings – also known as antiglare coatings – are often applied to the surfaces of lenses and other optical elements in order to minimize reflection.
Less reflection results in a significant increase in the quality of captured images. For this reason, tools such as binoculars, telescopes, cameras, and microscopes benefit from applying anti-reflective coatings to their surfaces.
The elimination of stray light improves visual contrast, resulting in a decreased amount of light reflection. This is the underlying principle by which anti-reflective coatings work.
Opticians can apply an anti-reflective coating to eyeglass lenses, which helps ensure that the faces of those wearing the glasses are visible to others. Additionally, this coating effectively minimizes the glare from sources like binoculars or telescopic lights, which are often used by covert observers.
Coating Technologies
A number of well-known processes can be used to apply optical coatings, including physical vapor deposition. Each of these processes offers distinct benefits, potentially making each one an ideal choice for a set of use cases.
Some processes may be suitable for more than one use case, but none of them is best suited for every single application.
The following are the optical coating technologies that are most commonly used.
Ion-Assisted Electron-Bean (IAD E-Beam) Evaporative Deposition
Electron-Bean Assisted Ionization (IAD E-Beam) Evaporative Deposition involves using an electron gun to bombard and vaporize source materials in a vacuum chamber.
This technique offers more flexibility in terms of coating design than any other method presented here. It is capable of employing the widest variety of materials while being the most cost-effective of the processes. This technique is also able to work with considerably larger sizes of coating chambers.
IAD E-Beam Evaporative Deposition represents the most effective option for applications prioritizing a low-cost coating option with great flexibility rather than focusing solely on performance.
Ion Beam Sputtering (IBS)
Ion Beam Sputtering (IBS) is a highly repeatable technique that results in excellent optical quality and stability coatings.
IBS processes allow users to accurately monitor and control parameters such as layer development rate, oxidation level, and energy input. The ability to manage these process parameters allows extremely reproducible coatings to be generated and is central to IBS’ capacity used to produce exceptionally durable coatings.
IBS also offers improved resistance to the effects of temperature and humidity versus similar technologies.
There are some negatives to consider with IBS, however. For example, IBS coatings have a significantly higher cost when compared to the other techniques discussed in this article. They are also hindered by slower growth rates, reduced chamber sizes, and an increased tendency towards UV spectrum stress and loss.
Plasma-Assisted Reactive Magnetron Sputtering (PARMS)
During the process of PARMS, a glow discharge plasma is used to accelerate positive ions onto the target. A magnetic field is used to restrict the plasma to an area surrounding the target, causing atoms to be ejected and then spread out to cover the optical surface.
This procedure offers smooth functionality even when used with low chamber pressure, so there are no additional setup requirements before commencing the coating procedure.
PARMS is somewhat less repeatable than IBS, but it does offer relatively high throughput and is significantly more repeatable than evaporative deposition. The coatings of films applied via PARMS are typically tough and dense.
PARMS offers a good balance between optical performance and volume throughput, making it ideally suited for application as a fluorescent optical filter.
Advanced Plasma Sputtering (APS)
Advanced Plasma Sputtering (APS) is another method of evaporative deposition. APS leverages the same underlying principle as IAD E-Beam, but this method benefits from more sophisticated forms of automation during the processing stage.
APS can be used to apply smooth, dense, and hard coatings, all of which offer more consistent optical qualities than those created using the IAD E-Beam technique.
APS and magnetron sputtering can be regarded as intermediate solutions for several characteristics that vary between IBS and IAD E-Beam evaporative deposition.
Conclusion
Developments in technology have led to a noteworthy increase in the market for optical coatings, not least because of a marked global rise in the consumption of electronic goods and semiconductors.
Optical coatings remain a vital part of many of the products and devices making up this market, with various applications from use in eyesglasses to use in lenses.
Acknowledgments
Produced from materials originally authored by Avantier Inc.
This information has been sourced, reviewed and adapted from materials provided by Avantier Inc.
For more information on this source, please visit Avantier Inc.