Tech Notes » Coatings Tutorial
General Coatings Considerations
Transmit, Reflect, Absorb, and Scatter ("T+R+A+S")
Anyone dealing with the design, manufacture, or application of optical components (including optical coatings) must consider the following equation:
T+R+A+S = 100%
This means that when an incident light source hits an optic, it can do four things: transmit (T), reflect (R), absorb (A), or scatter (S). This equation is the cornerstone of all RMI coatings and is the first step taken by our engineers when developing a coating solution
There are several basic coating types that clearly illustrate this formula. In an anti-reflection coating, the goal is to maximize "T" while minimizing "R+A+S". RMI's anti-reflection coatings can reach "T" specifications of 99.9%, keeping "R+A+S" to under 0.1%. For a mirror coating (Rmax), the goal is to maximize "R" while minimizing "T+A+S". For partial reflectors and beamsplitters, the goal is to achieve the desired T/R value while minimizing "A+S".
RMI's Techniques and Offerings
There are a number of techniques employed by RMI engineers to control the outcome of "TRAS". They include:
1) Selecting the right coating chemicals for both low absorption and optimal reflective index
2) Minimizing the number of coating layers on a given surface since scattering is directly related to the thickness of the coating
3) Selecting the optimal coating method to meet the particular customer requirement.
Below is a list of popular specialized coatings provided by RMI:
• Anti Reflection
• High Reflection
• Partial Reflection
• Metal Mirrors
• Phase Retarding
• Dichroic Filters (Short-Wave Pass / Long-Wave Pass)
Dielectric vs. Metal Coatings
RMI manufactures two types of mirror coatings: dielectric and metal. Dielectric coatings are constructed by layering multiple dielectric (non-conductive) chemicals in a formulated sequence. Some of the most complex dielectric coatings can have more than 100 layers of two or more coating chemicals. In general, dielectric coatings are harder, have higher reflectivity and higher damage thresholds than metal coatings; particularly for shorter wavelengths (UV and visible). Dielectric coatings are ideal for high power laser applications where high damage threshold and maximum reflectivity (R ≥ 99%) are desired for a single wavelength (i.e. 1064nm YAG lasers and Excimer lasers).
Metal coatings, in contrast, are constructed by putting a single layer of metal (aluminum, silver, gold) onto the substrate. Although metal coatings generally have lower reflectivity and lower damage thresholds than dielectric coatings, they have the benefit of providing good reflectivity (often ≥ 90%) over a wide range of wavelengths (UV, VIS, and IR). This is particularly true as the wavelengths become longer. These attributes qualify metal coatings as a simple solution for low-power UV and visible applications where absorption damage is not a critical factor. Additionally, metal-coated mirrors have the added benefit of being good conductors of heat while maintaining high reflectivity in the IR region. This makes metal-coated mirrors ideal for all types of IR applications, including high-power CO2 lasers where high damage thresholds are required.
UV, VIS/NIR, and IR Coating Considerations
There are some inherent challenges associated with coating UV optics. In general, the shorter wavelengths of ultraviolet light increase the scattering and absorption effect of optics and coatings. Also, there are a very limited number of suitable coating chemcials because many chemicals do not transmit well in the UV region (high absorption), and all materials have short wavelength cutoff (where "T"→ 0) at various points along the UV spectrum. While the best coating chemicals may still have some absorption to start off with, the chellenge has not halted the growth of new UV applications. In addition to being a shorter wavelength, ultraviolet light carries no heat and has higher interaction with the materials with which it comes into contact. These characteristics have made it popular in medical applications including LASIK eye surgery, in micro-machining, in IC lithography, and in diamond/glass scribing, etc.
VIS/NIR Coating Considerations:
The first optical devices operated in the visible spectrum due to the obvious nature of visible light. Even today, visible optics make up the largest portion of all optical applications and there are a wide variety of proven coating chemicals, as well as well-established coating techniques. The relative maturity of visible optics has driven the development of many complex optical devices with widely varying functions where demanding technology is required.
The infrared region is a relatively new area in optics and coatings. In recent years, however, some of the key IR materials such as Zinc Selenide, Zinc Sulfide, and Silicon have become readily available at reasonable prices. With a number of new developments have come new, highly sophisticated applications for the IR region that have increased the need for complex coatings solutions. The most common applications are CO2 laser welding and cutting (10.64 micron), and FLIR/Thermal Imaging (Atmospheric Transmission Regions: 3–5 microns and 8–12 microns).
As in the case of UV optics, there is a limited selection of coating chemicals that work with IR optics since all materials have a long-wavelength cut-off (where"T" → 0) at various points along the IR region. In general, RMI regularly works with about six optical substrates and another sic coating chemicals.