Lasers
HeNe | CO₂ | Argon Ion | Excimer

For me to explain the process of dielectric coatings, the method in which the coatings are deposited should first be discussed. My experience with dielectric coatings was with our Bendix Diffusion Pump System, which employed the thermal evaporation method technique. Figure 1, is a diagram of how thermal evaporation works.

Figure 1: This diagram illustrates the thermal evaporation method
Source: Thin Film Technology, 1968

Thermal evaporation is probably the most simple way of depositing material onto a substrate. One major disadvantage of this is that a lot of material is lost in the process. Say you have 10 substrates being coated with your evaporant, there will only be a small percentage of the total evaporated material actually deposited onto your substrate. This method of evaporation is often used with materials that arent overly expensive, such as Aluminum. Thermal evaporation often uses a filament of high current is passed through (10-40 Amps depending on the filament & material), the rate of deposition can be controlled by the current being passed through the filament. Current passing through the filament turns into heat, which heats up the material to be deposited. When the substance is heated adequately, it begins to evaporate and travel through the chamber, 'sticking' to the substrate (and many other things in the chamber). The higher the vacuum, the more efficiently material will be deposited to your substrate. In a higher vacuum there are less molecules in the chamber, which will increase the 'mean free path', a longer mean free path will allow the evaporated molecules to traver further before striking an unwanted molecule in the chamber.

In the systems I used in the labs, there were several different styles of filaments (standard spiral, basket, etc.). The standard spiral filaments were used in labs that aluminum was being deposited. Something that was not included in the diagram above is the shutter that is necessary in these systems to reach an exact deposition thickness. A shutter is basically a steel plate that swings over top of the filaments in the chamber, blocking the path to the substrates. This is necessary for two reasons:

1. When you are heating up the filaments, there will most likely be some outgassing of molecules that are undesirable. These cannont be deposited on the substrate, therefore the shutter should be closed when the filament is heating up.
2. When you have deposited the correct amount of material onto the substrate, simply turning off the filament will not stop the deposition immedeatly. A more precise way of controlling the deposition is to cose the shutter just before the required deposition is reached, then turn the filament off. The filament keeps on depositiong because it stays hot for a considerable amount of time under high vacuum. This is because there are few molecules in the chamber to dissipate the heat from the filament.

Another method of depositing material onto a substrate is called sputtering. This method was studied in class, since we did not have a working sputtering system.

Now that the deposition of materials has been covered, dielectric coatings can be explained, in this page a dielectric filter will be explained.

A dielectric filter is essentially a Fabry-Perot interferometer, there are only an integral number of half-wavelengths allowed in the interferometer, as in a dielectric filter. This is illustrated in Figure 2, below.
The Fabry-Perot interferometer is a folder Michelson interferometer, with the Fabry-Perot model an interference pattern is observed through two partially reflecting mirrors. In a Michelson interferometer the interference pattern of light is observed by reflected light.

Figure 2: Illustration of a Fabry-Perot optical cavity
Source: Optoelectronics and Photonics: Principals and Practice by S.O. Kasap, 2001

There are general relationships between index of refraction of the medium and thickness that dictate how incoming light is affected by a thin film. A phase shift of 180° will occur when light strikes a lower index of refraction that the medium in which it was propagating. Conversely, if light strikes a higher index of refraction, there will be no phase change in the light. An example of this is that no phase change occurs when light strikes the first aluminum layer when approaching a dielectric filter.
When light strikes a dielectric filter, its split into two components which later recombine and add in amplitudes or subtract; when the amplitudes are added, this is constructive interference which allows that light to propagate through. On the other hand when light recombines and destructively interferes, the amplitudes subtract leaving zero output light, destructive interference occurs when one ray of light is out of phase 180° its other ray of light.

Figure 3: Physical Components of a Dielectric Filter

As shown in Figure 3 above, a dielectric consists of at least three layers, the first being of a reflective material such as Aluminum, the second of a transparent material such as cryolite, and the third of Aluminum. The dielectric layer is usually designed to be the length of one wavelength of light, the same wavelength that you wish to filter. Theoretically, and multiple of the desired wavelength (from 2x etc.) can be used, but in practice one wavelength is most commonly used. This keeps the cost down on material deposited on the substrate and makes manufacturing faster.
The two layers of aluminum on each side of the dielectric material act as a reflector which allows only one wavelength of light into the dielectric medium. The thickness of the aluminum also dictates the finesse of the filter. The finesse is how selective a filter is of wavelengths of light; for example a large finesse filter will have extremely sharp peaks at the wavelength that fits in the dielectric, this filter will sharply select a wavelength of light and appear very dim. Conversely, a small finesse filter has a much broader peak, which spans across several wavelength of light, thus making it appear brighter when viewed. In Figure 2c above, finesse is illustrated, the solid line is of a filter (or interferometer) with a large finesse compared to that of the dotted line in that same chart. It is obvious that the dotted line will allow many more wavelength of light through compared to the solid line in that diagram.
Finesse is one of the most important things when manufacturing optics. In a laser with closely spaced emission wavelengths, it may be imperative to select only one wavelength and not allow the other to oscillate. To do this a very sharp peak at a specific wavelength must be produced, which requires a very large finesse.
The amount of finesse is dependent on the thickness of the reflective (usually aluminum) layer of the filter, the lower the reflectivity the smaller the finesse, likewise, the bigger the reflectivity the larger the finesse. This, of course has practical limitations, if the reflective layer is too thick light will not enter the dielectric layer and the filter is essentially a mirror, on the other hand if the layer is too thin light will hardly reflect and pass directly through filter not selecting any specific wavelength.
The finesse of a filter can be determined using the following equation:
F = (pi) · R^1/2 / (1 - R)
where R is the reflectivity of the filter.

Below are a few examples of dielectric filters we've made:

Here are some examples of the dielectric filters I've made

As a final project in our Vacuum systems class, a handfull of students got together and deposited dielectric filters on sunglasses. We bought the cheapest sunglasses possible, which means plastic lenses! These lenses would melt easily in the chamber while being deposited on, we did, however, get some good results!

Picture looking through a finished sunglasses lens, impractical, but very cool!