Optical Coatings
Optical Coatings
Optical Coatings
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Fundamental Optics<br />
Material Properties <strong>Optical</strong> Specifications Gaussian Beam Optics<br />
<strong>Optical</strong> <strong>Coatings</strong><br />
Multilayer Antireflection <strong>Coatings</strong><br />
Previously, we discussed basic principles of thin-film design and<br />
operation for a simple antireflection coating of magnesium fluoride.<br />
It is useful to discuss to also discuss layer antireflection coatings in<br />
order to understand the operation of multilayer coatings. It is beyond<br />
the scope of this chapter to cover all aspects of modern thin-film<br />
design and operation; however, it is hoped that this section will provide<br />
the reader with insight into thin films that will be useful when<br />
considering system designs and specifying cost-effective coatings.<br />
Two basic types of antireflection coating have been developed<br />
that are worth examining in detail: the quarter/quarter coating and<br />
the multilayer broadband coating.<br />
THE QUARTER/QUARTER COATING<br />
This coating is used as an alternative to the single-layer<br />
antireflection coating. It was developed because of the lack of suitable<br />
materials available to improve the performance of single-layer<br />
coatings. The basic problem of a single-layer antireflection coating<br />
is that the refractive index of the coating material is too high,<br />
resulting in too strong a reflection from the first surface which cannot<br />
be completely canceled by interference of the weaker reflection<br />
from the substrate surface. In a two-layer coating, the first reflection<br />
is canceled by interference with two weaker reflections.<br />
A quarter/quarter coating consists of two layers, both of which<br />
have an optical thickness of a quarter wave at the wavelength of interest.<br />
The outer layer is made of a low-refractive-index material, and the<br />
inner layer is made of a high-refractive-index material (compared to<br />
the substrate). As figure 5.14 shows, the second and third reflections<br />
are both exactly 180 degrees out of phase with the first reflection.<br />
As with any multilayer coating, performance and design are<br />
calculated in terms of relative amplitudes and phases which are<br />
then summed to give the overall (net) amplitude of the reflected<br />
beam. The overall amplitude is then squared to give the intensity.<br />
How does one calculate the required refractive index of the inner<br />
layer? Several methodologies have been developed over the last 40<br />
to 50 years to calculate thin-film coating properties and converge<br />
on optimum designs. The whole field has been revolutionized in<br />
recent years with the availability of powerful microcomputers.<br />
Among the most sophisticated and effective programs are those<br />
developed by Professor H. A. Macleod, which are used by<br />
Melles Griot.<br />
With a two-layer quarter/quarter coating optimized for one<br />
wavelength at normal incidence, the required refractive indices<br />
can easily be calculated by hand. The formula for exact zero<br />
reflectance for such a coating is<br />
2<br />
3<br />
= n<br />
2 0<br />
2<br />
nn 1<br />
n<br />
(5.25)<br />
where n 0 is the refractive index of air (approximated as 1.0), n 3 is<br />
the refractive index of the substrate material, and n 1 and n 2 are the<br />
refractive indices of the two film materials, as indicated in figure 5.14.<br />
If the substrate is crown glass with a refractive index of 1.52 and<br />
if the first layer is the lowest possible refractive index, 1.38 (MgF 2 ),<br />
the refractive index of the high-index layer needs to be 1.70. Either<br />
beryllium oxide or magnesium oxide could be used for the inner layer,<br />
but both are soft materials and will not produce very durable coatings.<br />
Although it allows some freedom in the choice of coating<br />
materials and can give very low reflectance, the quarter/quarter<br />
coating is very restrictive in its design. In principle, it is possible to<br />
deposit two materials simultaneously to achieve layers of almost any<br />
required refractive index, but such coatings are not very practical.<br />
As a consequence, thin-film engineers have developed multilayer<br />
antireflection coatings and two-layer coating designs to allow the<br />
refractive index of each layer to be chosen.<br />
quarter/quarter antireflection coating<br />
A B C<br />
AMPLITUDE<br />
Figure 5.14<br />
coating<br />
TIME<br />
air (n 0 = 1.0)<br />
low-index layer (n 1 = 1.38)<br />
high-index layer (n 2 = 1.70)<br />
substrate (n 3 = 1.52)<br />
wavefront A<br />
wavefront B<br />
wavefront C<br />
resultant<br />
wave<br />
Interference in a typical quarter/quarter<br />
5.12 1 Visit Us OnLine! www.mellesgriot.com