Thin Film Design & Applications  

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2 Layer AR Coatings (V-coat)

Benefits:  More material choices for index.  More refinement options.  Easy to

Limitations:  Narrow band.

Choosing Layer indices:

2lar.jpg (11422 bytes)

A straight forward way of determining if the materials you have on hand, combined, would make a decent AR, is to use a Schuster diagram. (reference 1)

Below, the Schuster diagram has been calculated
for glass (n=1.52) & germanium (n=4.0) in an air medium (nm=1.0).

shu_glass.jpg (16339 bytes)shu_germ.jpg (15622 bytes)

The calculation requires that of the following three expressions:

wpe15.jpg (2862 bytes)

All three must be positive
Any two are negative and the third positive.

Layer thicknesses that are equal optical thickness (reference 2)

To calculate the refractive index requirments
for a 2 layer AR coating where both layers
are of equal optical thickness (each are QWOT's,
see terminology)
then the use the following:

wpe17.jpg (2855 bytes)