|Thin Film Design & Applications|
An edge filter is a multilayer coating with a substantial passband on either designated side of the stopband. The edgefilter is meant to have a crisp boundary between wavelength regions that are reflected and those that are transmitted.
This section of the course will be divided into 3 areas:
The Core Stacks - the two basic multilayer edge filter designs.
Combining Core Stacks - this technique will broaden the reflectance area (combining stopbands).
Reflectance in the Passband - this technique will help the designer to eliminate
There are two basic multilayer designs that are currently used for edgefilters. One design has the passband on the high wavelength side of the stopband, the other on the low wavelength side. The designs are the following:
The designs are illustrated below:
Combining core stacks is a way that the designer can expand or broaden the high relectance region while maintaining the high or low wavelength pass band. The next example includes combining two high wavelength pass edge filters.
Consider the "core stack 1" as a high l pass
we look at the equivalent index vs. wavelength of the three layers, .5H L .5H,
We have a second core stack (red) where the design is:
As we scale the stack up in wavelength, the stopband will increase in size, slightly.
If you design an overlap of stopbands between each stack, you will
The combination of both designs gives the black line:
If we combine core stack 1 with another stack with less overlap (red) of the design:
We can see where the stopband will fall when we plot equivalent index vs. wavelength.
We can also see there is no overlap between stopbands for this example.
Since there is no overlap, a small leak will be created in the high reflectance zone (black curve).
As a designer works with edgefilters, there may come a time where your specifications dictate that the transmission in the passband be extremely low at a particular wavelength. Knowing that you will come across this issue in the future, it is important that you can complement your edgefilter with some simple refinements that can help you meet these requirements.
The rest of this section is under construction