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# RP Coating – Advanced Software forDesigning Optical Multilayer Structures

## Demo File: Short-pass and Long-pass Filters

This demo file provides a custom form with which one can easily design optical filters which have a high transmittance at short wavelengths and high reflectance at long wavelengths (short pass) or vice versa (long pass).

For such designs, it is often not practical to use purely numerical techniques, since many layers are required, leading to a huge parameter space. Instead, we use a design which is essentially that of a Bragg mirror, but with added λ/8 layers at the top and the bottom. In many cases, that is already sufficient. If required, however, the performance can be further improved with an optional local numerical optimization.

Our nice custom form allows you to simply enter the used materials, required parameters etc.:

You could further expand that form as required, e.g. for displaying additional calculated values of interest.

It is instructive to see the used script code for the definition of the coating, based on various variables which are entered in the form:

```beam from superstrate
substrate: (material_s\$)
if DesignType\$ = "Bragg" then
begin
for j := 1 to N_Bragg do
begin
* (material_l\$), l/4 at l_Bragg
* (material_h\$), l/4 at l_Bragg
end
end
else if DesignType\$ = "short-pass" then
begin
* (material_l\$), l/8 at l_Bragg
for j := 1 to N_Bragg do
begin
* (material_h\$), l/4 at l_Bragg
* (material_l\$), l/4 at l_Bragg
end
* (material_h\$), l/4 at l_Bragg
* (material_l\$), l/8 at l_Bragg
end
else if DesignType\$ = "long-pass" then
begin
* (material_h\$), l/8 at l_Bragg
for j := 1 to N_Bragg do
begin
* (material_l\$), l/4 at l_Bragg
* (material_h\$), l/4 at l_Bragg
end
* (material_l\$), l/4 at l_Bragg
* (material_h\$), l/8 at l_Bragg
end
superstrate: air
```

The first diagram shows the reflectivity profile of a short-pass filter, designed for transmitting pump light at 808 nm into a laser operating at 1064 nm:

A second diagram shows the reflectivity as a function of wavelength and angle of incidence:

As we are using a custom form, both the form and the underlying calculations can be modified by the user according to specific requirements.