n,k vs. temperature data are
generally not available and will
require a series of measurements. Click
here to learn more.
Temperature variations bring about n,k variations causing spectral shifts in optical coatings. Several approaches are
supported by FilmStar. We can use dispersive index files corresponding to
particular temperatures and/or implement temperature dependence coefficients dn/dT,
One approach is to simply create and store (using program INDEX) dispersion files
corresponding to different temperatures. A user might, for example, store data
for TiO2 at 150 deg as file TIO2-150. The dispersion files become
the Film Indices dialog as shown below for material H.
When considering index variations,
the design must be
characterized by physical rather
than optical thickness.
Facilitating switching between temperatures, entries displayed in the Film Indices dialog
can be stored on disk independently of the thin film design itself. This is accomplished in several ways:
Set DESIGN to use FILM mode instead of FILM Archive mode.
Then Film Indices are loaded and saved via File Open
and File Save As.
If FILM Archive mode is utilized (recommended), Film Indices can
still be opened with shortcut keys <Alt+Ctrl+I> and saved
with <Alt+Shift+I>. Or, if Augment File Open/Save is checked in File Configuration
Preferences, the usual File Open and File Save As dialogs
support Film Indices (.inw) files in addition to FILM Archive (.faw)
Other ways of including temperature dependence rely on User-Defined Index Functions. Suppose
we know that the index of refraction of a material at 20° is 2.453 and the temperature
dependence coefficient dn/dT = 11.5*10^-6/°C. If the wavelength range of
interest is restricted so that dispersion can be neglected, we might use the
following Index Function to specify material M at 90°C in the Film Indices
dialog (see above). While variable A could be replaced by constant 2.453 in the
formula, it is more general to utilize A as a coefficient.
Alternatively, suppose we need to use dispersion, but can assume
that the temperature dependence dn/dT is relatively constant over the wavelength
range of interest. Assuming once again that there is no absorption (k=0) we
define User Index Function TMPTR2 which specifies material N at 90°C in the
Film Indices dialog.
Suppose we have dispersion curves for 20°C and 150°C
and believe that intermediate values follow a linear relation. In this case,
Index Function TMPTR3 specifies material P at 90°C in the Film Indices dialog.
Approximate thermo-optic coefficients
are given below.
These were obtained from various sources and, in some cases, apply to bulk
Another approach is to use a FilmStar BASIC
program to generate new index tables. In the following example (included in
FilmStar installation and in
basic.zip), index file 'TiO2' (room temperature, say 23°C) is processed into
two new files: 'TiO2 +40C' and 'TiO2 -10C'.
' Program IndexTempVar.bas for FilmStar INDEX
' Copyright 2009 FTG Software
' Applies thermo-optic coefficient to n values
' NOTE: This model assumes a constant index dn/dT
' variation with temperature deviation from 23°C
Option Base 1
DefSng A-H, O-Z
Public fName$, Coeff
Const Title$ = "Index Adjust"
On Error Resume Next
fName$ = IndexName$
If fName$ <> "" Then
Coeff = Val(InputBox(fName$ & " thermo-optic coefficient?", Title$, s$))
If MsgBox("Apply T-O coefficient " & Format$(Coeff, "0.00E+0") & _
" to index table " & fName$ & "?",vbQuestion + vbYesNo, Title$) = vbYes Then
If nIndex > 0 Then
MsgBox "New files (+40°C, -10°C) saved",vbInformation, Title$
MsgBox "Unsuccessful operation",vbExclamation, Title$
MsgBox "No index file specified", vbExclamation, Title$
ReDim xIndx(1, 1)
xIndx = nkTable
' original index tables at 23°C (room temperature)
For i = 1 To nIndex
' increase 17°C
xIndx(i, 2) = xIndx(i, 2) + 17 * Coeff ' + 40°C
nkTable = xIndx
FileSave fName$ & " +40C"
For i = 1 To nIndex
' decrease -50°C
xIndx(i, 2) = xIndx(i, 2) - 50 * Coeff ' - 10°C
nkTable = xIndx
FileSave fName$ & " -10C"
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