Technical Issues -
Designs on Multiple Substrates II
of Design on Multiple Substrates I
Consider a design with 0.02%
transmittance. The same stack is deposited on the second parallel surface
of the nonabsorbing substrate. The total transmittance is 0.01% instead of
0.0004%. This apparent anomaly is caused by multiple internal reflections. One
way to increase blocking is to delete parallel from the above
description. While the example below refers to coatings on a single substrate,
it applies to assemblies with multiple substrates.
DESIGN 2.61.3101 adds
new Side 2 Compensation mode: FWD Wedge. The mode applies only to
designs with massive layers and is exactly the same as Mode 1 (FWD ignore
Side 2) otherwise.
By eliminating reflections from secondary surfaces, a wedged
substrate increases blocking of filters containing coatings on
multiple surfaces. While thin film manufacturers can
achieve similar improvement with a slightly
absorbing substrate, that is not always possible.
Wedge mode assumes there are no reflections between massive surfaces.
Reflectance is the reflectance of the first surface only while
transmittance is the product of each element. The
wedge angle must be sufficient for all reflections to walk off the
As example we consider the IR bandpass Fnum.faw
design (included in the FilmStar installation) centered at 1640 nm. A second
(Side 2) design is centered at 1630 nm. We set k = 0.0001 for both L and H
Wedge mode is equivalent to multiplying two separate filters in
FWD ignore Side 2 mode. Reflectance is Side 1 reflectance (R1) while
transmittance is given by T1*T2. The Side 2 calculation sets substrate thickness
to zero; otherwise substrate absorption would be counted twice. Example: Download
Fnum-Diab.faw (Side 1),
Fnum-1630.faw (Side 2)
and run BASIC program
TwoSideWedge.bas. Finally open
Fnum-Diab-1630-1640.faw and evaluate in Side 2 mode 1 (Parallel) and
mode 6 (Wedge). As seen in the plot below, Both Sides and Wedge
traces overlap as expected.
TwoSideWedge.bas provides an alternative for users who do not need to
simultaneously design coatings on both sides. Want to test absorption in the
substrate (massive layer)? Set SUB k = 0.00001 in Fnum-Diab.faw and Fnum-1630.faw. Set
massive material G k =
0.00001 in Fnum-Diab-1630-1640.faw and regenerate the curves shown below.
Not sure what a massive layer is? No idea how to run a BASIC
program? Contact FTG Software for an explanation using our
Networks facility. When substrate k>0 the
BASIC program requires DESIGN 2.61.3058 or newer. (Contact FTG for a
green Both Sides plot is covered by the
teal Wedge plot.
Using FSPlot zoom (see below) we find that blocking is improved by a factor ~200. If you try the suggested exercise (substrate k = 0.00001) you will
note much less improvement because blocking is already increased by the