Triple Bandpass Filter for OIC Contest
Every three years the OSA (Optical Society of America) holds a meeting called OIC (Optical
Interference Coatings). As part of that meeting, there is always a design contest. The 2007
meeting has two design problems, the first of which is discussed here. The second problem is
a Nonpolarizing Beamsplitter.
The challenge is to design a normal incidence bandpass filter having the requirements
We use the seven continuous optimization targets above, but with two slight modifications: (1) we use density targets in the stop bands and (2) to give us a little margin for error, we increase the passband targets to >91% and the density targets to >4.1, which corresponds to about 0.08%. When T is required to be low, density targets work better because they force TFCalc to design the blocking part of the filter before dealing with the passbands. Recall that optical density is Log 1/T.
After a little experimenting, we decided to attempt a fairly traditional two-material design using a high index of H=2.5 and a low index of L=1.45; there seems to be no benefit to using more materials. First we will create a design assuming that the back surface does not reflect. To find the thinnest possible design, we start with a single thin layer of H and use TFCalc's needle/tunneling optimization to design the filter from "scratch". This can be a slow process for such a complex design, but the results will be worth the wait. After an hour or two of optimization, we find a 75-layer design having a physical thickness of 6735 nm. The second step is to design a coating for the back side of the substrate. That coating needs to be antireflective across the three passbands. An 8-layer, 426-nm thick AR coating works nicely on the back surface. The final design consists of 83 layers, 7161 nm thick. Although the performance, shown below, meets all the requirements, it is not very attractive!
If you want a copy of the design file, please send e-mail to firstname.lastname@example.org. The contest results will be announced at the OIC meeting in Tucson, June 6, 2007. This design was not entered in the contest.
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