Most obtained filters aren't wide enough. Generally, in this kind of design, the central frequency is chosen. Is it possible to chose the cutoff frequencies instead of central frequency?
You should look for "chirped" quarter wavelength stacks. By deviating from the ideal periodicity in layer thickness it is possible to increase the bandwidth.
I am not a specialist in this field. I know that the technique is used for 'broadband' high quality mirrors as in optical resonators. I guess that the same approach is valid for interference filters as they are quiet similar.
You'll find a lot via google : "chirped quarter wavelength stack"
Her is an appetizer http://en.wikipedia.org/wiki/Chirped_mirror
Dear Kebab,
You should use "chirped" mirror for this aim.
See for instance this publication
http://szipocs.com/publications/Theory_and_design_of_chirped_dielectric_laser_mirrors.pdf
Often, optical filters are deliberately designed to have a narrow bandpass region, e.g. in certain hydrocarbon gas analysis applications, it can be desirable to filter out all IR wavelengths except, say, 3 +/- 0.2µm, and to maximise transmittance at this single wavelength.
It is possible to design wide bandpass filters and select the cuton/cutoff wavelengths; there are a number of ways you can go about it. You need to consider angle of incidence (and polarisation effects if at large AOI), the rejection/transmission bands you require, and what type of filter you need. (Could be wide or narrow bandpass, long wavepass, short wavepass, notch, edge, neutral density, beamsplitter, etc.)
Often, for a narrow bandpass application, a quarterwave stack will suffice. Bandwidth can be tuned to an extent by choice of materials in the multilayer (typically one low-index and one high-index material in most stacks).
Choice of substrate can help to eliminate higher-order transmittance bands, e.g. fused quartz will block IR wavelengths above ~5µm.
One method of producing a wide bandpass filter is to deposit two edge filters, perhaps one on each side of the substrate; you could deposit a long wavepass on one surface, and a short wavepass on the other.
I recommend "Thin Film Optical Filters" by H.A. MacLeod, IOP Publishing (2001), and
"Practical Design and Production of Optical Thin Films" by R.R. Willey, Marcel Dekker (2002).
- Badges
- Science topic
- Similar topics
- Mathematical Sciences
- Graphs