Ultra Narrow Bandpass Filters In High Demand

Dear Friends, Colleagues and Customers of Alluxa,

Thanks to all who came out and participated in our Free Coating Run Contest and microbrew tasting at Photonics West. The grand prize drawn was a free custom coating run of virtually anything we can make. Filter type, substrate, and nearly all the specifications were completely up to the winner to determine. We only asked that it obey our laws of physics.

So… Congratulations to our winner, Garrett Richards of Quartus Engineering!

Garrett decided to specify an ultra-narrow, very square, 532nm band-pass filter on a fused silica substrate to be cut to 0.5 inch diameters, with blocking to OD6 over the silicon detector range as shown in the table below.

Custom coating run specs
CWL 532 later transmit > 90%
FWHM < 1 nm
Squareness > 50% Figure of Merit (HPBW / 3OD BW)
Blocking 6OD average out of band 200 – 1100
Substrate Fused Silica
Diameter 12.5mm

The filter “squareness” requested by Garrett required our designers to use five separate Fabry-Perot cavities. Each cavity is composed of two 99% reflectors sandwiching a half-wave spacer layer. The cavities are coupled with a low index quarter-wave layer. We adjusted the Q of the cavities slightly to improve pass-band ripple. As a comparison, most 1nm and narrower filter spectra are made with a two or three cavity construction and figures of merit of 30% or less.

The results shown below met our expectations and look outstanding. The transmission peaks at 94% and the filter shape matches theory nearly all but exactly. Hopefully they will be put to good use, Garrett!

To learn more about our narrowband filter technology see this SPIE paper Advanced Plasma Deposition Improves Ultra Narrowband Optical Filters.

Ultra Narrow Bandpass Filter 01

We are not surprised that Garrett specified ultra-narrow filters as his free coating seeing as how our ultra-narrow filters have been in high demand lately. The plots below show examples of the ultra-narrows we have done over the past three months. Please let us know if you have inquiries or requirements similar to these – we’d love to help you improve your system performance.


Ultra Narrow Bandpass Filter 02

Ultra Narrow Bandpass Filter 03

Our future goal for ultra-narrow filters is to continue to improve the transmission and to push into the far IR and UV. Stay tuned!

Any questions or comments please send me an email at CEO_Notes@Alluxa.com or schedule a meeting during one of the upcoming industry shows.


Mike Scobey

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