Glue for tacking a filter onto a photodiode? - Page 1
Forum section: electronics / projects

author: mark03
author_rank: Frequent Contributor
timestamp: October 05, 2018, 10:09:29 pm
content: For my free-space-optical comms project, I need to attach a red interference filter to a medium-area photodiode in a big TO-8 package (Opto Diode Corp ODD-12WB)--see photo. I see that there are some UV-cure adhesives especially for optical use. But I am not cementing lenses together; I only need something relatively transparent in the visible, which will not harm the dielectric coatings of my filter. Ideally removable. An 80% solution, in other words.Would a thin layer of neutral-cure silicone do the trick?

author: tomato
author_rank: Regular Contributor
timestamp: October 05, 2018, 10:55:42 pm
content: Quote from: mark03 on October 05, 2018, 10:09:29 pmWhy don't you just use a few drops of 5-minute epoxy around the edges?Are you sure those are interference filters? They look like colored glass filters.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: October 05, 2018, 11:34:02 pm
content: Another option:https://www.newport.com/p/F-IMF-105

author: mark03
author_rank: Frequent Contributor
timestamp: October 05, 2018, 11:46:25 pm
content: You can't really tell in the photo, but the filters are shiny on one side when viewed at an angle; they're not just colored glass. Ebay surplus "SS-6600" by Corion Corp, Holliston, MA.I figured a transparent or semi-transparent adhesive would have better transmission than leaving an air gap. That index-matching fluid seems like a good option, especially if I can find it cheaper. I still wonder about the silicone though.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 06, 2018, 03:14:56 am
content: Unless you know the index of refraction of the filter and the window, it's hard to match. I doubt it would buy much. I'd go with the epoxy around the edge and black paint to mask the exposed part of the window.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 06, 2018, 03:38:05 am
content: What are you trying to accomplish?If all you need to do is put the filter in front of the detector, the normal way would be to use lens tubes. Just mount both the filter and the detector in the tube. There will be a few mm between the filter and the detector, so if that is a problem, then you will need something different. I would still recommend trying to mount them by the edges rather than using adhesive on the optical surface.Optical cements and index matching fluid are used to eliminate surface reflections when two glass surfaces are placed next to each other. This is especially important as it helps get rid of the interference patterns (like Newton's rings) between closely spaced glass plates or lenses. If your optics have AR coatings this won't work. The AR coating is designed to work with air as the second surface not index matching gel.

author: mark03
author_rank: Frequent Contributor
timestamp: October 06, 2018, 04:46:24 am
content: What am I trying to accomplish?1) Attach filter to photodiode so it doesn't fall off.2) Do this in a way that can be removed later if I change my mind.3) Bonus points: minimize light loss.Sorry, I thought all of the above was obvious. A mistake I make frequently.AFAICT the filter coatings are only on one side (or in the middle?), so the filter-to-photodiode interface is glass to glass.I think I'm about to do the classic "ask for advice and then ignore it"because something like GE Silicone II appears to meet criteria 1 and 2, and probably 3 as well because my googling suggests the refractive index of silicones is in the 1.3-1.6 range. I was half expecting an expert to chime in and say "don't do that!" but as that hasn't happened, perhaps my original instinct was ok.I am surprised by all of the suggestions for epoxy. Doesn't epoxy work best on rough surfaces?Thanks for your thoughts. And please tell me "don't do that!" if I'm off base here.

author: tomato
author_rank: Regular Contributor
timestamp: October 06, 2018, 05:14:45 am
content: Quote from: mark03 on October 06, 2018, 04:46:24 amYou want to use bathroom caulk to glue optical components together; maybe you need to consult an expert at Home Depot.QuoteIt will easily hold that tiny filter.QuoteDon't do that.

author: TERRA Operative
author_rank: Super Contributor
timestamp: October 06, 2018, 06:05:10 am
content: Maybe hold them together and put a tiny dab of hot glue on two sides?

author: cvanc
author_rank: Frequent Contributor
timestamp: October 06, 2018, 12:33:16 pm
content: Whatever you do, be extremely careful if you decide to use cyanoacrylate glues - they outgas when curing and will fog your optics. Better to try something different.

author: StillTrying
author_rank: Super Contributor
timestamp: October 06, 2018, 12:58:05 pm
content: A ring of blu-tack, it goes quite hard after a few years.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 06, 2018, 01:57:53 pm
content: https://en.wikipedia.org/wiki/Canada_balsam