outgassing

That's a real live issue. Crud gets pulled into the most intense parts of a beam by the photophoretic effect (which is how optical tweezers work).

Back in the day, there was a real problem with this in telecom lasers, which were hermetically sealed with an inert atmosphere, and kept dying from organic crud collecting on the laser and fibre facet. It was discovered that using dry air instead fixed the problem, by allowing the crud to oxidize instead of accumulating.

High powered lasers of shorter wavelength should have an even stronger self-cleaning effect, but it would be worth measuring to make sure.

Cheers

Phil Hobbs

Silicone based heatsink grease moves - it's usually a mix of several molecular weights to establish a desired viscosity and the low MW stuff is volatile. It can get onto optical surfaces and is very difficult to remove. Silicones are also bad around switch contacts and motor commutators - an arc will turn the film to quartz - abrasive and an insulator. Uncured epoxy and RTV silicones have volatiles - use stuff that is rated for vacuum use and follow the cure schedule. Some flexible plastics have volatile components - use crosslinked vinyl, formvar or ptfe insulation. Mechanical components (switches, motors, variable caps) may (will) have volatile lubricants. It is possible to get some components with vacuum-grade lubricants (trade names like Fomblin) but you'll have to negotiate a spec with the manufacturer. Board materials (FR4) are generally good. If very low contamination is required going to something like Rogers Duroid might be an option.

We're thinking ordinary parts (uP, ADC, Mux, maybe USB) on a conventional PCB, FR4 if that's OK. We already have customers in the semiconductor industry who let us use pretty ordinary construction, including ribbon cables and polycarb front-panel stickers, but no silicones.

I'd guess that vinyl wire insulation would be bad, because it smells like plastic. There is a basic nose test for things that outgass.

The wire I'm talking about is irradiated PVC (see Alphawire 7055 for example). It doesn't melt or burn with soldering iron contact, is tough as nails, strips easily and is a good bit cheaper than PTFE. I switched to it years ago.

I've used FR4, ribbon cables, normal connectors etc. for stuff that goes into SEMs for the semiconductor research guys - they're not terribly fussy. When you get to the production defect analysis types, they don't want anything in the chamber that can't be certified clean. If you get to where it's that sort of problem, you're probably looking at building everything into a washable hermetic hybrid before you'll satisfy that crowd.

When I worked for a big laser company, any intra-cavity PCB was teflon. It was carefully cleaned and de-greased using a vapor degreaser. Those only we nt into low energy systems, and only when absolutely needed.

Even a plasma cleaned, vacuum baked, hermetic sealed cavity made only of t eflon, optical glass, and 306 stainless eventually de-gassed from material s degradation under intense light. All the "gunk" landed right in the cente r of the beam optic. The photophoretic effect is amazing. The quickest way to degrade a bar arra y is to cause back reflections, or allow dust and coatings to land on the o utput faucet.

I've seen diode pumped lasers go both ways, FR4 and Teflon boards for the d iode and TEC connections. For very low powers, up to a couple of watts, y ou might get away with FR4 for a long time. Under any other circumstances, the cavity should be sealed and free of circuit boards. Only a very caref ully selected collection of materials and wire should be used.

I've seen selected Teflons, cleaned, hard radiation linked PVC, and Kynar used intracavity for wiring. Nothing else is worth the risk as far as I'm concerned.

Glues are a no-no. There are a few select dust loaded epoxies we use intrac avity, and again, only if there is NO other way.

You might have IR from the pump diode, but if the overall laser is UV or Vi sible, over time, very slow reaction rates add up.

Steve

iirc NASA used to have a lot of info about outgassing.

I have an epson scanner which has something on the moving part which slowly fogs the glass above it. I have cleaned the glass but it fogged it again due to whatever crud it is evaporating. I suggest you don't use whatever they used.

Chris

Part of the problem might be that the glass adheres to the residue; there are silicone-oil-containing treatments for glass, that might help. Xerox part # 8R3671 is one such.

The flexible drive belt, and the (unknown composition) plastic casing are likely contributors to the haze sometimes found inside scanners. The dirtiest I've ever seen was at a paper mill, though. Vapors from woodpulp?