The hermetically sealed module thread

Ok, so I have been playing around with Oclaro HL6385DG 150mW 642nm diodes and TEC cooling... Wow, these things like to be cold, at 350mA power rises from 140mW @ 25c to 157mW @ 18c. I would like to go colder however I run into the dew point issue which messes with the output due to condensation on the G2 lens.

My plan is to rebuild my main 638nm module with PBS instead of knife edging and then cool the diodes as much as I can. Hermetically sealed module is therefore a must.

A few questions for those that do this already (planters and co).

1. How do you ensure the internal air is moisture free at the time of sealing? Do you simply do this in an air conditioned (dry) room?

2. Is it worth placing silica gel inside?

3. What do you recommend for an exit window?

4. What is the best connector or cable exit strategy?

5. Any tips, tricks, traps to share?

Thanks in advance.

Dry nitrogen purge? We use a sealant that derives from Mill-s-11034 at work that I believe in but it's an AA part number now that I don't remember. If you use RTV to seal with get and use the correct primer too. Use a hermetically sealed connector or feed throughs. A dual vent flow through purge with sealing screws avoids the whole glove box thing...

This cooling and sealing is really easy and forgiving.

To start with, if you're interested in dropping the diode far below zero and in my experience this has always lead to more power, then use multiple stage TECs. You can build up a stack, wire them in series and fix them to each other (permanently) with thermal epoxy. Selecting the individual TECs is pretty simple just add the total heat from each stage and the load it cools to the TEC below it and so on. For a total V drop across the series stack you therefore select the lowest V drop for the top TEC and so on. A practical delta of 80 degrees is easy. This includes the diode heat load and heat from the surrounding environment. With a single stage the largest delta is about 30 degrees and keep in mind the base plate will heat up somewhat.

The most challenging task is to effectively remove the base plate heat from your cooler. If it runs hot then you undermine your cooling efforts. Use more heat sinks and fans than you think you'll need.

The cooling chamber I demonstrate in the C-mount video is what I recommend. It can be fabricated from metal or plastic, but the PVC I use is inexpensive, easy to machine and glue. The 1/4 wave, BK-7 window can be broadband or V coated. I glue this to the chamber with common aquarium, one part RTV silicone sealant. Just allow it to cure for a day to avoid acetic acid out-gassing.

The "O" rings on the top and bottom of the box should be thin enough that they track well around the screw studs, yet don't force them in toward the center under a lot of pressure. They should not be so thin that they don't sit outside the stud clearance holes and fail to seal the box. I use 1/16" cross section rings.

The feed through(s) work well through the base plate with a little RTV to seal the gaps with the wires. To minimize the gaps I install all fittings after the wire has passed the plate and the holes are positioned in the gap between the cooling stack and the inside of the PVC wall. You have to design for some clearance here.

You do not need to purge or use descent. The coldest part of the TEC stack will never be the warm diode. Even off its conductors warm it slightly from the relatively hot base plate. The coldest surfaces of the TEC act as a getter to condense the residual moisture within the chamber. That's why a good seal is important, but dehydration is not.

http://www.youtube.com/watch?v=g9z5ioZhNjE

Hey Wayne

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4. What is the best connector or cable exit strategy?

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Jaycar have these http://www.jaycar.com.au/productView.asp?ID=PS1210 that would most likely be suitable (not cheap but easily accessible)

Thanks Eric and loopee, great info!