Slow "analog" attenuation methods

[absolom7691]

Hey guys, I have a couple of 591nm laser modules that I would like to put into my lumia projector. I like to fade in/out my lumia effects and also control the brightness. These summation lasers don't work well, or at all, at low power levels so fading/attenuating via modulation is out. Since we're talking gradual fades over the course of a half second of more, I don't need fast "blanking". What's a good way to mechanically attenuate a beam? I know in the good old days it was either a galvo/retro reflector or an AOM. I have some of the old Stocker Yale GM20 knock-offs. I was thinking of just using a flag on one of those guys. Since the beam is going into lumia glass, I don't think I need to worry about aberrations from the edge of the flag, right? If anyone has a better idea, please let me know.

[buffo]

If you are OK with slower response, you can get completely adjustable attenuation using a pair of polarizing filters. Mount the first one in a fixed mount, and the other one in a rotating mount such that you can rotate it up to 90 degrees. Start out with both filters aligned in the same direction (both horizontal or both vertical). Then, as you rotate the second filter, the beam will slowly fade to zero as the second filter approaches 90 degrees. Speed limit is based on how fast you can rotate that second polarizing filter.

Flag blanking is much quicker, of course, but it won't give you smooth fading.

The other option would be to use an AOM; you can find cheap surplus units on Ebay from time to time.

Adam

[absolom7691]

If you are OK with slower response, you can get completely adjustable attenuation using a pair of polarizing filters. Mount the first one in a fixed mount, and the other one in a rotating mount such that you can rotate it up to 90 degrees. Start out with both filters aligned in the same direction (both horizontal or both vertical). Then, as you rotate the second filter, the beam will slowly fade to zero as the second filter approaches 90 degrees. Speed limit is based on how fast you can rotate that second polarizing filter.

Flag blanking is much quicker, of course, but it won't give you smooth fading.

The other option would be to use an AOM; you can find cheap surplus units on Ebay from time to time.

Adam

Thanks, Adam. I didn't even think about using polarizing filters. I'll most likely go down this route.

[buffo]

Thinking about this a bit more - you don't really need to secure the second filter in a mount... You could stick it on the end of a short servo arm and just let it rotate through 90 degrees that way. (Assuming your filter was large enough, that is.)

Adam

[laserist]

The flag will work just fine, and without the losses involved with 2 polarizers. Can the lasers handle pwm? A reasonably fast strobe won't hurt low speed lumia.

[absolom7691]

The flag will work just fine, and without the losses involved with 2 polarizers. Can the lasers handle pwm? A reasonably fast strobe won't hurt low speed lumia.

I was thinking about that, they might be able to. Summation lasers are pretty far outside of my knowledge as far as theory of operation. I know they are power/temperature sensitive so, it's possible that it will work. I'll see how the flag method works. I would like to preserve as much power as possible since these are only 30mw and I need every precious mw! Thanks again!

[CountFunkula]

Surprised nobody has mentioned ND wheels/variable beam splitters. Bonus: you get a waste beam to do "whatever" with.

[swamidog]

Surprised nobody has mentioned ND wheels/variable beam splitters. Bonus: you get a waste beam to do "whatever" with.

i put it in my pocket for later.

[lasermaster1977]

How about an open "V" slot cut into a metal rectangle and painted w/flat black Krylon based on something like this:



Although this shows an early, rotating beam chopper wheel of mine that was moved into and out of beam path with a motor driven lead screw, it worked great as a beam attenuator at high rotational speeds as the beam diameter entered the very narrow tip of the "V". This disk was made of 1/16th inch flat cardboard, painted flat black, hot glued onto a 12v motor shaft hub and used with a 1 watt mixed-gas ion laser. (Metal would be better, but I never had a blade catch fire in the two years of use in planetarium laser shows. A/O's eventually replaced this.

For a simple analog attenuator for one beam only one "V" is needed. The hard part is coming up with a way to move the open "V" slot in and out of the beam path.

hint: DC motor driven lead screw.

For attenuation the "V" would only need to be an inch long, with it's widest part just a bit larger than the beam diameter.

[absolom7691]

For a simple analog attenuator for one beam only one "V" is needed. The hard part is coming up with a way to move the open "V" slot in and out of the beam path.

I plan on using a Stocker Yale actuator (a poor man's GM20). I should be able to gradually apply voltage in small increments, inducing very minute deflection. This should give me decent control of the flag. I'm curious now about the shape of the edge of the flag. Perhaps I'm overthinking this for attenuating a beam that's only diffusing through lumia glass. Outside of modulation, I haven't had to mess around with the "old school" tech before!