Fading Lasers without modulation, optically

[polishedball]

I want to be able to fade in and out my lumia effects, but don't want to get into analog modulation. I have them TTL control currently. Does anyone make a wheel that is coated to increase reflectance and minimize transmission as spun? Looking at just putting it in the beam path and then using a stepper motor control to effectively fade the effect out or in.

Thoughts? How else can this be done.

Better yet tell me you have such a wheel and want to sell it to me, cause it is useless in the analog age

[danielbriggs]

You could make your own "variable beam splitter".
Laser + 1/2 wave plate in a rotation mount (driven by a stepper perhaps) + PBS cube.

As the 1/2 wave plate is rotated, the ratio of the power from each of the output beams from the cube will vary sinusoidally.

[polishedball]

very cool idea do you know does a BB 1/2 plate exist? seems there always individual colors.

[mixedgas]

I have a few broadband quartz plates but they only cover about 50 nm in the blue-green. Mine cover 488 to 532 very well. Very expensive to have made. I had the honor of scrapping about 20 of them, with the cubes, from semiconductor inspection gear. I have a few left in the rotating mounts.

For low energies good old crossed Polaroid polarizer works just fine, if it gets burned and starts doing a lumia you use two lenses to up-expand and then down collimate the beam.

I can order full spectrum polymer plates, but the cost is probably more then you want to spend. They can handle 10 watts in some cases. I used to work for the professor who invented them. The polymer plate covers 485 to 630, it works OK out to 647.

Steve

[danielbriggs]

Sorry, I didn't realise it was a polychromatic source.

Daft idea of the day: Would stacking 1/2 wp's work? Say all three zero-order plates 445nm/532nm/638nm "sandwiched together" in the same mount, all properly aligned?

[mixedgas]

Dan,

You could converge them at one and only one set point. Otherwise they would be known as a "Quartz Optical Depolarizer"
Or "Hybrid polarization Shifter"

Happy sticker shock, and note these don't span down to 445:

http://www.meadowlark.com/store/Spec...achromatic.pdf

I'm aware of a case where they were used to combine two big white light Ion systems. If one laser went down, the show stayed up.

Steve

[swamidog]

how about a variable neutral density filter wheel?

[mixedgas]

Those variable wheels are not cheap, about 800$ new. I've got two of them and they are precious.

Thorlabs has a linear strip variable density for 75$ to 96$ depending on model. Which, adding a surplus linear bearing off ebay, might be the way to go.

You can also use a galvo with a flag on it, and a pair of lenses. This is tricky to set up and needs a lot of optics mounts.

Steve

[Junktronix]

How about a galvo with a wide mirror and a retro-reflector like in the old XY+blanking scanners?

[Junktronix]

Also, I have a linear variable ND filter here. Smaller and cheaper than the circular ones. If I can find it, I'll get you the specs.