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Thread: Fading Lasers without modulation, optically

  1. #21
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    I found the linear variable ND filter. It's one of these:

    http://www.edmundoptics.com/optics/o...plitters/41960

    It's metalized so acts as a variable reflector. Just dump the reflected beam into a beam stop. Max ND is 2.0 so you'll get 100:1 attenuation at the high end. You could stick a 100% mirror or a beam block at the end if you need 100% attenuation.

    Make me an offer if you want to give it a try.

  2. #22
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    No-one's mentioned the soft start circuit. Even TTL-mod lasers usually have that. Given the cost and work of all other ideas so far, why not do what I posted about recently? I didn't describe any detail, and it does vary according to the laser controller but basically if you see the beam start dim and brighten up, you'll almost certainly find a resistor charging a capacitor, and the junction showing a rise from 0V DC to something else DC likely less than 5V. If you open the circuit on the OTHER side of that resistor, take the far side of the gap to a unity gain stable op-amp wired as a voltage follower (pretty much the simplest way you can use an op-amp!), that will buffer the signal and copy any detail in that signal you don't know about too, so it won't prevent proper operation. Put the output across a potentiometer, with a fixed resistor about one quarter of the value of the pot's total resistance in series with that pot on its ground side. Then feed the wiper terminal's output back to the open end of the original resistor.

    That will give you a 'volume control' that will be sluggish because it's smoothed by the original integrator that does the soft start. That's the safe way... If you want speed, and perhaps even to hack your own analog mod, you might buffer the post-integrator voltage, then use a difference amp to put an analog modulation signal into the input of the amp that drives the diode current control. I haven't done this on mine, I just wanted to fade the power, but as that's basically what you want too, you could do this. It's easy, safe, and while it voids a warranty, offset the value of that against the money you'll save. It's up to you...

    Edit: My buffer board was a tiny bit of FR4 stripboard mounted on the pins of a PCB-mount pot that also has chassis mounting hardware. Small hole in the case to mount the pot, and all you need is enough space to do that, and to link to the board by thin insulated wire. It's a very easy modification to do..

    Edit more...
    I know this isn't optical, but there is a big advantage to it.. My green DPSS lasers that I did it to run much more quietly and stably at lower powers. They run cooler too. By doing it this way you dramatically prolong their lives by running them gently a lot of the time. Everything, from pump, vanadate, KTP, bonding if any, it all gets an easy ride this way. I don't know about you, but I found that well worth the risk involved. Which is low so long as you keep grounded so you don't stick static charges into the drive circuit. There's usually a fair bit of buffering between the soft start circuit and the diode anyway.
    Last edited by The_Doctor; 09-25-2013 at 17:59.

  3. #23
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    Quote Originally Posted by Photonbeam View Post
    I have some of these 1/2 wave plates rotated by stepper motor. The parallel plates separate the two polarizations and the waveplate varies the ratio 0-100%. They are from Laserscopes - 532nm. Not sure how they would work for other wavelengths. Of course your beam must be polarized.
    Attachment 40110Attachment 40111Attachment 40112
    That is exactly what I was thinking. Rotating polarization filters bring me back to fun physics classes.

    Adam.
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    Laser (the acronym derived from Light Amplification by Stimulated Emissions of Radiation) is a spectacular manifestation of this process. It is a source which emits a kind of light of unrivaled purity and intensity not found in any of the previously known sources of radiation. - Lasers & Non-Linear Optics, B.B. Laud.

  4. #24
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    Pcaom

    Mod is the best wAy

  5. #25
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    yes... assuming

    1) your lasers are polarized
    2) you have sufficient space
    3) you can tune the pcaom driver for the right frequency
    4) you get lucky on ebay
    5) lattes!
    Quote Originally Posted by kecked View Post
    Pcaom

    Mod is the best wAy
    suppose you're thinkin' about a plate o' shrimp. Suddenly someone'll say, like, plate, or shrimp, or plate o' shrimp out of the blue, no explanation. No point in lookin' for one, either. It's all part of a cosmic unconciousness.

  6. #26
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    When scanning graphics I would stay away from using PWM/TTL control of brightness, it will leave dashes in the display - but - because you intend to have control over lumia that usually is a slow moving effect I will suggest that you PWM modulate your existing TTL control.
    This can easily be done with a single 555 timer if you look hard enough you will find a circuit, most of the circuits will be using two 555's or a dual 555, the 556 yet like I said.. it can be done with one.

    otherwise - Meredith has an LCD? optical type attenuator for $20, it says it has an attenuation ratio of 50/1
    http://mi-lasers.com/index.php?main_...products_id=87
    I just called Meredith up on the phone and they say that they have not tried putting 1W through this item.. I wish I had asked what the size of that hole that the beam would pass through..
    If the active area is large enough you could always try expanding the beam out so that there is not as much energy that would otherwise fry the optic.
    nmLaser has a product line of electro-optical attenuators :
    http://www.nmlaser.com/modulation.htm


    In the past I had taken a plastic polarizer film and a large LCD display that I had peeled away at its layers till I was left with just the liquid layer and its clear electrical contact points, put some DC voltage across that and I had control of light, yet this would not work for fast blanking as it was somewhat slow to turn on or off.

    I wonder if there is a way one could dissolve away a mirror coating gradually, Burn it away?

  7. #27
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    To fade lumia in and out just use a flag (or a pickoff mirror) on a gm20 or other slow galvo actuator... If you want to use a stepper ,put a disk mounted excentrically on the stepper. 0 degrees full on, 180 degrees full off...

    Quote Originally Posted by polishedball View Post
    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
    "There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso

  8. #28
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    Much older solution: mount an arm on the shaft of a GM-20 and epoxy a thin bit aluminum to it so it will block the beam. A small clipping from a soda can will handle 10 watts easily. For higher powers, use a chip of graphite. You can cut down on diffraction by placing an aperture with a hole just slightly larger than the beam diameter "downwind" of the shutter.
    You don't even need an amp to drive the GM-20. A 100-200 ohm wire-wound pot rated at 5 or more watts hooked to a 5 volt supply will work. Relatively cheap and no loss or pesky reflections.

  9. #29
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    Wow .. old thread resurrected ..
    I'm actually wondering if anybody knows of a simple triangle wave generator that will supply 0-5V in a slow slope .. say 5-10 seconds or so .. with 7 or less components .. ? (I'm getting lazy in my old age .. )

  10. #30
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    My first thought would be an ICL 8038. I'm not sure if they'll move that slow, but it's worth a look. Yes, they are obsolete, but you can find them on Ali Baba dot com in China for less that 2 bucks each, and I've seen them on fleabay.
    I'll look around my files, I may have something op-amp. Do you need variable frequency, and is 7 parts an absolute?
    Jay

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