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Thread: Working on a controlled Laser Vortex style thing... a little lost.

  1. #1
    Join Date
    Dec 2010
    Posts
    106

    Default Working on a controlled Laser Vortex style thing... a little lost.

    Hey All,

    Been a long-time member of PL, but been a away for many moons...

    Long ago, I was dabbling into a project, and now I am back on it.

    I want to make an old-school radar screen sort of thing. I have got a demo of sorts working as a programming bed, and I have all the nice tight timing under control.

    With a 3000 RPM motor and hall-effect-sensor, I have really tight control over the laser, but I see that there is a 'pre-tail'. More specifically, it seems my 100mw Greenie ramps up to full brightness under TTL control. The OFF works just fine, nice and sharp and abrupt, as needed...
    Originally I thought it was a programming or processor speed issue, but with all sorts of testing and programming tweaks, NOTHING I do clears it up...

    So I ask the wiser-than-I folks, what gives me this tail?
    Is it indeed a 'ramp-up" time? Seems on the order of 40-50 microseconds, but without a scope, it's not possible to determine that...

    Should I be looking at analog modulation, with a short, dim pulse prior to going to full brightness?
    Should I be looking at a beam shutter?

    I'm open to any advice or testing recommendations.

    Thanks all,

  2. #2
    Join Date
    Jan 2006
    Location
    Charleston, SC
    Posts
    2,147,489,446

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    Quote Originally Posted by GoolGaul View Post
    it seems my 100mw Greenie ramps up to full brightness under TTL control. The OFF works just fine, nice and sharp and abrupt, as needed.
    Unfortunately, the power ramping up that you describe is very common for DPSS lasers. In a projector this can be compensated for by adjusting the blanking offset and/or adding a separate delay to the modulation signals for just the red and blue lasers (which would otherwise come up to full power more quickly than the DPSS green laser would). That way the scanners hold in position until all the lasers are on before they start moving the mirrors again. (DZ's color board was designed to correct exactly this sort of problem.)

    However, it sounds like your application is using a constantly-moving mirror. In that case there's no way to pause the mirror in place while it waits for the green laser to come up to full power. In this case your only option is to leave the DPSS laser running all the time and use a different blanking method (either flag blanking or an AOM, for example).

    Should I be looking at analog modulation, with a short, dim pulse prior to going to full brightness?
    First, does your laser support analog modulation? If so, then you could try this, but honestly it is very unlikely to make much difference. The problem is that DPSS lasers in general have very poor modulation response. You would be much better off using a direct-injection diode laser (so 520 nm green instead of 532 nm green).

    Should I be looking at a beam shutter?
    If you have a very fast shutter, that could work. That's basically what flag blanking is. You have a flag mounted on an actuator of some kind that can rotate the flag into the beam path to block it. If you use a scanner as your actuator, the flag can move very quickly. If you use a rotary solenoid or a stepper motor, then the flag will not be fast enough.

    Adam

  3. #3
    Join Date
    Dec 2007
    Location
    Stockholm, Sweden
    Posts
    551

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    Do you really need to use a DPSS laser? You could get a modern green diode and get rid of the problem!

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