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Thread: P3 Flexmod

  1. #1
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    Default P3 Flexmod

    I was having a problem where in I could not get the green diode to completely extinguish with bias all the way down. This is fixed by the middle pot. Thought I'd pass that along.

  2. #2

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    Quote Originally Posted by kecked View Post
    I was having a problem where in I could not get the green diode to completely extinguish with bias all the way down. This is fixed by the middle pot. Thought I'd pass that along.
    The MIDDLE pot!? NNNOOOOOOO *head kersplodes*
    Neat!

  3. #3

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    Just wondering... what is this thing about extinguishing diodes? I thought they lasted longer and modulated faster if you only go down to just below threshold of lasing. I have since read of 'standby blanking' and such. If this doesn't mean putting drive current just below threshold, what does it mean, and why is the old standard good advice for laser diode drive and modulation changed?

    I understand that stronger diodes look brighter at threshold than weaker ones do, but context is everything, if the ambient light demands a stronger laser, then why isn't the threshold current's resulting output considered weak enough, given the context of the brighter ambient light? Blanking a diode to total darkness stresses it badly for fast modulations, so why are people insisting on doing this?

  4. #4

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    Quote Originally Posted by The_Doctor View Post
    Just wondering... what is this thing about extinguishing diodes? I thought they lasted longer and modulated faster if you only go down to just below threshold of lasing. I have since read of 'standby blanking' and such. If this doesn't mean putting drive current just below threshold, what does it mean, and why is the old standard good advice for laser diode drive and modulation changed?

    I understand that stronger diodes look brighter at threshold than weaker ones do, but context is everything, if the ambient light demands a stronger laser, then why isn't the threshold current's resulting output considered weak enough, given the context of the brighter ambient light? Blanking a diode to total darkness stresses it badly for fast modulations, so why are people insisting on doing this?
    Isn't the problem more with uncontrolled beams than brightness/contrast? With no input from a controller the laser shouldn't be producing light.
    Neat!

  5. #5
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    Quote Originally Posted by The_Doctor View Post
    Just wondering... what is this thing about extinguishing diodes? I thought they lasted longer and modulated faster if you only go down to just below threshold of lasing. I have since read of 'standby blanking' and such. If this doesn't mean putting drive current just below threshold, what does it mean, and why is the old standard good advice for laser diode drive and modulation changed?

    I understand that stronger diodes look brighter at threshold than weaker ones do, but context is everything, if the ambient light demands a stronger laser, then why isn't the threshold current's resulting output considered weak enough, given the context of the brighter ambient light? Blanking a diode to total darkness stresses it badly for fast modulations, so why are people insisting on doing this?
    That's interesting. I didn't know that. I am still undecided re. beam suppression on startup delay. Maybe I'll stick with the delay on the P3 for the DTR bundle.

    Keith

  6. #6
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    i'd opt for beam suppression over startup delay. you can always add global delay with a relay and a timer, but suppression will improve the quality of your output.

    Quote Originally Posted by Galvonaut View Post
    That's interesting. I didn't know that. I am still undecided re. beam suppression on startup delay. Maybe I'll stick with the delay on the P3 for the DTR bundle.

    Keith
    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.

  7. #7

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    Interesting. That too puzzles me. To modulate fast, a driver has to do things that make slow startup meaningless, it is enough to make the maximum slew rate no faster than will adequately modulate the diode at maximum rate, and to make sure that during startup the current can never peak even for an instant above maximum drive. This can be achieved with a 'slow' startup on microsecond time scales, as my own driver does, but if it ramps up like a domestic dimmer switch it's useless, it just means you have to wait, for no reason at all. I used to make very slow-start constant current drivers, but quit that the moment I designed a fast modulation driver, for obvious reasons as explained just now. Real slow starts probably are a hangover from days when constant drive was wanted for expensive diodes, so they were treated like very fragile babies to protect from any kind of shock, thermal, ESD, mechanical, etc.

    My question about beam suppression still stands. Why is simple reduction to below threshold not considered good enough, given that it used to be the only recommendation because any lower stresses the diode?

  8. #8
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    Slow start is needed with power supplies that have poor regulation at startup. Once the supply rail has stabilized, and the diode is conducting, a proper slow start circuit drops out of the picture. Slow start is good for home constructed systems, as to save cost they have poorly defined startup states. I've seen driver designs where the pass transistor starts conducting before the current sources in the driving op-amp have stabilized. This is bad.

    Slow start is good, and the slow start circuit should drop out once the diode is running.

    I'll also tell you that on a proper slow start system, it should not take longer then 30 milliseconds for the slow start to function. You should not see the slow ramp up in the diode light by eye.

    Steve
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  9. #9
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    1) diodes still glow even below threshold. the glow is bright enough to be projected beyond the galvos and it's quite distracting in graphic and abstract shows.

    2) the low end modulation response time of the diodes vary slightly, but noticeably on the low end. if you can unify the low end modulation characteristics of the diodes by adjusting the bias current on the diode drivers, it will be easier and more accurate to do palette training.

    the best solution is to match low end modulation response characteristics in the drivers and then set beam suppression for anything below the base level you want the diodes to come on it.

    Quote Originally Posted by The_Doctor View Post
    Interesting. That too puzzles me. To modulate fast, a driver has to do things that make slow startup meaningless, it is enough to make the maximum slew rate no faster than will adequately modulate the diode at maximum rate, and to make sure that during startup the current can never peak even for an instant above maximum drive. This can be achieved with a 'slow' startup on microsecond time scales, as my own driver does, but if it ramps up like a domestic dimmer switch it's useless, it just means you have to wait, for no reason at all. I used to make very slow-start constant current drivers, but quit that the moment I designed a fast modulation driver, for obvious reasons as explained just now. Real slow starts probably are a hangover from days when constant drive was wanted for expensive diodes, so they were treated like very fragile babies to protect from any kind of shock, thermal, ESD, mechanical, etc.

    My question about beam suppression still stands. Why is simple reduction to below threshold not considered good enough, given that it used to be the only recommendation because any lower stresses the diode?
    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.

  10. #10
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    Thanks Swami, I had decided on beam suppression then undecided again - Beam suppression it is!

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