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Thread: Modulation efficiency

  1. #21
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    Default I'm not doing it right...

    Well, I discovered the first problem with my method... I'm not using DZ's color correction board correctly (RTFM fail I guess). I forgot to set the gain so that 5V in gives 5V out for each offset...

    I'll update the results soon...

    Quote Originally Posted by Zoof View Post
    Tocket, very interesting! could you also show the modulation signal in the diagrams.
    The warm-up issue can also play part at a very small time scale, i.e., the length of a frame. It is the same problem but I previously characterized that as a duty-cycle dependency problem.
    As you point out the main issue is color: with a low green duty cycle* in a frame my yellow would indeed come out orange.
    I didn;t and don't have access to such a nice meter so I used the eye's sensitivity to color changes to eyeball the quality of modulation.
    (the eye is much more sensitive to color changes that brightness changes)

    *the green laser would be operating colder, reducing the optical output for the same modulation input.
    I have been trying to figure out how to acquire the modulation signal. I managed to find one of these here:
    http://sine.ni.com/nips/cds/view/p/lang/en/nid/14124
    I think that would be ideal, but it is used in a rather important system, so I'm trying to find another solution. Don't want to stir up a shit storm by breaking it.

    If anyone else here has a DAQ and FB3 I'd be really interested in obtaining the modulation signal.

    Alternatively, I have one of these:
    http://labjack.com/u12/specs
    That I can use freely. I suspect that it is too slow (1200 samples/s) to capture the signal unless I can average it somehow. I have no clue about how to do that though.

    Quote Originally Posted by danielbriggs View Post
    Could be wrong on this one, but:
    Do the TEC's on the KTP of the laserwave modules "heat and cool" or just cool?
    The graph would suggest only cool.
    My thought process is if the KTP is only cooled below set temp, then for all intents and purposes the KTP will be at room temp (of which I will assume is less than 25.6 deg C).
    The "heating" will therefore come from the waste pump power unconverted. I assume this will cause the KTP to heat up past 25.6 deg C and then the TEC's will kick in to drag down the temp to a stable value. That is the only reason I can think of as to why there will be a few mins of lag before the KTP warms up, and matches the 'pre-warmed' curve.

    Would it be possible to change the TEC driver to stabilse the KTP? (assuming it is not already)

    Tocket, what was the room temp?

    Dan
    I think they can only cool, at least that's how I understood it from my conversations with bridge.

    The ambient temperature was 21C when I did these measurements. Personally I think that's a very comfortable temperature. Apparently chinese lasers do not.

    Quote Originally Posted by -bart- View Post
    Do you have a way of sampling (and averaging) the color-modulation signals ?

    Showing the REAL relative power instead, which is simply the momentary power divided by the dictated power of the dac.

    You want to remove the "software part" out of the equation.

    How much of the available power is used by the show.
    In other words what's the efficiency of the show, if one only looks at the modulation outputs.
    Yes, I'm working on obtaining the modulation signal. However, if we assume that the diode is ideal with respect to modulation (My gut feeling tells me it's actually >90%, but I'm hoping drlava will give his view on this) then the measured power from the diode can be viewed as the time-averaged modulation signal.

  2. #22
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    Unlike lower powered single-mode diodes, multimode diodes (such as the one used in your tests) do not respond in an "ideal" fashion to modulation either. As current increases, more and more emission regions in the die become active. As each emission region becomes active, the power spikes upward, thereby departing from an ideal linear response curve.

  3. #23
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    Quote Originally Posted by ElektroFreak View Post
    Precise control of a TEC or oven can stabilize CW output no problem, but it cannot react anywhere nearly fast enough to cope with these sub-millisecond fluctuations. I don't know of a single option currently available that can.
    What we need is a photon capacitor!
    - There is no such word as "can't" -
    - 60% of the time it works every time -

  4. #24
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    Yes we do! That would be a cure-all for LOTS of laser related issues.

  5. #25
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    It would be interesting to see the modulation step response in the millisecond time scale using a fast photodiode such as in previous blue dpss reviews. Then there would be no guessing what the important timescale is in terms of modulation response to the signal. What you are seeing is pretty common, it shows the benefits that can be gained from optical feedback modulation control or better lasers!

  6. #26
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    It would be interesting to see the modulation step response in the millisecond time scale using a fast photodiode such as in previous blue dpss reviews.
    This pic was from quite a while ago, but it shows the modulation input (bottom) and what the optical sensor sees from a blue laser (top). Bit of a mess! If I get a chance, I'll post a new pic from all 3 lasers going through the sensor. Red looks good, green look ok and blue is a disaster!
    Attached Thumbnails Attached Thumbnails DSC09581.JPG  


  7. #27
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    Optical feedback is a good way to improve modulation response, and gives the best results available currently. It seems that Viasho has integrated it into their latest line of high-powered green modules.

  8. #28
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    Quote Originally Posted by ElektroFreak View Post
    Viasho has integrated it into their latest line of high-powered green modules.
    They did? That's a good start! Did you read that somewhere?

    One thing about the pic above, to make the light output absolutely stable, you'd have to set the max power output the lowest point on the top trace.

  9. #29
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    According to Laser Ben's review of their fresh new 4W design, optical feedback is available. Here's the thread: http://photonlexicon.com/forums/show...253#post126253

    Look in the "improvements" section.

    Optical feedback would require the laser to run at a maximum of the lowest power indicated on the scope. That's not really too much of a problem, though.. many people here would love to see a slightly less powerful, but more stable laser.
    Last edited by ElektroFreak; 11-30-2009 at 13:03.

  10. #30
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    unfortunately that review leaves out the very tests that I wanted to do on that laser when he posted about it, covering issues like this.

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