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Thread: Help with dual red setup.

  1. #1
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    Default Help with dual red setup.

    I have been making a dual red setup with a PBS. I have done what I thought was a perfect alignment. The beam was real nice for about 15min then the LOC on the straight through axis went LED on me. I replaced the LOC and within 5min it was a LED too.

    I searched through the forum and found many references to back-reflection. I believe this is the case here.

    In a couple of posts it was stated to turn the PBS slightly to make the back reflected beams not return to the LOCs. No where was I able to find how much to turn the PBS.

    When adjusting this dual setup, do you make it so the reflected beam does not return to the lens at all being just outside the lens area. Or is it ok to have the reflected beams enter the lens as long as it is not directly centered in the lens?

    I don't want to loose any more diodes due to back-reflection.

    Thanks!

    Magman116

  2. #2
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    are the diodes isolated from each other?
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  3. #3
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    Quote Originally Posted by andy_con View Post
    are the diodes isolated from each other?
    Yes, the diodes are isolated in their mounts using 5mil Kapton tape.

    I use Aixis modules with AR coated glass collimator lenses.

    I am running them in parallel using a Flex Mod P3 with a 1ohm balancing resistor on each diode.

    Drive current to each LOC is set @ 400mA.

  4. #4
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    back reflection does happen but its pretty rare
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  5. #5
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    Quote Originally Posted by andy_con View Post
    back reflection does happen but its pretty rare

    Back reflection is the only explanation I have. Current to each diode is good, isolation is good. temperatures at good. Power supply is very stable.

    I replaced the last blown LOC and had both LOCs running since last night (not through the PBS) and they are still running great right now.

    I am afraid, if I put them through the PBS then the through-axis LOC will blow again.

  6. #6
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    cod due to back reflection is indeed uncommon. Although it's more likely to happen when fiddling around with optics. Suddenly, for one split pico second all conditions are right, you create an external cavity, and poof !
    Due to fabrication tolerances, if you replace your diode your laser-die will never be in the same place twice, and point in the same direction. So a double backreflection death is extremely unlikely.

    Don't rule out esd, it's not always an instant killer, propagated delayed defects will kill eventually too. Short your diodes as soon as you receive them, if not already. And unshort them when connected to a lasorb.

    Swap your drivers to rule out one of them causing havoc.

  7. #7
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    The diodes you are using (LOC) are rated for 240mA CW. You drive them with 400mA this makes them very sensitive to for backreflection, and is only possible with good optics.

    Try running them at 300mA and if they still die replace the psu.

  8. #8
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    Quote Originally Posted by -bart- View Post
    cod due to back reflection is indeed uncommon. Although it's more likely to happen when fiddling around with optics. Suddenly, for one split pico second all conditions are right, you create an external cavity, and poof !
    Due to fabrication tolerances, if you replace your diode your laser-die will never be in the same place twice, and point in the same direction. So a double backreflection death is extremely unlikely.

    Don't rule out esd, it's not always an instant killer, propagated delayed defects will kill eventually too. Short your diodes as soon as you receive them, if not already. And unshort them when connected to a lasorb.

    Swap your drivers to rule out one of them causing havoc.
    Each time I replaced the diodes, I used a new Aixis module.

    My setup is on an ESD protected workbench designed for electronics manufacturing. I am an electrical engineer and have a pretty full electronics lab, taking into consideration ESD.

    I have two Flexmods. I guess I could run each LOC off its own driver. I would think that running them for 14+ hrs continuously on the same flexmod would indicated there wasn't an issue with the driver.

    So, Bart, should I not worry about the laser alignment to the PBS and get them as square as possible? Or should I factor in a small turn to the PBS to slightly offset the reflected beams?

  9. #9
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    I just inspected one of the LOCs under the microscope. I saw the junction wire-bonds were still intact. Then slowly brought the voltage and current up to about 300ma. This was more than enough to illuminate the die.

    Everything looked normal until I zoomed in. Once zoomed in, it looked like the very front of the die had a very small burn mark on it. It looked like this burn mark was preventing the light from emitting from the front of the die.

    This looks to be back-reflection damage. I will see if I can't scrape the burn spot and see if it will lase.

    Scraping was not a good thing. The die started to crumble where the burn mark was. If I chipped a small portion of the die away, I could get some lasing, but small. I tried to do more, but broke the die.

    Something still burned the end of the die.
    Last edited by magman116; 01-09-2011 at 10:09.

  10. #10
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    Quote Originally Posted by mccarrot View Post
    The diodes you are using (LOC) are rated for 240mA CW. You drive them with 400mA this makes them very sensitive to for backreflection, and is only possible with good optics.

    Try running them at 300mA and if they still die replace the psu.
    Been running them at 300mA. I have the PBS offset slightly with good near-field and far-field alignment. No problems yet. I do notice the through-axis LOC is drawing about 1.3mA more than the other, but that isn't a significant amount.

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