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Thread: Factory power grading of laser arrays?

  1. #11
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    Default A140

    The A140 I bought has an A written on it, along with a 7. I have yet to remove the diode and test them though. Is there really a difference between any of these arrays?Click image for larger version. 

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  2. #12
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    There is only one way to find out...

    I am hoping to get something other than a B grade in my next shipment, but if not would you be willing to sell the A graded one to me for testing?

    Per my graph, the values are currently +/-50%, typically I was seeing about 5-10% variations comparing a diode to itself, so the spread is significant.

    edit for catalanjo's edit
    the Y axis is not in mw because I have not decided how I want to rate the output power, coming from the raw diode, after a lens lens, etc. It is currently the raw reading from my meter.
    Last edited by krazer; 08-26-2010 at 18:35.

  3. #13
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    Default

    Quote Originally Posted by krazer View Post
    I will add that most of my XJ-A140's have had a B code on them, precisely one of them had an A code. They also have a number written in sharpie elsewhere on the array, usually 17

    Has anyone else noticed significant variation in the output power for diodes across an array? I thought some of them looked a little dim, so I took IU curves for an array and was amazed at the spread
    Attachment 19045

    There is some slight variation due to the test, but after spot checking a few diodes I would say its within ~+/-25%, so the factor of 2 difference is disturbing
    You're getting some pretty major heating on those diodes for them to be rolling over that much. Try doing it without the heating, meaning either 1) pulsed measurements, which you may very well not be set up for, or 2) more heatsinking or temperature control, longer cool-downs, etc.

    You just seem to be getting a lot of roll-over, that I haven't seen as much elsewhere, in addition to that huge variation, which I also haven't seen as much elsewhere.

  4. #14
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    The diode was clamped in a thorlab diode mount (LM9F) on a coldplate at 25.0C. The duration of the sweep is ~50ms, I tried taking sweeps with more points, but started to notice significant heating effects, the 15 point sweep didn't show significantly less power than a 3 point sweep. I did observe significant power loss when running CW, but when running the sweeps I graphed I do not believe heating to be a significant source of the variation.

    I am currently working on finding the sources of error, the thorlabs diode mount really just doesn't seem to cut it for a diode of this power. I was suspicious of my photo diode for a little while, but I added a od2 filter and the shape remained unaffected so I do not think it is due to issues with the detector

    Does anyone know of a good quick diode mount which has good thermal properties? I was hoping the LM9F would work well since it clamps on the whole outside perimeter of the diode quite securely, but the CW measurements don't lie. I have a big fat aluminum block style mount (nearly identical to that used on the projector) which I hope to compare to when I get my next lot of diodes in, but swapping diodes into it takes 4 screws and is quite tedious when testing whole arrays. I am seriously considering getting set up to test the diodes before they have been removed from the array; I have a few ideas to make that work but I am not sure I will have the time to get that set up in the upcoming weeks.

    I am also getting a grating based spectrometer in the next few weeks which will allow me to monitor the wavelength and calculate how much the die itself is heating, I was hoping to use my wavemeter but these diodes have such a wide spectral width (over a nm!) at high powers it doesn't have a chance of getting a reading.

    I was also considering looking at the voltage drop, the temperature of a diode does have a measurable effect on the voltage drop, but I am not sure how close to an ideal diode these diode are. It is a kind of chicken/egg problem, if you assume the diode is ideal you can easily calculate the die temperature, or if you assume the die is at a constant temperature you can easily measure n, but you can't do both at the same time

    One final observation, only the lower power diodes show significant amount of rolloff, the high performers just keep on rising.

  5. #15
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    Semi-related question, since you appear to be online right now...are you doing all this with home equipment, or in a lab at UCSB?

    And heating may not be the cause of the diode-to-diode variation, I don't understand what the difference could be there, but it is still the cause of the rollover. Test pulsed, and the rollover won't be there. It could be that some diodes are more sensitive to the heat than others in your tests? Curious results, none-the-less.

    Very nice that you do have some active cooling and other nice equipment, it seems.

  6. #16
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    I am doing this at home, I am a big fan of quality (used) test equipment. I have however been known to take diodes into the lab to use the OSA or take a nist traceable power reading...

    In any case, your comments inspired me to pull a diode out of my 'for projecting images use only' projector, and comparing a single pulse measurement to the 15 point sweep measurement there was a ~10% droop at 1.5A. I am not sure how I didn't notice it earlier, perhaps my test diode showed less of a variation with respect to temperature. Hopefully that will explain the rolloff, if not the variations in power.

    Time to go back to designing a better test setup...

  7. #17
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    Default A140 Array

    Hey Krazer,

    I want to keep the 'A' array, but would sell you some diode's if that would help in any way.

    -g

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