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Thread: Interesting kickstarter

  1. #21
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    Quote Originally Posted by mixedgas View Post
    SO WHAT did they do to get stable lasers?

    Steve
    The green is a DPSS pointer core. The red is a singlemode diode, as is the blue. The green appears to exhibit what I would describe as a wicked mode hop about ten minutes after it's powered up. After that, it's perfectly stable as far as I can tell. I used a microscope to measure the interference pattern produced when it went through a 1/8th inch glass slide- this is as close as I can get to a fabry-perot right now, as my lab is currently set up for electronics fabrication- and the fringes didn't move at all in 20 minutes.

    The blue appears to be one of the Nichia singlemode 450 blues, and the red looks like a 650nm diode of some kind. All three have transistor based current sources and run off of three volts.

  2. #22
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    Ok, so they current starved the green. That makes some sense.

    Steve

  3. #23
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    Heroic, Listen to the mode hops. Yes, use audio. Take a photodiode signal and run it into a transimpedance amp. Run that to another op-amp rigged as a comparitor. Split the signal from the PD. One side goes to a RC lowpass via a diode, Set the RC constant to say one second time constant. This sets the DC average level and goes to one input on the opamp. The other is the unfiltered raw dc light signal to the other input. AC couple the op-amp output and run it to the apartment amplifer du jour.... So now you have a constant fraction discriminator driving a speaker. Mode hops now show up as loud clicks.

    Redlum has a circuit here and video on his main page.

    http://redlum.xohp.pagesperso-orange...ser/noise.html

    Its a old trick, but it works.

    Steve

  4. #24
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    Alternatively I could just hook it to the oscilloscope.

  5. #25
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    Whats the fun in that? Cricket clicking is much more enjoyable.

    Steve

  6. #26
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    My scope's got an FFT and a multichannel analyser feature that works up to 1 GHz, might be some interesting data there

  7. #27
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    To see the mode beating your going to need a very tiny pd, on the order of .1 mm chip size or less. A of the better cheap fiber to the desktop modules have them. They need to back biased by 9 volts or more and terminated into 50 ohms, or terminated and amplified by a MMIC such as a MAR-4 or better.

    Steve

  8. #28
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    "Granny, mixedgas has some tips for sucking eggs"

  9. #29
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    Ah you guys are all to new world ! Just expand the beam on to a glass slide and look to the reflected interferomic fringes on the wall, its then very easy to see mode hops !
    Fizeau interferometer

  10. #30
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    That's what I was doing.

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