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Thread: Talk to me about back reflection

  1. #1
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    Default Talk to me about back reflection

    I had a 200mw red laser die recently. It was working fine at first. Then I noticed it was only putting out about 1mw. The only thing I was doing with it in between was measuring the beam height so I could make a spacer. I was measuring height by shining the beam onto the head of screw. The screw was stainless steel and shiny. I think the laser probably shined back into itself. So, would that kill it? If so, how careful do you have to be? Would shining onto a piece of white paper reflect enough to be a problem? Or does it have to be mirror-like? I think I learned my lesson. But I want to make sure it doesn't happen again so I'd like to know the facts.

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    My maXYZ died from reflections from a piece of white paper. Just standard, not glossy.
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  3. #3
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    Quote Originally Posted by Laser Ben View Post
    My maXYZ died from reflections from a piece of white paper. Just standard, not glossy.
    one, turn the power down when aligning.

    two a piece of quarter wave film and a linear polarizer make a cheap optical diode. maybe we could do a group buy on 1/4 wave polymer film for deep red. perhaps, since the diode is polarized, all we need is the 1/4 wave film. I dont know and dont feel like blowing a diode trying. A couple of companies make it, edmund used to sell small pieces. A QW plate turns the beam to circular polarization, which when reflected, comes back with the wrong "handedness" and wont interfere or make it through the launching polarizer. If it cant do interference, it cant blow the faucet off the diode so easily.

    what we really want is a faraday rotator (optical diode) but they not too cheap. like used 2 year old porsche with no rust and no miles not cheap.

    film comes from 3M (the minnesota mining and manufacturing company, for you triva buffs) as part # 3M PP2500

    or from like Optografix

    670/4 =167 nm of retardance
    655/4= 163 nm of retartdance

    A half wave plate as you know rotates the polarization, but if the beam reflects back through it, it has symetry and rotates it right back. So you need a 1/4 wave plate when testing
    note that circular polarized passes 50 % through a 50% cube =25% of the light, so only useful AFTER combining the reds or during test.

    perhaps marconi could ship a 2 mm square piece when selling units at a small additional cost.

    Steve Roberts

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    Quote Originally Posted by carmangary View Post
    I was measuring height by shining the beam onto the head of screw. The screw was stainless steel and shiny. I think the laser probably shined back into itself. So, would that kill it?
    Yes, it would. A shiny belt buckle, a shiny mirror mount, a shiny baseplate, a shiny screwdriver blade - they're all reflective enough to kill a diode.
    If so, how careful do you have to be?
    Damn careful, unfortunately. Have a look at this thread for more discussion about killing direct injection diodes with back reflection. Warning: There are some sad stories in there... (Oh, the horror!)

    Here is a section of Sam's Laser FAQ that spells it out quite nicely. (Read down at least 5 or 6 paragraphs from the start of the anchored text.) Note that even when not operating near the maximum power output, you can still cause a diode laser to act funny when you expose it to back reflection. But when you *are* close to the maximum, well, chances are the first time you do it will be your last, because the diode will be gone before you can react. (The damage happens just about instantaneously.)
    Would shining onto a piece of white paper reflect enough to be a problem?
    Depends, but it's certainly possible.
    Or does it have to be mirror-like?
    Would you call the attenuation window on a Coherent Lasercheck power meter wand "mirror-like"? I wouldn't, but I know people that have killed diodes from the back reflection off that attenuation window... Gotta be really careful when you're up near the maximum for the diode. REALLY careful...

    Best bet is to turn the power way down whenever you're aligning things. That way if something slips and you get a mirror (or a shiny mount) in the beam path accidentally, it's not a problem. The 1/4 wave plate is a nice idea, but you're going to get considerable loss across it. Better to design the layout to minimize the potential for back reflection in the first place.

    Adam

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    Best bet is to turn the power way down whenever you're aligning things. That way if something slips and you get a mirror (or a shiny mount) in the beam path accidentally, it's not a problem. The 1/4 wave plate is a nice idea, but you're going to get considerable loss across it. Better to design the layout to minimize the potential for back reflection in the first place.

    waveplate plus state of the art polarizer, maybe 75% gets through

    waveplate only, max 8% loss, less if ar coated.

    but still for alignment, I'd think about the pol/waveplate combo if I were working with a expensive diode, like a 3500$ blue one.

    Steve

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    I have a MaXYZ red coming from Cruch and I was wondering how you guys measure the power on these lasers without getting any back reflection?

    I will be using an LPM-1 meter and don't want to blow the laser. Do you just aim the laser in at an angle rather than directly head on?

    Cheers
    XG

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    When you put the power meter in the beam, you ought to be able to see the very low power reflection coming off the face of the power meter. Adjust the power meter face so it's nearly parallel with the laser output face, but watch for the spot of the reflected beam. Don't let it get too close to the output aperture. (An inch or two is close enough.) Take your power measurement there and you'll be close enough that it won't matter, and you'll still be OK because the beam can't get back into the diode.

    Adam

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