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Thread: using LSR (laser speckle reducer) with a pico projector, ideas about design need

  1. #1
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    Default using LSR (laser speckle reducer) with a pico projector, ideas about design need

    Idea was sparked from this topic: https://photonlexicon.com/forums/sho...e-have-spekcle
    There appear to be some affordable speckle reducers here: https://www.optotune.com/products/la...eckle-reducers
    There is one "but" here, the price of the despeckler depends on its aperture, so the smaller the laser beam the smaller the despeckler can be.
    The DMD chip I have is 11mm in one axis, sadly a bit more than the stock aperture they have. So the typical setup as in laser scanner projectors of combining R, G, B lasers with dichros won't do because the beams will be collimated before reaching the despeckler and slightly larger for it. Two lenses could be used before and after tghe despeckler to make the beam small to fit the despeckler aperture then back to the size needed, however this won't work with two element lenses as only one wavelength will be properly collimated. To work with 3 colors the number of lenses will take up space and be costy. Any workaround ideas to this?

  2. #2
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    ... you can use a piece of glass fiber with some centimeters length, to "homogenize" the output beam - simply focus the diode into the fiber and then colimate the output ...

    Viktor

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    Are you suggesting to use that as alternative to LSR? Never used fibers. Will that preserve the low divergence of lasers? And what do you mean by homogenization? Thanks.

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    ... a fiber will "break" the coherency of the laser beam by multiple reflections, what I'm calling "homogenizing"

    Here some images of a 405nm single-mode diode at maybe 50mW output power and totally specle-free.

    First image with attached 0.5mm polymer fiber:

    Click image for larger version. 

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    - second image - the "far field" of the not colimated fiber output in around 300mm distance:

    Click image for larger version. 

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    - third image - "collimation" to a spot in maybe 1,5m distance:

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    Viktor

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    Very interesting and outside of what I know about so noob questions ahead:

    1) Will breaking the coherency break the low divergence associated with lasers?

    2) If no, how can I get the beam inside the fiber? Anything special that needs to be done or just shine it into the surface of the fiber? For the projector I need an RGB beam with <150mW power. I can either have the fiber before the collimating lens of each or combine the collimated beams of the R, G and B lasers and pass that to the fiber instead. Which should I go with?

    3) What is the typical power loss?

  6. #6
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    Quote Originally Posted by joec View Post
    1) Will breaking the coherency break the low divergence associated with lasers?
    By passing through a fiber? Yes. That's why you need output optics at the end of a fiber optic cable to re-collimate the beam. (You can see in the pictures above that the beam coming out of the raw end of the fiber is highly divergent.)

    2) How can I get the beam inside the fiber? Anything special that needs to be done or just shine it into the surface of the fiber?
    You need a fiber-launch. You need something to securely hold the fiber in place while having adjustments to position the fiber's face to be perpendicular to the incoming beam. You also need a lens to focus the beam so that it fully enters the fiber. A fiber launch puts all this hardware in a single package.

    3) What is the typical power loss?
    Depends on how much money you spend on the fiber and the optics and how long the fiber is. Cheap solution: 50% loss. Best solution: 10% loss.

    For the projector I need an RGB beam with <150mW power. I can either have the fiber before the collimating lens of each or combine the collimated beams of the R, G and B lasers and pass that to the fiber instead. Which should I go with?
    Collimate and combine all beams to a white light beam first. Direct that to the fiber launch, which will couple the beam to the fiber. Run fiber to wherever you need the light. At the output end of the fiber you'll need another collimating lens, but after that you can send the beam from there to a set of scanners or whatever.

    Adam

  7. #7
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    Thank you again. 50% is very reasonable. I'm surprised why there isn't a pico laser projector already doing this I could buy. Maybe the UO Beam does that though, will check when it arrives.

    From what you described I can have the initial beams and their collimators before the fibers smaller in diameter than the DLP/LCoS chip and use the collimator after the fiber to get the correct diameter?

    Quote Originally Posted by buffo View Post
    By passing through a fiber? Yes.
    What I was wondering and worded poorly was not that whether a collimated beam will come out collimated from the end of the fiber but whether I can re-collimate it and expect the same laser-like collimation.

    At the output end of the fiber you'll need another collimating lens
    Won't a single lens for a combined laser beam refract each wavelength differently and properly collimate only one of them?

    And how would you determine how long of a fiber you need to get a proper "homogenization".

    This is getting pretty interesting.

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    ... fix the fiber near to the diode output, then you can avoid any focussing or colimating lenses on the entry side

    With the testing setup in the images the 0.5mm fiber was placed directly on the output window of the diodes housing.

    Fiber-coupled diodes with lower powers use 0.05mm or 0.105mm fibers, placed inside of the diode housing with roughly 5 to 10 microns distance to the crystals output to avoid Newton interferency ...

    Viktor

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    Quote Originally Posted by VDX View Post
    ... fix the fiber near to the diode output, then you can avoid any focussing or colimating lenses on the entry side
    I would do that but for video projector I have 3 R,G,B diodes, not just one.

    Quote Originally Posted by VDX View Post
    Fiber-coupled diodes with lower powers use 0.05mm or 0.105mm fibers, placed inside of the diode housing with roughly 5 to 10 microns distance to the crystals
    Okay. What tool would you use for that though and how would you fix it in place?

  10. #10
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    ... here some examples of "fiber-coupled" diodes - first with 5 and 9Watts of output power out of a fiber with 105microns:

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    - then a 25Watt-diode, built with tree 9Watt-diodes, colimated with cylinder lenses and focussed into a 105micron fiber:

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    - and here the "recollimated" output on an IR-indicator card:

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    Viktor

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