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Thread: FAC lens nubm44 correction.

  1. #1
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    Nov 2017
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    Default FAC lens nubm44 correction.

    I'm designing a laser to shine a dot at a breath-taking 68 kilometers over a frozen lake in Hungary.

    I have experience working with 445nm diodes, my current working laser uses an m140 diode, but I am hoping to engineer a more powerful laser using either singular nubm44's or multiple nubm44's using knife edge array configuration.

    I have noticed that almost all blue solid state laser diodes suffer from fast axis dispersion, which at 68 km would likely be hundreds or thousands of meters wide, which is no good for the task at hand.

    Here is a photo of my current laser:
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    Here's a video of the problem:



    I have come across a special type of lens known as the FAC08-600
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    I've been told that FAC lenses have not been used to date to correct the fast axis of neither the nubm44 nor the m140 laser diodes.

    Click image for larger version. 

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    I can already squish the beam into this rectangle using the G2 lens, however I feel the FAC lens might be able to squish the fast axis into a tiny square or a "dot" suitable for long range work. To my knowledge there is not an adapter for this, so I begun work on a custom, adjustable housing for a FAC lens to a laser pointer. My intention is to use grub screws to correct vertical alignment and a pair of worm-wheel/worm-gear combinations for micrometer adjustments to the lens plate.

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    The design is still a work in progress, I was looking for cheaper sources of brass wormwheel gears, but I feel there may be chatter/rattle. I figured this is most likely the best way to align the lens to within micrometers or precision. I have begun work on the roll design for the lens, no work done to the pitch, do you guys think I should do a yaw also?
    The plan is to machine the DTR laser shop copper 32mm housing so that the entire assembly can screw-thread onto the laser module housing and can use this to adjust distance from the FAC lens to the G2 lens. meanwhile using the grub screw to align the FAC lens vertically and the pair, possibly trio of wormwheels for roll, pitch and possibly also yaw of the lens.
    Should a single nubm44 laser diode be of insufficient power I am considering using 4 of them in a knife edge array also into the same design.
    The holders of the lens are also designed to dissipate heat.
    If you guys think this has a chance in hell of achieving what I think it will please say so.
    I know this design looks and sounds crazy so I'm all ears to your input.

    Thank you in advance.

  2. #2
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    I look forward to seeing the results.

  3. #3
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    One milliradian at 68 kilometers is 68 meters - if you expand a 3mm beam with one milliradian divergence to one meter diameter at the source your beam would still be on the order of 200mm across at 68 kilometers, and vibration would be a real problem...
    "There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso

  4. #4
    Join Date
    Jun 2007
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    Monroe, Mi USA
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    Well....a mRad of 0.45 has been achieved with the NUBM044 diode....using a trio of Cylindrical Optics from Opt Laser....but....at 68 KM....that still equates to a Far Field geometry of slightly more than 27 M !!!!
    .
    I seem to remember that FAC lens positioning....is very, very technically challenging....as in +/- micron tolerance. IIRC....th FAC lens must be optically glued as near as possible to the LD emitter....as is in direct contact....perhaps I am wrong ?????
    .
    Very, very specialized XYZ positioning units are used to place the FAC lens. IIRC....the service of FAC lens optical correction....is driven by volume....so....1000 LD units min order. SO...this is not an option !!
    .
    You may have to rethink and go with a Gas Laser....which has much tighter beam specs....but.....much, much more expensive....and much lower power. On Gas Lasers....some on PL may offer advice....not myself...Good Luck !!!
    .
    CDBEAM
    Beam Axiom #1 ~The Quantum well is DEEP ! Photons for ALL !!
    .
    Beam Axiom #2 ~Yes...As a matter of fact...I DO wear tinfoil on my head !!
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    Beam Axiom #3 ~Whe'n dout...Po ah Donk awn et !!
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    Beam Axiom #4 ~A Chicken in every Pot, and a Laser Lumia in every Livingroom !!
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    Beam Axiom #5 ~"Abstract Photonic Expressionism"....is "Abstractonimical" !!
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    Beam Axiom #6 ~ "A Posse ad Essea" ~ From being possible to being actual ...is the beam target !

  5. #5
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    Denmark
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    Why not add a telescope 10x or so, then the beam would get 30 or 50 mm i diameter at aperture. Then the divergence get really low. :-)

  6. #6
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    I'd love to see the results. That said and because of crazy distance, I'm not sure if you could achieve the required expansion with a FAC to get an 'acceptable' divergence on such project.
    With no FAC and G2 for example, you should measure divergence on both axis 1st (20meters or more). Then as CDBEAM pointed, correct fast axis with 3 cyls (2 CV, 1 CX) and expand until getting as close as possible the divergence on the slow (uncorrected) aixis. Most probably you end up with something about 4mm tall (uncorrected slow axis) x 8mm wide (corrected fast axis) nearfield beam...sorry not real numbers...but maybe about 0,5mrad both axis.
    Both axis matched in divergence means a 'more square' dot farfield. Then you should add a 2-4x telescope (probably more) to get 0.2mrad or less. Expect a very fat beam maybe 20mm wide or more...but this is the only way to get some kind of 'useable' energy density after such distance...mainly with M44.
    If you don't want to telescope, its better with longest FL colli as possible like 3-elements. So you have a good expansion on the slow 'uncorrected' axis as an initial point (about 1/2 divergence than with G2). Then expand fast axis as well to match slow axis. Expect >10mm beam expansion with the cyls....but telescoping is the only way to get very low divergence which seems essential on your project.
    Last edited by jors; 12-11-2017 at 01:19.
    Jordi Luque


    AtenLaser.com
    Barcelona

    "Let there be light"

  7. #7
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    Apart from creating a beam that has a reasonable spotsize @70km away, did you take the curvature of the earth into account ?

    A beam at 1m height, pointed parallel to the earths surface, can not reach a target lower than 326m 70kms away.

    Or

    You need to be on top of a 90m mountain on one side of the lake to be able to hit a 90m mountaintop on the other side, just skimming the center of the lake.
    Last edited by -bart-; 12-14-2017 at 09:55.

  8. #8
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    haha good point! Yes, in other words (following below calculator) you should fire the laser at least 385m height over the lake to target on the ground 70km away. And this is with NO divergence at all. So you should also consider the expanding beam. And maybe the extra curvature on the water surface due moon gravity influence?
    https://dizzib.github.io/earth/curve...85&unit=metric

    Quote Originally Posted by -bart- View Post
    Apart from creating a beam that has a reasonable spotsize @70km away, did you take the curvature of the earth into account ?

    A beam at 1m height, pointed parallel to the earths surface, can not reach a target lower than 326 70kms away.

    Or

    You need to be on top of a 90m mountain on one side of the lake to be able to hit a 90m mountaintop on the other side, just skimming the center of the lake.
    Jordi Luque


    AtenLaser.com
    Barcelona

    "Let there be light"

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