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Thread: fiber optic beam combining

  1. #21
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    I find cyl len just fine. You got me thinking though why not pass the diodes through the first concave lens of the set to make the line and then knife the diodes(one concave per diode and then send the result together through the convex second lens after the knife edge. Saves one optic per diode. Seems you can align the stripes much tighter. Just need to have the distance between the optic large enough for the extra path length.
    I have no plan to try this but if someone wants to let me know if it works. I doubt the trouble is worth it for two diodes but a stack of 4 might be worth the effort and eliminates the cubes and waveplates.
    The trade off for eliminating an optical component to save space and money is the loss of a degree of freedom when aligning. Personally, I have found the advantage of alignment flexibility allows me to produce more precise alignment. There is no free lunch. The smaller spot at a given point will not allow a tighter final result, it will simply require a higher precision knife edging to avoid wider spacing. This then brings up the issue of edge chips and coating taper and chamfer etc.

    reading about fiber lasers the limit seems to be 6 diodes in a combiner before you overload the core.
    Also, these diodes are not going to produce a brighter far field spot due to passing through the fiber. Of course, I am not talking about the fiber laser itself which is actually generated in the active fiber.

  2. #22
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    Can anyone explain to someone far less experienced, are there any advantages to using fibres instead of good-ol knife edging when it comes to the quality (beam profile and diameter) of the beam which goes to the scanners?
    If yes, why? (if possible to be explained in a nutshell)
    The answer is no, there is not. The fidelity of the ray paths is lost as the beam scrambles down the length of the fiber. A fiber pig-tailed diode can be used to homogenize the output of the diode and the resulting divergence will be dependent on the exit diameter of the fiber. Scrambling the ray paths reduces the quality of the diode output as it relates to the far field spot size. The argument is thermodynamic. The beam's entropy has increased. Information is lost (well, not lost, but transferred to the environment). The exception here is if a single mode laser is fired into a few micron diameter, single mode fiber then the beam remains effectively as it started when it emerges, but no better.

  3. #23
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    Quote Originally Posted by planters View Post
    The answer is no, there is not. The fidelity of the ray paths is lost as the beam scrambles down the length of the fiber. A fiber pig-tailed diode can be used to homogenize the output of the diode and the resulting divergence will be dependent on the exit diameter of the fiber. Scrambling the ray paths reduces the quality of the diode output as it relates to the far field spot size. The argument is thermodynamic. The beam's entropy has increased. Information is lost (well, not lost, but transferred to the environment). The exception here is if a single mode laser is fired into a few micron diameter, single mode fiber then the beam remains effectively as it started when it emerges, but no better.
    I see. So is it a fair assumption that they use fibers to simply speed up projector assembly?

  4. #24
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    I see. So is it a fair assumption that they use fibers to simply speed up projector assembly?
    I don't really know. It seems weird. It may be for that reason or for marketing or they may have a CN source of these pig tailed diodes and this makes the projector layout more compact.

  5. #25
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    Quote Originally Posted by planters View Post
    It may be for that reason or for marketing
    That makes sense now that you mention it.

  6. #26
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    Quote Originally Posted by planters View Post
    I do not understand the novelty of this approach. I do not disagree that it works, but how is this different than a FAC diode?............

    Price and availability

    Using a fiber laterally as a small, cylinder shaped lens to FAC a diode output is interesting, but remember that these fibers are not AR coated on the surface that is designed to retain light by total internal reflection. That is a loss of 4-5%/surface times two surfaces. ............

    Correct as usual, but as you point out, some empirical results would be quite interesting, ....... tricky bit would be positioning & fixing.

    The output from a multimode diode at any wavelength is a cluster of ray paths that is usually pretty symmetrical in each axis ie l ll l, but it is not a smooth and continuous transition like the Gaussian spread of a single mode beam. Trying to get the smallest focus which is the same as achieving the lowest divergence requires a complex lens shape. Even the main stripe is not the same as the single mode spot and has structure.

    Correct again as usual, but this is what makes FACs so horribly expensive !

    It should not matter what scale of the correction lens and spacing that you use, ie a 20mm lens at 10mm from the diode or 100um at 50um from the diode, the result should be the same. The rays will follow straight paths to each lens surface and the correction should be scale invariant. The larger lens will be less expensive and much easier to position. The divergence will be the same and so even if the FAC diode's small spot would seem to allow tighter knife edging (more beams/mm^2) the divergence will be proportionally higher and the far field brightness will be the same.
    Agreed again, ... but unfortunately this means a "bigger" projector and seems to be a big downside for many people.

    I happen to believe that knife edging gets MORE efficient when beam size is INCREASED due to the relative size of the "gaps"
    between beams,........... however the trade off losses created by the beam size reduction telescope turn out to be quite large, not to mention the increase in price of the cyl lenses involved, which is why I personally, am quite interested in Viktors suggestions.

    Cheers

  7. #27
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    I happen to believe that knife edging gets MORE efficient when beam size is INCREASED due to the relative size of the "gaps"
    This is very true!

  8. #28
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    Quote Originally Posted by kecked View Post
    I find cyl len just fine. You got me thinking though why not pass the diodes through the first concave lens of the set to make the line and then knife the diodes(one concave per diode and then send the result together through the convex second lens after the knife edge. Saves one optic per diode. Seems you can align the stripes much tighter. Just need to have the distance between the optic large enough for the extra path length.
    I have no plan to try this but if someone wants to let me know if it works. I doubt the trouble is worth it for two diodes but a stack of 4 might be worth the effort and eliminates the cubes and waveplates.
    This sounds like what I had in mind for the fibres but saving a bit of $ in the multiple cyl lenses and spending a bit more on a single convex recollimator (if necessary) after the knife edging to compensate for any abberations produced by the fibre.

    However it would seem that in Mr.P's opinion the losses involved (no AR coating) might well outweigh the advantages!

    Spose it's a question of suck it and see ....but how to position/fix the fibres so that the wide (but not tall) nearfield beams are as near as possible fast axis focused in the far field and can be still be stacked vertically has me stumped for the time being !

    Obviously the slow axis would still need sorting but I would still use wave plate and PBS to double up the output power.

    Cheers
    Last edited by catalanjo; 07-03-2016 at 19:11.

  9. #29
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    However it would seem that in Mr.P's opinion the losses involved (no AR coating) might well outweigh the advantages!
    There is another overhead that is probably more significant. Quality! Unless you have a micro manipulator that can position this tiny fiber segment in X and Y as well as rotation (and rotation is the secret killer), the off axis aberrations will degrade the beam. For example, spherical aberration will occur if the fiber is not EXACTLY parallel in its distance to the output face. Coma, if it's center is not in line with the junction and a twisted astigmatism with rotation (as a propeller rotates) in front of the diode. The magnitude of all of these errors increases as the inverse square of the focal length of the "fiber-as a lens".

    This is why a bigger projector layout can be made so much better.

  10. #30
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    ... it's not so hard to DIY a 3/4-axis mount or micro-manipulator - here's an image of a common type with two screws for X and Y (the skewed cylinder holds a counter-spring), rotating the complete mount for angular adjusting and the big thread (with a counter-spring too) can be used for Z:

    Click image for larger version. 

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    This can not only be used for positioning lenses, but with attached tweezers or other manipulators for all sorts of "manipulating" small objects with high precision.


    When building the assembly-stages for fiberscopes and micro-sensors, I've made some micro-positioners/manipulators with 4, 5 or 6 axes per block and sometimes 2 to 3 aditional axes as base for combining this-blocks to multiaxis modules for handling multiple fibers, lenses or electronic parts ...

    Viktor

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