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Thread: Compressing fast axis instead of expanding slow axis to correct diodes

  1. #1
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    Default Compressing fast axis instead of expanding slow axis to correct diodes

    Ok I was discussing cylinders in a thread and got to thinking why do we expand the slow axis to match the fast axis instead of compressing the fast axis to match the slow axis? My thinking is why match the worse parameter? There has to be a trade off here for the better divergence but not sure what it is as the beam is now both smaller and diverging slower. If it made sense it would be done. Ok school me.

  2. #2
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    @ My thinking is why match the worse parameter?
    We treat the fast axis first so as to stop the beam becoming too large to manage if not done within mm of diode output.

    We expand the slow axis due to the over correction of divergence caused by the fast axis collimator lens.
    This lens "could" be replaced by a cylindrical lens (so as not to affect the slow axis and somewhere there is a thread discussing this option.
    Someone suggested a fibre (VDX I think) but Eric blew the idea out of the water for very good reasons that I can't remember offhand, due to me becoming old and senile with a memory like a sieve!

    I do remember being 99% convinced by said arguments however !

    Cheers
    Last edited by catalanjo; 10-16-2016 at 15:11. Reason: access changed to axis = my mistake

  3. #3
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    ... I'm successfully using fibers for "refocussing" the injected and then scattered beam to a perfect round spot again

    But didn't get it good enough for beams yet ... maybe not the correct optics setup for far (infinite) beam shaping ...

    Viktor

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    Quote Originally Posted by kecked View Post
    why do we expand the slow axis to match the fast axis instead of compressing the fast axis to match the slow axis?
    Aren't we expanding the fast axis though, to make it expand, I mean diverge slower? Isn't the "fast axis" the one which has more divergence which can be reduced by expanding the beam on that axis?

  5. #5
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    No the fast axis is the one which is made almost parallel by the first collimator lens.
    The slow axis (thin section) then appears to be "fast" because it is "over corrected" but it is still the slow axis, although after this first lens it is expanding faster since the other axis (fat section in the near field) is focused in the far field and almost parallel.

    If we start swapping names after each optical correction everything will get too complicated to be able to discuss properly, so best keep original names despite local appearances. So fast means fast out of the raw diode and slow means slow out of the raw diode NOT the speed of divergence AFTER the collimator lens.

    This confusion often gives rise to misunderstandings when stacking methods (vertical vs horizontal) are discussed.

    Hope this helps !

    @ Viktor.....yes with that reminder I remember that Eric's reasoning had to do with precision positioning, and aberations which the fibre section could not correct adequately for long distance focusing.

    Cheers
    Last edited by catalanjo; 10-16-2016 at 20:06.

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