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Thread: Diffraction Losses

  1. #1
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    Default Diffraction Losses

    I am wondering if anyone here has a handle on cavity diffraction losses.

    I am setting up a cell where I can adjust the gain medium thickness (cross section) transverse to the oscillation axis. The cell walls are fused quartz and ground to a matte finish to prevent the walls from acting as a wave guide. The thinner the cell the lower the allowable ray deviation which will decrease divergence in the output. Also, a thinner cell allows higher dye concentrations and this increases efficiency. The concern is that at some point this plano-plano cavity with a 50% output coupling will have so much diffraction loss that there is no benefit to getting any thinner. The mirrors are 80cm apart.

    I am not looking for a rigorous treatment. I will be able to experiment by purchasing quartz walls of different thicknesses, but I am unable to find even a rough estimate of the magnitude and how it relates to this thickness. For example, with a 2mm thickness and 532nm light expect say 8% losses/pass from diffraction alone. Then, for a 4mm thickness expect say 2% losses etc. This would give me a relationship and an idea for where to start.

  2. #2
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    This really, really, really, depends on the grade of the Quartz. Gas laser Brewster windows used by Coherent, Melles, and Lexel are "Swept" Quartz or Fused Silica where a electric field is applied to Quartz at a high temperature to "pull" impurities to one side through electro-migration. The quartz is then checked with a HENE for scattering centers prior to polishing... Needless to say this is not cheap..
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    An AR coated, Swept Quartz or Superasil window would be ideal and very low loss...
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    Oh wait, I misread that... You want the natural diffraction losses from a short plano-plano cavity.. You need Kolgelnik's (spelling?) work on laser cavity modes and optics. I'll dig it up. Probably Dr. Anthony Siegman's book on laser cavities as well...

    Steve
    Last edited by mixedgas; 02-04-2016 at 09:07.
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    Fox and Li, Kolgelnik, and others are referenced in attached PDF...
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    See attached... PDf will be pulled after 2 days for Fair Use clause...
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    The Google search is "diffraction loss or losses in a plano-parallel cavity" or similar search terms...
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    For example:
    http://www.ece.umd.edu/~davis/chapter16.pdf
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    Good Luck,
    {
    Steve
    Attached Thumbnails Attached Thumbnails beams_and_resonators_1.pdf  

    Last edited by mixedgas; 02-04-2016 at 09:20.
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    Thanks Steve.

    A little more searching came up with this:

    http://eprints.soton.ac.uk/78842/1/4P.pdf

    As the gap can very from anything as low as 100um to several mm, I believe a 1mm gap will be a good start. Machining tolerances and the inability to maintain strictly parallel walls will make the lower limit more likely to create loss issues aside from only diffraction . The upper limit is basically a standard cell expanded in one dimension and this is not interesting.

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