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Thread: The Big Green Thread (I'm gonna regret posting this in the morning)

  1. #371
    Join Date
    Nov 2008
    Location
    Orkland, NZ or Bavaria, Germany
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    169

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    Quote Originally Posted by planters View Post
    I didn't know that CN modules were available with this kind of beam spec. In all my experience this is the best laser beam I have ever seen including the ions. But, when using the direct diode pump I can understand why. You actually have to align the cavity for each power level because unless you are incredibly fortunate the thermal lensing is not just a low order plain-o positive lens, but rather an asymmetrical, high order, un-aligned blob. This is much less with a Gaussian or bell shaped and focused fiber homogenized spot.

    The problem as I see it with the 888 is that aside from the real benefit of the lower quantum defect, the problem of the very low absorption coefficient is that in order to take advantage of the spreading out of the heat effected region you have to produce a very tight and well collimated pump beam to avoid wasting the pump in regions of the crystal that never "see" the mode.
    Defiantly not with your beam spec + power, as you say CN is mostly direct diode, best I've seen from there is a roughly Mē 1.2, 2W 3-4mm and 0.45-0.6mrad depending on each laser and how it is feeling on the day.. For a high CN budget that's about as good as it gets, not really comparable to your results over any reasonable distance, some years down the track however, maybe. To my knowledge in the mid-high single watt CN range currently, there is nothing within light years of your setup.

    I'd hazard a rough guess you are in the 1.1-1.05ish Mē range? About as close to perfect as it gets, only know of one commercially available laser capable of meeting that. I'd assume your measurement is within the Rayleigh length too then?

    Either way, such an achievement at home is highly commendable.

    With new fibre tech that pump beam is only getting better, have a feeling 888 will be very viable in years to come as a result.

    By focusing to below mode size, does this mean the initial area(s) influenced by thermal lensing is reduced in size, so the overall effect of lensing is reduced in comparison to total beam output, thus reducing the impact to the beam? I guess you run into power density limit easier with this approach though.

    Raman fibre lasers

  2. #372
    Join Date
    Feb 2011
    Location
    New Hampshire
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    3,513

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    I'd assume your measurement is within the Rayleigh length too then?
    Yes
    By focusing to below mode size, does this mean the initial area(s) influenced by thermal lensing is reduced in size, so the overall effect of lensing is reduced in comparison to total beam output, thus reducing the impact to the beam? I guess you run into power density limit easier with this approach though.
    Thermal lensing should be thought of as a map of the thermal distribution across the face and with 808nm and a few mm into the crystal. Even the energy drop off at the edge of the spot ie Gaussian vs. bell shaped matters. The size relationship is more subtle than just less vs. more thermal lensing. It is my understanding that by adjusting the size you control the amount of spherical aberration introduced by the thermal lens. I am now testing if by aligning the crystal you can use the thermal birefringence ( the crystal expands more in one axis than the other when it is heated) to compensate for the astigmatism caused by a Z or V fold resonator.

    On your second point,,,YES! I have already fractured a crystal with too small a spot.

    This is fun, but it isn't exactly easy.

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