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Thread: Help applying an AOM on a dpss.

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    Laser Warning Help applying an AOM on a dpss.

    Hi guys, I could use some help applying and AA aom.
    I want to use it to modulate my 2,5W halfnote laser.
    Problem is I won't get an efficiency above 70%. And I really would like to maximize my output.

    I suspect this is caused by the beam characteristics as it leaves the laser.
    The beam has a diameter of 3mm and a focal point of approx. 1000mm behind the laser, that makes it 1,5mrad if I'm right.
    Although this is small enough not to hit the aperture of the aom and pass through the crystal, the specs mention an active aperture of 2mm. They also give an increasing efficiency with an increasing diameter. Is my assumption true that, even if the beam physically fits the crystal, this doesn't automatically mean an optimal efficiency?

    If I really want that deflection efficiency of 90% I guess I'll have to collimate the beam down to 2mm. How do I do this as economically as possible ? Without spending >400 on an adjustable telescope ?
    A telescope can be created by any two positive lenses, but what are the pitfalls when constructing a telescope. What lenses are specifically suited?

    Other tips and tricks on applying these aoms are greatly appriciated.



    specs on the aom
    http://www.isomet.com/FinalWebSite/PDFDocs/AO%20Sheets/1205C-1(2).pdf
    specs on the driver
    http://www.isomet.com/FinalWebSite/PDFDocs/RF%20Sheets/230.pdf

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    Aside from changing the way the pump beam propogates through the SHG crystal, you will have to compress the beam. This may be attained with the lenses you mentioned...say around 140fl and 70fl. It will depend on the beam diameter and focal point of the beam.....should be infinity.
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    A down telescope can help.

    Or image the beam waist into the crystal then collimate it.

    Beam==== ()---AO---()===== scanners = down telescope.

    where () = a AR coated 50 mm FL lens as the "sweet spot" in the crystal is often smaller then the window. Put the two 50 mm lenses roughly 100 mm apart. Use a lens at least 15 mm in diameter so you avoid abberation. Or mech engs 140/75, or a cavity change operation, either will work.

    140/75 will get you a larger range of beam diameters.

    When I want to beat that 70% number, I have found it useful to have the AO on a mount that can tip and tilt as well as do height and bragg angle. Also don't overdrive the crystal. Use just as much RF as you need to get peak power, and then just a tiny amount more. Most drivers have a adjustment for that. The sweet spot is often closer to the transducer on the side of the crystal then in the center.

    Steve
    Last edited by mixedgas; 10-31-2009 at 12:29.
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    Hi -bart-,
    I think this the the correct link to aom data sheet: http://opto.braggcell.com/uploads/files/MTS110A3VIS.pdf
    couldn't find a data sheet on the exact driver, but this is the info i was able to gather
    driver:
    carrier frequency: 110MHz
    power: 28VDC
    Rise Time / Fall Time: <8ns
    Video in: 0-5v / 50Ω
    Operating temp: 10-40C (heat exchange though base plate)

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    I'm getting there; almost 80% now
    I am a little cautious focusing the beam into the crystal, the damage threshold is 5W/mm2. Exponentials you know... If the waist of my 2,5W beam just drops below 1mm, I'm screwed. Well at least the crystal.


    Can anybody shed some light on those saturn-like ellipses?
    Are they bad or good, and should I get rid of them ?

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    first of all the 5 W over one mm number is a bit low. You'll see blooming of the beam before you do any damage to the crystal. Blooming is a difference in the beam diameter caused by the crystal heating up at the focus. The beam swells to a big spot if the focus is too tight, long before there is damage.

    The stray light issues are caused by two things, diffraction by the hole the beam passes through, and the "Atoms" are other diffractive orders caused by the RF level being a bit too high or too much distorted square wave in the crystal. You can also get ultrasonic reflections inside the crystal as drive is increased and get multiple orders that way. Square waves cause Ramen-Nath diffraction in the crystal while Bragg diffraction which in theory puts out exacty only a first order and a zeroth order beam is by a pure sine wave. If the RF amp is overdriven or its gain set too high, it "squares up" its output with distortion.

    So back off the drive, find the sweet spot and the sweetest angle.

    You will find the atoms are only a few hundred microwatts to milliwatts and thus can be ignore when things are tuned properly.

    Of course no AO is perfect, so at some point you just have to live with it.

    Steve
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    What losses are to be expected on a good beam expander, one like dave has for sale ?

    Right now, I'm losing more on the collimating optics than I gain at the aom using those optics...

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    I assume 2% to 5% per coated surface? so if the assembly is 3 lenses, maybe 6-10%?

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    Quote Originally Posted by shrad View Post
    I assume 2% to 5% per coated surface? so if the assembly is 3 lenses, maybe 6-10%?
    Typical coated lens is 3% simple "V" coat, .5 % or less multilayer

    Steve
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