Page 2 of 2 FirstFirst 12
Results 11 to 16 of 16

Thread: Diode collimator determination - theories and methods?

  1. #11
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    If the input beam out of the aspheric collimator was perfectly parallel then the cylinder pair could be placed anywhere with the same 2:1 magnification. But, because the reds diverge so quickly, even when collimated, then the further the first (negative) lens is placed from the diode the more powerful the resulting divergence out of this negative lens, This is because the beam is intercepting the glass over a larger diameter and this means the more steeply curved glass is being utilized. Another way of saying this is that the lens will be acting at a shorter F ratio.

  2. #12
    Join Date
    Dec 2010
    Location
    DC/VA metro area, USA
    Posts
    554

    Default

    Ah, I see! OK, very good, thanks for the education.

  3. #13
    Join Date
    Dec 2010
    Location
    DC/VA metro area, USA
    Posts
    554

    Default Some initial measurements

    I had some time to work with my DTR RGB bundle today (PL450B, PL520, HL63603TG), which I ordered with the 3-element glass lenses. It appears that these lenses have a relatively long FL, on the order of 7 or 8 mm per some difficult measurements, and I was unable to get a beam at the front face of the lens that was less than about 5mm in width for any of the diodes. The blue was the tightest, the green was in the middle, and the red was the longest. The shapes were very rectangular, which surprised me a bit for these single-mode diodes, as for some reason I had been under the impression that they collimated up nicely without the need for secondary correction optics. Be that as it may, 5-6mm at the lens face is clearly not going to fit into a 3mm scanner aperture, so it seems that I need some shorter FL lenses (or bigger scanner mirrors?!)

    With some careful adjustment, it is possible to minimize the 'spot' at a particular distance by using the collimator as a focusing element, but this of course produces fat beams at any significant distance beyond that distance. Is there some optical arrangement one can use to get as close as possible to true collimation for at least one axis?

    My reading of the geometry is that if I use a collimator with half the FL, the beam width at the exit aperture will also be halved but the divergence would be doubled, is that true?

    What sort of beam shapes and diameters should I be expecting out of these single mode diodes? Pix below.

    Click image for larger version. 

Name:	red-crop.jpg 
Views:	12 
Size:	128.9 KB 
ID:	45336
    Click image for larger version. 

Name:	green-crop.jpg 
Views:	12 
Size:	101.8 KB 
ID:	45337
    Click image for larger version. 

Name:	blue-crop.jpg 
Views:	10 
Size:	110.5 KB 
ID:	45338

  4. #14
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    There is a property called the beam waist. The two "forces", focusing and divergence are warring with each other as the beam propagates. If you attempt to collimate the beam from the diode out so that the narrowest point is at the exit aperture then until you get very far from the diode the beam will be fatter at all distances in between those limits than if the beam was focused at any given distance in between. I believe that the far field limit depends on the quality of the beam and so will vary from one laser to another.

    I have had very good luck focusing the beams to a distance that significantly exceeds the anticipated projection distance (like 50%), but not at infinity. This has held up to 50M projections.

  5. #15
    Join Date
    Dec 2010
    Location
    DC/VA metro area, USA
    Posts
    554

    Default

    Thanks planters, appreciate the advice. Right now I have beams a little too big for my scanner mirrors. Would you swap scanner mirrors, or swap collimators? I'm leaning towards scanner mirrors, but it seems like the bigger hassle.

  6. #16
    Join Date
    Dec 2007
    Location
    Nottingham, UK
    Posts
    2,845

    Default

    Replace all three lenses with shorter FL (~4mm) You should get very nice beams (2-3mm) out of these at low divergences (<1mRad), with no real need for secondary correction. The aspect ratio of the emitters are still slightly rectangular, but nothing to warranty one axis beam shaping like the multimode R/G/B diodes.
    - There is no such word as "can't" -
    - 60% of the time it works every time -

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •