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Thread: Calculate the cylindrical lens for the blue 445nm

  1. #1
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    Default Calculate the cylindrical lens for the blue 445nm

    Hello guys,

    I found a document in Word on the internet to calculate the cylindrical lens for the blue


    Circularize the laser diode beam using cylindrical lens

    Cylindrical lenses focus or expand light in one axis only. They can be used to expand the output of the laser diode into a symmetrical beam. The rectangular cross-section of the laser diode cavity results in a beam shape with a wider angular output from the narrow cavity dimension and a narrower angular output from the wide cavity dimension. Laser diodes have an asymmetric radiation pattern which, when collimated, becomes an elliptical beam pattern. As a result, anamorphic prism pairs are often used to convert the elliptical collimated beam to a circular shape.

    One can also use a pair of cylindrical lenses to achieve similar results to the anamorphic prism pairs. One cylindrical lens can be used to collimate the laser diode’s slow axis and a second cylindrical lens can be used to collimate the fast axis. By simply matching the laser diode beam divergence ratio (vertical vs. horizontal) to the ratio of the focal lengths of the two lenses, a nearly symmetrical circular beam can be created.

    Since a laser diode is considered as a point source, the two lenses should be placed a distance equal to their respective focal lengths from the laser diode emitter orthogonally to create a collimated beam output. Make sure the beam size at each lens does not exceed the clear aperture of the lens. The output beam width is equal to 2f tan Θ (where Θ is beam divergence).

    Below is an example of how to match the laser diode beam divergence ratio with the focal length ratio of two cylindrical lenses to create a nearly circular beam.

    Specifications of the Thorlabs HL6320G, 635nm, 10mW Laser Diode
    G1 = 11 Divergence x-axis FWHM (deg)
    G2 = 37 Divergence y-axis FWHM (deg)
    Divergence ratio: 3.364

    Specifications of cylindrical lenses
    F1 = 50.0mm Thorlabs’ LJ1695L1
    F2 = 12.7mm Thorlabs’ LJ1942L1
    Focal length ratio: 3.937

    Specifications of output beam
    d1 = 2f1 tan (G1) = 19.44mm
    d2 = 2f2 tan (G2) = 19.14mm

    As you can see, the output beam is slightly asymmetric but with a great improvement on the beam shape quality.


    Maybe this could be of any help to calculate the cilindrical lens.

    You only have to fill in the formule for the x-as (fast axis)

    LarryDFW from LPF measured 40 degrees on the x-as (fast axis) and 12 degrees on the slow (y) axis

    Who is a wizzkid

  2. #2
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    I read that document a couple of months ago.
    I believe it is meant to collimate a diode with two cilindrical lenses instead of one asphere.

    You can't get as close with cylindrical lenses though, sou you end up with a very fat beam with low divergence ...

  3. #3
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    yes, it's the "two cylinder method" isn't it?

    do someone have an optics simulation software we could use to compute cylindrical lens requirements?

  4. #4
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    Or use a aspheric, then 2 cylindrical lenses set up as a beam expander to tame the fast axis..
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  5. #5
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    Quote Originally Posted by shrad View Post
    yes, it's the "two cylinder method" isn't it?
    You mean something like this?

    Click image for larger version. 

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ID:	17707

    From http://www.newport.com/Beam-Shaping-...3/catalog.aspx, also contains some useful formulas for this application.

  6. #6
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    Quote Originally Posted by skai View Post
    You mean something like this?

    Click image for larger version. 

Name:	1267183.gif 
Views:	34 
Size:	57.8 KB 
ID:	17707

    From http://www.newport.com/Beam-Shaping-...3/catalog.aspx, also contains some useful formulas for this application.
    Exactly, not quite usefull for us

  7. #7
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    Quote Originally Posted by dave View Post
    Or use a aspheric, then 2 cylindrical lenses set up as a beam expander to tame the fast axis..
    Stellar work again Dave

    Either that or a fairly short FL (around 5mm) collimator and a pair of cylinders to expand just the slow axis to reduce the divergence.

    Quite deep in testing at the moment and getting some very positive results, waiting on some more lenses and I think we'll be on for the small beam with less <1mR

    Maybe a GB will be in order to get the optics price down for the much desired small tight beam with these 445's...

  8. #8
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    ive been having some very good discussions with throlabs about lens.

    here is the conclusion from them



    LJ1598L1-A
    <http://www.thorlabs.com/thorProduct.cfm?partNumber=LJ1598L1-A> - f =
    3.90, H = 4.00, L = 6.0, N-BK7 Plano-Convex Cylindrical Lens


    With this you would get a beam of


    D1= 2* 3.90 mm *tan (20) = 2,8mm

    And as a second lens you could use:

    LJ1805L1-A
    <http://www.thorlabs.com/thorProduct.cfm?partNumber=LJ1805L1-A> - f =
    12.70, H = 13.00, L = 15.0, BK7 Plano-Convex Cylindrical Lens

    With this you would get a beam of

    D1= 2* 12,7 mm *tan (6) = 2,66mm

    This is not absolute symmetrical but a big improvement in any case.

    I used the angles for the typical divergence 40 and 12 for your laser
    diode.

    Not sure if this small beam is ok for your purpose. If you can also live
    with a bigger one then it would be good to recalculate it with other
    lenses with bigger focal length. The bigger the focal length of the
    lenses the larger is the beam diameter and also the smaller is the
    remaining divergence. The beam will never be 100% parallel but it
    becomes better the larger the focal length is.
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  9. #9
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    I have tried the LJ1598L1-A and it seemed the CA is a bit low for this level of divergence and it may not produce a very clean beam. I'm waiting for a few more lenses before seeing if the final collimated beam losses are significant or not though, hard to tell until then.

    There are several ways of doing this, I'm leaning towards collimating with a aspheric that suits the fast axis, then correcting the slow axis with a cylinder telescope and will know more when the glass arrives soon

  10. #10
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    Quote Originally Posted by p1t8ull View Post
    I have tried the LJ1598L1-A and it seemed the CA is a bit low for this level of divergence and it may not produce a very clean beam. I'm waiting for a few more lenses before seeing if the final collimated beam losses are significant or not though, hard to tell until then.

    There are several ways of doing this, I'm leaning towards collimating with a aspheric that suits the fast axis, then correcting the slow axis with a cylinder telescope and will know more when the glass arrives soon

    fair enough, ill hold fire from ordering until you have done some more testing.

    im assuming you got your lens kit back from rob mate?
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