Old 07-31-2016, 08:19 AM #1
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Default Accidentally posted in the wrong section guys, reposted in Optics section.

Edit: I accidentally posted this in the BST section, I didn't think it would make it through approval to end up here, just now see it did. Sorry guys, I later posted the thread in the Optics section titled as "Question for the optics savvy in regard to beam expansion to reduce divergence", after seeing my mistake. Thank you for your answers and help with this question, but I don't want to add to the thread here further, since it is in the wrong section.

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I'm finding it difficult to find cheap AR coated lenses of the correct focal length for use with beam expansion. However, I have an idea, when using a PCX lens which is too small for the size of the spot hitting it, at its focal length, wouldn't the lens produce the reduced divergence of the spot size it is collimating, even if part of the beam were cut off due to the lens being too small?

To exaggerate the situation, what if I put a 2 inch diameter lens in front of a 6 inch spot, wouldn't that 2 inch lens produce the same divergence of a 6 inch expanded beam, except most of the light is cut off? I'm thinking this might be a way to produce a low divergence beam using a relatively small lens diameter and if the power is high enough, such as from a multimode diode, the trade-off might be advantageous, if not in terms of power delivered at an extreme distance, in terms of reduced size?

Any one know the answer to this? This isn't something I'm finding talked about anywhere yet. Perhaps because it is a hair brained idea and won't work?


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Newbie advice: Please take the time to first make an introduction in the Welcome section before posting questions.


Divergence to spot size calc: - 1 mRad is about .057 degrees which expands to be very roughly ~10% the diameter of the moon or sun at their distances.

Divergence Calculator: pseudonomen137's JScript mRad Calculator - Measure your lasers beam dia. at 1 foot & then 20'.

Online calc. to determine spot intensity at different mRad's & powers: http://tinyurl.com/divergence-calculator

Laser Power Density Calculator: Laser Power Density Calculator - Ophir

Build a beam expander to reduce divergence: http://tinyurl.com/BeamExpander

University YAG Project The Professor's Homebuilt Lasers Site - YAG Lasers

YAG Power Calc. Laser Peak Power Calculator - Ophir

Angular Size Calculator; use with diode angle of radiation spec. for lens dia. at FL: Angular Size Calculator

Compare different colors: http://lsrtools.1apps.com/RelativeBr...&useRaleigh=on

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Old 08-09-2016, 05:16 PM #2
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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Originally Posted by Alaskan View Post
Any one know the answer to this? This isn't something I'm finding talked about anywhere yet. Perhaps because it is a hair brained idea and won't work?
This is out of my interest as I'm not interested in loosing power/energy. However you can look for "truncated Gaussian beams" - there are a lot of theory about it.
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Old 08-09-2016, 08:09 PM #3
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

The question you are asking is very similar to the one I ask here. The basic idea, if I understand what you're asking, is to get the "best" (smallest) dot at a distance by expanding farther than the size of your final collimating lens. Yes, if the focal lengths are the same, a big lens and a small one will produce for you the same divergence, just with some truncated.
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Old 08-09-2016, 08:40 PM #4
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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Originally Posted by Eracoy View Post
Yes, if the focal lengths are the same, a big lens and a small one will produce for you the same divergence, just with some truncated.
It would mean that you would get a smaller spot at a far distance. Make a tiny lens, and you would get a focused beam everywhere

That is not right.
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Old 08-09-2016, 08:50 PM #5
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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Originally Posted by lazeristasUVISIR View Post
It would mean that you would get a smaller spot at a far distance. Make a tiny lens, and you would get a focused beam everywhere

That is not right.
From what I understand, the size of a laser's dot at distance L from a source with divergence θ and size at aperture of D would be:

D + 2 * L * tan(θ/2)

So as D shrinks, yes the dot would get smaller, but the size of the collimating lens wouldn't affect the divergence, so the spot would still have plenty size at distance.
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Old 08-09-2016, 09:43 PM #6
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

Quote:
Originally Posted by Eracoy View Post
From what I understand, the size of a laser's dot at distance L from a source with divergence θ and size at aperture of D would be:

D + 2 * L * tan(θ/2)

So as D shrinks, yes the dot would get smaller, but the size of the collimating lens wouldn't affect the divergence, so the spot would still have plenty size at distance.
When you'll start clipping your beam, you need to account the diffraction. Formulas are not that simple.
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