Old 10-27-2010, 04:32 AM #1
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Default Lower limit on beam divergence?

Ahoy.

I've been wondering something a while: is there a theoretical lower limit to beam divergence? Would it be possible, given ideal manufacturing and assembly, to make a laser with zero beam divergence?

It wouldn't surprise me if lasers were diffraction-limited systems. In fact, that would make an enormous amount of sense, as I've seen people here claim that wider beams diverge less.

On the other hand, it's possible that the effect of diffraction is small when compared to the divergence caused by slightly misaligned/misshapen collimating lenses. If a focusable beam can have both positive and negative divergence, shouldn't it be possible to get very near the diffraction limit? Do the cheaper manufacturers who make fixed lens systems just generally say "eh, good enough" when they get <2 mrad and use a easy to control production process?

Always curious, I am.


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Old 10-27-2010, 04:35 AM #2
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Default Re: Lower limit on beam divergence?

It's impossible to have zero divergence as you'd have to have an infinitely small source of light. You can get very low divergences with a beam expander, but never 0.
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Old 10-27-2010, 04:41 AM #3
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Default Re: Lower limit on beam divergence?

So the fact that the diode isn't a point means that the lens can't focus from a non-point to parallel rays?
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Old 10-27-2010, 12:58 PM #4
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Default Re: Lower limit on beam divergence?

There is also diffraction limit. Larger aperture will make better divergence, but never zero. It's the same problem as with telescope. To see further, you need bigger lens (or mirrors).
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Old 10-27-2010, 01:27 PM #5
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Default Re: Lower limit on beam divergence?

You can have zero divergence but your laser beam has to be infinite wide...
There is a relationship between divergence and beam wide.
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Old 10-27-2010, 01:31 PM #6
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Default Re: Lower limit on beam divergence?

that's not very important if the lenses are not perfectly aligned.
it's important that they are placed at the right place (at focal lenght)
the precision of lens surface is important.

the shape of the lens is important too.
best lenses are aspherical lenses but they are much more expensive than simple spherical lenses. (AR coating is important too to avoid parasite light)

the shape of the source of light (and its quality) is important too: in ideal world the lens would be designed especially for one particular shape of laser source.
but even in ideal world zero div is physically impossible.

when you use (almost) perfect lenses you can come close to the diffraction limit: the divergence of the beam will be proportional to: wavelength / diameter of the beam

Beam divergence - Wikipedia, the free encyclopedia



cheap chinese manufacturers are not afraid to sell you sometimes 10 mRad lasers.
you can get a bad minimum div even with focusable lens

Last edited by BKarim; 10-27-2010 at 01:35 PM.
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