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Old 07-06-2010, 06:55 PM #1
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Default Calculating Beam Waist

So I have a HeNe laser, and I want to calculate the waist. I know how to calculate the divergence, using the knife edge technique, and I was under the impression that the waist could be calculated the same way.

I haven't been able to find too much info on it, however. Can anyone point me in the right direction?

Thanks!



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Old 07-06-2010, 06:56 PM #2
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Default Re: Calculating Beam Waste

There is a difference in WASTE and WAIST....
Which one are you asking about....


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Old 07-06-2010, 06:59 PM #3
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Default Re: Calculating Beam Waste

Oops, typo there. Waist is what I'm referring to. For curiosity's sake, what is the beam waste?
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Old 07-06-2010, 07:08 PM #4
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Default Re: Calculating Beam Waist

That is the part of the beam that is not usable for certain uses
that comes from internal reflections and undesirable beam artifacts..

What you seem to referring to is the focal point of a colimated laser
beam...

BTW...here is a divergence calculator from one of LPF's members....

pseudonomen137's JScript mRad Calculator


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Old 07-06-2010, 07:13 PM #5
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Default Re: Calculating Beam Waist

I was just calculating the divergence using this formula:
2arctan((D1-D2)/2L) where D1 and D2 is the diameter at the two places and L is the distance between them.

You are correct in stating that I refer to the focal point, but I don't think a HeNe is collimated unless you put the beam through a collimated lens, at which point it wouldn't have a focal point, as is my understanding.

Here's a good picture from Sam's FAQ to show what I'm looking for:

http://an.hitchcock.org/repairfaq/sam/rayleigh.gif
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Old 07-06-2010, 07:18 PM #6
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Default Re: Calculating Beam Waist

A HeNe beam is collimated by the 2 mirrors at each end of the tube..
That focal point is fixed. A HeNe beam will diverge as it leaves the
output mirror... The only way to change the Focus is with external
optics...

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Old 07-06-2010, 07:20 PM #7
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Default Re: Calculating Beam Waist

I'm building an external mirror cavity, so like you said I think that affects the focal point.
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Old 07-07-2010, 12:58 AM #8
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Default Re: Calculating Beam Waist

Here:


Psst! homepage

The laser cavity simulator lets you enter mirror radius and spacing. You get back beam size, waist, and cavity stability.

Otherwise you need a copy of Silfvast's book "Laser Fundamentals". It has all the math you need for hene design.
2nd edition is best.

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Old 12-15-2010, 11:27 AM #9
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Default Re: Calculating Beam Waist

I have seen that this thread is quite old - but probably someone else benefits from the following link:

Newport Gaussian Beam Optics Tutorial

The Newport Gaussian Beam Optics Tutorial is really great to get an overview and the most important formulas for gaussian beams.

E.g. from waist 2w0 = (4*lambda/PI) (F/D), you can easily see, that the larger the incoming beam (D), the smaller the focus/waist (F=focal length of the lens).
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Old 12-15-2010, 12:46 PM #10
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Default Re: Calculating Beam Waist

The beam width is the radial distance off the optical axis where the intensity of the light in the enclosed area contains 1 - e^(-2) or ~86% of the total intensity. You can measure beam waist with this knowledge. Calculating is easy.

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Old 09-14-2011, 02:01 AM #11
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Default Re: Calculating Beam Waist

Nice link to the Gaussian Beam Optics Tutorial. Never really understood Gaussian Beam Optics before, the tutorial gave a good insight in to how it all works

thx
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