# how to make beam waist (width) smaller??



## a0977 (Apr 30, 2006)

If laser beam of my system already in its beam waist (smallest width), but, if I'd like to make the width smaller, how could the laser head be modified?


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## comozo (Apr 30, 2006)

For what purpose?


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## a0977 (Apr 30, 2006)

The beam width I have in my machine which is a solid state laser is about 10mil (0.254mm), if I could make the beam smaller, it would helpful for small feature parts without purchase another small beam system.


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## jkaiser3000 (Apr 30, 2006)

I don't know what your intentions are, there are a couple options for doing this.

1. You could use a focusing lens to focuse the beam onto a target, making the spot smaller. But doing this will not make your beam smaller, only the dot. the divergence in this case will be too large, making the beam more like a spot light.

2. you could use a beam expander put in reverse, effectively making it a beam "compactor" of sorts. For this option, you can use a binocular or small telescope, let the laser enter from the big lens, and exit through the smaller one. With this option you'll end up with a smaller beam, but the divergence will be greater also.

hope this helps


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## comozo (Apr 30, 2006)

a0977 said:


> The beam width I have in my machine which is a solid state laser is about 10mil (0.254mm), if I could make the beam smaller, it would helpful for small feature parts without purchase another small beam system.



All you need to do if you are cutting flat parts is to expand the beam then focus the beam. This site altough it concerns co2 laser the same principles apply
http://www.parallax-tech.com/faq.htm


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## cbfull (May 4, 2006)

One of the problems with the typical low power green laser is the way the beam is focused. In every one of these lasers I have seen, the beam is focused very tightly and then there is a lens right where the beam is almost at its narrowest. I'm not sure why they do it this way, because it limits the divergence of the beam.

If I can try to clarify what I mean, if you have a crappy plastic lens with many imperfections, and you place it in the path of a beam that is very narrow (let's say a beam that is 100 microns wide, for example) you are maximizing the effect of the imperfections in the lens, and minimizing the effect of the overall shape of the lense. From the beams perspective, it's almost a crappy window. These imperfections have a drastic effect on beam quality. Each imperfection is like a little unpredictable lens in its own way.

On the other hand, if you place the lens in a position where the beam is the same diameter as the lens, you maximize the effect of the shape of the lens (convex or concave) and minimize the effect of the imperfections.

So my point is, it is very difficult to create a very thin beam that stays thin over long distances, because this requires very small, expensive lenses that have a near perfect surface (you usually order them based on a wavelength fraction of imperfections).

Does this make any sense?


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## comozo (May 4, 2006)

cbfull said:


> One of the problems with the typical low power green laser is the way the beam is focused. In every one of these lasers I have seen, the beam is focused very tightly and then there is a lens right where the beam is almost at its narrowest. I'm not sure why they do it this way, because it limits the divergence of the beam.
> 
> If I can try to clarify what I mean, if you have a crappy plastic lens with many imperfections, and you place it in the path of a beam that is very narrow (let's say a beam that is 100 microns wide, for example) you are maximizing the effect of the imperfections in the lens, and minimizing the effect of the overall shape of the lense. From the beams perspective, it's almost a crappy window. These imperfections have a drastic effect on beam quality. Each imperfection is like a little unpredictable lens in its own way.
> 
> ...




Well, you asked, in a word, No. Here's why. Because you did not answer the question. Your answer is generally difficult to understand, for instance; the you attempt to clarify the first paragraph with the second paragraph which I find difficult to understand, both of which leave me trying to figure out what you're saying. In the last paragraph you make some errors such as 


> it is very difficult to create a very thin beam that stays thin over long distances,


 not difficult, it's not possible, beam waist directly affects beam divergence 


> because this requires very small, expensive lenses that


 large diameter lenses can be used as well what is important is the focal lengths. 
I'm sure the original poster is more confused now than ever.


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## cbfull (May 4, 2006)

I agree, my explanation is quite messy. I've never tried to explain this idea before, I do not even claim that any of it is fact. I apologize for just throwing thoughts down, but that's the only way I can sort this out.

Yes, the diameter of the lens is irrelevant, it's really the focal length that is important, I only mention a small lens because it seems like a waste to have a 1" diameter lens when only a millimeter or less of it is being used. Using a small spot in the center of a lens is the same as using a small lens of a much greater focal length. This is just simple physics.

I think I am a little confused about the definition of "waist". I thought it was the narrowest point on a focused beam, creating a sort of elongated "X" if you will. If this is the case, the waist is the smallest diameter possible. If someone were to place a lens of the correct focal length at or very near this waist, it should be possible to obtain your "thinnest beam possible". To get good results so close to this waist, your lens will need to be very, very precise, with virtually no surface error.

Another factor that affects beam characteristics is source dimensions, which in greenies it tends to be a stripe 100 by 1 microns. But then, because of the crystal alignment, there is SOME bouncing going on between the crystal faces, and subsequent stimulated emission from the Nd:YVO4. This helps to increase the coherency of solid state lasers.

It is possible however, to improve the divergence of these green lasers, and it has been done. I am not going to try to explain how I think it's done because it has become obvious that my ideas on the subject need some cleaning up.

I do not have a clear cut answer to the question, but I can try to encourage questions and hopefully, a better understanding of the factors involved. That of course includes a better understanding for myself.

And by the way, my statement was not an error. It is quite within the realm of possibility to create a beam that is very narrow which stays narrow, as long as the beam is allowed to travel unobstructed (in a vacuum, for example). That is the definition of coherent light. The photons travel in parallel paths, which I am sure you are aware of. Laser diodes are not considered to be coherent sources, but they do have some degree of coherence. Gas discharge lasers on the other hand (HeNe), are highly coherent compared to their diode cousins.

So theoretically, if you wanted to, you could create a laser that is as thin as a single photon. The hard part is devising a source, arrangement or whatever that will allow you to accomplish this.


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## jkaiser3000 (May 4, 2006)

There's a small flaw in your thinking. It's certainly possible, at least theoretically, to focus a laser to a very tight spot (not sure about a photon in width, though). But even if you could place a "perfect" lens, you could not collimate it such that it will keep thin for long distances. You have to consider diffraction in that case too. Diffraction will induce the spreading of the beam, causing what we call divergence. From this, it should be clear that, no matter how well you collimate a laser beam, it'll always have some divergence.

There's a formula for determining the smallest spot size for a focused laser beam, and from that, you can also calculate the depth of field (DOF). This DOF is usually very small, arbitrarily set to the distance traveled by the beam while keeping a diameter of 1.4 times the smallest possible.

So, you see, it will require a long focal length lens to have a large DOF, but using this lens also creates a large spot (wide beam). So we are left to make a compromise between the two.


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## cbfull (May 4, 2006)

Thanks for helping. I know my thoughts on the matter are quite scattered, so your input is helpful.

I thought diffraction was only a factor in non-monochromatic (polychromatic?) light?

Do you recall if that formula included the size of the source when calculating smallest spot size? I've asked laser "experts" about the smallest possible spot from a given source, and the answers I got were vague at best.

Yeah, that idea about the beam that's only one photon wide is purely theoretical. I only mention that it should be possible, not necessarily practical. I don't think that it could be accomplished with any arrangement of lenses, I think it would have to be a unique property of the source itself.


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## comozo (May 4, 2006)

cbfull said:


> I agree, my explanation is quite messy. I've never tried to explain this idea before, I do not even claim that any of it is fact. I apologize for just throwing thoughts down, but that's the only way I can sort this out.
> 
> Yes, the diameter of the lens is irrelevant, it's really the focal length that is important, I only mention a small lens because it seems like a waste to have a 1" diameter lens when only a millimeter or less of it is being used. Using a small spot in the center of a lens is the same as using a small lens of a much greater focal length. This is just simple physics.
> The last sentence is not correct
> ...



 Here's an applet that you should experiment with the second site has an applet which provided more detailed info.
http://www.us-lasers.com/beam_divergence.htm
http://www.lightmachinery.com/gausbeam.html
http://www-ee.eng.buffalo.edu/faculty/cartwright/java_applets/gaussian/applets/index.html

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## jkaiser3000 (May 4, 2006)

> Do you recall if that formula included the size of the source when calculating smallest spot size? I've asked laser "experts" about the smallest possible spot from a given source, and the answers I got were vague at best.



Yes, it does. In fact, the spot size is related to the size of the beam hiting the lens, the lens' focal length, laser's wavelength, and the quality of the beam (how much it deviates from a gaussian profile):

spot dia. = 1.27 * focal length * λ * M2 / beam diameter

M is the term that relates to the deviation from the perfect gaussian beam (M=1 for perfect beam)


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## cbfull (May 4, 2006)

Wow great sites. I am going to take a look at those for a bit. Thanks!


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