Beam Expander

[CNCadmin]

Does anyone know, can a beam expander go right in front of the laser output?

[owhite]

Paul,

dont see why not. photons in, photons out, right? :-) In front of my laser I have something called a cut quality enhancer, which is basically a polarizer that's supposed to avoid letting the laser light reflect back into the tube. anyhow, after the CQE the next thing is my expander.

do you want to make a homebuilt expander or get new/used OEM? To make, they're basically two plano convex lenses facing each other. I can dig up a couple web pages that diagram how they work.

owen

[CNCadmin]

I was thinking about buying one but I would make one.

[owhite]

Sorry I wasnt able to find the diagram that I once saw on the 'net. To my knowledge there's a couple strategies for making a beam expander. The method I knew about is to have two plano-convex focussing lenses pointed towards each other. Sorry, but I wish I was able to find this dumb picture. The page also had information on how to calulate what the expansion is based on the focal length of the lenses.

my expander has two lenses and the side of the expander is labeled with 2.5 fl on the input lens, and 7.5fl on the output lens. Not that I can tell you why but I was told that the expected is expansion is 7.5/2.5 = 3.

The tricky part is figuring out the right distance between the lenses. The process is to put the lenses in a sliding holder and measure the spot size coming out of the expander. Then, adjust the distance between the lenses so the spot size stays the same at increasing distances away from the output of the expander/holder.

thats what I know. not enough for a definitive plan, sorry. maybe a friendly engineer could help at synrad or an optics place.

owen

[owhite]

oh yeah the other thing I could do if you go ahead with making one is I'll open up my expander and make certain about the orientation of the lenses. You have to know if the plano side or the convex sides are pointing at each other. (this is why I'd like to find that diagram, grumble). this is the type of documentation that would really help the next guy making a system.

[CNCadmin]

The negative is my focal length will be much shorter.

[owhite]

I'm sure I'm missing something, but why would the focal length be shorter? I would have thought that focal length is the same regardless of beam diameter. Perhaps you'd have to use a different focusing lens?

[CNCadmin]

Sorry I meant Depth Of Field.

[Ortin]

Originally Posted by CNCadmin Sorry I meant Depth Of Field.

May be you meant a "Depth of Focus"?
But this is optics. If you increase beam diameter, you decrease spot size and depth of focus, and vise-versa.
May I ask you, why you decided you need a beam expander?

[pixpop]

Originally Posted by Ortin May be you meant a "Depth of Focus"? But this is optics. If you increase beam diameter, you decrease spot size and depth of focus, and vise-versa. May I ask you, why you decided you need a beam expander?

Hope I'm not stating the obvious..

Main uses for a beam expander:
1. Reduces the divergence of the beam.
2. Lets you focus beam to a smaller spot size.
3. Reduces power density so you can pass high power beam through filters etc.