collimator at the laser head on CO2 laser?

[Tom Schulze]

Has anyone tried or thought of using a collimator in the laser head after focusing the beam?
Would that provide a highly focused straight beam out of the laser head?
Would that be too much power concentration for the optics to handle?

Just a thought.

[Dan Hintz]

Several threads on collimators (one within the last couple of weeks). Most of the Western-made machines offer it as an option in one way or another.

BTW, collimation doesn't mean "concentrate" (that's the focusing lens' job), it simply makes the beam edges parallel.

[Tom Schulze]

I know some offer it as an option, but all that I've seen offer it after the laser, most likely before the first mirror, not after the focusing lens. I'm thinking after the focusing lens at the focal point so that beam stays at the smallest size coming out of the collimator, thus having a farther reaching high intensity beam to cut/engrave with.

[Rodne Gold]

You want as BIG a beam as possible coming from the expander/collimator before the focussing optic , not small.

[Dan Hintz]

Ah, I see what you're saying now... no, that's the worst place to put it. When you focus the beam, you're attempting to get a spot size the diameter of one wavelength (10.6um)... while theoretically possible, in practice it's all but impossible for equipment any of us could hope to afford. And that's just one optic... imagine trying to get (and keep) such precision with multiple optics on a moving carriage stored in someone's basement. And as Rodney says, the intent is to get the beam as big as possible (within reason) and parallel so it's even across the entire engraving surface.

[Ernie Balch]

As mentioned above, collimators just don't work to as we would all like to imagine so you can't get a very tiny collimated beam. You have to use a focusing lens. I have used Million dollar CO2 laser drilling systems used for circuit boards and the smallest gaussian spot they could produce was about 50 microns (5X the 10.6 micron wavelength).

Laser beams are diverging as they come out of the laser but the focusing lens needs a fixed beam diameter and distance from the laser head or else the spot size and focus position will vary as the head moves away from the laser head. In order to fix this problem a collimator is used to make the beam parallel as possible. Often this is combined with a beam expander to make the beam as big as possible going into the focusing lens. Think of it as using all of the lens to focus to a smaller spot. If you have a small beam going into the center of the focusing lens it will not use much of the curved lens surface thus it will not be able to focus to as small of a spot.

ernie

[Tom Schulze]

Thanks for everyone's thoughts. Dan, at least I was able to get you to understand what I was getting at. I have a hard time of explaining things to others. Everything sounds great in my head, but I have difficulty describing it in words. :)

On the flip side of this question, lets say I have a thick, lets say 2" thick, piece of foam that I want to cut. Of course to minimize the focusing effect of the laser beam I'd find the longest focal length lens, let's say 100mm. Even at, if I set the focus to the middle of the foam I'm going to get a much larger kerf at the top and bottom of the foam cut. So now, if I put a beam reducer/collimator between the laser and the first mirror that will reduce the size of the beam hitting the lens and create a larger depth of field, smaller "X" effect. I know this would significantly reduce the cutting power of the spot and also make the spot size bigger, but this should also give me a more uniform cut on the edge of the thick foam, correct? Would the reduced beam size put too much stress on the mirrors and lenses?

Thanks again, hopefully I explained myself better for this question.

[Kim Vellore]

The power density at the focal point is so high, the optics you if you can find any that can take that kind of power will be very expensive.

Kim

[Dan Hintz]

Tif I put a beam reducer/collimator between the laser and the first mirror that will reduce the size of the beam hitting the lens and create a larger depth of field, smaller "X" effect.

I think terminology is killing your description. When you say "laser" you should mean the laser cartridge itself, or the output lens. The first mirror is the one that sits either at the back of the machine or along the X-axis bar, depending upon your machine design (it's the first mirror the beam hits after leaving the cartridge). You (the world) want a collimator right after the output lens.

Your particular desire is for a collimated beam right out of the focusing lens. The best you could hope for is a 10.6um-diameter beam with no divergence, but that's a pipe dream. In the practical world, you're an order of magnitude larger on beam size focal point. Even so, if you focus the beam onto a collimator's optics, you would need a very high-quality optics to prevent destroying it from the power density (as you surmised). In theory, this would work... but the practicality of it is not so nice, and the collimator would need extremely tight specs to get you down to (near)zero convergence/divergence. It's possible, so the question then becomes "How big is your bank account?".

[Tom Schulze]

Thanks for entertaining my nativity and ignorance when it comes to these machines. Again. Thinking in my head and not expressing. I probably should have started a different post for my second question. I meant to suggest only reducing the beam by half or so after the output lens to slightly reduce its size. Yeah. I don't have that big of a bank account.

[matthew knott]

Youve been watching to many "james bond" films as thats how they always show a laser, normally about to cut someone in half :)
What you need is a dual focus lens, these focus at 2 different points allowing you to cut thicker material
Heres a link http://www.sintecoptronics.com/FocusingLensDualFocus.asp
Have i tried one? Yes and no, not on a simple low power engraving laser but they do defo work on high power cutting machines! How going to be first to give one a try ??

[Dan Hintz]

I tried to get that company to send me one of those lenses for testing during one of our CNC/laser gatherings... no such luck. That said, I think the price wasn't too bad... around $1k, if memory serves. There's an old thread discussing it somewhere around here. About the only thing I can see it being used for is acrylic, however.


Tom, you want the beam to be as large as possible (within reason to avoid edge effects) when it hits the focusing lens. Shrinking it beforehand will reduce your focusing ability.

[Tom Schulze]

Thanks for everyone's feedback.