Theoretical question for laser DOF

[Dan Simone]

I've been trying to do some reading on laser spot sizes, DOF(depth of field), and focus length in relation to using a beam collimator. I was just wondering if my thoughts are correct based on what I have read. I'm thinking that if you were to use an adjustable beam collimator just before the last mirror of the laser path could you change the depth of field by adjusting the spot size with the collimator? I'm just wondering if it is possible to reduce or enlarge the final spot size and DOF using an adjustable beam collimator rather than changing lenses? If the answer happens to be yes does anyone know where I can get an inexpensive adjustable beam collimator. The least expensive ones I have seen so far are in the 600.00 dollar range.

[Rich Harman]

I think that is a lot of mass to place just before the last mirror. As long as the beam is collimated, it won't matter where it is placed.

When I get around to it I plan to install my 2x beam expander so that I can easily remove and replace it. When I need the greater DOF, I'll just pull it out.

[Steven Cox]

I've just started reading myself about beam expanders so I can't give any advice on them, but I did find the following webpage that was interesting reading showing specs on different types so I thought I'd share.

http://www.sintecoptronics.com/beamexpander.asp

Also $600 range ??? most I've seen are double that so I'd be interested to know where the cheaper ones are for a comparision.

[Rich Harman]

Oh, I see. You were talking about adjusting the focus of the beam expander. In that case it would need to be just before the last mirror. If you were to use a zoom beam expander then you could adjust the magnification of the beam just after it leaves the laser tube.

[Dan Simone]

Here is one slightly higher than the 600.00 dollar range.
[/URL]
[URL]http://www.edmundoptics.com/products/displayproduct.cfm?productid=3004 (http://www.beam-expander.com/?gclid=CKWI69a20q4CFeJe7AodG0L6IA)

I had to edit the post when I realized that the first link to a beam expander under 600.00 was the wrong wavelength for co2.

[Dan Hintz]

Yes, you could change the DOF that way... but the big question is "Why?".

There are two reasons for the use of a collimator:
1) Ensure a consistent beam width across the entire table.
2) Provide a wider entrance beam to the focusing optics, allowing for a higher power density.

If you place the collimator at the focusing lens, you definitely defeat 1, and you partially defeat 2 (the beam out of the collimator will vary with the carriage's location on the table, leading to variable power density).

[Rodne Gold]

There are threads about this that go into some depth
http://www.sawmillcreek.org/showthread.php?170130-Considering-adding-in-a-beam-expander-collimator
EG one I started
Search the boards for other threads with collimator or beam expander in them , there was one recently with good links
Theres an online calculator that allows you to see what happens if you add a colimator/expander and use different lenses.
http://buildlog.net/cnc_laser/laser_calcs.htm
I have some expanders/collimators from some scrapped lasers and plan to put one inbetween my tube and my 1st optic on an 89w RECI tubed chinese machine, havent done so yet. Will get back to you when I do.

[Ernie Balch]

Laser beams are not parallel when they exit the tube. Every laser has some beam divergence, (the beam grows slightly with distance). Usually this is specified in milliradians, and is of great interest for micro machining applications where sub micron tolerances are expected. A collimator beam expander should be at a fixed distance from the laser tube, then adjusted for best collimation.

In laser engraving the effect is ignored since nobody worries about tight tolerances. I don't think the change in beam size across the typical laser engraving table would be enough to notice. So putting a beam expander just prior to the final turning mirror would probably work but it looks big and heavy so, it is still better to put a beam expander collimator at the tube prior to the first turning mirror.

Expanding the beam 3 or 4 X will fill up your turning mirrors and make their alignment more critical. You should make sure that the focusing lens and mount is big enough to make use of the larger beam without clipping.

[Dan Hintz]

In laser engraving the effect is ignored since nobody worries about tight tolerances. I don't think the change in beam size across the typical laser engraving table would be enough to notice.
Everything else was fine, but I disagree with this. Some of us do care... and on larger tables (especially those guys who have tables measured in feet rather than inches), a collimator is essential equipment to getting a consistent cut from one corner to the other.

[Mike Null]

I agree with Dan. I do industrial work and tolerances must be held within +/- .003" on some jobs.

[Lee DeRaud]

Everything else was fine, but I disagree with this. Some of us do care... and on larger tables (especially those guys who have tables measured in feet rather than inches), a collimator is essential equipment to getting a consistent cut from one corner to the other.As I discovered (http://www.sawmillcreek.org/showthread.php?165138-Uneven-power), it matters even for smaller tables.

[Dan Simone]

As for the why, I was thinking this might be a way to change the spot size and DOF without a lense change. More or less just using predetermined notches on the adjustable collimator to quickly change. I'm new to all of this and especially to laser theory so please forgive my ignorance on the subject.


Yes, you could change the DOF that way... but the big question is "Why?".

There are two reasons for the use of a collimator:
1) Ensure a consistent beam width across the entire table.
2) Provide a wider entrance beam to the focusing optics, allowing for a higher power density.

If you place the collimator at the focusing lens, you definitely defeat 1, and you partially defeat 2 (the beam out of the collimator will vary with the carriage's location on the table, leading to variable power density).

[Dan Hintz]

As for the why, I was thinking this might be a way to change the spot size and DOF without a lense change. More or less just using predetermined notches on the adjustable collimator to quickly change. I'm new to all of this and especially to laser theory so please forgive my ignorance on the subject.
I understand where you're coming from as I'm always looking for ways to tweak, but in this case the benefits are outweighed by other things. Sure, you could quickly set a different DOF, but at what cost?
1) You add more weight to the carriage, which translates to slower engraving speed.
2) How often do you really change the lens? Is it worth saving 30-45 seconds?
3) You always want the largest DOF if you don't have to sacrifice power density... this is accomplished by putting the collimator at the laser output, not at the end of travel.

[Ernie Balch]

Sorry Dan, I should have explained things better. Fine tolerances are a matter of perspective, to a carpenter 6 mm is a fine tolerance, to a circuit board shop 20 microns is a fine tolerance, to a semiconductor fab 2 nanometers is a fine tolerance. In laser engraving, something that looks good at arms length is fine.

I come a background of 40 years in micro-electronics where a tolerances are measured in nanometers. To me the CO2 laser spot sizes are very large and the depth of focus is huge. You use a ruler to measure things instead of an electron microscope. The laser engraver equipment specs do not include any accuracy numbers, spot size, mode structure and divergence is not even discussed. People do not measure laser power or the writing speed, they don't even know the spot size.

When my system was installed Ray was surprised when I inspected parts with a microscope and actually measured my effective spot size at different power settings. (It was much worse than theory predicted for a gaussian, diffraction limited spot).

ernie

[Dan Hintz]

Sorry Dan, I should have explained things better. Fine tolerances are a matter of perspective, to a carpenter 6 mm is a fine tolerance, to a circuit board shop 20 microns is a fine tolerance, to a semiconductor fab 2 nanometers is a fine tolerance. In laser engraving, something that looks good at arms length is fine.

I come a background of 40 years in micro-electronics where a tolerances are measured in nanometers. To me the CO2 laser spot sizes are very large and the depth of focus is huge. You use a ruler to measure things instead of an electron microscope. The laser engraver equipment specs do not include any accuracy numbers, spot size, mode structure and divergence is not even discussed. People do not measure laser power or the writing speed, they don't even know the spot size.
Again, I'll have to disagree. One member here has extremely tight tolerances on parts he cuts thousands of per month for a customer, essentially pushes the tolerance limits of the motion system. If he were to "measure at arm's length", well, he'd lose that customer. Just because the spot size is 5 mils doesn't mean that's the tolerance... adjusting for kerf width and spot width in your design is sometimes part of the game.