Flying Optics and Lense Diameter

[HughRob]

Hi,

I am in the process of building my first laser and I am a bit confused.

1. If the laser tube is fixed compared to x, y and z axis will I need a collimator gadget due to the changing distance between mirrors?

2. Typical lense diameters are 20 and 25 mm, what determines lense diameter?

Thanks in advance

Merry Christmas

[bdring]

The beam is only about 5mm in diameter when it exits the tube and has a very low divergence of a couple milli-Radians. The beam is only going to be a millimeter or two larger after going to the workpiece.

You then need to focus the beam onto the workpiece through a lens. 20mm lenses and mirrors are fine.

I have boiled down most of the basic optics calculations into the simplae calculators on this page.

Laser Optics Calculators

Bart Dring
buildlog.net - CNC Laser Buildlogs
(everything DIY lasers)

[zax15uk]

HughRob,

1. No - you won't need a collimating lens, just a focus lens to concentrate the beam to a spot.

2.As Bart said 20mm is fine, but so is 25mm. There are some minor differences but the diameter doesn't really matter.

Zax.

[MonoNeuron]

The big mirror size makes it a lot easier to set your alignment of the beam. I have tried with 10mm mirrors and after about 3 hours gave it up as a bad joke. You need to be way too precise to do it. You get a bit of leeway with the larger sizes.

Also remember that the gold mirror surfaces are VERY soft and easily scratched so when cleaning them use a WET lens tissue and without any force on the tissue just slowly pull it across the mirror surface. Gold is very malleable and so is also very soft and the coating on the mirrors is vacuum deposited and so is very thin. Remember that at cleaning time. If you keep the dust out then you won't have too many problems.

I have been using Aluminium first surface mirrors from photocopiers for the last few years and they are so much more resistant to scratching than the gold ones. I am going to try them out with the 100 watt laser I have just installed before Xmas and see how the extra power goes with them or if they will melt after a while.

The first gold mirror I cleaned with a lot of alcohol and a cotton swab/Q-Tip scratched the surface and I was only using very light pressure on it for fear of scratching. I won't be using the Q-Tips again as I want the mirrors to last a good few years.

NEVER use Acetone on your laser output coupler as it will erode the special coating on the face of it and may even reduce the output from the tube and stuff it up completely. I go to Chemist shops and ask if they have any Isopropyl Alcohol that is out of date and I get it for about 1/2 price. I never knew pure alcohol had a use-by date. :-) I am still on my first bottle (500ml) after 3 years and have only used about 25mm out of it so it's going to last a long time for me.

That might change when I build my next portable 35 watt laser table.

Hope this helps.
Rich.

[HughRob]

Thank you for everyone's comments.

Can I use a 20mm lense for a 150W laser without problem?

[zax15uk]

Originally Posted by HughRob Can I use a 20mm lense for a 150W laser without problem?

Yes - but as Rich said, if you are building a machine with a long beam length the 25mm may make alignment easier.

Zax.

[MonoNeuron]

Anything over about 16mm will do just for ease of use mainly. If you use a bigger lens assembly or mirror assembly you add weight and take up more space. It's not a critical thing and I would use what I could afford.
The lenses I use on my home made lens carrier are all about 18mm except the new lens I just bought from CNColtech which, I think, uses 22mm lenses to fit their carrier assembly. (somewhere around there but I'm not sure of the exact size)
The 10mm lenses you see on eBay are mainly for laser light show stuff where the mirrors etc are very close to the laser head and don't pose an alignment issue.

[MonoNeuron]

On an aside note.
When you are setting up your alignment are you going to use a red pointer or are you just going to set up with the laser itself?
The red pointer really does make setting up easier and also setting up jobs after is much easier too. I would recommend a red pointer if you are building a laser as it makes things so much easier in the long run.
Just make sure you set it coincident with the laser BEFORE you put all the mirrors in place as it will make the alignment less difficult and you will know for sure you have the pointer right in the middle of the beam.

[Laserman FLE]

A collimator/beam expander is necessary for proper beam output.
The physics behind them is to collimate (flatten) out the beam as much as possible to reduce the divergence of the beam. A diverging beam will not fill the entire surface of the lens or mirror and will create aberrations in the final focused spot.

How large of a field are you going to be running?

For example, a 100W Universal tube emits a beam so large that we needed to use a 6x beam expander to collimate the beam properly trough a 5" focused height with a 1.5" dia lens.

Better beam output from the source will allow smaller optics through the overall system thus reducing cost and produce a smaller spot.

Hope this helps, you really need to do the calculations based off many different variables.