Considering adding in a beam expander/collimator

[Rodne Gold]

I have been doing some research recently about spot size and energy density , basically the spot size of the laser is a function of the lens focal length and the width of the beam coming out the laser tube itself. The simplified maths is : Spot size = 0.013 x Focal length/beam diameter.

If you take a 50mm lens and beam diameter of 5mm , the spot size will be 0.13mm (0.013 x 50/5)

If you expand the beam diameter to 10mm , then the spot size is 0.065mm (0.013 x 50/10)

Now the big advantage is this - 50w into a spot size of 0.13mm produces an energy density of 18000 watts per sq mm. if you have 50w into a spot size of 0.065mm , the energy density rises to 47 000 watts per sq cm !!!

So you now have a smaller spot with a LOT more energy in it , which will produce cleaner far more detailed engraving and work much better on almost all materials Ostensibly , you could run the laser a lot faster at the same power as well , kinda like getting 4x the power for "free".

Well its not all that free in fact , simply put the lens will lose about 1/2 its depth of focus if you expand the beam to double the width , IE it will not cut thicker stuff and your material and table will have to be dead flat.
My idea is this - My explorers , one of which is dead , have beam expanders and collimators built in (the collimator makes sure the beam does not diverge - ie get thicker over distance and the expander makes it be "thicker" than it is coming out the tube) and I am considering taking one out of the dead machine and using it in one of my chinese lasers.
It would sit between the tube and the first mirror.

The problem is that I want to use the laser for both cutting thick stuff and for fine detailed high power engraving (like on some uncoated metals, granite etc) so the optic would have to be removeable.
The optic would have to be aligned as well so the beam enters and exits straight.

Currently the tube output is VERY close to the first mirror as the closer the beam output is to the mirror , the easier it is to align the tube to it , putting it further away means that small movements of the tube result in large movements where the beam hits the first mirror.

There IS room to shift the tube backwards to accomodate the Expander which is about 3-4" long , so that CAN be accomplished.

The issue is to make the expander removeable and my thought was to use a thickish base plate that is adjustable and has 3 locating holes drilled into it and then to bolt aniother base plate with pins that locate in these holes onto the collimator itself.

Thus one could align the expander/collimator via the base plate and remove it when its not needed , and when needed , just plug it back into the baseplate?

Waddya all thunk?

[Scott Shepherd]

Rodney, your mileage may vary, but we put a collimator on our machine and I can see zero difference in it's performance. I have some parts I vector cut on a regular basis. The settings are exactly the same, the kerf appears to be the same. It could be smaller, but if so, it's not enough for me to detect. Rastering with it uses all my old settings. In fact, if you took it off tomorrow, I'm not sure I'd know it was missing.

I thought it was going to really help with some issues we were having, but it didn't change a single thing that I can see in results.

I don't notice any loss of depth of focus either. It might be there, but I haven't noticed it.

[Ken Shea]

Nothing to lose but some time, you have the parts to try and could be a real winner, I'd sure be interested in how it works out.


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[Rodne Gold]

Scott , was it just a collimator which will merely stop the beam diverging over distance and perhaps not change anything like the spot size or focus depth etc?
Was there a beam expander also involved , which would change the size of the beam?
what were the issues you wanted to solve?

[Richard Rumancik]

. . . The issue is to make the expander removeable and my thought was to use a thickish base plate that is adjustable and has 3 locating holes drilled into it and then to bolt aniother base plate with pins that locate in these holes onto the collimator itself.

Thus one could align the expander/collimator via the base plate and remove it when its not needed , and when needed , just plug it back into the baseplate? . . .

Seems like it would be possibility. You might try to see if you can get a look at a ULS machine that has the dual-tube arrangement. They have a mounting mechanism that allows the tubes to be removed from one machine to another without re-aligning the optics. I haven't seen the mechanics close-up but maybe you might get an idea of a quick-release device that could be applied to the collimator assembly.

Some thoughts . . .

1. You need a stable base to mount to, that is rigidly attached to the machine (no flex relative to laser tube and optics mounts). Seems you have this covered.

2. You need a defined mounting plane on the baseplate. If the baseplate was ground flat, that could work (infinite number of pick-up points) but usually it is done by providing 3 specific points in a triangle. That way dust and particles are less likely to mess up alignment. Three points define a plane. These three points could be three small metal pins or domed polished steel rivets pressed in or something similar.

3. I don't think you want three locating pins, however. That would be considered "over-contrained". One pin/hole pair will provide location, but not stop rotation. So a second pin/hole defines the axis of the optics uniquely. Three pins usually causes problems.

Because of tolerances on the machining operations, usually the second hole needs to be a slot. That is a pain, because it is easier to make accurate holes but not so easy to make accurate slots. One way around this is to match-drill the two holes (clamp carrier plate and base plate together and drill). You still might have to enlarge the holes by .001" (.025 mm) afterwards. This method does not allow interchangability between machines but might be okay for this case. If you want interchangability there are other ways to approach it.

4. You need some way of clamping the carrier to the baseplate. One screw near each of the three mounting pads would work.

5. You need some way of "releasing" the carrier off the fixture without twisting/yanking/prying etc. as it should be a tight fit if it is going to work right. So I would suggest a "jack" screw beside each of the two pins. Since the carrier will be fairly tight, it might bind, so you want to get it off without damaging the collimator assembly or the baseplate. The jackscrew would bear against the baseplate, and by screwing in the "jacks" in alternate fashion the carrier would slide off the pins. (Unscrew jacks before replacing the assembly.)

This concept does not address the last comment: "Thus one could align the expander/collimator via the base plate . . . ."

I am not sure how you visualized doing the alignment. One thought: instead of the three hard pins, use three threaded cap screws (hexagon socket-head cap screw, low profile head, finest thread available). Then there could be 3 small access holes on the carrier for the hex driver to allow adjustment of the three screw heights. Would be nice to have a spring (or three) to provide some preload. Sounds complicated but is actually pretty routine fixture design. Take a look at the GCC adjustable mirror mounts - if I recall the configuration, the concept I have described is similar - they adjust the alignment of the mirrors with a couple screws (and I think there is a spring in the middle too for preload). It may provide some ideas.

[Dan Hintz]

If you take a 50mm lens and beam diameter of 5mm , the spot size will be 0.13mm (0.013 x 50/5)

If you expand the beam diameter to 10mm , then the spot size is 0.065mm (0.013 x 50/10)
You have to be careful not to get too close to the edge of the final focusing optic, then... as you get farther away from the central axis, you have to deal with heavier fringing effects, which leads to a loss of focus. Your final focused beam may have a less-focused point if you're not careful.

Search back through some of the earlier threads... I think I discussed this in more detail and some ways to mitigate it. One way would be to use a larger final optic. Considering how cheap they are ($20-30?), that may not be a bad way to go.

[Rodne Gold]

Thanks Richard , some good tips there , I was thinking of preloading the base plate. The explorer has got a very solid mounting system that I can canibalise , I'll try use that as a basis.
Dan I use 20mm optics in my chinese lasers , they come stock with 18mm I think. As far as I know, the beam diameter out the tube is something like 6mm , what I dunno is what the expander does to the beam and how much it expands? I will just have to try it out and see.
Anyway , it will have to wait , I'm off to china tommorow and wont be back till next tuesday (assuming no problems with my order , shipping , payments , agents , supplier etc) I will stay there till I see my goods loaded on the container , watch it being sealed and driven to Ningbo , the port it leaves from , my dealings with my agents have been less than confidence inspiring and I'm a bit nervous...no balance of payment will be made till I'm happy

[Rich Harman]

Isn't a beam expander/collimator the same thing?

http://www.edmundoptics.com/products/displayproduct.cfm?productid=3004

[Michael Hunter]

Rich

Not quite, though they are often combined into one unit.


The expander takes in a parallel or slightly divergent beam and makes it more divergent (i.e diameter increases with distance).

The collimator takes in a divergent beam and makes it parallel.

So putting the expander into the beam, followed by the collimator, gives a larger diameter beam that is parallel.
The larger diameter beam puts less stress on the mirrors and optics (lower power density along the beam path), as well as the advantages that Rodney has mentioned.

[Rich Harman]

Understood about the difference between expanding and collimating. All the expanders that I have found are also collimators. Can you point me to an example of just a collimator? Or just an expander?

[Michael Hunter]

In the context of laser engraving machines, you probably can't buy the two functions separately.
If you go to a specialist optics supplier, you can get a pre-assembled collimator (only) or all the components separately.
The expander is basically just a concave lens.
e.g. Comar (.comaroptics.)