Optics of those Chinese laser engraver modules?

[dylanetaft]

On sites like banggood, there are cheap laser engravers. I ordered one. Anyone know the optics of the inside? Usually they have an adjustable focus ring.
Would I be correct in that there is a light collimating lens on the inside, and the little lens on the end that you twist is some sort of zoom lens?

From what I can see, the zoom lens might focus down to a 500um spot. I sort of want to get ~100um resolution. There are whole modules that can get down to that - but could I just do this with a convex lens downstream of the collimating lens that ships with the engraver?

If the laser is pre-collimated inside the module - would it be as simple as finding a lens on ebay with an appropriate focal length
and plugging it into a calculator?

https://www.ophiropt.com/laser--mea...focal-spot-size-calculator-for-gaussian-beams

If it's doable - do I need to worry about the type of glass - will it get hot if I encase it in a 3d printed case?

Basically, I am wondering if I can get like a 20mm focal length lens, mount it in a tube after the collimating lens that ends 20mm away from a workpiece to get a smaller beam?

The optics of these Chinese lasers are unspecified. It's a bit frustrating.

 

[dylanetaft]

So, I think the "focusing" lens is a aspherical collimating lens. I'm reading about g2, g7, 3 lens, seems like marketing gibberish from Sanwu? If it really was a good collimating lens, the focus wouldn't be drastically affected if the work piece moves closer or further away from the lens?

The optical properties on these so called "collimating lenses" seem undocumented. What is the focal length? (Distance from laser output to the collimating lens)

 

[dylanetaft]

A few of them show up as 8 - 8.3mm focal length. I see sites like Edmund Optics have experimental grade lenses. When I get it in, it might be worth experimenting with a bit.

 

[RedCowboy]

The divergence out of the diode is aggressive so for a hobby CNC if only 1 lens is used it's a short FL lens to grab the beam where it's as narrow as possible, the EFL of the G2 is 2.39mm

I know this can work for certain materials when the work piece is placed just a few inches from the lens ( setting the depth can be critical ) and the material being cut is not very thick, however it would be neat if a lens train was used to get a super tight beam, but I don't know as I have not purchased any of the Chinese heads, I don't know what's used in them although the early hobby type I have looked at used a G2 lens.

Also as the repurposed laser diodes evolved and got stronger the divergence of the aggressive axis increased, the 2,3, and 5 watt heads used diodes such as the ndb7875 and ndb7a75, however if using the nubm44 now the more aggressive axis could be a real factor as you want to cut with a square beam and not a long rectangle unless it can be focused down so tight that it won't make any real difference.

I am curious to know what you discover, please post your findings when you get your head.

---edit---

Also you should add to your post like I am doing here and don't double/triple post as people get upset about it and it's actually against the rules, that may sound picky but it cuts down on clutter and makes for a better reading experience. Just use the edit feature bottom left, Thanks.

 

[diachi]

Basically, I am wondering if I can get like a 20mm focal length lens, mount it in a tube after the collimating lens that ends 20mm away from a workpiece to get a smaller beam?

A longer focal length results in a larger beam waist (focal point) for any given input beam.

https://www.ophiropt.com/laser--mea...focal-spot-size-calculator-for-gaussian-beams

Shorter wavelengths can produce a smaller beam waist for any given input diameter/divergence/M2 factor.

Single mode (round output, Gaussian/near Gaussian beam profile) 405nm diodes (BDR-209 does about 900mW or so) are your best bet for achieving a small spot size.