Any suggestions for getting a shorter focal distance?

[whamodyne]

All,

I'm making another CNC laser engraver and would like to have the laser module closer to the material. How close? I would love a focal distance of 1 to 2 inches (25 to 50mm). The engravers I've made in the past had the focal distance (end of the laser module to the material being burned) at 4 to 6 inches but this is a new design and needs to be all close and personal. Currently to get that close focal distance requires moving the lens farther and farther away from the diode to the point the lens part falls out of the housing module. Need to fix that.

I've been using 405nm lasers in the 200 to 300mW range with a standard acrylic lens. Engraving wise, they work great. The blue looks cool and I like a visable wavelength for engraving. I can switch wavelengths if that would help but this feels more like an optics/setup issue.

Is there any suggestions for lens or setup? Should I hack the stock acrylic lens housing and move the lens forward or back? Get some of those extended lens housings that DTR has and work with those? A different type of lens you can get from this place over here?

I would like not to have the lens cost more than the diode and housing if possible, this is something I would like to turn into a product at some point and keeping costs down is key.

Thanks!
- Ray

 

[Marco Polo]

Using a collimator with higher numerical aperture would work, I think. I say higher N.A. rather than shorter focal length because a high N.A. collimator has higher lens power at the edges of the clear aperture and it is this higher lens power that you are exploiting to focus to a point. Read on to see what I'm getting at (and if I'm wrong, someone feel free to correct me).

I've been thinking of this lately in terms of burning potential but it might be relevant here as well.

It seems to me that moving the lens farther from the diode wastes laser power and actually decreases the maximum power on target, i.e. burning/engraving ability. For optimal collimation (infinity focus) the lens should be placed where the beam diameter is equal to the clear aperture of the lens, isn't that so? Numerical aperture is basically another (dimensionless) way of stating the angle of divergence, and the lens is designed such that when its N.A. matches the beam's N.A. all of the rays are adjusted to be approximately parallel (to within the diffraction limit of gaussian divergence). When you move the lens away from the diode, only the central part of the beam is captured, and since those rays are diverging at a lower angle, the higher lens power at the edges of the clear aperture over-corrects, resulting in focusing to a point. The rays near the outer diameter of the diode output miss the lens and never reach the target.

It seems to me that if one wants maximum efficiency in terms of input power versus power on target, it would be better to use a lens with a large numerical aperture so that more of the beam can be captured without needing to move the lens as far away, thereby "clipping" the output less. I would not use one of the acrylic AixiZ-type lenses because those lenses have a lower N.A. than even a G2. This is why they "clip" the beam as they do (and also why high power lasers tend to melt their black plastic housings.)

In your case, you want to focus to an engraving point and you want that point to be closer to your lens. You should be able to do this even with a lens of longer focal length by exploiting the higher lens power near the edges of a high-N.A. lens. A lens with effective F.L. = 10mm and N.A. = 0.8 would focus to a point nearer to the lens due to the higher lens power at the lens's edge; the only difference between using that and, say, a G2 is that the lens would need to be a few mm farther away from the diode. You'd probably want use a custom module and lens barrel sized to fit the lens that places the lens where you want it. That would avoid the need to use spacers or whatever in the lens barrel, and also would avoid having to unscrew it so much that it is barely hanging in there. I'm sure one of our machinist members could create one as easily as they could replicate an AixiZ (M9x0.5 threaded) module.

The numerical aperture for the G2 lens is either 0.6 or 0.62 (depending on what spec sheet you read), so you would need to use a lens with a higher N.A. Looking on Edmund Optics I'm finding lenses with up to 0.66 N.A., and on Thorlabs up to 0.68 N.A. This lens is the highest N.A. (0.8) I could come up with on short notice. Be prepared to fork out though, they are pricier than the G2 but they should be able to do exactly what you want. If you're willing to fork out for one of those then the custom module and lens mount should be cheap by comparison.

Also, since it is an engraving machine and not a handheld, there is no need for the lens to be as small as a G2 or comparable lens.

I'm at work right now so I feel like this is a bit of a sloppy discussion on my part, but am I making sense, or am I way off? It seems to me that simply moving the lens farther away would clip the beam and reduce the power on target.

ETA: If you want to do it on the cheap you could use two lenses and "stack" them... maybe take an AixiZ lens and sit it right above the diode window so it captures all of the output and compresses the divergence a bit, and then aim the compressed beam into your G2. I'm pretty sure that would work. I think you would still want to use a custom module so that it fits together without needing to jerry-rig it.

Or, I know that DTR sells "extended" lens barrels that are longer than usual, they might be long enough that you could mount both lenses in the same barrel, which would avoid the use of custom parts entirely. Thinking on this a bit more, maybe use the meniscus lens from a 3-element, followed by an appropriately sized spacer, and then the G2 in an extended barrel. This would result in an all-glass setup where all the lenses are AR-coated. I have an LPM on order, when it arrives I'll test this if I can remember to.

 

[lazeristasUVISIR]

Is there any suggestions for lens or setup?

1. A different lens holder
2. You can try to collimate your laser and put an additional lens with f=25-50 mm.