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For my engraving application, I use a standard laser engraver in 1064 nm. The laser system is equipped with galvo scanning head and a f-theta lens with marking range of 100x100 mm Since I need to mark very small objects (like jewelry) I do not need such a large marking area, but an area of 25x25 mm would suffice. On the other side, I would benefit from faster processing time. Right now I need to keep the laser spot for some time in the single pixel I want to engrave, in other to produce a noticeable engraving. This is affecting engraving speed. If I engrave the object at higher speed, the image will not appear. Thus, I am planning of using an additional lens to make the laser working area smaller and increase the spot power at the same time. Which would be the best set-up to use?

  • Change the current f-theta lens with another one with smaller working area, however this option is complicated since a need to modify the machine and the f-theta lens is very expensive
  • Add another lens in front of the f-theta, such as a plano-convex lens, to focus the laser beam. This could be a good option as I would just need to add an extra element, which does not seem to be very expensive. However, how should I choose this lens? Which size, magnification factor should I choose to achieve a smaller working area? How much would the efficiency of the system decrease? I mean, adding a lens some laser power will for sure be lost and dissipated in some other form of energy. What about the focus distance of the laser? This would decrease the laser focus distance, which is indeed a good thing for my application as I can work closer to the object. Moreover, at which distance from the original f-theta lens should I install the new plano-convex lens?enter image description here
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  • $\begingroup$ For one thing you should be aware that how much material the laser can ablate depends primarily on the laser power. All else being equal a laser with twice the power will ablate twice as much material per unit time. If you are expecting wonders from a change of optics, then you are probably in for a nasty surprise. Having said that mmesser's detailed comment is right about one thing: if the manufacturer of your laser did a poor engineering job, then you can improve the machine. If they didn't, then you will simply be wasting more money on the wrong machine for your purpose. $\endgroup$ Nov 6, 2022 at 15:57

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To make a spot etch faster, you need a higher power density at the focal spot. There are two ways to get this - get a more powerful laser or make the beam focus to a smaller spot.

Two things limit the focal spot - diffraction and lens aberrations.

We usually approximate light by treating it as rays that travel in straight lines. But light is a wave, and waves spread out when they pass through a pinhole. You can think of the laser aperture as a giant pinhole. Light spreads out just a little. Rays follow paths the shape of a hyperbola, almost but not quite straight. You can make the situation worse by using a lens with a small aperture that restricts the beam. To learn more about diffraction as applied to laser beams, see Gaussian Beams It has a link on focus.

Another place that diffraction matters is when you bring the beam to a focus. Idealized ray optics would lead you to think that all the rays cross at a point - infinite power density. In reality, the spot is something like a pinhole. Light spreads out when confined to a spot. This limits how small the spot can get. To learn more about this, see Lenses.

Scroll down to Focusing a Collimated Beam. You will see the best possible spot size is given by

$$w = \frac{\lambda f}{\pi w_0}$$

There are things you can do here to make the radius of the focal spot, $w$, smaller.

  • Choose a laser with a shorter wavelength, $\lambda$. Probably not your favorite choice.
  • Choose a lens with a shorter focal length, f. Or as you propose, shorten the focal length by adding another lens element.
  • Make the beam radius, $w_0$, larger. A larger diameter beam can be focused to a smaller spot. You can do this with a Beam Expander. But again, this is not a cheap solution.

The bad news is lens aberrations. Lenses are designed using ray tracing. The goal here is to make all the rays come to a focus at the same spot. Diffraction prevents perfection of course. But a lens that isn't designed for this will add to the problem.

Lenses are ground to spherical or flat shapes because it is possible to make these shapes with the extreme accuracy required by lenses. But while a sphere is very close to the ideal shape, it isn't exactly right. The result is a plano convex lens has a slightly different focal length in the center and at the edge.

Lens designers deal with this by using more than one lens element in a compound lens. For a really good quality lens, the errors from the different elements cancel so well that performance is diffraction limited.

By adding a plano convex lens to a well designed lens, you will add an uncorrected aberration. You might not get good results.

If a plano convex lens is cheap, you could just try it out.

You can also buy more expensive lenses with two elements (a doublet lens) or three elements (a triplet). You can call the company that made your engraver and ask for advice.

The things that make diffraction small are a large diameter and short focal length. These make aberrations big, and a well designed lens with minimal aberrations expensive.

Camera lenses are cheap because they are produces by the thousands. Lenses for lasers are expensive because they are produced in much smaller batches. Doublet and triplet lenses are in between.

The RP Photonics Encyclopedia is full of useful information. Check the articles on Laser Marking, Laser Optics. And browse around.

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  • $\begingroup$ This partly answers my question, but it is not a complete answer. Given that I need to add a lens, as I already wrote in my question, which kind of lens should that be? Which magnification factor? I provided real life data, in the starting condition I got 100x100 mm marking area and I want to go down to 25x25 mm $\endgroup$
    – Francesco
    Nov 6, 2022 at 17:45

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