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I have a laser beam which is focused to a point at a certain distance. I'm then going to use a galvanometer to scan that beam across a plane. Obviously, as the beam scans across the plane, the distance between the beam source and the plane will vary and so the beam will only be focused on some of the plane (a circle equidistant from the central axis of the galvo). I want to minimize this effect.

Obviously the further away the galvo is from the plane where I want the beam focused, the less significant this effect will be. However, that makes for an awkwardly large machine.

Is there some sort of optics I can use to correct this so that the beam will be correctly focused across the target plane?

Edit I wanted to add this as a comment but I don't think I can put images in the comments. What about this arrangement:

enter image description here

This relies on being able to form a collimated beam from the laser - in practice I think it will have some noticeable beam divergence but I'm not sure how bad it would be. It focuses the beam on the target plane whatever steering is given from the galvo:

enter image description here

Obviously I'll have to correct for the deflection introduced by the lens, and it's a bit of a pain because it increases the mirror deflection required for a given beam deflection. But it's a lot easier than eg a moving laser or a custom-printed lens.

Comments?

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  • $\begingroup$ Can the plane become curved so that the distance is sensibly constant? $\endgroup$
    – Farcher
    Commented Apr 11, 2016 at 11:40
  • $\begingroup$ No. The plane is the bottom of an SLA printer - a flat print surface is more or less a requirement. $\endgroup$
    – Tom
    Commented Apr 11, 2016 at 12:16
  • $\begingroup$ Can you tell us more about your application ... especially why the native spot size of the laser is too large. Knowing what you are trying to accomplish will help. $\endgroup$
    – garyp
    Commented Apr 11, 2016 at 12:30
  • $\begingroup$ Program a servo to move the lens forward as the beam travels a longer path; you will have to calculate the path length as a function of the angle, and use that for the program. $\endgroup$
    – Peter Diehr
    Commented Apr 11, 2016 at 12:47
  • $\begingroup$ @PeterDiehr: Not a bad thought, though since the galvo is capable of ~30kpps it'd have to be a very fast servo loop moving a relatively heavy laser module (the lens is built into the laser module). $\endgroup$
    – Tom
    Commented Apr 11, 2016 at 12:58

3 Answers 3

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If the angle of deflection is at all large I think the corrector plate will be very thick at the centre, possibly a large fraction of the focal distance. If the beam was parallel rather than focussed before the mirror, the corrector would become a large lens which focusses the beam on a flat field.

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  • $\begingroup$ ..though I'm not saying that would be easy to design! ... $\endgroup$
    – Peter Fry
    Commented Apr 15, 2016 at 10:06
  • $\begingroup$ I don't think you can design it so that the beam enters and leaves the material normally. Then the beam would not be deviated on entry and in order to be normal to both surfaces the surfaces would have to be parallel, i.e the two faces would be concentric spheres therefore the material would have uniform thickness and no effect. $\endgroup$
    – Peter Fry
    Commented Apr 15, 2016 at 10:19
  • $\begingroup$ Thanks, Peter. Yes, I think trying to get a (near-) parallel beam before the mirror and then a large(ish) lens is looking like being the way o go. The longer the focal distance, the nearer the lens can go to the mirror, and so the smaller diameter the lens can be. I'll have to think again about the laser module and try to come up with something with collimating optics rather than focusing optics (beam diameter is not a problem, of course, since I'm just going to focus it to a point later on anyway - in some ways larger is better to reduce power density in the optics). $\endgroup$
    – Tom
    Commented Apr 15, 2016 at 10:34
  • $\begingroup$ Yes. Of course I wasn't quite right about "no effect". What I meant is it would just move the focal surface from one unwanted curve to another. If you have to make the lens large you may want to do it as a Fresnel lens, so it doesn't get too thick, although this might create blips as the beam passes over the steps. Good luck $\endgroup$
    – Peter Fry
    Commented Apr 15, 2016 at 10:41
  • $\begingroup$ @Tom by all means! My word is not law, this just came up as a flag in the review queues and my quick assessment was that it didn't seem like a comprehensive answer. But if you feel your question has been answered, you're free to mark it as the accepted answer! $\endgroup$
    – Kyle Oman
    Commented Apr 15, 2016 at 13:43
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The problem is to deliver properly focused light to a planar surface from a lens with a fixed location. Any method which keeps the optical path length constant, while not refracting the beam, would work.

Therefore introduce a carefully designed transparent phase plate, where the optical thickness at each point is intended to compensate for the shorter path; thus the plate would be thicker in the center of the field, and thinner towards the edges. The minimum thickness is determined by structural requirements, so that it can be held rigidly in place.

Ideally the beam would enter and leave the material normal to the surface, so that there are no focusing effects; the calculated distances for each of the galvo angles, and the location where the beams would intersect this plate provide most of the information required.

This is a passive element; no more moving parts; it just has be kept clean.

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  • $\begingroup$ After considering this for ten seconds, I'm left wondering if it is possible to do this without refracting the beam. The optical path of the optic would have to change continuously with angle, which seems to imply a curved phase profile. $\endgroup$
    – garyp
    Commented Apr 11, 2016 at 15:53
  • $\begingroup$ @garyp: Correct! It's like designing a cam, but 2D. They could print molds with their 3D printer. For a standard system this would be much cheaper and more reliable than adding more moving parts. $\endgroup$
    – Peter Diehr
    Commented Apr 11, 2016 at 16:40
  • $\begingroup$ But can it be done without refracting the beam? Perhaps I should add the word "significantly" to the end. Modeling is needed, I think. $\endgroup$
    – garyp
    Commented Apr 11, 2016 at 16:46
  • $\begingroup$ @Garyp: I agree; changes to the focal spot size would be the measure of acceptable variation. I imagine a very smooth surface, with a slow change in shape. But it all starts with a model which shows the optical path length as the two galvo angles are changed in order to reach each point of the plane. This in turn depends on the actual geometry of the setup. $\endgroup$
    – Peter Diehr
    Commented Apr 11, 2016 at 16:50
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Depending upon the overall size of the layout, you might want a field flattener lens. I don't know how large your screen is, but for Petzval curvature (the natural field curvature for all optical systems given by P = Sum(poweri/ni), where power is 1/(focal length) and n is the refractive index of the glass. But this is not field curvature. I'm just saying that to correct for curvature you can place a lens on the screen. For a curved field that is concave you would use a concave/plano lens. For a convex curved field, you would need a positive power lens. You want to make the focal length longer as a function of field angle so the light focuses on the flat screen instead of in front of it on the sphere. Thus, you would need a concave/plano lens on your screen. Probably not applicable in your situation. Depending up the size of everything you might consider an F/theta lens, or a telecentric setup. Place the galvo in the front focal plane of your lens. The collimated light is focused by the lens and will focus a distance f away (same distance from the lens as the galvo). But since the galvo is in the front focal plane, that means your aperture stop is effectively in the front focal plane as well and all your converging beams will be parallel. The spot will always be focused on the screen. You might consider using an off-axis parabola instead of a lens if the element is large. Effectively, your imaging element (lens or mirror) will be the same size as your screen. But it works very well. I have a simple Power Point drawing but cannot figure out how to attach it here.

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