Questions on Laser Optics & Focal planes

[Galvonaut]

When a lens system is placed in front of a laser - or in my case, in front of a set of XY scanners, where does the image/input plane need to be focussed?

I ask as I understand laser light to be a point source - so behaving as if it comes from infinity. Does this mean that the object plane(?) needs to be focussed at infinity or have a focal plane at the mirror surface? Does this change if the light comes directly from a source?

Is a collimating lens the same as a telecentric lens or 'telescoping' as I've read about here?

Thanks for any enlightenment - I'm not getting far with these questions on the interweb and I'm not really up on my optics terminology yet

Keith

[steve-o]

You are focusing after the scanners? I dont know if this is possible. Laser light is different emitted from a gas laser than a diode laser. A gas laser needs no corrective lenses, where a diode shoots out a wide "V" shaped beam that needs extreme correction. It is a "point source" I believe and what we are doing is focusing the image of the emitter at far-field on a distant surface for alignment. I don't know if this clarifies anything or not, and if my theories are flawed experts please step in and add some enlightenment.

[planters]

To make it easier to advise you, can you tell us what you are trying to accomplish? A collimnating lens changes a converging or a diverging beam into a "columnar"/parallel beam. A telecentric lens is a more complex design that effectively prevents the foreshortening that occurs when imaging. It corrects the parallax. All parts of the image irregardless of their distance from the lens are imaged at the same magnification. This is useful for machine vision.

A focusing lens can be placed, at any distance you like, in front of the scanner and this can be used to focus the parallel laser beam down to a work surface . This could be used for machining, but I believe there will be more aberrations secondary to this lens position do to off axis operation at the edge of the field than if the lens was placed prior to the scan system. In this latter case scanners often incorporate a third "Z" Galvo to correct the curved focal plane and insure the laser is in focus across the entire flat workpiece

A telescope placed in front of the scanner decrease the divergence at the same ratio that it decreases the scan angle/field of view. If this telescope is used to modify the beam size and the scan angle in this way it is operating as an a-focal telescope. This is what I am using with my spatial filters.

[Galvonaut]

Thanks steve-o. Interesting point there. I will be using DTR's single modes, so will be starting with nice beam specs. I have no idea how the collimation of the diode beams will interact with my lens set-up. I guess I will need to experiment.

I don't think I want to focus after the scanners but I do want optics which give will change the oblique angles of the beams entering the wide angle lens into a collimated format - so all of the beams enter it parallel with each other. In theory, I think, I need afocal optics after the scanners. In other words, set at infinity.

I could very well be wrong though - I'm really trying to figure this out with no where near enough knowledge or experience - I'm hoping to gain some on the journey though!

Keith

[Galvonaut]

Thanks Planters - I basically want to project abstracts at 360x180 degrees using an afocal wide angle lens.

I think most of my difficulties are that the optics I have available to me are all designed for imaging - so with a flat image plane, rather than a set of xy scanners. My goal is to correct this to reduce chromatic and geometric aberrations.

Keith

[planters]

Woa! This sounds like a job for Code V. When you say most of my difficulties..how far have you gotten using a wide angle camera lens? I have heard of an imaging devise called an "all sky camera" that is used for comet detection and weather monitoring. It must be taking parallel light over this field of view and focusing it at a flat focal plane, probably just a very wide field "fish eye" lens. This is also probably what a video projector lens is doing over a smaller field of view and in reverse by substituting a DLP for the CCD. In both cases the lens must operate in reverse. There cannot be a non-reciprocity depending on whether it is imaging or projecting. Therefore, the scanner needs to fool the lens so that the abstracts have a finite area at the image plane. Could you project the abstracts onto a ground glass screen at the focus of the lens?

[Galvonaut]

Just googled Code V, looks like a handy bit of software - if you know what to put into it

All I've done with the wide angle lenses I currently have is place them on top of a cheap (unaligned) laser. The results are promising (360x180 degrees give or take a few) but the beam is not acceptable for my intended use. My difficulties are my knowledge gaps and avoidance of maths. My problems are beam divergence and chromatic aberration and the ready made optics available to me to use with limited knowledge.

I've been looking at different types of lenses and how they work all week and had come across the All Sky Camera. The lenses I think are too small.

I want to project inside a dome, so I don't think the ground glass screen would help this.

My knowledge of optics is severely lacking and I need bite the bullet and spend some more money on optics to play with. I will see if I can pick up a cheap f-theta lens and a mono-telecentric lens and see what happens with the beams.

[planters]

For the color issue you might try an apochromatic wide angle. Or if it is just the extremes of your field, could you set up two projectors with less ambitious fields? If you have access to each of the lasers can you adjust the collimating lenses to compensate for the beam divergence or do you mean beam distortion rather than divergence?

[Galvonaut]

Yes, been looking for apochromatic afocal wide angles but they are thin on the ground - if not non-existent! Even if I found one, it would need to be corrected for laser wavelengths to be ideal.

I guess it is distortion I'm seeing. I wondered if re-focussing the beam when it is near the edge of lens with one of these might help at all?

Keith

[planters]

Those are very expensive. And I think the complexity of the control as well as the broad band operation would likely interfere with it's application.