Question about anamorphic prisms

[steve-o]

it's got a large convex side facing outward, and a much smaller convex bump on an otherwise flat side facing inwards.

Yep, that about sums it up. The very small "bump" is facing toward the laser diode.

@Slogan- not sure about the price question, but I prefer the 9mm blue diode for power and decent beam . Some others here may be able to answer your question about which has the best beam quality. There may be a lower power single mode 445-450nm out there; single-mode diodes usually have the best beam quality.

-heh-oops- posting same time a s Absolom --

[slogan]

ok thank you for your answers,
I'll find out more about the OSRAM and Nichia

[The_Doctor]

Yep, that about sums it up. The very small "bump" is facing toward the laser diode.

Ok, thanks, as I have three projector diodes in a tin waiting for the day.. To make sure I'm not missing something, this is diode first, prisms second? Got to be, I imagine. Also, as the tiny bump intercepts light very close to the diode I wonder if a good way to clean up light spill would be to find a tiny washer the same diameter as the lens rim, and fit that first, though it may be that trying this will throw more spill from diffraction than it would clean up. Not to mention being fiddly beyond imagining. As I prefer single mode for a beam, I will probably omit the prisms assuming the multimode projector diode is single mode in one axis, and put a cylinder lens after the asphere to make a static fan.

[planters]

I have found that the Optima 4mm aspheric lenses work just as well as Dave's 2mm lenses. They just require less beam expansion. The near and far field beams are the same and the throughput is the same. Dave can provide a 9mm x 0.5mm mount for the Optima or a pre-glued 2mm lens in the same size mount.

In general, with imaging systems there will be smaller aberrations (spherical aberration) if the lens surface with the greatest curvature is oriented toward the more parallel beam.

I have been a very strong advocate for spatial filtering to clean up these poor quality diode laser beams. I use this in all my projectors. It works very well and is easy to do, but it requires some space within a projector for the added beam length. It must be done at a focus and it woks better and is easier to do the longer the focal length of the identical positive lens pairs used to create this focus. Just clipping/masking the roughly collimated beam works poorly if at all.

Before you construct a projector or even lay out the components set up some simple experiments with a diode in a mount with a diode driver controlled by a 5V analog PS. Get a lens and focus the output then try a pair of prisms vs a cylindrical lens pair. This is a lot of fun, has a minimal investment in time and money and will get you comfortable with the essential aspects of the beam manipulation that comprises most of the effort in the building of a projector. If you go single mode you can still benefit from control of beam size and use spatial filtering to clean up this better beam. A static, clean, collimated, 2.5W 445nm beam is pretty neat bouncing around your workshop and knowing how to knife edge another to turn this into a hot 5W beam...well.

[The_Doctor]

I have found that the Optima 4mm aspheric lenses work just as well as Dave's 2mm lenses.

Interesting. Are they glass, or acrylic? I bet glass for withstanding the power, but I ask because they seem similar in intent and effect as the 4mm acrylic CAY033 lenses I have. I tried one in a test with the projector lens. Wasn't full power, but it showed no sign of collapse with 500mW going through, so maybe it's ok. And if it's like Opnext diodes now being Oclara and all, then keepign track of these names is going to be tough. CAY033 was Philips, last time I looked..

Re spatial filtering, Sam (Goldwasser) described that to me on Usenet, and it's a great idea. Expense and loss of power from small diodes put me off it at the time, but things change.. I do trim spill after collimation in a small projector I use at home (self-built thing), and it works ok, but as you say it isn't easy to get much out of that. I'm guessing that's what the tiny bump on the asphere is doing anyway, collecting with enough precision to reduce the need to do anything else to get a decent result. (Whatever gets through the flat part will be so diverged by the front curve that it is weak, wide, easily blocked). I don't think I'll try blockign light that could hit that flat part. Too tiny to do well, and scatter off the edge of a hole in a washer before a lens will likely make a pig's ear of whatever good does come out of it.

[planters]

The Optima lenses are glass and I have had them to 3W with a cooled P73 without a problem. $18 from Optima precision Optics CA. But, for what it's worth I use Dave's 2mm pre mounted lenses in most of my set ups.

If Mr Goldwasser described this I hope he did not suggest a microscope lens or a precision pin hole. All you need is a pair of positive lenses and they can even be of different FLs. Set them up, facing each other at approximately the sum of their FLs say 125mm for a 50mm and a 75mm lens pair. Now just insert a small metal strip ( a razor blade works great) slowly BY HAND at the beam waist between the lenses. You'll be amazed at how easy this is and how crisp the contrast is between the far field spot and the surrounding, now dark, projection screen.

[The_Doctor]

No, probably I was just fussy and wanted it simple and cheap. Sam's always advise me well and in simple pragmatic terms. I'm guessing that like Einstein, he beleives if it can't be expressed simply and in plain language it's not understood well enough. He was great for advice on safe diode drives especially when it cane to ESD protection. I heard of that laser edge trick, that might have been him, or another Usenet regular 7 years ago. That trick is going to make a mighty long laser module by my intentions, but if I like the result I can rethink that.

3W with ONE cooled diode? No PBS? Ok, that has me fascinated. I have some tiny and efficient TEC's but I suspect I'll want more power than they have. They're of unstated spec but a few quick tets suggest they're more than adequate for a250mW red, but the Vf of that would be much lower.. But it also depends on how cool they must get. A red could go easily sub-zero on mine. (Which was one of the holdups, I never resolved what to do there before life demanded big changes. I'm looking at lasers again now though.

[polishedball]

I have been a very strong advocate for spatial filtering to clean up these poor quality diode laser beams. I use this in all my projectors. It works very well and is easy to do, but it requires some space within a projector for the added beam length. It must be done at a focus and it woks better and is easier to do the longer the focal length of the identical positive lens pairs used to create this focus. Just clipping/masking the roughly collimated beam works poorly if at all.

Before you construct a projector or even lay out the components set up some simple experiments with a diode in a mount with a diode driver controlled by a 5V analog PS. Get a lens and focus the output then try a pair of prisms vs a cylindrical lens pair. This is a lot of fun, has a minimal investment in time and money and will get you comfortable with the essential aspects of the beam manipulation that comprises most of the effort in the building of a projector. If you go single mode you can still benefit from control of beam size and use spatial filtering to clean up this better beam. A static, clean, collimated, 2.5W 445nm beam is pretty neat bouncing around your workshop and knowing how to knife edge another to turn this into a hot 5W beam...well.

Is there a thread explaining "spacial filtering"?

[planters]

Is there a thread explaining "spacial filtering"?

I don't think so even though I show how I have done it in some of my videos covering my projectors.

If you want the deep answer it has to do with Fourier transformation. The spatial filter acts as a low pass frequency filter. The lowest frequency, corresponding to the TEMoo mode, focuses to the smallest spot for a given FL lens. The aperture is selected/adjusted to exclude all the higher frequencies. With geometric optics what you are doing whenever you "collimate" a laser beam is really focusing the beam at the far field screen. So, what the lens pair in the spatial filter is doing is forming a preliminary focus between the lenses and then once again refocusing the beam at the far field screen. This preliminary focus is a perfect copy, at a small scale, of the far field focus with all the blemishes, wings and scatter spread out around the central spot. It is trivial to block these imperfections at this first focus before the second lens can send them down field.

Another way of thinking of this is that unless the beam is brought to a focus all the rays of light that will slowly diverge to end up in the imperfections surrounding the main spot are not necessarily arranged around the main beam near the projector. They may lie within the diameter of the main beam but be very slightly divergent and only can be intercepted far down field. This is why when you try to block this undesirable light, you become more and more successful the farther from the laser you attempt this.

I have some free time next week and I will make a video demonstrating this technique specifically.

[absolom7691]

Is there a thread explaining "spacial filtering"?

There is now! Thanks, Eric!

http://www.photonlexicon.com/forums/...iltering-Video