Two anamorphic prism pairs for 445nm diode

[Colin Kaminski]

Divergence will be an issue if we are trying to pass three colors through a single spatial filter and have even color across the entire plate but it seems like we are a little ways away from that.

[Joe Farina]

Divergence will be an issue if we are trying to pass three colors through a single spatial filter and have even color across the entire plate but it seems like we are a little ways away from that.

Yes that's quite true Colin. Or even with two colors. However, there's a fairly easy way to control divergence and beam diameter, thanks to a tip from Ed Wesly. This is a zoom telescope arrangement, known as a Donders zoom telescope. It's just a three-element system, a negative lens between two positive lenses (or alternately a positive lens between two negative ones).

I have these three lenses (mounted on posts on a rail for covenience) in front of my 315M. The divergence and diameter are adjusted by varying the positions of the lenses, particularly the middle one. The beam goes through the backside of a dichroic beamsplittler and then to the table. The dichroic is at 45 degrees, and the HeNe beam reflects off the front surface. So the two beams are combined.

The zoom telescope makes it possible to not only change the divergence, but also the spot size at any remote point (in my case this is about 6 feet from the lasers). Without much trouble, I was able to get a beautiful spatially-filtered yellow disc of light on the wall.

So for a color setup with the 445nm diode, I think the zoom telescope might be something of a prerequisite.

[DJ Mathson]

Joe,
Do you know a way to calculate which lenses to use, and the distance between the lenses in the Donder´s telescope, to get a certain size and divergence of the output beam, depending on color, size and divergence of the input beam? Do you have a photo of your setup?

[Joe Farina]

DJ, there are so many variables involved that I think it's best to plunge right in without any calculations, with a good supply of lenses at hand. That's what I did, and it didn't take very long to get good results. But I suggest getting a healthy supply of lenses in the 10 to 18mm range or thereabouts. The best source for these lenses is Surplus Shed, I would get about 15 of them (they are cheap enough), mostly double convex with a smaller number of double concave, but maybe throw in a couple plano lenses (concave and convex) for good measure. But I do suggest using a rail, it makes it a lot easier to slide the lenses back and forth. I used a Thorlabs rail.

Here are a couple photos of my setup. Just ignore the first optic (the rectangular beamsplitter right after the 315, it's just there to attenuate the beam). The lensholders look a little grungy because I made them myself

[DJ Mathson]

Thanks for the images. Nice setup. You´re probably right that it is best just to try some lenses. So far, the papers I´ve found are heavy on mathematics. A bit too heavy for my taste. :wall: Just found the book Lens Design Fundamentals talking about the Donder's system. Here is the book Ed talked about.