Getting a dot out of a 808nm bar diode....

Hey there.
opening this thread during lunch break on the job, so I hope there isnt already a thread about this topic. Didnt find any...so...

I got some REAL nice IR diodes...up to 7W...but all I can get is a bar...focussing isnt that hard with the right lens, but if you wanna do some burning, the distance of 2inches sucks a bit. Is there an easy, cheap way to get a nice spot?

Thought about fibres...but they have to be at least 500mm long I guess. Isnt there a prism which can do the trick or something?

A fast-axis lens?

You will not get beam quality anywhere close to that of a typical laser pointer though.

We need a good thread about this. It comes up a lot, but I still have yet to see how these lenses are used, and how they are set up to make a dot. I would like to see what some of the lens characteristics are, and how things need to be mounted. Anyone care to share their experiences with this?

Phew....thought there might be a thread about this.

A fast-axis lens cant be used to get an actual beam.
Must be possible to get a coherent beam out of those little suckers. Dont need a ROUND dot, but a beam...burning on a distance.

To focus multimode diodes (bars, c-mounts, 9mm, 5.6mm, doesn't matter) you need a convex collimating lens and a cylinder lens if the diode doesn't already have a FAC (fast-axis collimator) lens installed. A FAC lens is a tiny piece of singlemode fiber that is glued horizontally in front of the diode's output facet so that it acts as a cylinder lens:



Here's what a cylinder lens looks like:



All laser diodes have two axes of divergence since the light output is not perfectly circular. One axis diverges faster than the other, hence the name "fast axis". With multimode diodes this is even more pronounced with the output resembling a line rather than an elliptical dot. The purpose of fast axis collimation is to compress the faster diverging axis so that it's divergence is similar to the slower diverging axis. Here's a diagram describing these axes in a single-mode diode: (you can see that the output is an oval rather than round, meaning that one axis diverges faster than the other)

and here's a more advanced paper on how to correctly collimate a laser diode:
The aberration correction of a Diode Laser

If the diode does not have a FAC lens installed, just about any cylinder lens will do, but the focal length (distance at which the beam is focused to a point) will vary from lens to lens. Experimentation is key to figuring out what will work best for you. This arrangement will also give you a square beam rather than a focused spot as long as the lenses are correct and properly positioned. The way you want things arranged is like this:


BUT you can swap the positions of the two lenses if necessary, with the convex collimator first and the cylinder second. This works best if you have the diode mounted in a module that already has a collimating lens in it. Then you just need to place a cylinder lens in the output beam from the module in such a way that you get a focused spot, which would require the cylinder lens to be oriented at 90deg from the fast axis of the diode. In the c-mount diode in the first picture in this post, the fast axis is vertical, which is why the FAC lens is horizontal. Experimentation will yield good results.. If the diode already has a FAC lens installed, then all you need is a convex collimating lens, just the same as the singlemode diodes we're all used to.

Here's a beam profile pic of the best case output from a c-mount 650nm laser diode collimated for a low-divergence beam. This pic is from a CNI 650nm red unit which utilizes anamorphic prisms and a plano-convex lens:

You can use cylindrical lenses to bring the beam into a more round shape then use anamorphic pairs or a beam expander to expand the beam a bit to control divergence then finally into a focusing optic.

There are also optics designed for diode bars called light ducts. These are used to pump the beam into a fiber. Once in the fiber, it pretty much circularizes the beam. At the end of the fiber you can attach any necessary focusing optics.

I found this info on Light Ducts from a company that makes/sells them
They have a bunch of other loptics as well

http://www.vloc.com/PDFs/Optics/LensDucts.pdf


Jerry
(6400-287-26034)

Light ducts and fiber are one of the best ways to smooth the output from multimode lasers of any kind, but they can be very expensive and technically challenging to set up properly..

EF, many thanks for that awesome post up there! I've never seen it explained so well! The system won't let me rep you again, but I'll give ya what I can. :beer:

This info is going to come in handy! :eg:

Now we need to find a somewhat reliable source for some generic cylinder lenses..

Hey

You could try these , if thats what u need TECHSPEC High Performance Cylinder Lenses - Edmund Optics

ElektroFreak - Would a rod lense work on a 20 watt diode bar do you think ? >> http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=2092

Thanks

^can't think of any reason why not..

ElektroFreak said:

^can't think of any reason why not..

Click to expand...

Ok thanks , ill give it ago , 15mm wide lense for 10mm wide bar emmitters

Electrofreak, was that C-mount with the fiber "cylinder lens" yours? Got any output shots? I'm curious why more people don't just use fiber scraps if it works decently... Seems like a great idea.

Is there a way to tell by looking if a diode has a FAC lens or is it too small to see? One I'm looking at says the beam divergence is 42x10 degrees which makes me think it is not, but the seller is saying it is.

BShanahan14rulz said:

Electrofreak, was that C-mount with the fiber "cylinder lens" yours? Got any output shots? I'm curious why more people don't just use fiber scraps if it works decently... Seems like a great idea.

Click to expand...

It is mine, but it's just a dead 808nm pump. I don't have any FAC corrected diodes that aren't pumping crystals in a DPSS laser of some sort..

@Pointiac: A FAC lens can be seen, but it is quite small. A magnifying glass will help.. I can't remember what some typical divergence numbers are for FAC diodes off the top of my head, though.. Who's the seller?

Here's the link, the sellers name is snoctony. The listing makes me nervous as it is, but I want a strong 635nm handheld laser, I love the color!!!!

If you look at the picture you can see what looks like a fiber, but I might be wrong.