Collimating theory

[LaserGuns]

I'm trying to figure out what is required to get a nice straight beam. I have a few average laser pointers in red and green, and the beams are fairly acute in their angle. On the other hand, I have a blue-ray diode that I put in an aixiz housing. With this thing if I focus it at one range, the point is fairly large at other ranges.

What is the best way to get the beam angle as acute as possible? Should I just use a single lens and focus it from as far away as possible? Or is there some way to use two lenses, first to focus the wide angle light coming from the diode into a point just past another lens that will "unbend" it into a parallel beam. Sort of like this Diode-> []<|>|= If that makes any more sense:thinking:

Ideally, I would be focusing my IR diode into a perfectly parallel beam about .1" diameter... In a perfect world...

Thanks for any help!

 

[ReNNo]

To get the best divergence without complication use only aixiz plastic lens to focus BR.
Just focus it to infinity and divergence will be just fine.

With IR diodes it's more complicated.
Divergence of IR lasers is worse because of it's wavelength.

And if you have multimode IR laser diode situation is worse.
At this point you need correction optics (the easiest way is combination of cylinder and aspherical lens)

 

[LaserGuns]

Sorry, I forgot some information. I will be using IR diodes that are already FA Corrected. Blue Sky Research makes them. They already have a near circular beam shape, so all I need to worry about is collimating it into a beam. I guess having it focused far out will work because I'm not too worried about having the beam large up close. I'm still curious how they get it like that with my green pointer...

 

[ReNNo]

Green pointers use crystals that make perfect round TEM00 beam, that is why they can collimate beam easily.
As I told you, IR light is difficult to collimate because of it's wavelength.

But I think beam expander can help you to have smaller dot at higher distances.
I know it works great with green laser....but don't know about IR.

 

[Dvorhagen]

Regarding the BluRay you mentioned - BluRay diodes (I'm assuming it's a PHR) have to be set deeper in an AIXIZ than other types. If you're unable to push it far enough into its setting, try removing the knob on the housing's lens nut so that you can screw the lens closer to the diode. You should be able to properly focus it then.

 

[ReNNo]

Yeah, try to unscrew focusing ring a bit, and glue it to plastic lens holder.
These new Aixiz lens are just fine for PHR.

 

[Cyparagon]

IR light is difficult to collimate because of it's wavelength

Only marginally. It has much more to do with the larger emitter area and multi-mode nature of a typical high power laser diode. The same is true of a high power red >300mW or so.

A 2.5mm beam on a multi-mode diode will inherently have high divergence. There's nothing you can do about that short of pricey optics.

 

[ReNNo]

Yeah that is true...
Besides wavelength difficulties to collimate beam, chip size is much bigger problem.
And multimode diodes are different story To successfully collimate multimode IR laser you need correction optics.

 

[LaserGuns]

I'm not really concerned about the BR pointer, but I have been able to get it focused fairly well by removing the metal cap.

I don't have the space to use a beam expander, so that leaves me with a little bit more limited options. The diode is only 50mw so I'm pretty sure it is single mode. And the beam is already Fast Axis Corrected in the diode.

Do you think I would be able to collimate this at all with an off the shelf uncoated plastic lens? I was told the plastic ones would be better than the glass ones.

Thanks for the help!

 

[ElektroFreak]

if the diode is single-mode, you should have no trouble collimating it with a simple plano-convex collimating lens. Plastic or glass, it doesn't matter too much.

 

[ReNNo]

50mW diode is probably single-mod, you don't need correction optics inside of diode.
You only need one lens, plastic or glass.
If you have coated glass lens for that IR wavelength that lens should be better, otherwise plastic would do good job.

 

[LaserGuns]

The 850nm diodes that I was looking at seemed to be particularly bad with the fast axis to slow axis ratio. They were about 8.5 degrees on the slow axis, and 30 on the fast axis. However, I found a company that sells laser diodes with the fast axis correction built in. The spec sheet says they are tuned to about 10 degrees.

I guess I'll give it a try with the plastic lenses, I have 5 of the 99 cent ones in the mail from modwerx. I may also try one of the coated plastic ones from thorlabs, but those are about $10 each.

Thanks

 

[LarryDFW]

LaserGuns;

If you want the tightest beam, you have to have a short F.L. aspherical glass lens.

It will focus to infinity much closer to the laser diode, before the beam has enlarged in both axes.

It is the best beam you can do with one lens.

LarryDFW

P.S. My dictionary confirms the plural of axis.

 

[Cyparagon]

It won't look like a laser if it has the divergence that comes with a beam that small.