Collimating a laser beam with a fixed focal point?

[SirLollington]

Hey,
Not sure if this is the correct sub-forum to post this question. I'm still kinda new here, so sorry if I got it wrong

I have a laser module with a fixed focal point of 16 mm and I'm trying to collimate that light into a beam. The point of focused light is at most a quarter millimeter across and I'm trying to get a thin beam of light that has minimal spread. I'm trying to stay below a beam diameter of 5 mm.

My approach so far has been to assume the focal point of the laser as a "point source" and to use a convex lens with a focal length of 8 mm at a distance of 24 mm from the laser module. This worked for me in theory, but somehow it just doesn't work all that great in practice. The beam spreads significantly, so the spot on a wall that's just 5 meters away is already 20x20 cm large. That's about as small as I can get it.

The conclusion I've come to is that I simply don't know enough about the subject and took a wrong approach. Does anyone know where I went wrong here?

 

[mfb]

Looks like an optics question, I moved the thread to general physics.

Did you move the lens around to see if there is a better distance? How did you measure the distance? At such a short focal length even half a millimeter can matter.

 

[SirLollington]

I mounted the lens on a plate that's suspended on a threaded bar. I positioned it at roughly 24 mm from the laser and then used a nut to make fine adjustments.

 

[mfb]

And the 20 cm spot was the best you got? Before and after the spot was even larger?
If yes: That is weird. How large is the spot without the lens?

 

[SirLollington]

I should probably mention that the module outputs a line that's roughly 5 times as long as it is wide (makes sense because that's the shape of the actual laser diode). Without the lens in front of it, it makes a cone with an opening angle of around 20 degrees, and the line covers around half of the wall (1.5 meters-ish, no good way of measuring it because at that point there are obstructions in the way).

With the lens, if I move it too close, I get a line that has a similar shape to what I'd get without the lens, just rotated at a 90 degree angle. If I move it too far away, I get a similar result to not having a lens at all. The 20x20 centimeter spot I described is as small as I can get it, and when I have the lens at that distance, the spot is pretty close to a perfect square.

According to the information given here: https://www.newport.com/n/focusing-and-collimating I should be able to get a beam diameter of roughly 3.1 mm with an opening angle of 1.8 degrees - which is much better than what I'm seeing here.

EDIT: This is roughly the setup I have, except it doesn't work:

 

[mfb]

If your focal point has the laser beam collimated to 1/4 mm (how accurate is that number?), then you have at most 1/32 spread as seen by the lens at 8mm distance. Over 5 meters that is 16 cm. If your focal point is smaller, that number gets smaller.

A lens with a larger focal length could help.

 

[SirLollington]

(how accurate is that number?)

Not very. I've basically made an estimate by shining it onto a sheet of paper with fine lines every 0.5 millimeters, and the diameter of the spot was significantly smaller than half of that. Unfortunately, I have no good way of measuring the size of things that small. I'll see if I can find a way to get a more accurate measurement in the morning.

EDIT: Actually, I went ahead and did it just now. It's actually kind of interesting, now that I was able to look at it in a dark room. I repeated the same thing as above, except on a black plastic surface.

The laser seems to have 2 'separate' focal points, which are separated by about 1 mm. The first one is roughly 0.25 mm across, and is the spot where all of the light of the laser is collected in the smallest spot.
About 1 mm behind that, I can make out the 'line' image of the laser diode again, but there's a very bright spot in the middle of it, which is significantly brighter than the 'first' focal point. It's also significantly smaller, and I have no way of accurately measuring the size of it. My equipment is very limited, sadly. :/

 

[Tom.G]

Put an aperature (pinhole in aluminum foil?) at one of those "bright spots" then image the aperature with lenses of various focal lengths until you find the magic combination.

 

[DaveE]

Diode lasers have different divergence for their two axes (called the fast axis and slow axis). You can't really collimate the beam with a normal lens since you need a different focal length or position for each axis. In the laser world they would do this with special lenses that are a mix of cylindrical and spherical (aspheric). This is similar to correcting astigmatism in glasses.

https://integratedoptics.com/laser-beam-collimation

 

[Tom.G]

"Festbrennweiten" = Fixed Focal Lengths (Norwegian to English)

Adjust the lens to the focal length (brennvidde) you want, then shine the Laser from the back of the lens (the end that mounts on the camera).

Change the distance between the Laser and lens until collimated.

The hard part will be setting the lens aperture (blenderåpning) because the camera usually controls that. There are adapters available that allow manual (Håndbok) aperture setting but I do not remember the details.

(translations by Google Translate)

Cheers,
Tom