Help with basic optics

[Meismewhoisme]

Hi everyone,

I am very new to the site and inexperienced with lasers.
I have currently got a project where I am using an ir camera (Kinect) to see an infrared laser (850nm 50mw laser diode).

My issue is that my setup produces a very large beam at around 2m away. (With or without lens) I have tried to correct this however the only lens I have has a focal length of 140mm. I believe I understand that the smallest point for the beam is at that 140mm -away from the lens but don’t know where to buy a lens that brings it closer. I am currently driving my laser with a cheap dc buck converter with current limiting.

Here are my supplies:

| TO18-5.6mm Laser Diode Housing Case Laser Module Heatsink Set with Fan G7 Focusing Lens
(Not enough points to link)

And

| JDSU Infrared IR φ5.6mm 850nm 50mW Laser Diode (Brand New)
From aliexpress (not enough points to link)

I do have a fairly limited budget ($10 -$30).

Any help on what kind of lens I need or what to search for is greatly appreciated.

I have purchased a couple pairs of safety goggles from a reputable source and run a quick test to be sure they work so I am not in any danger of injuring my eyes when using this laser.

Thanks for reading and sorry if this is a stupid question.

 

[RA_pierce]

The "G7 focusing lens" that comes with your heatsink should be able to collimate your laser diode if installed properly. A common mistake people will make is to install the lens backwards.
If you want the focal point to be closer to the aperture of the laser, use a lens with a shorter focal length.
You're right that for a collimated laser beam, you can create a focal point at a distance from a lens equal (approximately) to the focal length of the lens.

If you explain more clearly what you are trying to accomplish we can provide more helpful advice.

 

[paul1598419]

You should be able to find a cheap G2 lens on eBay. They have a ~2.39 focal length and I have used them on my 808 nm builds.

 

[Meismewhoisme]

The "G7 focusing lens" that comes with your heatsink should be able to collimate your laser diode if installed properly. A common mistake people will make is to install the lens backwards.
If you want the focal point to be closer to the aperture of the laser, use a lens with a shorter focal length.
You're right that for a collimated laser beam, you can create a focal point at a distance from a lens equal (approximately) to the focal length of the lens.

If you explain more clearly what you are trying to accomplish we can provide more helpful advice.

I would like the beam diameter to be smaller than 100mm at about 2 metres away.

With my current lens it is at about 14mm away. I have tried installing the lens in both directions. To move the focal point further away should the lens be close too or far from my laser?

 

[paul1598419]

I would like the beam diameter to be smaller than 100mm at about 2 metres away.

With my current lens it is at about 14mm away. I have tried installing the lens in both directions. To move the focal point further away should the lens be close too or far from my laser?

The focal length of a G7 lens is 7 mm. That is how far the lens should be from the diode's emitter face.

 

[RA_pierce]

I would like the beam diameter to be smaller than 100mm at about 2 metres away.

This should be easy to accomplish with the components you already have. It's not clear to me what the problem is.
Try this (maybe you already have):
0. Check that the camera you are using to view the beam is not distorting or "blooming" the image which could result in measurement error.
1. Install the lens with only a few threads engaged. Check the beam diameter at 2 meters.
2. If the diameter is too large, thread the lens in further by 3 full rotations and check the diameter again.
3. Repeat this process until threading the lens in further starts to increase the beam diameter. At this point, you've identified that the position of the lens for the best collimation is somewhere between the current and previous settings.
4. Thread the lens in the opposite direction (counterclockwise) by 1 or 1/2 turn and check the diameter again. Repeat until you get the desired result.
If doing this does not resolve your issue, you can try again after flipping the orientation of the lens. Without an image I can't tell you which is the "correct" orientation. If this still doesn't work, we will have to try something else, and we may need more information or pictures of your setup.
It may be helpful to use an IR detection card. You can get these on eBay for around $10.

The focal length of a G7 lens is 7 mm. That is how far the lens should be from the diode's emitter face.

Yes. To elaborate on that: When the lens is at a distance from the laser source that is greater than the focal length of the lens, it will converge the beam to create a focal point, after which the beam will diverge again. If the lens is closer to the laser source than the focal length of the lens, it will create a diverging beam with no focal point. When the lens is at a distance from the lens that is equal to the focal length of the lens, it will create a "parallel" beam with a divergence angle that is at the minimum possible with the optics being used.

 

[Meismewhoisme]

This should be easy to accomplish with the components you already have. It's not clear to me what the problem is.
Try this (maybe you already have):
0. Check that the camera you are using to view the beam is not distorting or "blooming" the image which could result in measurement error.
1. Install the lens with only a few threads engaged. Check the beam diameter at 2 meters.
2. If the diameter is too large, thread the lens in further by 3 full rotations and check the diameter again.
3. Repeat this process until threading the lens in further starts to increase the beam diameter. At this point, you've identified that the position of the lens for the best collimation is somewhere between the current and previous settings.
4. Thread the lens in the opposite direction (counterclockwise) by 1 or 1/2 turn and check the diameter again. Repeat until you get the desired result.
If doing this does not resolve your issue, you can try again after flipping the orientation of the lens. Without an image I can't tell you which is the "correct" orientation. If this still doesn't work, we will have to try something else, and we may need more information or pictures of your setup.
It may be helpful to use an IR detection card. You can get these on eBay for around $10.


Yes. To elaborate on that: When the lens is at a distance from the laser source that is greater than the focal length of the lens, it will converge the beam to create a focal point, after which the beam will diverge again. If the lens is closer to the laser source than the focal length of the lens, it will create a diverging beam with no focal point. When the lens is at a distance from the lens that is equal to the focal length of the lens, it will create a "parallel" beam with a divergence angle that is at the minimum possible with the optics being used.

Thank you so much. I’ll try to do this later and get back to you.

Thanks everyone for your help.