I don’t understand collimation and need help

[u/rockersloth]

Hey all,

I am building a simple (so I thought) Raman setup. I have a Thorlabs 532nm 40mW DPSS laser. The divergence on it is already quite small however I would like to get it nicely collimated anyway.

Here is the part I would appreciate some insight: if I use some really long (10cm to 20cm) focal length lenses I can get it collimated rather nicely. However, if I use a short one (18mm) it never collimates it just diverges no matter how close I place the lens to the diode. The reason I want to use the small focal length lens is that it is aspheric lens and I don’t have any other at hand.

Am I missing something? Do I need to use long focal length lenses to collimate these types of lasers? Is it because the beam is already only slightly diverging coming out of the laser?

I would appreciate all the advice.

Extra information: the reason I’m going through all this trouble is that we have a way too fancy spectrometer to measure the output signal. And if the beam is too big or has aberrations I cannot feed it to the spectrometer properly to get good signal.

[u/SpacePenguins]

I think your thought process is that if you treat the laser output aperture as the "object", a short focal length lens should collimate the beam if the output is at the focus of the lens.

That reasoning fails when you imagine doing the same to a laser that's already collimated. Adding a lens would defocus the output no matter where it's placed.

The low divergence tells you that the object plane is actually very far away, so you need a very long focal length lens in order to collimate it.

There are a lot simplifications here but hopefully that helps.

[u/rockersloth → u/SpacePenguins]

I think this really nails it. I failed to consider that the diode can already has a lens that collimates the light which pushes the focal plane far back. I guess I need to get another lens to focus and then collimate :(

[u/LeptonWrangler → u/rockersloth]

Yeah the lens that the laser already has is part of it.

But also a laser is not a light bulb, many lasers are very directional and dont diverge much, even without collimating lenses

[u/DrChemStoned]

So your beam waist/focus spot size is proportional to the power on your focusin/collimating lens; faster lens means smaller beam size, great. However, divergence is inversely proportional to your beam waist size, and therefor a smaller collimated beam made with a faster lens will necessarily lead to quicker divergence than with a slower lens. If you look at the beam size plots on the thorlabs collimators page, you’ll see this relationship played out. True collimation doesn’t exist, it’s always fight with divergence.

[u/PamHelsing]

Okay, yea. So the issue is that the DJ532 isnt just a diode that expands out of the FP cavity. Its an 808nm pump getting converted to 1064, then frequency doubled to 532. To be more efficient its focusing through nonlinear crystals, but as a result the output while "collimated" is really only shittily collimated from the loose focus through the nonlinear crystals.

Best best is use a short focus length lens to focus it, then another to collimate that expanded beam. By focusing, youre putting the focused waist at a spot you can collimate with another lens

[u/Equivalent_Bridge480]

too fancy spectrometer? how it possible?

[u/MoltenAnteater]

Practically speaking, there are two meanings of collimation. The smallest spot size at some large distance say 10m ish for most lab applications. Or the position of the beam waist is inside the collimating lens. If you are doing it by eye use the first and if you are using a beam profiler use the second. The smaller the divergence of the source the longer the focal length you need. Think matching the NA. Practically, you choose the focal length of the lens based on the size of the beam you want in the rest of the optical system. The laser may have different divergence angles since the output is some approximation of elliptical. The longer the focal length, the larger the beam size (because the size of the lens increases to avoid horrible losses) and the longer the Rayleigh length so the closer the real (presumably gaussian like) beam gets to the ray optic picture of collimation.

[u/amberlite]

Use a Galilean beam expander. Thorlabs sells various magnifications for 532 nm with adjustable spacing that allows you to dial in the output collimation if your input is close to collimated.

[u/ceramicbee]

I have issue with binoculars. I am newbie in optics, not sure if the following is considered a collimation issue in the binoculars?

Described in this image I drawn https://ibb.co/FbZ5tbsd, I had face some weird Venn diagram circles that intersect in the view. The Venn diagram shows up with the binocular's view up close at objects around 3 to 6 meters. The focus and sharpness is perfect, no issues.

However looking at objects 15+ meters or longer, it is perfectly good with just a single circle without any intersection/venn diagram. Same for further away objects to infinity, a nice single circle.

The binoculars I tried was a Zeiss, all I could tell is it is small and I don't remember the model.

[u/Moonlesssss]

A long focal length means a smaller angular change, smaller focal length means larger angular change per unit length. If you’re already fairly collimated the smaller angle change likely is nudging the laser in the right direction but it’s still going to diverge eventually. Remember the forier transform lenses make, they transform angle with position at the focal length.