How to focus 405nm laser diode to small round spot?

I'm trying to get the smallest possible round spot on a 405nm laser diode. I bought a 405-G-2 lens (haven't got it yet) and I'm confused by the specs.

The lens is supposed to be a glass "collimator". So it takes a diverging beam and makes it parallel? How does spinning it in and out make it focus to a small spot?

Is there a place where I can get a lens that takes a parallel beam 405nm and focuses it down to a diffraction limited spot size (eg like plano-convex) some where cheap? Focal length around 300mm prob would be ideal.

Thanks for the help.

It's against the laws of physics to get a diffraction limited spot size with a multimode diode. What exactly are you trying to do here?

laserlightguy said:

The lens is supposed to be a glass "collimator". So it takes a diverging beam and makes it parallel? How does spinning it in and out make it focus to a small spot?

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Yes, if focused to infinity. However, as with any light source, you can never have a parallel beam of light. Light will always want to spread out. You can change the focal point of the lens by adjusting how close it is to the source.

Is there a place where I can get a lens that takes a parallel beam 405nm and focuses it down to a diffraction limited spot size (eg like plano-convex) some where cheap? Focal length around 300mm prob would be ideal.

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We use specific lenses for lasers. Most often used are the 3 element lens and the G1/G2. A focal point of 300mm should be no problem with these lenses.

I want to focus the beam onto a plane with a spot size of say 25um or less to make a small spot on a focal plane for burning stuff. How do I do the calculation of the focal length using the G2 lens? It seems to have a fixed FL of around 4mm. So if I used 2 G2 lenses, one to make it parallel, and then feed it into the second G2 lens then it will focus down to 4mm FL with back FL of around 2.3mm. I see that some people only use a single G2 and it just screws down. However, the beam coming out of the diode is already diverging so shouldn't it just collect less light and focus it parallel with a single G2 regardless of where I put it with the screw down? Sorry for the nube questions.

What? Just use a single lens.

laserlightguy said:

I want to focus the beam onto a plane with a spot size of say 25um or less to make a small spot on a focal plane for burning stuff.

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Physics forbids this.

Bluray player spot size is 580nm and CD players focus to 2um. Why can't I get a 25um spot size. Seems the best way is to use the collimating lens with NA of > .5 to bring most of light to parallel first then another lens to focus to a smaller spot.

laserlightguy said:

Bluray player spot size is 580nm

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:wtf:

Why can't I get a 25um spot size. Seems the best way is to use the collimating lens with NA of > .5 to bring most of light to parallel first then another lens to focus to a smaller spot.

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Then use a blu ray drive's lens! Besides, why do you need such small spot? You can burn stuff without the need for super tiny spots.

Base FL of 4mm and BFL of 2.37mm prob is intended for bluray players. NA of > .5 is prob needed to capture all the light which has to be short FL for a small lens. Again I want to project to a smaller spot further out say 300mm. My calculations show I prob need to expand the beam to 5-10mm diameter before refocusing to 300mm to get smaller spot size. Anyone else do this before or know a place to get 405nm optics cheap to build a beam expander/long final focus lens?

the g series lens will focus a pretty small spot at 30cm. what kind of numbers are you getting?