Best lens setup for smallest most perfect focus dot possible at around 300mm distance

[lazeristasUVISIR]

Now I'm really confused, because what lazeristasUVISIR said about the longer focal length is the opposite of what Cyparagon said:

I'll leave to Cyparagon to explain his thoughts regarding a whole system :shhh:

 

[AngelG]

I'll leave to Cyparagon to explain his thoughts regarding a whole system :shhh:

The things are even more fuzzy - the lens systems we're talking about are being sold as "collimating lens" with different front and back focal length. The focus in a spot at given distance is accomplished by changing the distance between the L.Diode and the lens system....

 

[Cyparagon]

If shorter focal length makes smaller spot...

Not the focal length of the lens itself, the focal length to the target. Focusing a large beam down in a short distance makes for a smaller spot. If you insist on a 30cm distance to target, you won't get a small spot.

If you're also limiting your lens size to something that fits in a 12mm module, your options are down to just a handfull, all of which will not produce the best spot.

 

[AngelG]

Some 1200dpi LASER printers manage to focus using such small lens.
At a few cm then there are beam shaping cylindric lens (before the mirror) and then after the flat mirror, additional layer of lens which some people call "theta" lens. They're bigger.
I have such lens (beam shaping, theta from printer), but they are acrylic, for 780 nm. Probably the cylindric/beam shaping will help most, but I'm not sure how well this will work at 350 .. 650 mW with 405 nm wavelength. If the distance between the beam-shaping lens is critical, then mine won't work because they're in a monolithic block.

 

[Exerd]

I can help.

Instead of using words like "small", if you simply specify the requirements using a measurement system, we could turn this into physical understanding using the math. That should be straightforward.

I.E. Maximum spot size at target = <0.25mm ⌀

You cannot make an image projection that is more intense or tightly focused than the source image. (While it's easily possible to decrease projected image size by inserting a negative value lens with lens1[-FL]>lens2[+FL], this is merely increasing system FL with same diameter lens, so numerical aperture and efficiency is lowered at that cost.) I use the word 'image' to mean in this case, the laser die. So the intensity at the laser die face always sets the intensity maximum which will never be exceeded at the target. If you take a multi-mode diode, you cannot focus its dot down to a smaller size using optics to compress the energy more than the energy started as. The 405nm single-mode diodes have very high die intensity, so this is of no concern really. It is for your understanding that I added those details, so you can fully appreciate how great a 20X single-mode 405nm could be which is stable at ~1.25W. Maybe next Christmas.

Every lens will have a cross-over point at some distance away where the beam switches from what we call 'finite-conjugate' focus and turns into 'infinite-conjugate' focus (of the projection side that you see as the beam), as you move the lens nearer or further from source laser. That range is a product of focal length over source area. As source area increases, finite-conjugate length decreases. This is seen as you move the same lens from a multi-mode laser over to a single-mode laser. The single-mode has smaller area, and you can see that you can make the beam very narrow a few feet away easily out to maybe 6 feet with G2. The multi-mode cannot reach that far with a G2, its limit is maybe 2 feet before dot is focused to infinity.

I am slightly worried you are designing this system, simply because you should see that a G2 can easily focus to a very fine dot at 300mm with 405nm. I'm kidding, but only slightly. :eg:

 

[AngelG]

@Exerd: "So the intensity at the laser die face always sets the intensity maximum which will never be exceeded at the target."
I don't feel it so true. The people use 40W lasers to cut metal. These would destroy themselves if the energy density at the mirror was greater than at the spot...
As for the 1.25 W single-mode 405nm diodes, I don't believe they'd happen, because:
1. It's too difficult;
2. It depends on the demand (and prices) of the Blu Ray devices. There is a reason only 2 companies to be able to produce a 16-x capable drive.
For example I still don't own one (but I have at least 6 x BDR209 SLEDS - LOL).

 

[Julian95]

hello..i can help you with this..first you need to spatially clean the output of the diode, to get an output as gaussian as possible thus eliminating any stray light that generates problems around the focal point. what you need is a "spatial filter" like setup (look it up on google). that is, 2 lenses and a pinhole in the middle(20-200um diameter, if its bigger it wont work perfectly) where the laser focuses. once you are filtered, you can do the galvanometer thing with parallel rays(the output lens of the SF can be set up to do that) and then you need an f-theta lens to focus on the surface, the bigger the numerical aperture of the last lens (that is, the smaller the distance between the last lens and the focal point on your surface) the smaller your focal point will be..and smaller your depth of field will be so it gets harder and harder to focus. it's a compromise.

 

[rajhlinux]

On Ebay, you can get 3-element coated lens for 405..465nm LASER at prices starting from $3.50 (to around $15). There are also uncoated acrylic lens, but I'm not sure how they perform at P>0.5W.
Which lens would make smaller spot at 30cm (300mm=30cm). It might be the most expensive, but it might be the cheapest - no guarantee here. Talking about smallest spot, the best known are the 3-element glass lens. But the brands and quality of the lens are highly mysterious.
Also, I've found 3 kinds of lens with specified something called "F" (focus distance?):
F=8.3, F=9.8 and F=11, they are being sold at ~ $7.8 on ebay.
I guess F=11 stands for F=11 mm.

As for the Sanwu's lens, they re here: Sanwu shop - lens.
The Gxxx ones are single-element and more expensive, $15 at the time of writing vs $8 for his 3-element lens. Their pros are they reportedly allow for up to 10-20% more energy to pass. Their cons - higher price and slightly inferior focus than 3-element.

I asked staff from techhood about the difference between their $13 UV lens and another, unknown $3USD glass coated 3-element UV lens, sold on taobao. What they know is that they are from different factory in China and different coating. So the information is next to nothing :can:.
I have bought the $3.5 ones from another ebay supplier (eama/nanma) but they are in transit for the last 20 days...
I can compare the spot only with acrylic uncoated (when they arrive), but I don't know how to shoot it - on white paper it has lot of glare, probably on black paper or best - direct expose the 1/2.4 inch CMOS matrix (of web camera).

There is some additional stuff like beam expanders, which may help you to maintain low divergence at distances at the cost of expanded beam, but then you'll need additional focusing lens near the subject to focus the low divergence expanded beam in a tiny all-vaporizing spot :yh:

You have any recommendations for lens that provides ultra small laser spots? It is 3 element lens right? Laser wavelength will be the sony blu-ray laser diodes at 405nm.

 

[Unown (WILD)]

AngelG was Last seen Jun 27, 2017. Please don't necro post.