Balloon popng

Leodahsan said:

@DrSid
Well I was thinking about something you didn't have to put a lens in front of the target to destroy it...

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Where am I suggesting that.

That idea with Fresnel lens could work though .. that are rather cheap.

DrSid said:

Formula for spot size at distance is:

spot_size = distance * wavelength / aperture_size

and you can derive from it:

aperture_size = distance * wavelength / spot_size

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what is "apperture size"?
i know apperture in photography , and its not measured in meters (f stops) .

and telescopes are measured by focal length , right?
so what is apperture size in meters?

No, it's just a diameter of output element .. it's the same as diameter for telescopes.

DrSid said:

No, it's just a diameter of output element .. it's the same as diameter for telescopes.

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so why do you need a huge diameter output element , if the beam itself uses only a fraction (the middle part) of the lens ?
unless you spread the beam over the entire lens , why is the need for huge lens ?

sss said:

so why do you need a huge diameter output element , if the beam itself uses only a fraction (the middle part) of the lens ?
unless you spread the beam over the entire lens , why is the need for huge lens ?

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Of course, if the beam does not use whole diameter, it has no effect. I'm not saying you just have to slam large lens in front of a pointer.
The simplest way how to achieve large output diameter is to match single element (let's say lens, but it's basically the same with mirrors) directly to diode/crystal.
Let's say diode has 90 degree output cone (for simplicity). Let's say you have single convex lens 10cm in diameter. To cover the lens you have to place the diode 10cm form it. Let's aim first for collimated beam, ie. focused to infinity, best possible divergence. For that, the diode has to be in focal point of the lens. So the focal point of the lens has to be 10cm. Focusing at distances like 100m will require only slight shift of the lens, so it won't affect the design.
Since the rays will hit the diode at quite steep angle, there will be quite large spherical aberration. That is a problem for burning (and basically everything). With this setup it can be easily removed by using plani-convex lens, with flat side toward the diode, and curved side toward parallel beam (where it does not matter that much).
You will need more elements if output cone of your laser does not match your output lens. You might need to expand the beam first, or tighten it first.
Also the elements could be coated to reduce reflections for given wavelength.

Besides building your own optics, there are some other options.

First you can simply buy beam expander. Some laser companies sell them for their lasers. They usually fit over actual pointer lenses, and are usually about inch in diameter. This can give you burning at tens of meters.
Then you can use photographic lens. They usually match well common diodes (red, 445nm, 405nm) output. You just put the diode so it covers as much of the lenses inner hole (which is output aperture in photographic sense, but it is input aperture in laser sense). Then you can focus the lens using it's focusing ring, to achieve collimated output. Some lenses might not achieve it, and you might have to move the diode a bit, which will make the optics not use whole beam or whole lens diameter, but most lenses do just fine.
Problem with this setup though is that such lenses have lots of elements, and thus rather large losses. You can loose easily 50% of power in such lens. Also of course, with higher power laser, you can damage the lens, so I suggest using something you do not intend to use on camera again. It makes sense with only really large (thus expansive) lens.
And then there are telescopes. Large Newton with mirrors only is basically perfect system. Except they usually have very long focal length, which does not match well with diode lasers. You will need additional optics. In many cases you can use optics you have with your laser. So let's say I have 114mm diameter Newton with 910mm focal length. You have to set your beam so it is 114mm at 910mm distance. If your pointer optics can do that, you can simply put the laser instead of eye piece, and it should work just fine.
It works in similar way with refractor telescopes or with binoculars. With binoculars, you typically cannot remove the eye piece. But it works even with eyepiece. I just never tried it, I have no idea what angle the beam cone should have.

And then there are those very large Fresnel lens used to cook food. Now that should do just fine ! No spherical aberration, only two surfaces, large, cheap. The beam quality might be crap I'm afraid, but it has to be tested. Anybody knows how to get them in Europe ?