I've seen the designs for low-cost home-built nitrogen lasers. But these were for unfocused beams. Is there a low-cost method to focus the beam to a spot in the range of say a few hundred microns wide?
Bob Clark
I was going to mention mirror quality and how they're positioned vs. cavity proportions...
Mark L. Fergerson
337 nm. Fused silica, alkali halide, or alkaline earth fluoride lens. A meniscus lens is preferred to lessen aberrations. Down to microns is gonna take some work - superradiant lasers are not coherent and you'll need a large diopter rating (thick lenses are problems on several fronts). I doubt Fresnel or binary optics configurations can pull it off, especially if you need imaging in addition to concentration.
-- Uncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/qz.pdf
Superradiant nitrogen lasers have messy output, as such or one pass with a rear mirror. The cavity is irrelevant. The output is not coherent. You might as well try focusing a flashbulb to a few microns image radius.
-- Uncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/qz.pdf
The problem isn't the lens material as much as the beam quality. It will be hard to focus the typical home-built N2 laser's output to a very small spot.
--- sam | Sci.Electronics.Repair FAQ Mirror:
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More than that. It depends on the mode structure of the beam.
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Never actually played with one of these, but my hypothesis would be that if you used a long cavity -- longer the better -- with flat or long-radius mirrors (radius longer than the mirror spacing) and equal sized apertures at each end, the angular spread in the output would be roughly equal to the angular spread of either aperture seen from the center of the aperture at the other end. Would be a bit delicate to get and keep the mirrors aligned, however.
of
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regards Raymond
Quartz lens?
-- Dirk The Consensus:- The political party for the new millenium http://www.theconsensus.org
Depends on the length of the laser cavity I assume, since its single pass.
-- Dirk The Consensus:- The political party for the new millenium http://www.theconsensus.org
-- Mirrors??? Apertures???
Cannot you by placing the lens far enough away from the laser obtain a better focus? The distance will correlate off-line output with lateral displacement. A lens of sufficient quality and wide enough for the spread beam will then redirect more accurately to the focus.
Think of it in terms of the focused image for the distance to the laser will move closer to the focus from infinity as you move the lens farther away. (Again assuming no spherical aberation in the lens.)
Regards, James Baugh.
Well, it's not quite that bad. Most of the light is confined to the area of the the long narrow discharge gap.
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You're not imaging the output aperture of the laser, you're trying to focus the beam from the laser. As such, how far away you are isn't very relevant.
However, if you're implying that a large f-number lens is better. Sure.
I'm not sure I'd go so far as to say they aren't coherent asn other posts have stated, but there is a most one bounc from a rear mirror (which isn't essential) and the light makes at most two passes through the laser. This doesn't set up a nice mode structure. As everyone's stated, it is messy.
Maybe he'd be better off finding a surplus excimer laser. :)
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As a young boy, about 8 or so, I read about lasers, and spent many weeks pocket money buying up loads of toy magnifying glass lenses, stacking them in a piece of PVC waste pipe, and fitting a torch at one end. Can you imagine my disappointment when I didn't end up with a laser gun from 'Star Wars'?
Uncle Al overstates his case a bit. Why dont we try numbers instead of words.............. Forget 'mode structure' for most N2 lasers, there is essentially none, they are single pass. But neither is it *quite* 'a flasbulb'. To a pretty good approximation crude nitrogen laseers have a divergence which is about d/L where d is the 'tube' (discharge, often transverse) diameter & L the length. If you focus it with focal length F your spot diameter is just F*d/L
So, if you want 0.1mm, d is maybe 5mm & L maybe 200mm you would need a 4mm focal length, which is operating at F~0,8 You might get a UV transparent microscope objective (at a price) that gets near that, but its pretty challenging to put it mildly.
With a longer, thinner laser, and relax it to 'a few' hundred um spot, and you would get into just about achievable regimes. With longer focal lengths, the F number falls, & aberrations are rapidly less of an issue - its rather far from diffraction limited! Lens UV transparency at 337nm is an issue, especially for a thick short focus lens.
Harvey
For roughly collimated to focus, a plano-convex lens is near optimum for n~1.5; thats why they are so commonly available. Put it the right way round, students often dont! (The exact solution is indeed a near plano/vex meniscus; but the plano vex is what gets used.) (Meniscus is optimum for higher index material, such a Ge in the IR.) Silica or CaF2 would be the usual, available choices, (LiF & MgF2 also.) The chlorides & bromides are OK in the IR where scatter is much less critical, pain in the UV, & often form colour centres under UV irradiation, dependent on the source.
You need a *UV grade* silica.
Down to
See rough sum in the other post. Basically I agree, but it is just about 'do-able'
Harvey
No I was thinking for a fixed focal length but farther from the source. As long as the diameter of the lens catches the width of the spreading beam. Seems to me you could acheive focus up to the order of the wavelength this way... assuming you could build perfect lenses of a given focal length to arbitrary diameter.
Right you would get a random mix of coherent "packets", each the amplification of a single spontaneously emitted photon.
Of course, if this is the Scientific American laser, it's a monster, producing millijoule pulses a nanosecond wide, which will cause nice air plasmas near focus. Focusing tighter than the air breakdown limit will be a challenge even with good lenses.
The plasma will move rapidly towards the lens, to the point of drilling holes in it if it's too nearby (I used to have a 40x microscope lens with a nice 1-mm hole drilled in the front element from this effect).
Cheers,
Phil Hobbs
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The application I had in mind is indeed for micromachining.
This page discusses laser micromaching using UV lasers:
Small Excimers Opening Up New Industrial Applications.
And here's a report discussing micromachining with femtosecond laser pulses; but it gives examples showing that nanosecond pulses, the length of the pulses for the home-made nitrogen laser, can also be using for micromachinging:
Femtosecond Laser Micromachining: Current Status and Applications.
Bob Clark
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