Focused homemade nitrogen laser?

[[email protected]]

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

 

[Dirk Bruere at Neopax]

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?

Quartz lens?

--
Dirk

The Consensus:-
The political party for the new millenium
http://www.theconsensus.org

 

[Sam Goldwasser]

Quartz lens?

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: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

 

[Dirk Bruere at Neopax]

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.

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

 

[Sam Goldwasser]

Depends on the length of the laser cavity I assume, since its single pass.

More than that. It depends on the mode structure of the beam.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

 

[Mark Fergerson]

More than that. It depends on the mode structure of the beam.

I was going to mention mirror quality and how they're positioned
vs. cavity proportions...

Mark L. Fergerson

 

[Uncle Al]

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?

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]

I was going to mention mirror quality and how they're positioned
vs. cavity proportions...

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.

 

[Gary Cavie]

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.

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'?

 

[AES]

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.

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.

 

[John Fields]

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.

 

[Raymond Yohros]

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

Everithing you ever want to know about lasers:
http://repairfaq.ece.drexel.edu/sam/lasertoc.htm

Everything about nitrogen lasers:
http://repairfaq.ece.drexel.edu/sam/lasercn2.htm#cn2toc

regards
Raymond

 

[Baugh]

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.

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.

 

[Sam Goldwasser]

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.

Well, it's not quite that bad. Most of the light is confined to the
area of the the long narrow discharge gap.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

 

[Sam Goldwasser]

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.)

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.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

 

[Harveyx]

Well, it's not quite that bad. Most of the light is confined to the
area of the the long narrow discharge gap.

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

 

[Harveyx]

337 nm. Fused silica, alkali halide, or alkaline earth fluoride
lens. A meniscus lens is preferred to lessen aberrations.

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

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.

See rough sum in the other post.
Basically I agree, but it is just about 'do-able'

Harvey

 

[Baugh]

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.

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.

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.

Right you would get a random mix of coherent "packets", each the
amplification of a single spontaneously emitted photon.

 

[Phil Hobbs]

See rough sum in the other post.
Basically I agree, but it is just about 'do-able'

Harvey

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

 

[[email protected]]

The divergence of a nitrogen laser can be as low as 1 mrad if I recall
correctly. LSI used to sell a "near diffraction limited" version of their
sealed tube nitrogen laser (they may still). It use an unstable resonator
with the output mirror covering only 1/4 of the output aperture and a I'm
not sure if the rear mirror was flat or curved. I did some work using such
a laser focused to about a 50 micron diameter for laser ablation.

Bret Cannon

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.
http://www.resonetics.com/SMexcimer_default.htm

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.
http://www.exitech.co.uk/pdffiles/Femtosecond Laser Micromachining Current Status and Applicat.pdf


Bob Clark