Renato's N2 laser page.
(by Renato Salles)
High voltages and UV radiation may be present, so only experienced people should try it. Do it at your own risk.
Beam from my N2-Laser crossing a water-rhodamine mix (green line) and striking the flask label (white spot)
- Front view of laser channel
- Top view (note how the capacitors "role up" to make a compact system)
- Spark gap
- Inductor
- High reflector mount
- Laser in action
- More laser in action pictures !
Did you ever want to build a laser from scratch? Having found this page I would assume that you do. Nitrogen lasers are rather simple devices to build, because plain air (which is about 70% N2) may be used as lasing gas and all other components can be found at your garage or the next door hardware store. Nitrogen lasers work only in pulsed mode and if properly designed, can deliver a few hundred kilowatts per shot. The pulses are quite short, about 5ns or so, but for something built out of scrap that is not so bad at all! Moreover you can use it to excite a fluorescent dye like rhodamine to produce a laser beam at a wide range of visible wavelengths!
This page is not supposed to be the only source of information for people willing to build a nitrogen laser. It is not a complex job, but the very sense of doing it is to learn, at least I think so. Just to copy and paste a project won't teach much any way. This page is supposed to be a practical guide reporting usual problems and possible solutions to them. I will try to show every important detail of the laser I did. It is a somewhat different design that I believe to be cheaper and more flexible with components that are easier to find on the market. If any information is unclear or missing please report by e-mail. This site will be very alive from now on , I will change it frequently as suggestions come by e-mail.
My first ideas came
from the Scientific American article (of coarse J). A little library research helped a lot to open my mind and change a
lot to the design . Really cool stuff I found there, but no complete practical
guide to an effective inexpensive homebrew nitrogen laser that would work with
air could be found. Well, that is
why I decided to write down all my experiences with my air laser. I will only
report here things that I did and are known to work.
The design was always based on a Blumlein transmission line. Why a Blumlein?
Because it is the easiest way to create a very fast rising pulse. The upper
lasing level of N2 molecules is very unstable and lasts a few nanoseconds, after
that time a more stable lower level is heavily populated absolving the laser
been that was just created. It is like the gas becoming “black” to the laser
beam. The only solution to that is to excite the gas so fast that a nice laser
beam is out of there before the N2 molecules start blocking it. OK, it’s not a
very scientific language, but I don’t want to get people bored with theory.
Capacitive charge transfer circuits and other stuff may work, so be free to experiment!
A low inductance Blumlein transmission lines can be easily made with tree flat plates and an insulator (dielectric) forming two condensers with a common base plate. If we can name an enemy here it is inductance! The inductance must be kept really low or else you will never see a laser beam coming out of your N2 laser. How to do that? Simple, keep current very close to its return path, that is why most Nitrogen laser Blumlein circuits are made with very tin (0.15mm) PCB, not only to increase capacitance but more important, to keep inductance low!
One thing I knew for sure, 0.15mm PCB are expensive, hard
to find and a pain in the neck to prepare (try to mess with ferric-perchlorate and
see for yourself), that in why I never used it. Pure nitrogen was only
used a few times, air was the main laser gas filler used for all experiments. My
first bet was to use glass plates with aluminum foil glued to it as to form two
capacitors. The aluminum foil is a great thing to use, I used it all the way and
no problems with it whatsoever. Unfortunately I can’t tell the same about the
glass dielectric. It was far too thick for the job, nothing close to a laser beam
would come out of there. Another mistake was the electrodes shape. I used
sawblades hoping to get a homogeneous discharge… very wrong! Unless you have
lower pressures (<10torr) very sharp sparks will appear. Note that a smooth
corona discharge is very desirable for this laser to work well, sparks in the
laser channel are very bad. I don’t have a sure way to measure it, but
estimates are that my refrigerator pump gives me about 30 torr vacuum. At
that pressure you will need a very smooth electrode, high voltage and fast
discharge to avoid arching.
Section of the laser channel showing how the Al-foil is connected to the electrodes. (click to see detailed pictures of the assemble)
The plastic dielectric only pinched once during operation,
a good inspection should be a good idea before start gluing the foil. Small
defects on the surface will be enough to cause dielectric breakdown during
operation. A nice inspected 0.2mm plastic sheet will last many thousand hours of
operation. Maybe the ozone will corrode it after a long time, mine is on duty
for 6 months with no problem. I don’t use this plastic any more in my high
voltage capacitors, the one I’m doing now is made with 5 layers of 0.03mm
garbage bag. There are two main advantages, it is cheaper and less likely to
suffer dielectric breakdown; that will be very unlikely that two consecutive sheets have weak points at the same exact place.
Next step was to create a sealed laser head. It may sound simple if you never tried it (just like in any other business…). Most people would say that refrigeration is the main issue, but as far as my experiments went, that did not seem to be the nastiest problem. I made a laser in the above described fashion and then I sealed it. The laser functioned for 10 minutes and then stopped sending a beam. The pressure was fine; the temperature was the same… so what could be the problem? After days over it I got the bug. The laser after all is very sensitive to the presence of any organic molecule. The oil used at the vacuum valves on the tube would evaporate over time, mix with the gas and prevent lasing. Even the epoxy glue has a small outgas that may poison the chamber. It is finely working on sealed mode now for more than a week! If you want to make a sealed laser head, be very careful with possible contamination sources, organic molecules seem to be the worst contaminants. Besides, God knows what bizarre compounds will be born after those molecules get exposed to the ionizing radiation from the powerful discharge...
Don’t bother looking for expensive PCB, be original!
Keep inductance as low as possible, small spark gaps, discharge closer to
the ground plate…
Don’t bother getting pure N2, you may have a good energy output with “air”.
Using a fast Blumlein circuit, pure N2 gives no more than twice the power of
old plain air, is it worth?
Use smooth electrodes to avoid arching.
Make the laser cavity transparent so you can see what type of discharge you are getting, and correct it.
Oxygen prevents laser action, so air wont work.
It does work and very well!
Making a N2 laser is easy, even a child can make it.
Well, as anything in life, it is easy when one gets to knows how to do it. If you grab any old PCB's, solder it together and apply high voltages, it wont work for sure and may easily kill you. A bit of research over it is essential.
N2 laser may very easily work with air on atmospheric pressure
(TEA ).
Maybe its
possible, but it is sure very difficult to avoid arching, I never got one to
work. I suppose preionisation electrodes are required for corona forming when
you get near 600 torr. (?). When they work, witch is difficult,
they give up very inefficient picoseconds pulse, sure not good por amateurs
first laser. What is the problem getting an old refrigerator pump for vacuum
anyway?
An atmospheric discharge during a thunderstorm may have a strait path somewhere
and generate a laser beam.
This kind of discharge is far to slow for that! We are talking about nanoseconds, a lightning is fast but to compare the Blumlein transmission line required for air lasing with it would be the same as comparing a jet plane with a worm. So don’t worry, you will never be killed by a UV laser beam during a thunderstorm unless some Klingon ship is after you.
Take a look at my dye laser project too. New ideas are welcome!
HAVE FUN!!!!
Renato B. K. Salles 02/01/2005
e-mail me at:
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