A new field for me but with some familiar territory such as high voltage power supplies.
(click to enlarge)
This is a home made laser in free air with no mirrors that gives an ultra short ultraviolet pulse. Typically at atmospheric pressure in air (78% nitrogen) the laser pulse discharge is only about 1 nS and occurs at 337.1 nm. This means that the light pulse is only 1 foot (30 cm) long. After this there are more nitrogen molecules in the lower than the higher energy state and lasing ceases. The lower level lasts very much longer before lasing can take place again. These ultra short discharges mean that they are extremely sensitive to any inductance in the path.
The graphic (borrowed from my wife's unrelated webpage) gives a sort of idea of what is happening in a superradiant laser (ie light has only one pass with no mirrors) and has a beam from both ends.
The power in each pulse however is typically 155 uJ measured on Joshua's power meter on his similar laser. At 1 pulse per sec this means that each pulse delivers a peak power of 155 kW during the nanosecond that it is on. At 6 pulses per second this is 1 megawatt peak power (sort of) and average power of 1 mW which is the same as a handheld diode laser.
Lower pressure and pure nitrogen lasers are much less critical with discharges of typically 20 nS.
Nitrogen laser Mark 1
2004 My first (unsuccessful) attempt at a
transverse excited atmospheric pressure (TEA) nitrogen laser and why it
The main gap is shown firing and the paper coloured with a fluorescent marker is shown at the top of the picture lighting up but not lasing.
I subsequently learnt that this would not work for several reasons.
Nitrogen laser Mark 2 2004 My second (successful) attempt at a transverse excited atmospheric pressure (TEA) nitrogen laser and why it worked.
Success! The little green dot in the photos below say it all.
On the left is a clearer shot of the setup without the weights that press the capacitor plates together. It is extremely simple but the devil is in the details.
I had gone back and re read everything I could on the net and as usual it was all there.
For the successful model above I made the following changes:
Earlier and later versions with a close up of the spark gap. The later version works better and has wider cut down aluminium angle without the glass backing. The spark gap goes directly onto the main aluminium plate rather than onto an electrode via the foil. Hopefully this allows higher currents. The inductor is aluminium soldered directly to the electrodes. The final capacitance for the setup is 17nF.
In action there is a glow between the two plates with some white high
The beam unfocused at 4 feet (1.3m) above, left, then focused with one and then two plastic lenses out of toy binoculars. Note the striations and significant divergence. Scale is cm.
These pictures show a collinear laser diode beam down the nitrogen laser
channel. The left shot is of the beams at 4 feet (1.5 m). the laser
diode beam has spread due to reflections. The right shot is reflected
off a fragment of a CDROM (Mario teaches typing - see his eyes). This
gives a central reflection and first order diffraction with the ultraviolet
being closer and the red diode beam being farther from the central
reflection . This confirms it is monochromatic.
Nitrogen laser Mark 3 2004 I have made a third much smaller laser but with higher power and it can be fired a lot faster. In addition I have a dye to highlight the beam. This was made by dissolving some fluorescent yellow marker pen dye in water.
Note the absence of sparks in the channel, just the glow of ionization spread fairly evenly. The beam is being reflected off some shiny aluminium foil. The spark gap is made of sections of brass tube.
I have taken oxygen from a cylinder, blown across the laser channel. It completely extinguishes lasing for 4 or 5 seconds and it is not restored for almost 20 seconds. Oxygen is poison! It also increased the intensity of the sparks in the channel. Note that none of these lasers, at least so far, have used pure nitrogen.
An interesting fact. I have made all my nitrogen lasers with no new expenditure. All equipment was available as scrap. In other words, they cost nothing. A pleasant change for laser stuff.
I am curious about my laser output. Certainly the output from yellow marker fluorescence is much lower than a 1mW diode laser. Granted the UV output may be brighter depending on the efficiency of the fluorescence. A UV LED is not that bright either but I don't know the output of the one that I have although typical eBay ones are 2000mcd.
Here is a comparison of a N laser beam of about 2mm compared with a 20mm
beam of a 5000mcd blue LED (running at its rated 20mA). The brightness is
about equal but the area of the LED is 100 times greater. Hence
the brightness of the UV paper fluorescence is about 50mcd for my laser.
I have a photodiode linked to my frequency meter tonight to count the flash
rate. The photodiode wavelength response is down to only 5% at 400nm and
presumably much lower at 331nm but still seems to function directly
sufficient to trigger the meter. In practice it is easier for aiming to put
some yellow marked tissue paper in front of it and the yellow fluorescence
will be much closer to it optimum wavelength.
laser Mark 4
2004 Ideal shaping of the laser channel electrodes has been investigated with
various profiles (Chang,
Rogowski etc) to give a uniform discharge. Other amateur experimenters
such as Ralphoo have used tubing for the laser channel electrodes. I
thought I would try too. Aiming for a larger-than-I-thought-reasonable
size, I used 5mm round thoriated tungsten rods. These are very finely
finished. No joy with them until they were very firmly pressed down then
they really shone and were much better than a beam from the plain aluminium
angle I had used before on my test bed laser (Mark 2).
My favourite related web pages for Nitrogen lasers are:
Dye laser 2004 I have some Rhodamine 6G dye which is one of the easiest dye's to lase. It is an intense pink and fluoresces yellow shown on the left below. The right picture shows the UV beam passing from right to left. It passes through some colour TV phosphor dusted on the side of a plastic jar and then on through some Fluorescein in alcohol. Note that this is not lasing, just fluorescing.
I have made a dye cell with a small plastic cylindrical lens to focus the Nitrogen laser into a very thin line which it does well but not enough to give superradiant dye lasing. I am having problems with leakage both using silicone and epoxy. More attempts soon.
This page was last updated January 30, 2011