The N2 TEA laser is most likely the easiest to build laser. No mirrors, no
optics, no vacuum work, no exotic gases - just some plain old air, a few parts
from the local DIY store and a high voltage source is needed to build one. For
the latter one the rectified output of a flyback supply can be used.
The difficult thing is to produce a very fast discharge between two parallel
electrodes spaced some millimeters apart. Unlike most gas lasers the discharge
is not longitudinal but transversal (hence the name Transversal Excited Atmospheric
- TEA). This is done using the so called Blümlein Circuit. The schematic
below shows the construction of a nitrogen laser:
The Blümlein Circuit consists of two capacitors with one common plate.
Plate 1 and plate two are connected via an inductor (an air-core coil with a
few windings is sufficient).
Plate 1 is charged by a high voltage supply (at least 10kV) and due to the small
inductance of the inductor plate 2 is charged as well. Once the breakdown voltage
of the spark gap is reached it triggers and short-circuits plate 1. This happens
so fast that no current flows trough the inductor, so there's a fast voltage
rise between the electrodes connected to the plates. This leads to a discharge
between the electrodes and a laser pulse is produced.
In order to achieve a fast discharge the capacitors must have little inductance.
This is done by using a thin dielectric and thick capacitor plates. The capacitor
plates of my laser are 0.75mm thick alumnium, the common plate ist 333mm x 333mm
the other two plates 140mm x 320mm. Plate one and two are slightly smaller than
the common plate and have rounded edges to minimize field strengths.
I'm using a stack of two 0.2mm thick polyethylene foils as the dielectric.
The electrodes (see foto) are made of an aluminium profile. It has very smooth
edges and is bent so the discharge doesn't damage the dielectric.The profile
is anodized to make the surface more durable, this anodized layer is insulating
and has to be ground away on the electrode surfaces and the surfaces connecting
the profiles to the capacitor plates.
On the foto you can also see the 21 turn inductor.
The next foto shows the spark gap with the high voltage connectors. Building it took as long as constructing the rest of the laser...
The finished nitrogen laser with all components clamped together. I'm using a steel bar to press the spark gap contact to capacitor plate 1. You can see the laser in the "adjustment mode" - one electrode is pressed to capacitor plate 1 using a steel weigt so I can tap the "adjustment screw" next to the weight with a plastic rod while the laser is running. This may not be the most elegant way to find the optimum electrode angle but it surely is the most inexpensive...
The laser in action. I've found the best electrode space to be 1.5mm. The banded
blue discharge ist where the laser light is produced. Unfortunately there are
always some white sparks. This is a bad thing because those sparks heavily absorb
the laser radiation. As mentioned above I'm aligning the electrodes to a slight
angle so the sparks concentrate at one end of the laser channel. This way a
strong beam leaves the laser to the left side (plus I don't have to worry about
two laser coming out of the laser :-)
Although the nitrogen laser works with the upper capacitor plates charged to
positive as well as negative voltages I've found to be less sparks at negative
voltages.
The output of the N2 laser is in the UV range (337.1nm) but can be made visible
with fluorescent materials. A piece of paper is sufficient. Below you can see
a single shot and a multi shot picture with the piece of paper about 1m away
from the laser. The lines are spaced 5mm apart.
The output of the N2 laser is in the UV range (337.1nm) but can be made visible with fluorescent materials. A piece of paper is sufficient. Below you can see a single shot and a multi shot picture with the piece of paper about 1m away from the laser. The lines are spaced 5mm apart.
With the flyback supply as high voltage source the firing rate of the laser is pretty erratic, so I built a HV pulse generator which provides a stable firing rate.
The nitrogen laser's output is ridiculous even compared to that of a laser pointer but it's a cool construction project ('cause it envolves high voltage!) and the monochromatic light can be used for a big variety of experiments. Furthermore only the average output is wimpy - the pulse output power ist in the range of tens of kilowatts to some megawatts! Sadly the lasing action can only be sustained for some nanoseconds, afterwards the nitrogen needs some milliseconds to de-excite - limiting the pulse frequency to about 100Hz.