Jochen's High Voltage Page

Project: N2 Laser

Work in progress!

This is probably the most simple to make laser one can think of. It has been described in various scientific papers, popular science articles and web sites. There is an extensive literature database at Thomas Rapp's Pulsed Lasers site [and also a much nicer example of the N2 laser].

The nitrogen laser emits (more or less) coherent pulses of ultraviolet light (337nm). It is excited by a high voltage discharge. An excited nitrogen molecule decays spontaneously to a lower energy state, emitting a 337nm photon, within several ns (1ns = 1/1,000,000,000 s). For laser operation, it is necessary that more molcules are in the excited state than in the lower state (inversion). This situation can only be reached when excitation is faster than spontaneous decay. This means that the electric discharge has to provide a high current within a few ns. Special techniques are required to make a discharge this fast. The pulse of light emitted from such a laser is just about 1-3 meters long (!).

The ultraviolet light of the laser is invisible. However, most white paper is fluorescent at this wavelength. Most types of glass are transparent, some are fluorescent. Clear plastic seems to be opaque to near UV. Water with a small amount of fluorescein or other fluorescent dye makes the beam visible. It can be focused with a glass lens. A CD can be used as a grating for diffraction experiments, for example one could compare the wavelength to that of e.g. a HeNe laser (630nm) or a (semiconductor) laser pointer (around 670nm).

This variant of the nitrogen laser works with the 80% nitrogen available in air. It needs neither a bottle of nitrogen gas nor a vacuum pump, just plain air. It also has no resonator (no mirrors), it's a superradiant laser. Excitation is so fast and so massive, that a single photon gets amplified to a pulse of light in one pass. The fast discharge is produced by a standard Blumlein circuit [More information on this to follow].

This my first (and up to now only) N2 laser - and it did work.

Click on image for larger version.

The following pictures are taken with a newly built laser, but the construction is basically the same as above. Again, getting it to work was rather easy. Click on images for larger versions.

New laser setup.

Close-up of the aluminum L-profile used as electrodes in both the above setup and the first laser. Note the way it is slightly sharpened at the edges - it comes this way, you just have to cut it and round off the corners.

Beam profile at about 1.3m from the electrodes (fluorescence on white paper). Beam diameter is something like 2cm there. Note the vertical line structure - caused maybe by lack of coherence across the laser channel? If so, why not in the other direction? The pictures show two shots. Each shot produces a slightly different pattern.

Laser in operation. To the left, you can see the extra setup used to demonstrate diffraction of the laser beam, basically just a folded (for mechanical stability) piece of white paper with a hole in it, plus a Compact Disk (not visible). Note the corona along all edges of the capacitor plates.

Diffraction of the UV laser beam by a CD (Simon and Garfunkel :). The beam comes in through the hole in the paper screen and is diffracted from the CD back onto this screen. The camera was open for ten shots of the laser. The bright big spot is 0th order (not diffracted, just reflected). To the left and right of it, the first order diffracted spots are visible, and further to the left even a second order spot is clearly observable, although rather weak.

The diffraction experiment proves that the beam is highly monochromatic and at least partly coherent, and therefore deserves the name "laser".

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