Nitrogen laser

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A Nitrogen laser is a gas laser operating in the ultraviolet range, using molecular nitrogen as its gain medium, pumped by electrical discharge.

Contents 1 Gain medium 2 Typical devices 3 Application 4 External links

[edit] Gain medium

• Nitrogen molecules

• 3-level laser. In contrast to ruby or to a typical 4-level laser, the upper laser level of nitrogen is directly pumped, imposing no speed limits on the pump. Pumping is normally provided by direct electron impact; the electrons must have sufficient energy, else they will fail to excite the upper laser level. Typical reported optimum values are in the range of 80-100 V per Torr·cm.

• 40 ns upper limit of laser lifetime at very low pressures; the lifetime becomes shorter as the pressure increases, and is only 1-2 ns at 1 atmosphere. (t = 36/ (1+12.8* (p/bar))*ns; see [1],[2]).

For various reasons the efficiency of the nitrogen laser is low, typically 0.1% or less, though nitrogen lasers with efficiency of up to 3% have been reported in the literature.

• wavelength: the strongest lines are at 337.1 nm (ultraviolet). Other lines have been reported at 357.6 nm, also ultraviolet. (The information on this page refers to the second positive system of molecular nitrogen, which is by far the most common. There are also lines in the far-red and infrared from the first positive system, and a visible [blue] laser line from positively ionized molecular nitrogen.)

• metastable lower level lifetime t=40 μs; see [3]. Thus, the laser self-terminates, typically in less than 20 ns. (This type of self-termination is loosely referred to as “bottlenecking in the lower level”.) Thus, in contrast to the Helium-Neon laser, the nitrogen laser can only operate in pulsed mode. Many nitrogen lasers terminate even more rapidly, but this is usually because the drive circuitry cannot continue to provide sufficiently energetic electrons for more than a few ns. (Many people think that the short pulsewidth of the nitrogen laser is directly and solely caused by the short lifetime of the upper laser level, but that is demonstrably false: several organic dyes with upper level lifetimes of less than 10 ns have been lased CW. The upper level lifetime does limit the attainable pulsewidth in the case of the nitrogen laser; but this is caused by the bottlenecking issue, and is not an inherent feature of short upper-state lifetime.)

• Repetition rates: These can range as high as a few kHz, provided adequate gas flow and cooling of the structure are provided. Cold nitrogen is a better laser than hot nitrogen, though, and this appears to be part of the reason why pulse energy and power usually start to fall off as the repetition rate increases to more than a few pulses per second. There are also, apparently, issues involving ions remaining in the laser channel.

[edit] Typical devices

• Gas pressure: some mbar to several bar. Air = 78% Nitrogen can be used, but more than 0.5% oxygen poisons the laser, and air provides much less output than pure nitrogen or a mixture of nitrogen and helium.
• Energies: µJ to mJ
• Power: some kW to more than 3 MW
• Pulse length: between a few hundred picoseconds (typically at 1 atmosphere partial pressure of nitrogen) and a maximum of approximately 30 nanoseconds at reduced pressure, typically some dozens of Torr, though FWHM (Full Width at Half Maximum) pulsewidths of approximately 6 or 8 ns are typical.
• Nitrogen lasers can operate superfluorescently (without a resonator cavity), though it is quite common to put a mirror at one end so that all output can be collected from the other end.

The medium is usually pumped by a transverse electrical discharge. When the pressure is at (or above) 1013mbar, this is called a TEA laser Transverse Electrical discharge in gas at Atmospheric pressure.

[edit] Application

• pumping of dye lasers
• measurement of air pollution (Lidar)
• scientific research.

[edit] External links

• Professor Mark Csele's Homebuilt Lasers Page
• Example of TEA Laser prototype
• Sam's lasers FAQ/Home Built nitrogen (N2) laser
• dye pump