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Molecular Lasers

Definition: gas lasers where the laser-active gas consists of molecules rather than separate atoms or ions

More general term: gas lasers

German: Moleküllaser, Molekül-Gaslaser

Category: lasers

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Molecular lasers are gas lasers where the laser-active constituents are molecules rather than separate atoms or ions. Examples for such molecules are CO2 (carbon dioxide), CO (carbon monoxide), N2 (nitrogen), HF (hydrogen fluoride), DF (deuterium fluoride), NH3 (ammonia) and CH3OH (methanol). One exploits excited states of such molecules which can involve not only a purely electronic excitation (as of atoms or ions) but also vibrations and rotations of the molecules. The excitation energies are in most cases relatively small, leading to laser emission with long wavelengths in the mid or far infrared spectral region.

There can be a substantial number of rotational–vibrational lines on which such a laser can be operated (→ vibronic lasers), but single-line emission and even single-frequency operation is not difficult to achieve with an intracavity bandpass filter, since the Doppler broadening of the laser transition is quite weak. If the pressure is relatively high, however, the different lines may overlap due to pressure broadening, resulting in a larger gain bandwidth.

Common Types of Molecular Gas Lasers

The most common types are the following:

  • Carbon dioxide lasers (CO2 lasers) use a gas mixture of CO2, helium (He), nitrogen (N2), and possibly some hydrogen (H2), water vapor, and/or xenon (Xe) for generating laser radiation mostly at 10.6 μm. They have wall-plug efficiencies above 10% and are suitable for output powers of multiple kilowatts with fairly high beam quality. Some high-power CO2 lasers use a system for quickly circulating the gas (forced convection, fast flow). They are widely used for laser material processing, e.g. cutting, welding and marking, but also in laser surgery. They can operate continuous wave or in pulsed mode, e.g. with Q switching.
  • Carbon monoxide lasers (CO lasers) can have wall-plug efficiencies of the order of 40%, thus being substantially more power-efficient than CO2 lasers. They can emit on various lines between 4.8 μm and 8.3 μm and are mostly used as light sources for laser absorption spectroscopy. Following technological advances concerning the device lifetime, CO lasers emitting around 5.5 μm might also become interesting for laser material processing (e.g. cutting of glasses); in comparison with the widespread CO2 lasers, they offer better absorption in many materials and better focusing capabilities.
  • Nitrogen lasers are another type of pulsed ultraviolet laser, based on pure nitrogen, a nitrogen–helium mixture, and sometimes even simply air (with lower performance). Emission typically occurs at 337.1 nm in the form of short pulses; a self-terminating laser transition is used. The high gain leads to relatively efficient superluminescent emission even without a laser resonator. Nitrogen lasers are relatively easy to build and operate, and have been made by many hobbyists without refined laboratory equipment.

Some less common molecular gas lasers:

  • Acetylene (C2H2) and hydrogen cyanide (HCN) lasers are used for laser spectroscopy.
  • There are e.g. chemical hydrogen-fluoride (HF) lasers, fueled with H2 and F2, which is converted to HF, and oxygen-iodine lasers (COIL). Such chemical lasers are mainly studied for military purposes, e.g. as anti-missile weapons, to be operated even on board large airplanes.

Pumping Molecular Gas Lasers

There are different ways of pumping molecular lasers:

  • Many molecular lasers are electrically pumped with a gas discharge (partly using radio frequency excitation), and sometimes with energetic electron beams. The most prominent example with gas discharge pumping is the CO2 laser, which can produce many kilowatts of output power with high beam quality and a power conversion efficiency which (as a consequence of a favorable excitation pathway) is at least substantially better than that of many other gas lasers. Also, excimer lasers are electrically pumped.
  • For some molecules, chemical pumping is an option (→ chemical lasers). Here, the excited molecules, supplying the needed energy for laser operation, are generated in an exothermic chemical reaction. For example, hydrogen fluoride lasers (HF lasers) and deuterium fluoride lasers (DF lasers) utilize the combustion of ethylene in nitrogen trifluoride, followed by the injection of helium and hydrogen (or deuterium) in a facility which has some similarities to a rocket engine. That technology has the potential to provide enormously high output powers (sometimes several megawatts) for military applications like destroying rockets; chemicals can store substantial amounts of energy and release them quite quickly.
  • In some rare cases, optical pumping is used. For example, the output of a CO2 laser can be used for pumping an ammonia (NH3) laser, and high-power ammonia lasers can also be pumped with HF lasers. Some molecular lasers have been pumped with laser diodes.

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Bibliography

[1]O. R. Wood, “High-pressure pulsed molecular lasers”, Proc. IEEE 62 (3), 355 (1974), doi:10.1109/PROC.1974.9429
[2]I. Mukhopadhyay and S. Singh, “Optically pumped far infrared molecular lasers: molecular and application aspects”, Spectrochimica Acta Part A 54 (3), 395 (1998), doi:10.1016/S1386-1425(97)00230-8
[3]B. Wellegehausen and W. Luhs, “Diode-pumped CW molecular lasers”, Appl. Phys. B 122 (5), 133 (2016), doi:10.1007/s00340-016-6409-9

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See also: gas lasers, CO2 lasers, mid-infrared laser sources, excimer lasers
and other articles in the category lasers

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