FAR-IR  LASERS

Gases other than carbon dioxide may be made to lase in the infrared, many in the far-IR region of the spectrum (generally defined as wavelengths greater than 20 μm). While a few are pumped by an electrical discharge, most are excited optically by a carbon dioxide or, less frequently, a nitrous oxide laser, so far-IR lasers represent an application for IR gas lasers. Most far-IR lasers use molecules such as alcohols (e.g., CH₃OH) or other organic compounds. It is rumored that Scotch whiskey will lase when used in such a configuration. Scotch, like other liquor, contains a number of organic compounds that can lase.

        Far-IR laser tubes are simple structures: a glass tube into which the lasing species (most of them liquids) are injected at low pressures of around 0.1 torr. Liquids vaporize readily at low pressures, forming the gaseous lasing medium. Because of the low pressures involved, far-IR lasers require a high-vacuum system using a diffusion or turbo molecular pump to evacuate the tube of all contaminants. Optics for a typical far-IR laser consist of a pair of mirrors that are movable, to produce a variable cavity length. The laser is pumped in an end-on configuration in which the pump beam (usually from a CO₂ laser) passes through the rear optic to excite molecules in the tube. The front optic (the OC) must reflect 10.6-μm pump light back into the laser tube but pass far-IR radiation. In the example of the CO₂ laser, we found that the FSR of the cavity exceeded the linewidth of a single emission line, so only a single longitudinal mode will oscillate when a wavelength selector is used with the laser; an etalon is not required for single-mode operation. In a far-IR laser, the FSR of the cavity (the distance between resonant peaks) can be many times greater than the gain curve of the lasing species, so it is likely that the cavity will not be resonant for emission at the required wavelength. For this reason, the cavity length must be adjusted until a resonant peak corresponds to a wavelength at which the lasing species exhibits gain.