Overview

The far-infrared is the part of the electromagnetic spectrum loosely defined as extending from about 10 micrometers ( ?m) to 1 millimeter (mm). Neither endpoint is firmly or formally defined, and the longer-wavelength part of that region, beyond about 40 ?m, is sometimes called the submillimeter region.

The haziness of the definition reflects the fact that the far-infrared had been little explored until recent years, when far-infrared laser sources became available. In these lasers, molecular gases emit at wavelengths of 10 ?m to about 2 mm, on vibrational-rotational transitions at the short-wavelength end, and on purely rotational transitions at the longer wavelengths. Laser action has been demonstrated experimentally on well over 1500 far-infrared lines. Not all of those lines have been reported from commercial lasers, partly because manufacturers have used only a limited number of gases.

In practice, two commercial lasers which emit at the short-wavelength end of the far-infrared are not usually labeled far-infrared lasers. These are the carbon dioxide laser (Chap. 10) and the nitrous oxide (N ₂O) laser (Chap. 16), both of which can be excited electrically to emit in the 10- m region. Most other far-infrared lasers require optical excitation with either a CO2 or N2O laser, although a handful emitting at longer wavelengths can be excited electrically.

Commercial far-infrared lasers emit at wavelengths from about 28 ?m to 2 mm. A single laser cavity can house many different gases, and typically each gas can emit at a number of...