Color-center lasers are types in which optical pumping of a crystal generates wavelength-tunable output in the near-infrared. They sometimes are called F-center lasers from the German word for color, Farbe. Some of their characteristics resemble those of continuouswave dye or tunable vibronic solid-state lasers, but there also are important differences. Color-center lasers have an intrinsically narrow spectral linewidth that makes them attractive for spectroscopy. Experimental versions have operated near 0.4 micrometer ( ?m) and at 0.8 to 4.0 ?m, but commercial models operate in a more restricted range, near 1.5 ?m and beyond 2 ?m.

Continuous-wave color-center lasers differ markedly from the family of flashlamp-pumped crystalline or glass solid-state lasers described in Chaps. 22 to 24 and 27. The active medium is a thin crystal, into which point defects have been introduced deliberately. The resulting microscopic defects in the crystals tend to absorb light, coloring the normally colorless crystals and earning the name color centers. Color centers are found in many types of normally colorless crystals. Laser action in color centers, first seen in 1965 (Fritz and Menke, 1965) is mostly in alkali halide crystals.

Like continuous-wave dye lasers, color-center lasers require pumping with another laser. The usual choices are ion, dye, or neodymiumYAG lasers. Normally operation is continuous, but color-center lasers also can be synchronously pumped by modelocked lasers. Pulsed operation of color-center lasers has been demonstrated experimentally with both laser and flashlamp pumps. However, only continuous-wave and modelocked versions are marketed.