Active Medium. The active medium in a dye laser is a fluorescent organic compound (a dye) dissolved in a liquid solvent. Intense illumination by light from a separate source another laser or a flashlamp excites the dye molecules, producing a population inversion. The dye then produces stimulated emission, generating a laser beam. The process is called optical pumping, and inherently involves some energy losses, so the output wavelength is longer than the absorbed wavelength. The dye solution is housed in a transparent cell or flowed in a jet through the pumping region.

Dyes are large molecules containing multiple ring structures, and they have complex spectra. Most important dyes fall into a number of families with chemically similar structures. Members of these families differ in the end groups attached to their outer edges, and these chemically superficial differences lead to important differences in characteristics such as laser emission wavelength, tuning range, absorption wavelengths, and tolerance of operating conditions. Listings of dyes in which laser action has been demonstrated go on for many pages (Maeda, 1984; Steppel, 1982). Many of these dyes are offered commercially for laser use.

Dye degradation is an important practical issue. Complex dye molecules can be decomposed by the pump light, with the degree of de- composition increasing with pump intensity and with shorter pump wavelength. Short ultraviolet wavelengths, present in the light from flashlamps and from ultraviolet pump lasers, trigger some photochemical reactions. Some degradation is thermal; and some, a combination of photochemical and...