Since most organic dyes have a large range of wavelengths over which amplification can occur (called the gain bandwidth), lasers built around them can be composed of light waves spanning a range of colors in the spectrum. This makes possible the ability to select the color (or wavelength) of the laser light through the adjustment of a prism or grating. This tunability feature permits certain applications (such as delicate laser-surgery operations) to be performed at minimal cost and risk rather than having many different monochromatic lasers, each of which can only give forth beams of one specific wavelength. An outline of a tunable dye laser is shown in Figure 4.
Since the dyes used in tunable dye lasers are fluorescent, another light source is almost always used to 'pump' the dye and cause the population inversion. The characteristics of the light used in the excitation will determine the characteristics of the laser. If a flashlamp is used to pump the dye laser, the beam will be pulsed like the lamp, whereas if the laser is pumped by a continuous-wave laser, the dye laser's beam will also be continuous. In other words, graph of the dye laser's intensity versus time will be similar to that of the pump beam.
The pump beam used to excite the large dye molecules and produce the population inversion is a strong light source -- either a flashlamp or another laser -- which is focused on the dye stream. The dye will absorb those wavelengths of light which are shorter (and carry more energy) than those which it emits, since some input energy will always be absorbed in the form of vibrations or heat. In the case of Rhodamine 6G, this spectrum of usable wavelengths is rather large, about 130 nm. The energy absorbed by the dye creates a population inversion, moving the electrons into an excited state. Typically, the dye molecule de-excites spontaneously into a relatively long-lived 'metastable state', where it awaits stimulated emission from the de-excitation of other molecules in the dye. Since the dyes commonly employed are large and have many lower-energy states available for the excited electron to decay into, a large range of de-excitation energies (and, hence, a large range of output wavelengths) is available to the dye. See the energy-level diagram below (Figure 3) for a more visual representation of the energy-level scheme of fluorescent dyes.
The dyes most commonly and successfully used in dye lasers are dissolved in a liquid and pumped through the optical cavity in order to prevent any one part of the dye from becoming exhausted during the excitation process. Sometimes, solid dye cells are employed in order to increase convenience and eliminate hardware. In spite of the large gain bandwidth of organic dyes, no one dye can cover the entire visible spectrum; some lasers make use of a mixture of dyes, such as Rhodamine 6G combined with Coumarin, to achieve the desired effect, although this approach is not recommended.