Fig. 1 Diffraction grating
Dye lasers have the broadest range of output beam wavelengths of all lasers. The organic dye used as the active lasing medium is dissolved in a condensed liquid solvent. Due to the large, complex nature of the dye molecules, there are many vibrational and rotational states. These broadened electronic states are what allow the laser to be tuned. There are over 200 organic dyes in use and each covers a portion of the spectral range from ~200nm to ~1000nm (ultra violet to infrared).
The first experiments in tunable dye lasers were conducted in 1964. These first dye lasers were excited by giant-pulse ruby lasers and they were tuned by varying the concentration of the dye in the cell. This method of tuning was soon replaced when it was found that by substituting one of the mirrors in the cavity with a diffraction grating, the laser could be tuned over a range seven times greater than the concentration method. The diffraction grating works by using constructive interference to allow only one wavelength to propagate in the cavity (see in Fig 1). By changing the angle of the diffraction grating the output wavelength is changed; this is called spectral condensation. Using this method, most of the original energy is retained in the resulting narrow band output.
It was also discovered that while pumping with a laser gives the widest range of output properties, it is more economic to pump it with a flashlamp. This discovery made tunable dye lasers accessible to many more scientists in the early days of laser research. Though for today s scientific uses the laser is the most desirable pumping method.
If the input pump light irradiates the dye for too long, the dye will become bleached, and will no longer fluoresce due to a reduction in the population inversion. This can be avoided by