Solid state bulk lasers having a very thin Disk of laser-active material as Gain medium.

The thin disk Laser (sometimes called thin Disc Laser or active mirror laser) is a special kind of diode-pumped high power solid state laser, which was introduced in the 1990s by the group of Adolf Giesen at the University of Stuttgart, Germany. The main difference to conventional Rod Lasers or Slab Lasers is the geometry of the gain medium: the laser crystal is a thin disk, where the thickness is considerably smaller than the Laser Beam diameter, SO that the generated heat is extracted dominantly through one end face, i.e., in the longitudinal rather than in the transverse direction. The cooled End Face has a Dielectric Coating which reflects both the laser Radiation and the pump radiation.

The thin disk is also often called an active mirror, because it acts as a mirror with laser gain. Within the laser resonator, it can act as an end mirror or as a folding mirror. In the latter case, there are two double passes of the laser radiation per Resonator round trip, so that the gain per round trip is doubled and the threshold pump power is reduced.