Saturable absorber

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A saturable absorber is a passive optical switching element that is used as a simple form of a Q switch in laser resonators . It consists of a material with an intensity-dependent absorption coefficient . Usually this is a dye solution, the absorption maximum of which is at the laser wavelength, or a semiconductor component ( SESAM , semiconductor saturable absorber mirror ).

With increasing population inversion in the laser-active medium, the number of photons in the optical resonator also increases. When a certain threshold is reached, the absorber material becomes permeable for the laser radiation and the laser starts to oscillate. The coupled-out light has a very high intensity, since a certain number of photons in the resonator must first be exceeded so that the absorber becomes transparent. If the inversion is largely eliminated, the absorption increases again after a certain relaxation time and the quality of the resonator “switches” back below the laser threshold.

The saturation intensity should be reached at the time of maximum inversion. This is e.g. B. to influence the concentration of the dye solution. The result is a short, high-power laser pulse.