Depopulation Losses

One of the main considerations in the design of an amplifier or amplifier chain is its stability under the expected operating conditions. In a laser operating as an amplifier or as a Q-switched oscillator the activematerial acts as an energy storage device, and to a large extent its utility is determined by the amount of population inversion that can be achieved. As the inversion is increased in the active material, a number of different mechanisms begin to depopulate the upper level. Depopulation can be caused by amplified stimulated emission (ASE). A process that will be enhanced by radiation from the pump source falling within the wavelength of the laser transition, or by an increase of the pathlength in the gain medium either by internal or external reflections.
Also, if sufficient feedback exists in an amplifier because of reflections from optical surfaces, the amplifier will start to lase prior to the arrival of the signal pulse. If the reflections are caused by surfaces which are normal to the beam direction, the amplifier will simply become an oscillator and prelasing will occur prior to Q-switching or the arrival of a signal pulse. Reflections that include the cylindrical surfaces of a rod will lead to parasitic modes, propagating at oblique angles with respect to the optical axis of the laser