Active resonator

In this section we will analyze the changes that occur if alasing medium is inserted into the resonator. In cw and high-average-power solid state
lasers the dominant effect that distorts the mode structure in a resonator is
thermal lensing. Heat removed from the rod surface generates a thermal gradient.The thermally induced spatial variations of the refractive index causes the laser rod to act as a positive lens with a focal length that depends on the power dissipated as heat from the pump source.
As an introduction to the effect of thermal lensing on the resonator geometry we will first consider a thick lens. This is followed by considering a thick distributed lens that represents a laser rod. Finally at the end of the section the sensitivity of different active resonator configuration to mirror misalignment will be discussed. Perturbation of the thermal lens of the laser rod leads mainly to changes in mode
structure and beam divergence, whereas a misalignment of the resonator mirrors causes a lateral displacement and angular tilt of the output beam, which causes an increase of the diffraction losses and therefore a reduction of output.