Particularly in high power lasers, the heating of the gain medium (e.g. a Laser crystal) often causes a significant thermal Lens through the following mechanisms:
The gain medium is hotter on the beam axis, compared with the outer regions, typically causing some transverse gradient of the Refractive Index (thermooptic effect, quantified with the coefficient dn/dT).
Further index changes can be caused by thermally induced mechanical stress (photoelastic effect, quantified with photoelastic coefficients Ïij and the thermal expansion coefficient a).
Mechanical stress can also lead to bulging of the end faces of the gain medium, SO that these also cause lensing. This effect can be important for short laser crystals.
Depending on the situation, these effects can have different relative strength and even sign. In many cases, the first mentioned effect (temperature dependence of the refractive index) is most important.