Thermal lensing effects

Let us get closer to real life : during the pumping, a lot of energy is gathered on a very small area inside the crystal. This leads to local heating, and as the refractive index varies with temperature, an index gradient appears. In the same time, the thermal dilatation of the crystal leads (in most cases) to a kind of bulge of the crystal. This last effect transforms for example the plane mirror into a spherical one.

The index gradient could be simulated in a first approximation by a converging (in most cases, but sometimes it could be a diverging one) lens positioned at the pump focusing point inside the crystal.

We will not here perform the calculations : it is fastidious and does not add anything to the lesson as the principle is exactly the same as before (but with more lenses inside the cavity !) Fortunately, this kind of matrix calculation is a very simple game for computers and several software (some of them are even free !) exist on the market.

We can then get as close as possible to real resonators (other points can be taken into account : absorption, polarization...) and accurately predict the beam characteristics before building the laser.