The resonator of a solid state laser is a very well aligned optical system. As the resonator may be
as much as a meter or more in length and precise alignment of the optical elements is required, the
whole system needs to be exceptionally stable in order to guarantee reliable function.
On the other hand, the laser crystal is pumped by light from a focused laser diode with significant
optical power. This energy is only partly converted into optical energy as the required laser light.
Most of the pump energy is lost as heat. The heat is dissipated within the laser crystal and into the
mounting system of the crystal. Due to the thermal expansion of material, the crystal and its
mounting system are deformed. The deformation of the crystal will cause a so-called thermal lens
effect while movement of the mounting system can lead to disalignment of the laser resonator.
In this paper, we describe an experimental set-up to measure the thermal lens effect by means of
digital holography. A digital holography microscope was positioned above the laser resonator. The
surface of the laser crystal was observed by the microscope via a selectively reflective mirror while
the crystal was being pumped and the whole laser system was operating. By changing pump
current and laser power, it was possible to monitor both deformation of the crystal surface and
deflection of the crystal holder. We present the results of this experiment including an estimate of
the stresses and temperatures on the laser crystal induced by its thermal deformation.