Articles techniques et présentations

High average power lasers are currently being designed for laser fusion applications. Single beamlines with laser amplifiers should be able to deliver up to 10 kW average power at 10-Hz repetition rate. For achieving a 5 to 10 % wall-plug efficiency, this means that at least, 50 kW will be dissipated in the laser amplifier. The deposited heat in the laser medium will introduce wavefront distortions that will decrease the beam quality, its ability to propagate and to be focused. It is nowadays completely impossible to design such amplifier without the support of a full thermo-mechanical-optical software model. Our model follows the propagation of light from the source (flash-lamps or laser diodes) to the laser medium in order to assess the deposited heat inside the laser medium (rare-earth doped glass, crystal or ceramic). Then the laser beam from the front-end or any probe beam will propagate throughout the heated laser medium. The temperature gradient inside the laser medium will introduce wavefront distortions and birefringence. The first part of the model’s architecture follows the propagation of light from the pumping source to the laser medium, then calculating the deposited heat and assessing stored energy, gain and amplified spontaneous emission. The second part deals with the thermal management of the deposited heat in the different volumes (laser medium with or without cladding, windows, coolant channels and additional parts included in the boundary conditions). Follows a thermal resolution (transient or steady state) for calculating temperature 3D map(s), stresses, strains and bouncing / bowing of optical surfaces. The third part is the classical propagation of a probe beam inside the laser medium and the final result is the optical path difference or wavefront distortions of the probe beam. We will detail the different parts of our software model in Comsol Multiphysics with the support of different packages (Heat Transfer, Structural Mechanics, Ray Optics) and show the different contributions of the deposited heat to the final optical path difference of the beam (thermal index gradient, local birefringence, inhomogeneous beam propagation).