Laser Performance of 1lcurlyb\%rcurlyb at. Yb: Ceramic

We present the performance of a laser prototype based on 1% at. Yb？:？ ceramic, longitudinally pumped in quasi-CW regime at 968？nm. A slope efficiency of 49% with respect to the absorbed pump power was obtained for laser operation at 1032.5？nm. We studied the effects on the laser oscillation due to the reinjection of the residual pump. The thermal behavior of the sample was investigated by means of numerical simulations, accounting for the different thermal load resulting from spontaneous and laser emission. Finally, we report the measured level of the Amplified Spontaneous Emission, which is found to be less than 0.1？mW. 1. Introduction Since their appearance on the scene, polycrystalline transparent materials of cubic structure doped [1–4] with rare earths as Nd3+ [5–7] and Yb3+ [8–10], are found to be appealing as laser gain materials showing all their potentiality in term of shortness of the pulse [11–13], average power level and beam quality. In consequence of that, in the past years many efforts have been made to improve and refine the techniques of fabrication paving the way to a new class of diode-pumped solid-state lasers. In fact, the excellent goals achieved in different host materials [14–19] testify that lasers based on ceramic materials exhibit performances comparable with the corresponding single crystals. The reasons underlying these successful results are multiple and closely related to both the optical and the physical properties of the host ceramics. The host material plays a crucial role to reach an efficient laser action. In fact, it modifies the intrinsic properties of doping ions as the lifetime of the states involved in the laser action as well as the Stark level structure and the absorption and emission cross-sections. Moreover, it determines by its own thermal properties the thermal dissipation of the whole gain medium, and then the magnitude of the thermal gradients due to the pumping process, which induces stress and strain inside the lasing sample determining a modification of the refractive index. The undesirable consequences are the depolarization and degradation of the laser beam quality, the change of the resonator stability condition, and an increase of losses. It has been demonstrated that ceramics show better thermomechanical properties [20, 21]; indeed, they withstand higher thermo-mechanical stresses with respect to the crystals. The lower temperatures of sintering have allowed, for instance, the growth of good optical quality sesquioxides as Y2O3, Sc2O3 and Lu2O3, which are very hard to grow as single crystals