Quasi-three-level solid-state lasers in the near and mid infrared

Abstract

Quasi-three-level lasers based on trivalent rare-earth ions are the most important types of solid-state lasers covering the near to mid infrared from 0.9 µm in the Nd3+ ion up to beyond 7 µm in the Pr3+ ion [1] and they are the most important sources especially in the eye-safe range > 1.4 µm. Although for a long time being regarded as less efficient and not so well suited for high-power operation due to the presence of reabsorption on the laser emission by its own active medium, these lasers made an enormous progress in the last years. This progress has been enabled on one side by the availability of high-power pump sources like laser diodes with now high spectral and spatial brightness, allowing operating the laser at high pump intensity far above its threshold. Often, resonant direct-diode pumping is the key to high efficiency and high output power. On the other side, however, the population dynamics and the spectroscopic properties of these lasers need to be very well understood in order to obtain efficient lasing and to fully exploit their possibilities. The unique properties of these lasers will be addressed based on two examples: Recent advances in the description of the spectroscopic properties of these lasers on the example of Er3+ allow us now to incorporate the laser medium's temperature as a real-time variable into laser modeling and simulation [2,3]. These new theories, together with a novel description which takes into account the influence of the laser medium's own fluorescence on the laser performance [4], e.g. opened the way to design and realize one of the most powerful solid-state lasers in the eye-safe spectral range: The Er3+:YAG solid-state heat-capacity laser (SSHCL). Fig. 1 shows the most recent results with an output power of 4.65 kW at an emission wavelength of 1.64 µm and an incident pump slope efficiency of 51.4 % (58.4 % with respect to the absorbed pump power). Over 435 J of pulse energy at a moderate temperature rise of only 56.7 K/s are achieved, currently limited by the influence of increasing thermal inhomogeneities destabilizing the laser cavity.