This review is devoted to summarizing and discussing the scientific progress achieved during the past decade in diode-pumped ytterbium-doped solid-state lasers. The output radiation centered on the 1-μm region is of particular interest for the well-known applications of previously developed Nd3+ lasers. Several hosts have been studied and tested in recent years. Due to the enormous number of papers dealing with ytterbium, Yb3+ doped hosts, we will limit our report here to materials based on transparent polycrystalline sesquioxide laser ceramics, such as Lu2O3, Sc2O3 and Y2O3. The reason is basically due to the wider emission band of ytterbium-doped sesquioxides as compared with more commonly used hosts such as YAG. The higher thermal conductivity with respect to hosts such as CaF2 makes them very interesting for applications requiring both good thermal conductivity and a wide fluorescence emission band, as well as for short pulse generation at high average power levels. Furthermore, the replacement of trivalent Yb with sesquioxides does not require any charge compensation (as is the case of the replacement of divalent Ca in CaF2), which makes their fabrication easier, in particular at high doping levels.

We present a high-power, high-efficiency and low threshold laser prototype based on doped ceramic Yb³⁺:YAG. We achieved an output power of 9 W with a slope efficiency of 73% and a threshold of 1 W at 1030 nm in quasi-Continuous Wave (QCW). Moreover, we obtained an output power 7.7 W with a slope efficiency of 60% in Continuous Wave (CW). Finally, a characterization of a low losses tunable cavity for several laser wavelengths with an output power exceeding 5 W is reported.

We report the tunable, CW and quasi CW laser operation at room temperature of an highly doped (30% at.) Yb:YLF crystal longitudinally pumped by a fiber coupled laser diode array. The CW output power is 1.15 W vs. an absorbed pump power of 6 W, with a slope efficiency of 31%. In quasi-CW operation (20% duty factor @10 Hz) an output power of 4 W with an absorbed power of 9.5 W, and a slope efficiency of 62.8% were obtained. The tuning range spans from 1022 to 1075 nm. To our knowledge, these are among the best experimental results obtained at room temperature with Yb doped YLF.

Thermal effects in transparent laser crystals and ceramics are generally an unwanted consequence of the pumping process: temperature gradients give rise to an unevenly distributed refractive index variation and a distortion of the optical surfaces crossed by the laser beam (thermal lens); birefringence due to thermomechanical stress can cause depolarization losses; and absorption from the ground level usually increases with temperature in quasi-three-level systems. All these effects can seriously impair laser performance, especially in high-power devices. Layered structures with a tailored modulation of the doping level can be used to reduce the peak temperature, the temperature gradients and also the thermally induced deformation of the laser material, thus mitigating the overall thermal effects. In the present work, structures comprising two and three layers of different compositions (pure YAG/10 at% Yb:YAG and pure YAG/10 at% Yb:YAG/pure YAG) were designed with a view to control deformation and stresses, and to reduce the thermal lensing effect. The multilayered samples were assembled by linear and cold isostatic pressing, and co-sintered under a high vacuum in a clean-atmosphere furnace. The microstructure of the layered samples obtained was characterized by FEG SEM, ESEM and TEM. The Yb diffusion profile across the doped/undoped interface was identified and related to the laser's output power. An internal optical transmittance up to 96% was obtained. A laser output power up to 5 W, with a slope efficiency as high as 74.3%, was also achieved.