New active three-level laser materials and their use with innovative geometries for efficient continuous-wave and mode-locked coherent sources

There is an increasing demand for efficient, reliable and compact diode-pumped solid-state laser sources in both the continuous-wave and pulsed operation regimes. In the 1 μm spectral range, Yb3+-doped gain media are ideally suited as the absorption band is covered by highpower InGaAs laser diodes. Moreover, the very simple electronic configuration of Yb3+, based on two multiplets, reduces the number of parasitic spectroscopic processes and provides a low quantum defect, reducing the thermal effects. Finally, depending on the host, Yb-doped materials exhibit broad emission bands that enable the production of femtosecond pulses in the mode-locked regime. This thesis investigates two extremely promising groups of doubles tungstate crystals as laser media; the monoclinic double tungstates KT(WO4)2 and the tetragonal double tungstates NaT(WO4)2 , where T is a trivalent metal or rare-earth cation. The potential of monoclinic double tungstates as laser materials has not only recently be confirmed by the scientific community. As a matter of fact, the strong anisotropy of the monoclinic double tungstates results in the cumulation of the favorable features, a large and broad emission cross section particularly, for one crystal orientation. Moreover, some of these crystals can advantageously tolerate extremely high doping concentration up to the stoichiometric level (100% doping), indicating potential development of exceptionally thin active structures. The anisotropy of their thermo-mechanical properties introduces, however, some limitations and actually prevented till recently the manufacture of advantageous composite structure. In this thesis, laser experiments based on several of such composite structures are performed for the first time. In particular, composite geometries including planar waveguides and composite thin disk were studied and slope efficiencies close to the theoretical limit were achieved. Such structures were also investigated for passive mode-locking and pulse durations as short as 66 fs were notably obtained with a diode-pumped diffusion bonded Yb:KY(WO4)2 /KY(WO4)2 crystal. This corresponds to the shortest pulse ever achieved with monoclinic double tungstate lasers and any Yb laser based on a composite crystal. With Ti:sapphire laser pumping and external compression, the pulse duration could be even further shortened to 62 fs. The other group of crystals investigated, the tetragonal double tungstates, are characterized by a local disorder. Thus, when doped with rare-earth ions, the transition cross sections exhibit significant inhomogeneous broadening leading to broad and smooth absorption and emission bands. This denotes, on one hand, relaxed requirements for diode pumping and, on the other hand, a high potential for ultrashort pulse generation. For the first time, continuous-wave laser operation has been investigated with Yb:NaY(WO4)2 and a slope efficiency of nearly 70 % has been achieved. The pulse duration in the passive mode-locked regime was as short as 53 fs, which corresponds to one of the shortest pulse generated from any Yb laser.