We report on a novel approach to fabricate channel (ridge) waveguides (WGs) in bulk crystals using precision diamond saw dicing. The channels feature a high depth-to-width aspect ratio (deep dicing). The proof-of-the-concept is shown for a Tm:LiYF4 fluoride crystal. Channels with a depth of 200 μm and widths of 10–50 μm are diced and characterized with a confocal laser microscopy revealing a r.m.s. roughness of the walls of about 1 μm. The passive waveguiding properties of the channels are proven at ~815 nm showing almost no leakage of the guided mode into the bulk crystal volume. The laser operation is achieved in quasi-CW regime. The maximum peak output power reaches 0.68 W at ~1.91 μm with a slope efficiency of 53.3% (in σ-polarization). The laser mode has a vertical stripe intensity profile. The proposed concept is applicable to a variety of laser crystals with different rare-earth dopants and it is promising for sensing applications.
We systematically study cross-relaxation (CR) and ion clustering in Tm3+:CaF2 crystals using a spectroscopic approach. For this, the luminescence from the 3H4 and 3F4 states was monitored for a broad range of Tm3+ doping concentrations, from 0.01 at.% to 7 at.%. The decay curves were fitted using a model of two ions classes, namely isolated ions showing no energy-transfer processes and ions with neighbors exhibiting both CR and energy-transfer upconversion (ETU), and accounting for energy-migration. The fraction of ions with neighbors and the microscopic concentration-independent CR and ETU parameters are deduced. The critical Tm3+ doping level for which at least half of the active ions are clustered is only 0.7 at.%. The obtained results are relevant for achieving efficient laser operation of Tm3+:CaF2 crystals at the 3F4 → 3H6 (at ~1.9 μm) and the 3H4 → 3H5 (at ~2.3 μm) transitions.
Yttrium orthoaluminate (YAlO3) is an attractive laser host crystal for doping with thulium (Tm3+) ions. We report on the absorption and stimulated-emission (SE) cross-sections of this orthorhombic (sp. gr. Pnma) Tm:YAlO3 crystal for the principal light polarizations, E || a, b and c. Polarized absorption data lead to the Judd-Ofelt parameters Ω2 = 1.46, Ω4 = 2.82 and Ω6 = 1.09 [10-20 cm2]. In particular, for the 3H4 → 3H5 transition, it is found a stimulated emission cross section of 0.86×10-20 cm2 at 2278 nm corresponding to an emission bandwidth of ~12 nm (for E || b). Continuous-wave laser operation on this 3H4 → 3H5 transition is achieved with an 1.8 at.% Tm:YAlO3 crystal under laser-pumping at 776 nm. The mid-infrared Tm:YAlO3 laser generated 0.96 W at ~2274 nm with a slope efficiency of 61.8% and a linear laser polarization (E || b). Tm:YAlO3 is promising for mode-locked lasers at ~2.3 μm.