To mitigate these issues, cooling the active medium down to cryogenic temperatures is a proper solution. However, Tm ions suffer from reabsorption losses due to the quasi-three level absorption–emission transitions at room temperature and, in addition, they suffer from other parasitic processes such as excited state absorption (ESA) and energy-transfer upconversion (ETU), limiting the power scaling capabilities and hindering to achieve high beam quality. This will result in much higher slope efficiency (~ 80%) than the quantum defect limited value (~ 40%). In addition, one important advantage of the Tm-doped materials is that they experience the so-called two-for-one cross-relaxation mechanism, meaning that, a pump photon can excite two Tm ions in the 3F 4 emitting level. This also increases the compactness of the laser setup. The 3H 6 → 3H 4 transition in Tm absorbs effectively around 793 nm, which can be directly pumped by the commercially available AlGaAs laser diodes emitting at 793 nm. However, in this work, we give more emphasis on the former one. Both active ions have their own merits and demerits. In this aspect, two kinds of rare earth ion-doped active materials are preferred: thulium (Tm 3+)-doped active materials (typically emitting at slightly shorter than two microns) and holmium (Ho 3+)-doped active materials (emitting at slightly longer than two microns). Emerging applications, such as laser-induced damaged threshold measurement, polymer material processing, debris removal from space, pump source for Mid-Infrared (Mid-IR) lasers, including ultrafast optical parametric oscillators based on non-oxide nonlinear crystals, etc., require compact high average and peak power (HAPP) laser sources emitting at the two-micron spectral range.
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