Mid-infra-red solid state lasers based on thulium and holmium-doped crystals are of increasing interest in applications in medicine, material processing and particle physics. Thulium-doped lasers can be pumped at wavelengths around λop = 780 nm and diode laser pumps with high conversion efficiency ηE and high intensity are therefore sought at this wavelength . However, high ηE here is more challenging to realize than around λop = 940…980 nm (for pumping Yb:YAG), due to the available semiconductor materials. High aluminium-content waveguide and cladding layers are needed (to suppress carrier leakage), whose low mobility leads to either high electrical resistance or requires high doping, leading to increased optical loss αi. In addition, the active region is realized using either compressively strained high-aluminum content (> 10%) InAlGaAs (which leads to challenges with facet passivation, limiting reliable power, and is prone to oxidize, limiting optical performance and lifetime) or tensile strained GaAsP (with high transparency current, limiting efficiency, and tendency towards rapid defect growth, limiting reliability).
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