Efficient, High Power Pumps for Mid-IR Solid State Lasers Enabled by 200 K Operation of 808 nm Diode Lasers

Abstract

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 λ op = 780 nm and diode laser pumps with high conversion efficiency η E and high intensity are therefore sought at this wavelength [1]. However, high η E here is more challenging to realize than around λ op = 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 α i . 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). Commercial long resonator (L = 3 mm) GaAsP-based bars at λ op = 780 nm operate with peak η E ∼ 50% [2], significantly less than the η E > 65% achievable for λ op = 940 nm [3]. Although improvements in η E for GaAsP-based diode lasers with λ op = 780 nm are the subject of intense study, an alternative approach is to use longer wavelength devices at reduced operating temperatures, potentially enabling pumping at 780 nm with improved powers and efficiencies [3]. We therefore assess the performance of efficient 808 nm diode lasers at T HS = 200 K, testing their suitability as efficient 780 nm sources.