Efficient resonant diode pumping of 2 µm thulium lasers at 1.7 µm

Tm³⁺-doped media are actively researched due to the 2 μm laser transition ³F₄ →³H₆ of Tm³⁺ⁱon. A diode pumping of the ³H₄ manifold has become a standard excitation method, utilizing the availability of 0.8 μm GaAlAs-based diodes and efficient cross-relaxation energy transfer. An essential drawback of this scheme results from a strong inter-ionic distance dependence of the cross-relaxation, which therefore requires sufficient Tm³⁺doping to achieve the desired quantum efficiency. This can in turn result in an increased probability of up-conversion losses, clustering of Tm³⁺ions, increased generated heat, more difficulties with material growth, and less favorable thermal properties. In this proceeding, we aim to bring attention to the resonant diode pumping of the 3F4 manifold in the 1.6-1.8 μm region and its feasibility. This excitation scheme has a low quantum defect, it circumvents the cross-relaxation requirements, and it is supported by broad absorption peaks. The commercial availability and output power of such diodes is already adequate for a solid-state laser pump source. To illustrate the feasibility, we summarize and expand our results with lasers based on Tm:YAG, Tm:YAP, Tm:YLF, Tm:GGAG and Tm, Ho:GGAG. Crystals were pumped using a 25 W fiber-coupled 1:1 focused diode laser (core diam. = 400 μm, waist diam. = 376 μm, NA = 0.22, M2 = 52) emitting in the 1.68-1.71 μm region. Despite the relatively low spatial and spectral quality of the used 1.7 μm diode emission, favorable results were obtained, such as an efficiency of 80% with respect to absorbed power, multi-watt output power in CW regime, or efficient operation of low-concentration crystals.