Compact chirped-pulse amplification systems based on highly Tm³⁺-doped germanate fiber

We report the fabrication of a dual cladding large mode area thulium-doped germanate fiber (TDGF). The fiber has a core diameter of 20 µm, a high Tm³⁺ ion concentration of 3cm³×10²⁰/cm³, and a hexagonal inner cladding to enhance pump absorption when cladding-pumped. Using a short fiber length, we demonstrate a compact 300 fs chirped-pulse amplification system operating at 1925 nm, investigating both core- and cladding-pumped implementations. By cladding pumping a 65 cm long fiber we produced an average power of 14.1 W (limited by thermally induced damage) and a peak power of 2.17 MW at a pulse repetition rate of 15.7 MHz. Core pumping a 19 cm length of TDGF produced 2.3 W of average-power and 16 MW peak-power pulses at 0.39 MHz. The performance is already comparable to the state-of-the-art success achieved with flexible silica fibers. Considering the rapid improvements in glass quality and the scope for further increasing the doping concentration, this fiber type holds great potential for pulsed fiber lasers in the 1.5-3 µm wavelength region.