Multi-kW Yb-doped aluminophosphosilicate fiber

Abstract

For directly characterizing photodarkening effects, we demonstrated long-term laser stability of a 5kW-level fiber amplifier based on a Yb-doped aluminophosphosilicate fiber with a 30µm-core and 600µm-clad. The molar ratio of Al3+/P5+ was designed to be 1.1 ± 0.05 for remarkable photodarkening suppression, suitable numerical aperture, and central dip mitigation of refractive index profile. A modified chemical vapor deposition system combined with an all-gas-phase chelate precursor doping technique was applied to fabricate this fiber, doped with 0.15mol% Yb2O3, 1.7mol% Al2O3, and 1.4mol% P2O5. Tested at a master oscillator power amplifier laser setup, the 18m-long 30/600 Yb-doped aluminophosphosilicate fiber allowed for 6.14kW aggregated pump power at 976nm, and then showed a 5.19kW laser output at 1064.4nm with a high optical-to-optical efficiency of 85.2%. Up to this power level, an output spectrum without any sign of stimulated Raman scattering and amplified spontaneous emission was obtained. The fiber amplifier setup was kept at a 5.16kW output for over 600 minutes without power degradation, justifying a remarkable suppression of photodarkening. The results indicated that the all-gas-phase chelate precursor doping technique is highly competitive for low-photodarkening Yb-doped aluminophosphosilicate fiber fabrication towards a 5kW-level commercial high-power laser.