Watts-level, short all-fiber laser at 1.5 μm with a large core and diffraction-limited output via intracavity spatial-mode filtering

We report over 2 W of single spatial-mode output power at 1.5 μm from an 8-cm-long, large-core phosphate fiber laser. The fiber has a numerical aperture of ⋍0.17 and a 25-μm-wide core, heavily doped with 1 % Er⁺³ and 8% Yb⁺³. The laser utilizes a scalable evanescent-field-based pumping scheme and can be pumped by as many as eight individual multimode pigtailed diode laser sources at a wavelength of 975 nm. Nearly diffractionlimited laser output with a beam quality factor M² ⋍; 1.1 is achieved by use of a simple intracavity all-fiber spatial-mode filter. Both spectrally broadband and narrowband operation of the laser are demonstrated.