Towards a holmium doped multimode fiber amplifier with high quality focused output using wavefront-shaping

Lasers and amplifiers at 2.1 mu m window are of great interest for applications that require high atmospheric transmission. To date, fiber lasers and amplifiers operating at this wavelength are based on single-mode holmium doped fibers (HDF) so that a high-quality output beam can be obtained. However, as can be referred from the case of ytterbium doped fiber power amplifier, limiting nonlinear and thermal effects such as stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) will become obstacles in scaling single-mode holmium doped fiber amplifiers into the multi-kW power regime. The use of multimode HDF can help to mitigate the SBS and TMI effects, facilitating future power scaling of HDF amplifiers (HDFA). Here we propose and experimentally demonstrate a multimode HDF amplifier where the typical speckle pattern output is shaped into a quality focus by wavefront-shaping the amplifier's input seed.