5 kW all-fiber amplifier based on homemade spindle-shaped Yb-doped fiber

To enhance the thresholds of both stimulated Raman scattering and transverse mode instability, we proposed a novel active Yb-doped fiber with a spindle-shaped core and inner-cladding. An all-fiber main oscillator power amplifier (MOPA) was experimentally established based on this homemade fiber. A maximum power of 5 kW was achieved with the optical-to-optical efficiency of 66.6%, Raman-suppression ratio of ＞45 dB and M2 factor of about 2.0. We believe that the brightness and the efficiency of the laser can be improved by optimizing the structure of the Yb-doped fiber.