5 kW single-mode oscillating-amplifying integrated fiber laser through tightly bent confined ytterbium-doped fiber

We demonstrated a 5 kW single-mode oscillating-amplifying integrated fiber laser by tightly bending a homemade 25/400 µm confined YDF. Simulation results showed that tightly bending the confined YDF could decrease the mode coupling coefficient and increase the TMI threshold. By optimizing the doping ratio and bending radius of the confined YDF, a TMI-free 5.1 kW laser output was achieved based on the confined YDF and non-wavelength stable 976 nm LD. The beam quality factor M 2 was 1.2 and maintained in the 5.5 hours full power operation. In comparison, the 976 nm pumped oscillating-amplifying integrated fiber laser with the conventional 25/400 YDF was limited by the quantum defect induced TMI at 3.1 kW, while the 915 nm pumped oscillating-amplifying integrated fiber laser with the conventional 25/400 YDF experienced the photodarkening induced TMI at 3.8 kW. To our knowledge, this is the highest output power of the single-mode confined doped all-fiber laser. This work also proves that tightly bending the confined YDF is an effective method to suppress the TMI induced by the quantum defect and the photodarkening.