Demonstration of a bidirectional oscillating-amplifying integrated fiber laser with two ports times 2.5kW

Bidirectional Oscillating-Amplifying Integrated Fiber Lasers (B-OAIFL) combine the advantages of Oscillating-Amplifying Integrated Fiber Lasers (OAIFL) and bidirectional output fiber lasers, including high efficiency, simple control logic, good anti-back reflection, low cost, and small volume. In this work, we introduced the (2+1)×1 Side Pump Signal Combiner (SPSC) between the Output Coupler Fiber Bragg Grating (OC-FBG) and the amplifying sections to address the temporal instability of the laser at low power levels under the counter pumping. Two groups of wavelength-stabilized 976 nm Laser Diodes (LDs) were directly coupled into the oscillating section to increase the laser power within the cavity, and then, a 60 W-level time-stabilized threshold signed as photodetector-detected no-pulse characteristics was achieved. We compared the output characteristics with two different fiber-coiling structures in the oscillating section (STR1, STR2). STR1 was coiled in a single-track-shaped fiber groove with a diameter ranging from 8.0 cm to 10.7 cm, while the coiling diameters of the two output ports were different. For STR2, an '8'-shaped coiling was adopted with gradually varying diameters from 8.0 cm to 9.5 cm and the same coiling diameter of 8.0 cm at both ports. The Transverse Mode Instability (TMI) threshold under the unidirectional pumping at end A and end B were 2271 W and 2530 W, respectively, with Structure one. With Structure two, the thresholds at end A and end B were 2735 W and 2508 W, respectively. Based on STR2 under the bidirectional pumping with a total pump power of 6997 W, we finally achieved a laser output of 2655 W at the end A and 2491 W at the end B, with a total power of 5146 W and a good beam quality of M² to approximately 1.32 at end A and M² to approximately 1.45 at end B, respectively. The Stimulated Raman Scattering (SRS) suppression ratio exceeded 40 dB for both end A and end B.