Objective High-power all-fiber lasers hold significant application value in industrial processing and scientific research. To achieve high-power single-mode laser output with high stability and a high threshold for nonlinear effects, this study begins with the optimization of pumping schemes and combines theoretical and experimental approaches. Firstly, based on the theoretical model of laser rate equation, the relationship between the power in the cavity and the length of the gain fiber, the distribution of the number of inversion particles along the fiber length, and the simulation of stimulated Raman scattering under different pump structures are simulated and analyzed. Second, fiber oscillator lasers with different pumping schemes were constructed experimentally. The study found that different pumping schemes exhibit significant differences in thermal management, nonlinear effect suppression, and output performance: the forward-pumping scheme resulted in uneven thermal distribution in the gain fiber, causing transverse mode instability to occur earlier and limiting output power scalability; the bidirectional-pumping scheme offered more uniform thermal distribution but still faced challenges such as nonlinear effects and feedback light management; the backward-pumping scheme demonstrated excellent performance in suppressing nonlinear effects in high-power fiber lasers. Ultimately, the backward-pumping scheme was adopted to achieve laser output with a operating wavelength of 1080 nm, an output power of up to 2.16 kW, an optical-to-optical conversion efficiency of approximately 79%, a beam quality factor below 1.2, and a power instability of less than 1% over one hour. This research holds significant importance for further enhancing the output power and system stability of high-power all-fiber lasers.

Methods The research methodology combines theoretical simulation and experimental construction to evaluate different pump structures (Fig.1). Theoretically, the study employs laser rate equations implemented in SeeFiberLaser simulation software to model a 1080 nm Yb-doped fiber laser (Fig.2, Fig.3). Key parameters include pump wavelength of 976 nm, pump power up to 2800 W, gain fiber dimensions of 20/400 μm, and fiber length of 20 m. The simulations analyze intracavity power distribution and population inversion along the fiber for forward, bidirectional, and backward pumping schemes. Experimentally, three laser oscillators were built using the same 20/400 μm Yb-doped fiber, high-reflectivity and low-reflectivity fiber Bragg gratings centered at 1080 nm, and 700 W LD pump sources at 976 nm (Fig.4). Critical components such as cladding power strippers and quartz block heads were used to ensure signal purity and output stability (Tab.1). Output characteristics including power, spectrum, beam quality, and power stability were measured using an optical spectrum analyzer, power meter, and beam profiler.

Results and Discussions The experimental results demonstrate distinct performance characteristics for each pump structure. Under forward pumping, the output power reached 1.78 kW at 72% efficiency, but TMI occurred abruptly near 1.7 kW, limiting further power scaling without any SRS observed. Bidirectional pumping achieved 2.1 kW output with 76% efficiency; however, SRS emerged at 2 kW with a Raman suppression ratio >34 dB (Fig.5). Additionally, feedback light in the forward combiner reached 27 W, posing a risk to pump diodes. In contrast, backward pumping yielded the best performance: output power of 2.16 kW at 79% efficiency, with no observed TMI or SRS (Fig.6). The efficiency curve showed a sudden rise due to LD wavelength shift to 976 nm, enhancing pump absorption. Beam quality measurements confirmed near-diffraction-limited output, and power stability was within 1% over one hour (Fig.7). These results align with theoretical predictions: forward pumping causes high initial inversion and thermal gradient; bidirectional pumping offers flatter power and heat distribution but suffers from nonlinearities; backward pumping maintains low power along most of the fiber, minimizing nonlinear accumulation and enabling cleaner high-power operation.

Conclusions In conclusion, this study comprehensively evaluates the impact of pump structures on the performance of high-power all-fiber oscillators. The backward-pumping scheme proved superior in suppressing nonlinear effects such as TMI and SRS, enabling stable and efficient operation at 2.16 kW with 79% optical-to-optical efficiency and excellent beam quality (M² < 1.2). Forward pumping, while simple, induces uneven thermal loading and early TMI. Bidirectional pumping improves thermal uniformity and allows higher power but introduces challenges like SRS and feedback light management. The research underscores the importance of pump structure selection in balancing thermal management, nonlinear suppression, and output performance. The successful implementation of a backward-pumped oscillator with high stability and efficiency meets the demanding requirements of industrial and scientific applications, providing a valuable reference for future designs aiming to push the power limits of single-mode fiber lasers.