We demonstrate an all-fiber 7 × 1 signal combiner with an output core diameter of 50 μm for high power incoherent beam combining of seven self-made Yb-doped single-mode fiber lasers around a wavelength of 1080 nm and output power of 2 kW. 14.1 kW combined output power is achieved with a total transmission efficiency of higher than 98.5% and a beam quality of M² = 5.37, which is close to the theoretical results based on finite-difference beam propagation technique. To the best of our knowledge, this is the highest output power ever reported for all-fiber structure beam combining generation, which indicates the feasibility and potential of >10 kW high brightness incoherent beam combining based on an all-fiber signal combiner.

Anti-resonant hollow-core fibers have emerged as an important medium for high-power laser delivery due to their low optical nonlinearity and high damage threshold. However, current delivery systems of high-power laser based on anti-resonant hollow-core fibers mainly rely on free-space optical components, which limits long-term stability. Here, we report an all-fiber delivery of 2 kW laser with 85.4% transmission efficiency over 2.45 km, using a self-fabricated hollow-core fiber with a record low transmission loss of 0.168 dB/km at 1080 nm. Notably, we observed the phenomenon of stimulated Raman scattering amplified within the silica nested tubes for the first time. By effectively suppressing the Raman noise from the laser source, we achieve an all-fiber laser delivery without stimulated Raman scattering of silica glass. This work marks a significant breakthrough in multi-kilometer and multi-kilowatt power delivery that is potentially useful for industrial manufacturing, nuclear decommissioning, laser drilling in oil and so on.

We investigate a high-beam-quality $(6+1)\times 1$ signal and pump combiner with 50 μm core diameter of the output fiber based on the tapered-fused fiber bundle technique for the first time. The influence of taper ratio of the taper bundle for different mode proportions of the output fiber and the transition efficiency of the signal light are analyzed numerically. In the experiment, a signal fiber core diameter matching from 10 to 50 μm in the combiner is achieved. We experimentally demonstrate that the total 1018 nm pump efficiency of the combiner is $>98\%$ and the signal light insertion loss is less than 2%, with a beam quality factor ${\mathit{M}}^2$ of 1.59. Also, we have set up an amplifier based on the combiner, whose output laser power can achieve 2340 W with the slope efficiency of 73.76%. The beam quality factor ${\mathit{M}}^2$ of the output laser is 2.83 at the maximum power output, and a high beam quality is maintained when a large-mode-area fiber is employed in the laser system.

At present, increasing the output power of a single fiber laser is a problem worthy of attention, and a fiber signal combiner which can carry high power and high beam quality is a solution to this problem. In this paper, a high beam quality 4×1 fiber signal combiner was developed. Firstly, the simulation model of a 4×1 fiber signal combiner was established, and the factors affecting the beam quality and the transmission efficiency of combiner were simulated. Secondly, based on corrosion technology, a high beam quality 4×1 fiber signal combiner with an output fiber of 50/400 µm (NA=0.12) was fabricated by using the taper-fused fiber bundles technology according to simulation results. Finally, four Yb-doped fiber lasers with a central wavelength of 1080 nm were used to test the output power and beam quality of combiner. The test result showed that the total output power of the signal combiner was 12.03 kW, the overall transmission efficiency was more than 96%, and the M² factor was measured to be 4.03.

Compared with end pumping fiber combiner, one of the advantages for side pumping combiner is the unlimited pumping points, which means multi-point or cascaded side pumping can be realized. However, the loss mechanism of the cascaded structure is rarely discussed. In this paper, we present the numerical and experimental investigation about the loss mechanism of a two-stage-cascaded side pumping combiner based on tapered-fused technique. The influence of loss mechanism on the coupling efficiency and thermal load of the fiber coating is analyzed according to simulations and experiments with different tapering ratios for the first stage. Based on the analysis, a cascaded component with total pump coupling efficiency of 96.4% handling a pump power of 1088 W is achieved by employing 1018 nm fiber laser as the pump source. Future work to further improve the performance of a cascaded side pumping combiner is discussed and prospected.

Side-pumping combiner is used for pumping double-clad fiber in various fiber laser schemes. However, its coupling efficiency and temperature characteristics suffer when pumped via a large numerical aperture (NA) pump light. We investigated the method of optimizing the coupling efficiency of a (2 + 1) ×1 combiner under a large NA pump light injection. After optimization of taper ratio and length of the pump fiber and fusion area between pump and signal fiber, the coupling efficiency increased and the temperature characteristic improved, which could be useful for fabrication of a side-pumping combiner for high-power fiber laser applications.