Beam quality degradation of signal light in a side pumping coupler with a large-mode-area signal fiber.

In addition to the pump coupling efficiency and power handling capability of a signal/pump coupler, it is of great importance for high power fiber lasers and amplifiers to maintain the optical properties of the signal light propagating through the component. We report on the comprehensive theoretical and experimental investigations on the beam quality degradation of the signal light in a side pumping coupler with large-mode-area (LMA) signal fiber, fabricated by tapered-fused technique. For the purpose of further understanding the mechanism of beam quality degradation of a side pumping coupler, the impact of micro deformation and thermal dopant diffusion in the signal core is analyzed and discussed theoretically through Beam Propagation Method (BPM). The beam quality and signal insertion loss of several home-made couplers with different fabrication parameters are measured experimentally. We show theoretically and experimentally (using a hydrogen/oxygen flame) that the beam quality degradation and the signal insertion loss are dependent on the temperature and duration of heating process, as well as on the size of the heating region. Accordingly, several solutions and suggestions about the component design and fabrication with the consideration of beam quality are proposed.

High-beam-quality signal and pump combiner with large-mode-area fiber for high-power fiber laser and amplifier.

We investigate a high-beam-quality (6+1)×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 M2 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 M2 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.

Incoherent beam combining of fiber lasers by an all-fiber 7 × 1 signal combiner at a power level of 14 kW.

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.