Fiber lasers have been widely used in various fields including medicine, industrial processing, and national defense, because of their excellent beam quality, high conversion efficiency, straightforward heat management, and flexible operation. The pump-signal combiner occupies a crucial role in efficiently coupling the pump light to the double-cladding fiber, for signal light transmission, and is one of the most critical fiber laser components. Transverse mode instability (TMI) and nonlinear effects have been identified as bottlenecks for further improvement in fiber laser power. Counter-directional pumping and large-mode-area double-cladding fibers are helpful for suppressing nonlinear effects, which are beneficial for fiber laser power improvement. Most existing research on pump-signal combiners focuses on the few-mode signal fibers with core diameters smaller than 30 μm. In this study, the fabrication method for a counter-directional (6+1)×1 pump-signal combiner based on a large-core (50 μm) multimode signal fiber is introduced, with higher thresholds for nonlinear effects. The proposed pump-signal combiner achieves high pump-coupling efficiency alongside high-beam quality.

By conducting numerical simulations and experimental validations, the effects of the taper length and ratio, and refractive index of the glass tube on the coupling efficiency of the pump light were analyzed. The effect of the core axial offset on the transmission efficiency and beam quality of the signal light was also investigated. Consequently, the optimal parameters for fabricating the pump-signal combiner were obtained. During the pump-signal combiner fabrication process, signal fiber tapering was avoided by pre-tapering the pump fiber and signal fiber corrosion. While optimizing the cutting and fusion parameters, the tapered fused bundle (TFB) and output fiber were spliced using an inline feedback alignment. Subsequently, the pump and signal arm performances were tested using laser diodes (Reci, DAB 1200, 915 and 976 nm wavelengths) and a 3 kW fiber oscillator, respectively. Finally, an integrated device based on the proposed pump-signal combiner was fabricated and applied to a narrow-linewidth laser system, which included an end cap and a cladding light stripper.

Numerical simulations and comparative experiments show that selecting an appropriate taper length, reducing the taper ratio of the pump fiber, and using a low-refractive-index glass tube can improve the pump-arm performance of the pump-signal combiner (Figs.3 and 4 and Table 1). To achieve this, a pump-signal combiner was fabricated using a semi-fluoride thin-walled glass tube. The proposed pump-signal combiner achieved a pump coupling efficiency of over 98.5% and a temperature rise coefficient below 10 ℃/kW without active cooling. It could be observed that the fiber core offset during multimode signal fiber fusion results in fundamental mode conversion to higher-order modes. Although this may not significantly impact the overall signal light passing rate, the M² factor, which is more sensitive to the axial offset, was selected as the feedback alignment indicator (Fig.5). During the M² feedback fusion process, fusion quality and strength are ensured by maintaining the angle of cleavage within 1° and controlling a slight collapse of the TFB (Fig.8). Consequently, the beam quality degradation ratio is only 3.4% (Fig.9). The integrated device based on the pump-signal combiner effectively reduces the number of splice points and the transmission fiber length, thus, enhancing fiber laser system compactness and stability. When applied in a narrow linewidth system based on a simple MOPA structure, at 4182 W output power, the beam quality is Mx2=1.48, My2=1.36, 3 dB linewidth is 0.44 nm, 20 dB linewidth is 2.14 nm, and the Raman suppression ratio is 40.5 dB (Fig.11).

The proposed pump-signal combiner fabrication method enables signal and pump fiber matching of any size without the necessity for signal fiber tapering. Hence, the beam quality of the pump-signal combiner can be effectively maintained. Through theoretical analysis and experimental verification, it is demonstrated that the use of a semi-doped fluorine thin-walled glass tube can improve pump arm performance. Additionally, the M² factor is confirmed as a suitable indicator for aligning large-core multimode signal fibers. Consequently, development of a (6+1)×1 pump-signal combiner was achieved with a pump coupling efficiency of over 98.5% and beam quality degradation of only 3.4%. The temperature increase coefficient was maintained below 10 ℃/kW without active cooling. Based on the proposed pump-signal combiner, an integrated device without splice points was fabricated to reduce fusion loss and to ensure a more compact system. The proposed solution has broad application prospects for high-power, high-beam-quality fiber laser systems.