The transverse mode instability (TMI) effect in single-fiber fiber lasers is one of the main factors limiting their power scaling. In this paper, we design an industrial-grade 7 kW all-fiber laser oscillator using a hybrid pumping scheme, which effectively mitigates the occurrence of TMI. Based on traditional large mode area ytterbium-doped fibers (LMA-YDFs) with a core/inner cladding diameter of 30/400 μm and commercial unstable wavelength pump sources, the TMI threshold is significantly increased under dual-wavelength hybrid pumping conditions at 915 nm and 976 nm. The output power of the single-stage fiber laser is increased to 7.06 kW using a bidirectional pumping structure, with a Raman suppression ratio greater than 42.23 dB. This laser operates stably at an output power of 7 kW with a power fluctuation within 1% during a 110 h aging test. Additionally, high-reflection resistance tests are conducted, and the laser maintains good stability when perforating high-reflection materials, indicating its readiness for use in most industrial applications.

In this study, we design a high-power fiber laser oscillator based on double-clad ytterbium-doped fiber with a core/clad diameter of 30/400 μm and an absorption coefficient of 0.65 dB/m at 915 nm. The fiber is fixed onto a water-cooled plate with a minimum coil diameter of 13 cm. By utilizing both 915 nm pumping and a combination of 915 nm and 976 nm pumping, we compare the laser output characteristics under different pumping conditions. First, in the case of forward pumping, a 976 nm laser diode (LD) with an unstable wavelength is used as the pump source to test the threshold of TMI. Second, the pump wavelength is changed to a mixed pump wavelength configuration, and its output characteristics are compared with those of pumping solely at the 976 nm wavelength. Finally, a bidirectional pumping approach is employed to achieve a higher power output.

When pumping with a single 976 nm wavelength pump source, the forward pumping TMI threshold of our system is tested to be 1.36 kW. By solely changing the pump wavelength to a mixed wavelength configuration, the forward TMI threshold exceeds 2.12 kW (Figs. 2 and 3). This indicates that the mixed pumping method effectively suppresses TMI. After confirming the advantages of mixed pumping, the pumping configuration is changed to bidirectional pumping, which increases the pump injection power while mitigating the thermal effects generated in the fiber. Ultimately, at a pump power of 9.3 kW, a laser output power of 7.06 kW is achieved, with an optical-to-optical efficiency of 75.9%. In the absence of TMI effects, the Raman suppression ratio is 42.3 dB. At the maximum power output, the beam quality factor (M2) is approximately 2.16 (Fig. 4). Finally, power stability tests and high-reflection resistance tests are conducted on the designed fiber laser oscillator to verify its feasibility for industrial applications.

This paper presents an industrial-grade end-pumped all-fiber laser oscillator that achieves a high output power of 7 kW and demonstrates superior stability. Commercial high-power LDs with unstable wavelengths of 915 nm and 976 nm are employed as pump sources. The forward TMI thresholds of the fiber laser oscillator are compared under a single 976 nm pumping scheme and a hybrid pumping scheme using both 915 nm and 976 nm. Experimental results indicate that the hybrid pumping method significantly enhances the TMI threshold. On this basis, a bidirectional hybrid pumping scheme is ultimately adopted to achieve a laser output power of 7.06 kW, with an optical-to-optical conversion efficiency reaching 75.9%. Furthermore, the laser oscillator maintains a Raman suppression ratio of 42.23 dB in a 15 m long output pigtail fiber with a core/clad diameter of 34/250 μm, and a beam quality factor M2 of 2.16. In terms of industrial application stability testing, after operating at a high output power of 7 kW for 2 h, the laser superior performance against back-reflection light is verified using copper as the high-reflective material in the high-reflection resistance test. By optimizing the pumping wavelengths of the fiber oscillator and balancing TMI and SRS, further increase in the output power of this oscillator is expected in the future. This paper provides a reliable solution for constructing end-pumped bidirectional fiber lasers with output powers of 7 kW and above for industrial applications.