5-kW single stage all-fiber Yb-doped single-mode fiber laser for materials processing

Stimulated Raman scattering gives back reflection sensitivity to a high power fiber laser. Therefor SRS suppression is necessary in order to realize stable laser processing by a high power fiber laser. A 5-kW single-mode ytterbium doped fiber laser with a 20-m long delivery fiber has been realized. The fiber laser is an all-fiber single-stage Fabry-Perot system in a co-pumping configuration. The optical to optical efficiency was 80% at the output power of 5.0 kW. And the M-squared figure of 1.3 was obtained. The Stokes light by SRS is suppressed to 45 dB below the laser output by using fibers with the effective mode area of 600 μm² . While SRS was well suppressed, four wave mixing was observed with the frequency shift of ~6 THz. Four wave mixing between the fundamental mode and the secondary modes is believed to take place. Four wave mixing is believed not to give back reflection sensitivity to the fiber laser. The 5-kW single-mode fiber laser was applied to laser processing. Bead-on-plate tests were carried out with a galvanometer scanner. The laser ran without stopping nor damaging the laser system even during processing highly reflective material. This implies that our SRS suppressed single-mode fiber laser can be used practically in most of processing systems.