Performance of 915 nm Pumped LMA Yb Fiber Designs for Long-term Reliable Multi-kilowatt Operation

We report the performance of new LMA Yb fibers with increased cladding absorption for pumping in the 915 nm absorption band. A 0.5 dB/m cladding-absorption Yb20/400 fiber showed negligible photodarkening loss in 400-hour laser operation at 3 kW, with 77% optical-to-optical efficiency. Low-SRS and TMI-free operation at 3.5 kW signal power was achieved with a 0.65 dB/m cladding-absorption and 20.2 µm mode-field diameter Yb fiber, tested in a co-pumped amplifier. The Raman peak was 31 dB below the signal peak at the maximum power.