Counter-directional pump/signal combiner based on a triple-cladding fiber and its application in a tandem-pumped fiber amplifier

Triple-cladding fibers (TCFs) facilitate high-power handling, high absorption coefficient of pump light, and ease in thermal management. However, passive components based on double-cladding fibers (DCFs), particularly pump/signal combiners (PSCs), constrain the full potential of TCFs when integrated into triple-cladding fiber lasers (TCFLs). Here, we report the first fabrication of a (6 + 1) × 1 counter-directional PSC using a TCF matched to the gain fiber, achieving a 99% pump coupling efficiency under laser diode pumping and a 98% signal light transmission efficiency. When each (a TCF-based PSC and a DCF-based PSC) is incorporated into a 6-kW counter-directional tandem-pumped fiber amplifier, the TCF-based PSC significantly outperforms its DCF-based counterpart. The TCF-based PSC achieves a remarkable 40% reduction in temperature rise of the output fiber and a 3.8% increase in slope efficiency (from 79.8% to 83.6%), attributed to the efficient coupling of pump light into the TCF's inner cladding. Additionally, the combiner enhances the M² factor by 15%, optimizing from 2.89 to 2.45, owing to the excellent compatibility between the delivery TCF and the gain fiber. These findings highlight the superiority of special-designed combiners in high-power TCFLs.