100 W All-Fiber Side-Pump Combiner on a Passive Large-Pitch Photonic Crystal Fiber

Pump combiners are essential components used to merge the power from pump diodes in high-power fiber laser oscillator and amplifier systems. A popular method for fabricating these pump combiners, which includes signal feedthrough, is the fused taper fiber bundle. This technique utilizes an end pump approach where the signal fibers are also tapered alongside the pump fibers [1]. However, with this type of combiner, pumping can only occur in one direction. Recently, it has been demonstrated that counter-pumping or bi-directional pumping mechanisms allow for increased power scaling and lower nonlinear interaction thresholds. These interactions include phenomena such as Stimulated Raman Scattering and, more critically, Transverse Mode Instability [2]. An alternative to the end pumping technique that facilitates both counter and bi-directional pumping is the side coupler technique. In this method, the fiber core remains uninterrupted, allowing for greater flexibility in positioning the pump fiber points compared to the end pumping approach. The most widely used fabrication technique for side pump combiners is the direct fusion method, which is favored for its suitability in high-power laser operations [3], [4]. This technique effectively combines the advantages of side coupling with the ability to scale power while maintaining beam quality. With this motivation, we aimed to fabricate a high-power side pump combiner using a homemade passive large-pitch photonic crystal fiber (LPF).