Fabrication of a Counter-Directional Pump/Signal Combiner with Built-In Mode Field Adapter

Abstract

This article introduces a novel counter-directional pump/signal combiner with a built-in mode field adapter. This combiner offers additional functionality to address the problem of mode field mismatch between the gain fiber and delivery fiber, which occurs when using a large-core (50 μm) signal fiber to suppress stimulated Raman scattering. The combiner exhibits negligible degradation in beam quality as a result of implementing mode field matching and employing an effective feedback alignment technique. The beam quality of the mode field adapter integrated in the combiner is improved by 10%, compared to the conventional approach, thanks to the reduction in splicing points and the prevention of fusion loss caused by tapering-induced cladding diameter mismatch. The combiner demonstrates an average pump coupling efficiency of 98.5% and a temperature rise coefficient of less than 10 °C/kW without active cooling. Furthermore, an integrated device based on the combiner is fabricated to shorten the length of delivery fiber, therefore mitigating the effects of stimulated Raman scattering. All those techniques mentioned above are utilized in a narrow linewidth laser system, resulting in an output of approximately 5840 W with a nearly single-mode characteristic.