Monolithic all-glass device combining pump coupling and end cap scheme for high-power fiber lasers

We report on a novel concept for monolithic pump combining technology to integrate efficiently multi pump fiber channel into a double clad ytterbium doped fiber. The proposed structure consists of a dichromatically coated planar convex lens spliced to an Ytterbium-doped double-clad photonic crystal fiber surrounded by multiple pump fibers. The lens is also used as a protecting end cap where the laser beam expands before exiting the surface. The pump fibers are also attached in this lens circularly surrounding fibers. The lens images these pump fibers end facets into the pump core, where the lens surface is coated by a dichroic mirror (reflective for 980 nm, transmissive for >1030 nm). The allglass structure, assembled by laser splicing, makes the system stable, efficient and suitable for high power operation. We selected 5 channels as testing channels among 14 pump channels (200 µm, NA=0.12) in order to confirm reliability of the system. The coupled pump power efficiency into the 500 µm core with NA=0.5 was over 80% and typical slope efficiency of the laser output is over 70%. Theoretical analysis was discussed in order to get optimized parameters and scaling this type of coupler to higher average powers is considered. With the monolithic pump combining technologies, we confirmed that the proposed device has a potential application not only in kW range high power fiber lasers but also compact photonic devices.