Effective all-fiber pump recycler for kilowatt fiber lasers

We report an effective pump recycler for industrial kilowatt fiber lasers. The pump recycler is a (6+1)×1 tapered fiber bundle, with signal ports of Ge-doped fiber (GDF) with core/cladding diameters of 20/400 µm and pump fiber ports (PFPs) with core/cladding diameters of 135/155 µm. By splicing PFPs in pairs, 77.9% of the residual pump light reaching the pump recycler is sent back to the cladding of the GDF. The insertion of a pump recycler increases the power conversion efficiency (PCE) of a fiber laser using an Yb-doped fiber (YDF) from 61.0% to 70.5%, with a maximum output power of 2.78 kW. The laser with a 20 m long YDF and pump recycler compares well to another laser using a 40 m long YDF without pump recycler. In both cases, the PCE is comparable but the laser with a 20 m long YDF and pump recycler benefits from reduced stimulated Raman scattering (SRS), thus enabling an 80% increase in Raman threshold. By giving access to short YDF length, the tapered fiber bundle represents an effective pump recycler since it enables reducing SRS while keeping a large PCE.