Effects of ion clustering and excited state absorption on the performance of Ho-doped fiber lasers.

The effects of ion clustering and excited state absorption occurring in holmium-doped fiber lasers are investigated experimentally and theoretically. It is found that the slope efficiencies of holmium-doped fiber lasers are reduced by inhomogeneous upconversion associated with the clustering of Ho ions. Via theoretical analysis based upon Judd-Ofelt theory, it is also found that the effect of excited state absorption on the performance of Ho-doped fiber lasers is negligible, a fact indicating that ion clustering is the dominant cause of the lower-than-expected slope efficiencies observed in holmium-doped fiber lasers. We argue that ion clustering is an intrinsic flaw of holmium-doped fibers and is difficult to eliminate, because our research efforts are based on commercially available low-concentration fiber, which is fabricated with state-of-the-art techniques.