Fusion splicing of highly rare-earth-doped optical fibers

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A method and arrangement for achieving low splice-losses when connecting Highly Rare-Earth-Doped (HRED) optical fibers and dissimilar optical fibers having a large Mode Field Diameter (MFD) mismatch. Warm images are taken during a pre-fusion process to capture thermal light emissions and determine an arc-center position. The end-surfaces of the fibers are abutted and longitudinally offset from the arc-center, based on the light propagation direction and the MFD-mismatch. The fibers are then asymmetrically heated with different fusion temperatures during the main fusion processes. An MFD-match is achieved with well-defined fusion currents and fusion time. To maintain the same offset distance in a sequence of splices, the main-fusion arc-center position is determined by a process of direct arc-recentering. Variations of fusion temperature caused by changes of electrode conditions and the operating environment are monitored in real-time, and an arc-check process is invoked to compensate for the large variations of fusion temperature.