  Fiber fuse effect can occur spontaneously and propagate along optical fibers
to cause wide-spread damage; it threatens all applications involving optical
fibers. This paper presents two results. First, it establishes that the
initiation of fiber fuse (IFF) in silica fibers is caused by defect-induced
absorption. Critical temperatures and critical optical powers for IFF are
simulated for the first time using a 3D solid-state heat transfer model with
heat source generated by defect-induced absorption. In this method, formation
energies of the defects can be uniquely determined, which offers critical
information on the chemical reasons for fiber fuse. Second, this paper offers a
method to evaluate operating temperatures of fiber lasers. General analytical
solutions of the operating temperatures along gain fibers are deduced. Results
of 976-nm laser-diode-(LD)-pumped and 1018-nm tandem-pumped ytterbium-doped
fiber (YDF) amplifiers using 10/130-{\mu}m YDFs are calculated. Potential
limits caused by fiber fuse are discussed.
