We study inteal features of fiber fuse in a Yb-doped double-dad fiber.The samples of fiber fuse are acquired at the power level of 3 kW in an all-fiber forward-pumped master oscillator power amplifier configuration fiber laser that is built specially for fiber fuse analyses.At this high power level,drastic refractive-index redistribution arises in an expended high refractive index area around the bullet-shaped voids of fiber fuse.Electron spin resonance analyses on post-fiber-fuse samples of the Yb-doped double-clad fiber indicate rising Frenkel defect concentration,meanwhile showing a new resonance center that is different from the ones of the Ge-doped fibers studied previously.This new resonance center probably suggests the generation of Al-oxygen hole center,a kind of defect formed during the catastrophic fuse process.