We report on the fabrication of filament damages in Yb-doped phosphate glass by femtosecond laser pulses. The uneven index modification was obtained using a 20× optical microscope objective with 0.4 numerical apertures. The multimode fields of original channel waveguides were investigated and in good agreement with the simulation results, whose structures can be regarded as a kind of beam splitters. Under the optical pump at 976 nm, the end region of the original filament shows continuous wave laser oscillation. The elongated and uniform filaments were fabricated by introducing a circular aperture, so that the guiding performance of tradition-channel single mode was achieved. Both large-mode-area waveguides are expected to find applications in optical communications and high-power lasers and amplifiers.
