Optical fibers and optical fiber coupler using the same erbium doped optical fiber amplifier, an optical waveguide

Provided at the center, a core made of silica-based glass containing at least germanium, an inner cladding large diffusion coefficient provided around the core in the core and concentrically, diffusion coefficient provided around the inner cladding smaller outer in the optical fiber having a clad, a core, by containing germanium 200% of more than the concentration of the inner cladding, even for MFD different optical fiber, such as single-mode optical fiber or an erbium-doped optical fiber, it can be connected in and with sufficient connection strength low connection loss.