The evanescent optical coupler is constituted by an IOX waveguide and an optical fiber. The IOX waveguide is formed in a glass substrate and has a tapered section that runs in an axial direction. The IOX waveguide supports a waveguide fundamental mode having an waveguide effective index NW0 that varies within a range ΔNW0 as a function of the axial direction. The IOX waveguide can also support a few higher-order modes. The optical fiber supports a fiber fundamental mode having a fiber effective index NF0 that falls within the waveguide effective index range ΔNW0 of the waveguide fundamental mode of the tapered section of the IOX waveguide. A portion of the optical fiber is interfaced with the tapered section of the IOX waveguide to define a coupling region over which evanescent optical coupling occurs between the optical fiber and the IOX waveguide.
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