GAIN-EQUALIZED FEW-MODE FIBER AMPLIFIER

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PROBLEM TO BE SOLVED : To provide a few-mode rare-earth-doped amplifier fiber having equalized gain for a supported signal transmission mode. SOLUTION : A fiber according to the present invention has a raised-reflective index core surrounded by a lower-reflective index cladding region. The core has a radius a1 and refractive index difference Δn1 relative to the surrounding cladding region, and is configured to support, at selected signal wavelength, a set of lower-order fiber modes having an optical field with a diameter greater than 2*a1. The fiber further includes an active region, doped with a rare-earth dopant, comprising an inner portion that is coextensive with the core and an outer portion that surrounds the inner portion and extends into a cladding.The active region has an outer radius a2 greater than a1 and encompasses the optical field of the set of lower-order fiber modes at the selected signal wavelength. SELECTED DRAWING : Figure 1 COPYRIGHT : (C)2017, JPO&INPIT