An
optical fiber comprises a singlemode core, and an inner cladding disposed around said core, an intermediate cladding disposed around said inner cladding and an outer cladding disposed around said intermediate cladding thereby forming a triple-clad geometry. The core is doped with rare-earth elements and represents the
waveguide section of higher
refractive index through which the
signal is transmitted and amplified. A
fiber coating is provided around said outer cladding. The core has a higher
refractive index than said inner cladding, said inner cladding having a higher
refractive index than said intermediate cladding, and said intermediate cladding having a higher refractive index than said outer cladding. The inner cladding has a fairly high refractive index to limit the
numerical aperture (NA) of said core. The inner cladding is within said intermediate, larger, cladding which is made of glass, that is used to guide multimode, high-power pump, said inner cladding acting as an
intermediate level between said core and said multi-mode pump guiding intermediate cladding, with said outer cladding being present in said
optical fiber for allowing said intermediate cladding to guide the pump power properly. The outer cladding is made of glass or of a low-index
polymer material. The inner cladding is adapted to trap some of the pump
signal into a smaller section in order to accelerate the absorption of the pump power.