Multi-clad optical fiber with delocalization of pedestal modes

A cladding-pumped multi-clad optical fiber comprising, concentrically and radially outwards from a center axis of the optical fiber:

• a core configured to guide a light signal in a core mode, said core having a core diameter dcore smaller than about 30 μm, the core being doped with at least one rare-earth dopant material providing amplification of the light signal, the core having a refractive index ncore;
• a pedestal cladding structure supporting light propagation in a plurality of pedestal modes, the pedestal cladding structure having a pedestal cladding structure diameter dpcs, the pedestal cladding structure comprising:
• a pedestal layer contiguously surrounding the core and configured to confine the light signal in said core mode, the pedestal layer having a refractive index nped smaller than the refractive index of the core ncore;
• and
• a raised index layer contiguously surrounding the pedestal layer and having a thickness Δw, the raised index layer having a refractive index nril larger than the refractive index of the pedestal layer nped and smaller than the refractive index of the core ncore, the thickness Δw and the refractive index nril of the raised index layer preserving a confinement of the core mode in the core;
• an inner cladding having a refractive index nicl smaller than the refractive index nril of the raised index layer;
• and
• an outer cladding having a refractive index nocl smaller than the refractive index nicl of the inner cladding;
• wherein the pedestal cladding structure is a thin pedestal cladding structure characterized by a ratio of the pedestal cladding structure diameter dpcs to the core diameter dcore smaller than about 3, and the thickness Δw and the refractive index nril of the raised index layer minimize an overlap of one or more modes of the plurality of pedestal modes with the core to reduce a probability of amplification of the pedestal modes without requiring extraction of said pedestal modes.