Patent ID: 7064889

Claim:
A double-clad optical fiber comprising: a core doped with an optically active dopant, the core having a core refractive index; an inner cladding surrounding the core, the inner cladding having an inner cladding refractive index, the core refractive index being greater than the inner cladding refractive index, and the core supporting at least one core mode; an outer cladding surrounding the inner cladding, the outer cladding having an outer cladding refractive index, the inner cladding refractive index being greater than the outer cladding refractive index, and the inner cladding supporting at least one cladding mode; and at least one long-period fiber grating formed in the fiber by a substrate applied against the fiber to perturb the refractive index of at least one of the core and the inner cladding, the long-period fiber grating coupling light from the at least one cladding mode to the at least one core mode.