Patent ID: 8000576

Claim:
A double-clad optical fiber comprising a core, an inner cladding and an outer cladding wherein: the core comprises a radius of less than about 5 μm, a first index of refraction n 1 and does not contain any active rare-earth dopants; the inner cladding surrounds the core and comprises a radial thickness of at least 25 μm, a numerical aperture of at least about 0.25, and a second index of refraction n 2 such that n 2 <n 1 , wherein a relative refractive index percent (Δ%) of the core relative to the inner cladding is greater than about 0.1%; the outer cladding surrounds the inner cladding and comprises a radial thickness from about 10 μm to about 50 μm and a third index of refraction n 3 such that n 3 <n 2 , wherein a relative refractive index percent (Δ%) of the inner cladding relative to the outer cladding is greater than about 1.5%; and the core, the inner cladding and the outer cladding are formed from silica-based glass, wherein said fiber has attenuation of less than 1 dB/km at the wavelength range of 1060 to 1700 nm.