A double-clad optical fiber includes a core, an inner cladding and an outer cladding of silica-based glass. The core may have a radius of less than about 5 .mu.m, a first index of refraction n.sub.1 and does not contain any active rare-earth dopants. The inner cladding may surround the core and includes a radial thickness of at least about 25 .mu.m, a numerical aperture of at least about 0.25, and a second index of refraction n.sub.2 such that n.sub.2