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Oscillation and amplification have been observed in a fiber in which the core is passive glass and the cladding is neodymium-doped glass. As in conventional fiber optics, the refractive index of the core is greater than that of the cladding, so that light is carried in the core by total internal reflection. The results are interpreted as meaning that the reflection coefficient at the interface between passive core and laser cladding is greater than unity. Expressed in waveguide terminolgy, modes which are guided by the high index passive core, but have a substantial portion of their energy in the cladding, can be amplified by means of an active cladding.