An optical system comprises an optical fiber with gain producing core with an index of refraction n.sub.1, surrounded by at least one cladding with an index of refraction n.sub.2, said cladding including at least one index reduced area with an index of refraction n.sub.2, such that n.sub.1>n.sub.2>n.sub.2, the core propagating a signal at a spatial fundamental mode at a signal wavelength .lamda..sub.1 and at a power level sufficient to generate optical power at a wavelength .lamda..sub.2, where .lamda..sub.2>.lamda..sub.1, and the at least one index reduced area in combination with the core provide has at least one cut-off fundamental spatial mode wavelength .lamda..sub.C, and .lamda..sub.1<.lamda..sub.C and .lamda..sub.2>.lamda..sub.C.