Four-wave mixing in non-step-index optical fibers: a sensitive technique for measurement of refractive-index profiles. 
Investigation of four-wave mixing 6.4-mum core-diameter fiber with 520-610-nm pump light is used to develop a new technique for characterizing the refractive-index profiles of non-step-index fibers.  Deviation from the step-index profile is measured.  Small variations in the refractive index between different segments of the same commercial fiber is measured with a sensitivity of 0.0001.