Measurement of effective refractive index differences in multimode optical fibers based on modal decomposition

We demonstrate the nondestructive measurement of the effective refractive index difference of two arbitrary modes within a multimode optical fiber by utilizing a tunable fber grating. We use a mechanical grating of variable period to couple the respective modes and measure the mode content at the fber output based on the correlation filter technique. From the dependence of the coupling effciency on the grating period, the effective index difference of the modes can be extracted with high accuracy.