Refractive-index profile measurement of preform rods by a transverse differential interferogram

A nondestructive refractive-index profile measurement of optical-fiber preform rods by means of a transverse-differential-interference method is given. The error resulting from the data reading of a fringe center is investigated by computer simulation and is confirmed to be less than about 1% of the index difference between the core and the cladding. The refractive-index profile of a preform rod was measured, and it almost coincided with that of the fiber drawn from the rod. The elliptic deformation of the core and its variation along the longitudinal axis were measured.