Focusing method for nondestructive measurement of optical fiber index profiles

A new method to determine the refractive-index distributions of the cores of optical fibers is presented. The fiber is immersed in index matching oil to eliminate light refraction at the outer core boundary. Collimated light is passed transversely through the fiber core and is detected with a microscope focused just above the plane of the core boundary. The intensity distribution in the image plane is recorded with the aid of a computer-controlled video-analysis system. The refractive-index profile of the fiber core is then computed from this intensity distribution. In this paper we present error estimates and show how well the focusing method agrees with the interferometric slab method. The repeatability of the index measurement is approximately 2%, an error which is easily attributable to incident light level fluctuations.