Determining Index Data from Refracted/Diffracted Rays

Abstract

An optical fiber is a cylindrical waveguide of visible (or near visible) light composed of silica doped with germanium oxide (Ge02). The guiding is accomplished by varying the level of Ge02 in the fiber to create an index of refraction in the fiber that varies with the radius of the fiber. The fiber is manufactured by creating a large cane with a radius on the order of centimeters that goes through a sequence of heatings and extrusions until it reaches the finished size, which has a radius on the order of microns.

To assess the quality of optical fibers during their manufacture, it is common to measure the index of refraction of a cane during an intermediate step of the process. The index of refraction varies with the radius of the cane, and is written n(r). The desired profile varies depending on the future use of the optical fiber, but a standard profile is a simple parabola.

The actual profile in an optical fiber does not match the desired profile due to the way in which optical fibers are manufactured. A glass blank is spun on a lathe while a flame that is fed an appropriate level of silica and Ge02 moves rapidly back and forth along the cane. Soot from the flame is deposited on the spinning blank. Naturally the deposition will create spiral patterns of doping on the cane. This creates oscillations in the level of Ge02, and therefore in the desired refractive index. Because soot is being deposited at a constant volumetric rate, the wavelength of the oscillation decreases as the radius of the cane increases. The flame travels up and back along the cane in each layer, so the layer structure has two local maxima in each full oscillation.

Because the oscillatory behavior of n(r) is unimportant in the final product, Corning asked to determine a way to remove the noise in the measurements of n(r) caused by the oscillations, and determine the background profile.