The Centering of Optical Fiber Coatings by Monitoring Forward Scattering Patterns - Theory and Practice

The goal of placing an axially symmetric coating around an optical fiber has been experimentally approached through various techniques1-4 and theoretically analyzed with various models.4"7 There is ample reason for this interest. Good centering has been shown to increase fiber strength.8 It also eases the fabrication of connector arrays for splicing and provides maximum abrasion protection for a given coating. Furthermore, an asymmetric coating may add to fiber loss.29 For a coating that is fairly transparent and has a refractive index more than that of the fiber cladding, a light beam incident essentially transverse to the optic axis has been found to have a notably charac313 teristic forward scattering pattern. Its shape and position are easily monitored and provide unambiguous information about both coating thickness and eccentricity for any degree of eccentricity. The backscatter pattern has been previously analyzed and used as an inline centering monitor.3,7 In this paper, we describe the pattern, recount our experiences with it when used as a coating monitor, and present a ray trace model of the forward scattering. We use this model, together with a more limited closed form expression, to obtain a better understanding of the sensitivity and operating range of this monitoring technique. The pertinent parameters are coating refractive index, coating thickness, and eccentricity.