CAN THE CURVATURE OF AN OPTICAL-GLASS FIBER BE DIFFERENT FROM THE CURVATURE OF ITS COATING

The study is aimed at the evaluation of the relationship between the m easured (or imposed) constant curvature of the coating of an optical g lass fiber and the elastic curve of the fiber itself. It is shown that the buffering effect of the coating is different for different points along the curved area and depends on the length of this area, and the compliance of the coating. In the case of a very short curved area an d/or a very compliant coating, the curvature of the glass fiber is sma ller than the curvature of the coating and increases with an increase in the length of the curved area and the coating stiffness. In the cas e of a long curved area and/or a stiff coating, both curvatures are pr actically the same for almost the entire curved area. Only when approa ching the ends of this area, the ratio of the curvature of the glass f iber to the coating curvature somewhat increases (by a factor of 1.043 ) and then rapidly drops to unity at the ends. There are, however, som e ''intermediate'' unfavorable combinations of the lengths of the curv ed area and coating compliances that result in curvature ratios exceed ing (by up to a factor of 1.086) the coating curvature in the midporti on of the curved area. It is shown that such a paradoxical situation i s due to the redistribution of the interfacial radial load at certain combinations of the lengths of the curved areas and spring constant of the coating. For a current AT&T dual-coated fiber design with a 30 mu m thick silicone primary coating, the curvature ratio is greater than unity when the lengths of the curved area fall within the range betwee n 1.84 and 4.27 mm, and reaches the 1.086 value when the length of cur ved area is about 2.44 mm.