Microdeformation losses of single-mode fibers

Abstract

In this paper, we consider the scattering losses of single-mode fibers that are caused by microdeformations such as microbends of the fiber axis and random fluctuations of the fiber core diameter. Since very little is known about the statistics of microdeformations of actual fibers, we assume that the autocorrelation functions of random bends and random core diameter fluctuations are Gaussian, characterized by the rms deviation and the correlation length of the random function. We consider single-mode fibers with step, parabolic, and triangular (linear) refractive-index profiles and reach the following conclusions: (1) Whereas for equal (large) mode radii the microbending losses of all three fiber types are the same, losses due to random core diameter fluctuations can be three times as high in step-index fibers as in triangular-index fibers. Since triangular-index fibers have sometimes been observed to have lower scattering losses than step-index fibers, one might conclude that, in these cases, excess losses may be caused by random radius fluctuations rather than by microbends. (2) Radial refractive-index ripples, which tend to be present in the deposited claddings of single-mode fibers, seem unlikely to be a major source of microdeformation losses. (3) The wavelength dependence of microdeformation losses depends strongly on the value of the correlation length of the Gaussian autocorrelation function of the fiber deformations. If the correlation length is of the same order of magnitude as the fiber radius, the losses are only slightly wavelength dependent. For very long correlation lengths the losses are very much smaller (for the same rms variation of the random functions), but they become strongly wavelength dependent, increasing sharply with increasing wavelength.