Effect of noncircular cross section on the forwardscattering pattern of side-illuminated unclad fibers

The geometric ray-trace theory has been modified to predict the intensity maxima and minima of the forwardscattering pattern for unclad fibers with noncircular cross sections. The cross section is represented by a Fourier series, and the effect of the presence of various foils (harmonics) is demonstrated. The results show that the magnitude of low-order foils, specifically the second, third, fourth, fifth, and seventh, can be determined by observing the pattern at ~12 degrees scattering angle and monitoring the interference fringe movement as the fiber is rotated axially. The effects of elliptical fibers on fiber diameter determinations from measurement of the scattering pattern are discussed.