A modified focusing method for measuring simultaneously the refractive-index profile and the cross-sectional geometry of an optical fiber preform is presented. In this method the preform is immersed in indexmatching liquid, and collimated light is made to impinge on the preform laterally. The intensity distributions of the transmitted light are detected on two planes inside the preform core. From the recorded light intensity distributions, the deflection function is calculated. After the preform has been rotated through 180 degrees and the necessary data have been collected, a numerical inversion is performed to obtain the 2-D index profile. In this paper, the principle, the experimental setup, and results are described. The sensitivity of the method to measurement errors and the accuracy of the inversion formula were investigated by computer simulations. It was found that the error of the index profile measurement was <1%.

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A new nondestructive technique is presented for determining the refractive index profile of an optical fiber from its backscattered pattern arising from a normally incident laser beam to the fiber axis. The proposed method requires no sample preparation or index matching liquid. The principle of the method is to construct a deflection function from the measured pattern. The index profile can then be determined by the inversion of an Abel integral equation. Good agreement is obtained between the index profile determined by this technique and that measured by the near-field scanning technique.

The backscattered pattern of an optical fiber with arbitrary refractive-index profile is used to determine the fiber diameter. Two methods are presented: the fiber diameter can be determined either by noting the position of a particular fringe maximum or by counting the total number of fringes in the pattern. This method is fast in that it does not require any mechanical moving part, and the measurement error is 1%. This method is based on the prior knowledge of the refractive-index profile of the fiber, but it is shown that the error incurred by assuming the wrong profile is less than 1%. It is also shown in this paper that the method can be used to monitor the diameter variation while the fiber is being pulled. This is done by tracking the displacement of a particular fringe. It is shown that this method is also applicable to fibers with elliptical cross section.

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