Determination of the refractive index profile of fiber preform by lateral shearing interferometry

For determining the refractive index profiles of the fiber preforms several methods have been developed. These methods are divided into direct and indirect measurements of the refractive index profiles. In indirect methods the preforms are treated transversely and the index profiles are usually obtained by means of an Abel inversion. The indirect methods includes, transverse interferometry(1), holographic shearing interferometry(2) and ray deflection(3) These techniques suffer from some shortcoming. For example; they require large volume of computation, errors easily appear in calculation of Abel inversion, and mismatch occurs between the immersion oil and the preform. In the direct methods a slice of the preform along axial direction is treated. Atomic-force microscopy(4,5) and direct deflection method(6) are examples of direct methods. The direct methods are destructive. The atomic-force microscopy need specialized equipment and the direct deflection is a geometrical and scanning method. We introduce a direct method in which the lateral shearing interferometry (LSI) is exploited and the fringe patterns are analyzed by the phase step method. The report is organized as follows. In section 2 the theoretical aspects of the work are discussed and the required relationships are established. The experimental procedure and results are included in section 3.