Interferometric determination of the principal refractive indices for light vibrating parallel and perpendicular to the fiber axis and the birefringence for Acrilan® have been carried out. Multiple-beam Fizeau fringes in transmission were used for fibers of irregular cross sections. A new method suitable for evaluating small birefringence in fibers was applied. Variation of refractive index with temperature was also determined. The fringes of equal chromatic order were applied for determining the birefringence of the fibers with diameters of 15-40 microns.

Multiple-beam Fizeau fringes revealed the existence of successive layers forming a graded-index fiber core.Both thickness and approximate refractive index graded from one layer to another have been estimated. The fiber is found to be formed from a succession of step-index layers, n(r), which remains constant over the thickness Deltar, yet follows the known function relating n(r) with r in terms of n(r = 0) and alpha. The index profile for graded-index fibers showing such microinterferograms.

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Multiple-beam Fizeau fringes in transmission have been applied to gamma -irradiated blue Cashmeline fibres, to determine their refractive indices for plane-polarised monochromatic light at gamma =5461 AA, vibrating parallel and perpendicular to the fibre axis, and the birefringence over the range from 5 Mrad up to 200 Mrad.

Multiple-beam Fizeau fringes in transmission have been used to measure the refractive indices of water, glycerol, dimethyl formamide and three mixtures of liquids at a wavelength of 546.1 nm and temperature T = 23 • C. A small drop of the liquid under study is introduced between the two silvered surfaces of a wedge interferometer illuminated by a parallel beam of light. The refractive index of this liquid is the quotient of the separation between two successive fringes in the air region to that in the liquid region. The relative error in measuring the liquid refractive index using this method ranges from 0.02 to 0.0006 depending on the type of reading instrument and interferometer wedge angle used.