Application of Fizeau fringes with bending technique to determine the shear modulus of the cladding material of single-mode optical fibers through the mechanically induced change of refractive index

In the present study, multiple-beam Fizeau fringes combined with the bending technique are used to determine the shear modulus of the cladding material of a single-mode optical fiber with accuracy of 1 x 10(-4) for the index measurements. The described method requires no precise polarizing optics, or complicated mechanical equipment, or variation of angle of incidence, or accurate light intensity comparisons. A mathematical expression describing the modulus in terms of the fringe shift that is due to the induced-bireftingence in the bent fiber has been derived. An asymmetric distribution of the shear modulus in the two cladding sides is seen experimentally. Applying the forward scattering technique confirmed that the asymmetry distribution in the modulus value due to asymmetric index profile could be attributed to a shift in the fiber centroid rather than a deviation in the circular fiber crosssection to a deformed elliptical crosssection under the effect of bending. The determined value of the shear modulus in the tensile side of the cladding is 3.4 +/- 0.2 x 10(4) N/mm(2) while its value in the compressed side is 4.3 +/- 0.2 x 10(4) N/mm(2). The modulus mean value, in the present study, is in good agreement with the literature. As far as the author is aware, it is first time that the radial profiles of the shear modulus of the cladding material of an optical fiber have been determined interferometrically. (C) 2002 Elsevier Science B.V. All rights reserved.