M A Mabrouk

On the determination of the refractive index of a fibre. II. Graded index fibre

Two- and multiple-beam interference fringes are used to measure the refractive index of graded index fibres. A general expression considering the refraction of the beam by the graded index fibre has been derived, from which expressions for homogeneous and skin-core fibres can be obtained. Comparison between the results with the method used when neglecting refraction is dealt with using Philips, LDF (TM) optical fibres and drawn polypropylene. Considering the derived expressions in the refractive index determination gives a correction even when a matching liquid is used, this suggests that the refraction method should be used especially for graded index fibre.

Determination of GR-IN optical fibre parameters from transverse interferograms considering the refraction of the incident ray by the fibre

Abstract The core–cladding refractive index difference Δn and the index gradient profile parameter α of the graded index (GR-IN) optical fibre has been determined. The curve representing the real path, in the core region, due to refraction of the beam and the change in the exit wavefront are considered. Multiple-beam Fizeau fringes and two-beam interference Pluta polarizing microscope, are used for this investigation. A new method, based on a derived mathematical expression, is used with a prepared computer program to estimate the fringe shift inside GR-IN of the fibre core. The estimated and experimental values of the fringe shift, along the core radius, are used to obtain the refractive index profile of the optical fibre. A comparison between the new method and a previous model considering the refraction has been shown. Microinterferograms are given for illustrations.

On the determination of the refractive index of a fibre: I. Skin-core fibre

Two- and multiple-beam interferometric methods are used to study the refractive index profiles of cylindrical fibres having skin-core structure. The refraction of the incident beam inside the fibre is considered. Mathematical expressions are derived for this aim, and applied with two- and multiple-beam microinterferograms of cuperamonium, polypropylene, Roman nylon-6 (synthetic polymer fibres) and step index optical fibre. A comparison between the results and that in the case of non-distorted immersed wavefront is dealt with. The application of the derived expressions results shows a good correction of the results due to the mismatching effect. Microinterferograms are given for illustration.

Effect of gamma irradiation on the optical properties of fibres

Multiple-beam fringes of the Fizeau-type transmission and on reflection were utilised for the studies of the optical properties of gamma -irradiated Dralon fibres. Irradiation was carried out over a period of 24-1948 h. The dose rate was maintained at 14.7+or-0.2 rad s-1. Empirical formulae were suggested to represent the variation of refractive indices and birefringence with the dose. Variation of the mean refractive index with the wavelength of light has been studied, and the constants A and B of Cauchys dispersion formula were calculated. This multiple-beam interferometric method was also applied to determine the thermal coefficient of refractive index of the fibres. Micro-interferograms are given for illustration.

Interferometric measurement of some structural parameters of drawn polyethylene fibres

Multiple-beam interference Fizeau fringes are used to determine the optical orientation function and molecular structure of drawn polyethylene fibres. The optical orientation function across the fibre radius is obtained at a certain draw ratio, and the measurements of at different draw ratios can be used to give useful information about the fibre structure. Empirical formulae are suggested to correlate the birefringence with the refractive indices and with the draw ratio. Some parameters dependent on the fibre structures are also determined. Illustrations are made using microinterferograms.

A Multiple-beam Interferometric Method for Refractive Index Determination of Graded Optical Fibres

Abstract The mathematical expressions for the multiple beam Fizeau fringes were solved analytically. Using a computer program the refractive index profile of graded index optical fibres is given by two different methods; using the measured values of interference fringe shifts or the area enclosed under the interference fringe shift. These methods assume that the graded index fibre consists of very thin layers, each of constant refractive index. Both methods give the same refractive index profile. A discussion of the accuracy of these methods is given. The rate of change with temperature of the refractive index of a point at the middle of the fibre is also calculated, and microinterferograms of multiple beam Fizeau fringes are illustrated.

Interferometric studies on polymer structure

Abstract A multiple-beam interferometric method is used to study the change of optical orientation function and molecular structure of nylon-6 fibres due to γ-irradiation under vacuum. It was found that γ-radiation causes alignment to the fibre chains in the direction of the fibre axis, this alignment gives an increase in the optical orientation function. A two-beam interferometric method is used to study the changes in optical orientation function on drawn polypropylene fibres. Empirical formulae are suggested to correlate these changes in the optical orientation function with the dose and with the draw ratio. This study aims to show that the multiple-beam and two-beam interferometric methods can be used to study the changes that take place in polymers and fibres by irradiation or drawing.

Some parameters affecting the determination of the refractive index profile of optical fibres by interferometry

The authors use multiple-beam Fizeau fringes to determine the refractive index profile of graded index optical fibres. Some parameters affecting the determination of this refractive index profile are discussed. The dispersion of the material of the optical fibre has been studied, and the constants of Cauchys equation for fibre cladding are also determined.

Optical anisotropy in ion-beam irradiated fibres

Multiple-beam interferometric methods of the Fizeau-type in transmission and on reflection were utilised for the determination of the effect of ion-beam irradiation on the refractive indices and birefringence of Dralon fibres. Nitrogen gas was used to produce a continuous ion-beam current density of N2+ ions equal to 100 mu A cm-2 at a reduced pressure of 5.6*10-4 Torr. Fibre samples which are anisotropic were exposed to the ion beam for the time intervals: 1.2, 1.8, 2.4, 3.6 and 4.8*103 s. Empirical formulae are suggested for the measured variation of optical properties with time of exposure. The wavelength dependence of refractive indices of the irradiated fibres was studied interferometrically. Illustrations are given by microinterferograms.

Optical anisotropy in gamma-irradiated fibres under vacuum

Abstract The change of optical anisotropy in polymeric fibres by γ-irradiation is suggested to develop a simple method for γ-ray dosimetry. Multiple-beam Fizeau fringes in transmission and in reflection are used in this study for the determination of the refractive indices and birefringence of γ-irradiated Dralon fibres. Variation of the refractive indices of these fibres with the temperature and wavelength of light are also studied. Empirical formulae are suggested to correlate the optical properties of the irradiated fibres with the applied dose. Lorenz-Lorentz equation is applied to calculate the polarizability of the irradiated fibres per unit volume. Illustrations are given by multiple-beam microinterferograms in transmission and in reflection.