Comparative study on determining the refractive index profile of polypropylene fibres using fast Fourier transform and phase-shifting interferometry

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The refractive index profile measurement is desirable for fabricating and characterizing fibres. The fast Fourier transform method (FFT) and phase-shifting interferometry (PSI) were used to determine the refractive index profiles of polypropylene (PP) fibres. Interference fringe patterns produced by Mach–Zehnder interferometer have been analysed using the suggested methods. Wrapped phase maps and contour lines of the obtained fringes were produced. The methods used are relatively simple, accurate and are capable of conducting fully automatic measurement of refractive index profiles for fibres. By comparing the obtained refractive index profiles using the two methods, it is found that there is good agreement between them and also with the literature.