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Determining small refractive index contrast in chalcogenide-glass pairs at mid-infrared wavelengths

A two-composition thin film (Ge20Sb10Se70/Ge20Sb10Se67S3 atomic % core/cladding glasses) was fabricated using a hot-fibre-pressing technique in which both glasses follow the same post-fibre processing. A simple approach is proposed that uses normal incidence transmission spectra to determine their refractive index contrast over the wavelength range from 2 to 25 µm with an error of less than ± 0.002. Using an improved Swanepoel method, the calculated numerical aperture of these two compositions was within ± 0.011 of that obtained from prism minimum deviation measurements. Results show that introducing 3 atomic % S into the Ge-Sb-Se glass system lowered the refractive index and blue-shifted the visible optical bandgap, the far-infrared fundamental vibrational absorption bands and the zero-dispersion wavelength.