Computerized refractive index measurements for bulk materials at UV, visible, and IR wavelengths

A computer-controlled apparatus was built to measure refractive index and its temperture coefficient to fifth decimal place accuracy at UV, visible, and IR wavelengths. Deviation of monochromatic radiation through a prism having a reflecting rear surface is measured by the autocollimation method as in a Littrow prism spectrometer. Entrance and exit rays at the prism are superimposed, so that a fixed entrance/exit port permits convenient temperature-controlled enclosures for measurement of dn/dT. Angle measurments involve only rotation of the prism, making use of a 20-bit digital angle sensor suitable for computer readout. A geared stepping-motor drive permits computer control of data acquisition and processing. Useful wavelength coverage has so far included the range from 0.25 to 8 μm, but is limited only by source intensity, detector senstivity, and sample prism transparency. Index results for four germano–silicate glasses at 22 wavelengths between 0.36 and 1.69μm are reported and values of dn/dT for pure SiO2 to 373 K (100̊ C) are given. Sellmeier coefficients from fitting a three-term formula are listed, and determination of material dispersion values from the Sellmeier coefficients is discussed in terms of their use in optical fiber modeling calculations.