Measurement of refractive index and dispersion in axial gradient material using prism refractometry

Abstract

Axial gradient material is now fabricated via controlled diffusion of glass plates for a wide variety of optical applications. The variation in both refractive index and dispersion, through this new optical material, challenges current refractive index measurement techniques. To provide accurate characterization of refractive index, as a function of thickness, at multiple wavelengths, a prism refractometer was constructed. Multiple laser lines are spatially filtered and passed through a right-angle prism of gradient material. A movable exit slit selects the region under test along the angled back face of the prism. The selected slit of radiation is then retro-reflected back through the prism and the refracted angle is measured directly. By scanning the exit slit along the prism back face, the refractive index variation through the prism is measured at multiple wavelengths. This paper describes, in detail, the test method summarized above and reports on the precision and accuracy of this laboratory test.