Assessment Of The Optical Performances Of Radial Gradient-Index Lenses

Abstract

An experimental study was carried out to evaluate the optical performances of radial GRIN lenses. The GRIN materials were fabricated by the CGE Research Center through the vapour deposition process, which generally involves index profile perturbations. We purposed to test the feasability of making lenses from such materials. Specific evaluation means were developped for the lens characterization. The wavefront was obtained from phase interferometry, also leading to the refractive index profile. A set-up was realized for the assessment of chromatic variations of the index maximum slope. Dependence of birefringence as a function of index variation was investigated. Automated measurements of line spread functions and modulation transfer functions were also implemented. We particularly investigated, both experimentally and through computer raytracing, the effect of macroscopic and microscopic perturbations of the index profile on imaging. We showed up the quite different influences of low frequency and high frequency fluctuations. The ray trapping predicted in a previous paper was observed as a depression of the on-axis illumination. We evidenced the correlation between the amplitude of the high frequency fluctuations and the observed drop of illumination. These fluctuations could be cut down to a negligible level. In return, the low frequency fluctuations, due to a want of precision in the fabrication process, could not be easily eliminated, leading to a drop of the MTF.