Non-invasive and in situ measurement of a refractive index gradient profile of one-dimensional GRIN materials

Abstract

Refractive index (RI) distribution plays a crucial role in the propagation of light trough any transparent medium except a vacuum. Gradient-index (GRIN) optical components made from heterogeneous materials with the continuously varying RI are finding an increasing role in optical communications, imaging, biomedical and industrial sensing. A diverse majority of existing methods for RI measurements are intended to perform point-wise RI measurements of homogeneous media. Withal only a few methods are devoted to measure RI gradient profiles (RIGP) of a GRIN medium. In this work we present a design of a noninvasive method aimed at the measurement of the RIGP of GRIN materials with the onedimensional RI variations. The physical principle of the proposed technique is based on the relaying a line structured laser beam displacements, which occur due to the difference of the refraction across medium, to the corresponding RIGP. Two of the main advantages of the proposed method are that the RIGP is rendered from a single measurement and without knowing of any RI value of a sample under the test, respectively. We show that the precision of up to 10 -4 in RIGP measurement can be achieved due to the ability of localizing laser stripe displacements with the subpixel accuracy, as well as using of a machine vision based triangulation system and corresponding image processing algorithms. The simplicity, robustness and measurement speed (indeed, real time) of the proposed method confirm that it can be utilized for real-time quality control of GRIN materials during the manufacturing process.