"This thesis introduce a highly sensitive optical technique to measure refractive indexes of liquid transparent or turbid samples, based on measuring the heights of the primary side-lobes of the normalized intensity profiles of a focused Gaussian beam with high spherical aberration, measuring the refractive index of the samples with high accuracy and repeatability. The sensitivity can be easily modified. We demonstrate analytically and experimentally that, since the technique relies on the diffracted properties of the Gaussian beam, the measurements are immune to noise. We exemplify the use of the technique by measuring glucose concentrations."