Differential refractometry by an integrated-optical Young interferometer

Abstract

Highly sensitive refractive-index determination is used for process monitoring and quality control, e.g., in the food industry, as a detection principle for HPLC and for applications in research. Because of the large temperature dependence of the refractive index of liquids, sensitivities below 10 −5 are obtained only by differential refractometers. A new detection principle is based on the interference of two divergent rays, produced by an integrated optical splitter. These two rays pass the two flow cells and overlap on a CCD array, producing an interference pattern. The problem of ambiguity of the interference signature is overcome by using partially coherent sources, yielding a modulation of the fringe visibility. The position of fringes on the CCD array is evaluated by Fourier analysis of the intensity distribution. The spacial resolution is about a hundredth of a pixel width, resulting in a refractive-index resolution of 1×10 −7 at a measuring range of 5×10 −3. The level of signal noise (peak-to-peak) is 3×10 −8. Flow-cell volumes are in the range 15–140 μl. Tests have been performed with regard to process control and HPLC detection, showing the applicability of the system for these different areas.