Absolute Refractive Index Determination by Microinterferometric Backscatter Detection

Microinterferometric backscatter detection (MIBD) has previously been shown capable of measuring changes in the refractive index of liquids on the order of 10-7. The MIBD technique is based on interference of laser light after it is reflected from different regions in a capillary. These reflections generate an interference pattern that moves upon changing refractive index of the liquid in the capillary. The small-angle interference pattern traditionally considered has a repetition frequency in the refractive index space that limits the ability to measure refractive index-to-refractive index changes causing such a repetition. Such refractive index changes are typically on the order of three decades. Recent modeling and experiments with the MIBD technique have shown that other intensity variations in the pattern are present for larger backscattered angles. By considering these variations, we have shown two methods by which it is possible to extend the dynamic measurement range to make an absolute refractive index measurement. One method utilizes variations in the Fresnel coefficients while the second approach is based on the refractive index-dependent onset of total internal reflection angles. With the second approach, we have been able to measure the absolute refractive index of a liquid with a precision of 2.5 × 10-4.