Capillary-scale refractive index detection by interferometric backscatter

We describe a novel approach for measuring fluid bulk properties such as refractive index (RI) and temperature changes in tubes of capillary dimensions based on a simple optical configuration which uses an unfocused He-Ne laser, a cylindrical tube, and a photodetector, Side illumination of a fused silica capillary produces a fan radiation in a 360 degrees plane normal to the central axis of the capillary that is spatially well defined in the lateral direction, It is shown that, upon viewing a high-contrast interference pattern contained in the beam profile on a hat imaging plane in a direct backscattering configuration, the physical characteristics of the capillary and the bulk properties of the fluid contained within the tube can be determined, Position changes in the maxima and minima in the intensity-modulated profile (interference fringes) are directly related to the refractive index of the fluid in the tube, Measurement of the fringe shift by a slit-photodetector assembly facilitates the determination of dn/n at the level of 1.8 x 10(-7), while temperature changes can be measured using the fringe centroid position at a level of 5.8 x 10(-5) degrees C (DL = 3 sigma), Each determination can be made within a probe volume of 2.6 nL. Also, a wide range of tube diameters can be employed (75-775 mu m i.d,) with no modification to the optical train or measurable degradation in system performance. We report results for an optical ray trace model for predicting fringe pattern energy distributions, optimization of the optical configuration, and general system performance predictions, Comparison of our four-beam interference model with experimental results illustrates the utility of the model.