Optical sensor apparatus to detect light based on the refractive index of a sample

An optical sensor apparatus, comprising:

• an optically transmissive light guiding structure having planar first, second, and third faces;
• two or more light sources located outside the light guiding structure adjacent the first face;
• a photodetector array located outside the light guiding structure adjacent the first face, wherein the light guiding structure, light sources, and photodetector array are configured such that light from the two or more light sources that is totally internally reflected at an optical interface between the light guiding structure and a sample outside the light guiding structure proximate the second face is reflected at the third face and incident on a portion of the photodetector array that depends on a refractive index of the sample,
• wherein the two or more light sources are positioned with respect to the light guiding structure and photodetector array such that light from each of the two or more light sources that is totally internally reflected at the interface and reflected at the third face corresponds to a different range of refractive index of the sample and maps to a corresponding portion of the photodetector array.