A system and method of non-intrusively measuring characteristics of a fluid in a substrate are provided. The fluid is located in a fluid channel in a substrate, and an electromagnetic radiation source emits an electromagnetic beam toward the fluid and the substrate such that a first part of the beam reflects off a first surface of the substrate and a second part of the beam travels through the fluid and reflects off a second surface of the substrate. A sensor detects the resulting interference pattern produced by the two parts of the beam. Changes in fringes of the interference pattern, such as shifting positions or intensities, relates to a change in the refractive index of the fluid. Since a change in refractive index relates to the density and pressure of the fluid, the changes in interference patterns can be used to discern changes in characteristics in the fluid such as pressure.