A sensing system for monitoring a composition of a downhole fluid in a well, where the sensing system includes: a light source, an optical waveguide, an evanescent field sensing element that is indirect contact with a downhole fluid, and a detector. The light source is operable for emitting a beam and includes a frequency comb generator configured to modify at least a portion of the beam into a sensing comb beam. The evanescent field sensing element provides attenuated internal reflection of the sensing comb beam at the interface between the evanescent field sensing element and the downhole fluid, and the portion of the sensing comb beam interacts with the fluid to form at least a portion of an interacted beam. The detector obtains a spectral distribution of the interacted beam.