1. A method for estimating a property of a formation fluid, comprising: exposing a tunable optical filter to a narrow light beam from a light source; transmitting a selected wavelength of light from the light source to the tunable optical filter; tuning the tunable optical filter having a first member and a second member that are substantially parallel and not directly in contact with one another, by modulating a distance between the first member and the second member to a wavelength that overlaps a wavelength for the narrow light beam to the selected wavelength of light passed by the filter; exposing the fluid to light output from the filter; measuring a light output interaction with the fluid; and estimating a property of the formation fluid from the measured interaction.