A signal conditioner to measure the length of an interferometric gap in a Fabry-Perot sensor (interferometer). The invention includes a light source, a Fabry-Perot interferometer capable of spanning a range of gaps in response to physical changes in the environment, a second interferometer that is placed in series with the Fabry-Perot interferometer which does not filter any particular wavelengths of light but acts as an optical cross-correlator, a detector for converting the correlated light signal into electronic signals, and an electronic processor which controls system elements and generates a signal indicative of the length of the gap spanned by the Fabry-Perot sensor.