A system and method includes a tunable light source and a gas cell configured to detect the presence of gases at terahertz frequencies. The light source is operable to emit a light signal at terahertz frequencies. The gas cell includes at least two high frequency mirrors adapted to reflect signals at terahertz frequencies. The gas cell is adapted to be tuned based on the frequency of the emitted light to obtain a Fabry-Perot resonance of the reflected light signal. A pair of detectors are operable to detect the original light signal and the light signal reflected through absorption paths in the gas cell. The system and method are operable to determine a presence and identity of a gas present in the gas cell.