A system includes first and second gas cells each comprising a respective sealed interior waveguide; first transmit antenna coupled to the first gas cell to provide a first electromagnetic wave to the first gas cell along a first direction; second transmit antenna coupled to the first gas cell to provide a second electromagnetic wave to the first gas cell along a second direction opposite the first direction; third transmit antenna coupled to the second gas cell to provide a third electromagnetic wave to the second gas cell; first receive antenna coupled to the first gas cell to generate a first signal indicative of an amount of energy in first electromagnetic wave; second receive antenna coupled to the second gas cell to generate a second signal indicative of an amount of energy in second electromagnetic wave; and processor to calculate a background-free signal based on a difference between first and second signals.