Parts per trillion sensitivity for ethane in air with an optical parametric oscillator cavity leak-out spectrometer

Spectroscopic detection of ethane in the $3-\mathit{\mu }\mathrm{m}$ wavelength region was performed by means of a cw optical parametric oscillator and cavity leak-out. We achieved a minimum detectable absorption coefficient of $1.6\times {10}^{-10} {\mathrm{cm}}^{-1}/\sqrt{\mathrm{Hz}}$, corresponding to an ethane detection limit of 6 parts per trillion $/\sqrt{\mathrm{Hz}}$. For 3-min integration time the detection limit was 0.5 parts per trillion. The levels are to our knowledge the best demonstrated so far. These frequency-tuning capabilities facilitated multigas analysis with simultaneous monitoring of ethane, methane, and water vapor in human breath.