Trace detection of C2H4 by photoacoustic spectroscopy with a compact pulsed optical parametric oscillator

A periodically poled lithium niobate (PPLN) optical parametric oscillator using a simple grazing-incidence grating cavity configuration (GIOPO) for broad and fast wavelength tuning has been developed for the mid-infrared region. The GIOPO is capable of producing ∼100 mW of infrared power with a bandwidth of 0.2 cm^-1 at 3000 nm. Since pressure-broadened linewidths are in the range of a few tenths of a wavenumber, the GIOPO output bandwidth is suitable for exciting single well-separated rotational lines of many gases under typical atmospheric conditions. We have used the highly sensitive technique of photoacoustic detection coupled with this compact all solid-state GIOPO developed in our laboratory to achieve ppb level detection and analysis of gaseous molecules. Result shows a photoacoustic spectrum taken with 1 ppmV of C₂H₄ in comparison with a FTIR spectrum. The PA spectrum has a resolution of 0.2 cm^-1, clearly distinguishing the rotational structure of the molecule. A plot of the PA signal as a function of C₂H₄ gas concentration is also obtained. A linear relationship between the C₂H₄ concentration and the PA signal is established over at least two decades of gas concentration. A minimum sensitivity of 10 parts in 109 by volume (10 ppbV) C₂H₄ in nitrogen was obtained in the direct measurement. The current limit on the detection sensitivity is determined by the background, noise and the accuracy is mainly determined by calibration errors involved in the gas mixing procedure. In order to improve on the sensitivity, a multipass cell arrangement is being designed.