Optical parametric oscillator-based photoacoustic detection of CO2 at 4.23 μm allows real-time monitoring of the respiration of small insects

A continuous wave, single frequency and continuously tunable optical parametric oscillator is used in combination with photoacoustic spectroscopy to detect trace emissions of CO2 from insects under atmospheric conditions. The optical parametric oscillator (OPO) contains a periodically poled lithium niobate crystal and is tunable over the 3.9 to 4.8 μm infrared wavelength region. With the strong rotational-vibrational absorption band of CO2 at 4.23 μm, it is possible to detect CO2 down to 7 parts per billion volume using 20 mW of the OPO beam. This detection sensitivity was achieved by adding 4% of SF6 gas to the atmospheric gas mixture to overcome the slow vibrational relaxation of the excited CO2 levels. The usefulness of this system is demonstrated by real-time measuring of the fluctuations of the CO2 concentration in the breath of a single ant (Lasius niger) and individual fruit flies (Drosophila melanogaster).