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). [ABSTRACT FROM AUTHOR]