Enhanced photoacoustic spectroscopy sensitivity through intra-cavity OPO excitation

We report an optical molecular gas sensor exhibiting high levels of selectivity and sensitivity. The outstanding sensitivity demonstrated by our technology is rooted in a novel combination of photoacoustic spectroscopy (PAS) operated within the cavity of a continuous-wave, intra-cavity Optical Parametric Oscillator (OPO). We exploit the very high circulating field present within the resonant down-converted cavity as the excitation source of the photoacoustic effect, conferring orders-of-magnitude improvement in optical excitation power. Additionally, the wide selectivity of the system arises from the inherent broad tunability and narrow optical linewidth of an OPO. Here we report the use of this technology for the detection of ammonia (NH3) as a simulant target molecule. A 3-D printed miniature PAS cell with microelectromechanical systems based (MEMS) microphone is used for the gas detection. The resonance frequency of the cell was measured at 17.9 kHz with a Q-factor of 9. The down-converted signal wave resonating within its optical cavity was tuned to 6605.6cm-1 (corresponding to a strong local NH3 absorption line) through a combination of phase matching and intra-cavity etalon control. The laser was amplitude modulated at the resonance frequency of the PAS cell, producing an average optical excitation power of ~10W in the signal arm of the OPO, to induce the photoacoustic effect for only 4W of primary diode pump power. In this work we show detection limit at the level of single parts-per-billion (ppb). Additionally, we will discuss how this technology could be readily refined to potentially demonstrate a sensitivity of tens parts-per-quadrillion.