Versatile photoacoustic spectrometer based on a mid-infrared pulsed optical parametric oscillator

We demonstrate the usefulness of a nanosecond-pulsed single-mode mid-infrared (MIR) optical parametric oscillator (OPO) for photoacoustic (PA) spectroscopic measurements. The maximum wavelength ranges for the signal and idler are 1.4 μm to 1.7 μm and 2.8 μm to 4.6 μm, respectively, with a MIR output power of up to 500 mW, making the OPO useful for different spectroscopic PA trace-gas measurements targeting the major market opportunity of environmental monitoring and breath gas analysis. We perform spectroscopic measurements of methane (CH4), nitrogen dioxide (NO2), and ammonia (NH3) in the 2.8 μm to 3.7 μm wavelength region. The measurements were conducted with a constant flow rate of 300 mL/min, thus demonstrating the suitability of the gas sensor for real-time trace-gas measurements. The acquired spectra are compared with data from the HITRAN database, and good agreement is found, demonstrating a resolution bandwidth of 1.5  cm1. An Allan deviation analysis shows that the detection limit for methane at optimum integration time for the PA sensor is 8 ppbV (nmol/mol) at 105 s of integration time, corresponding to a normalized noise equivalent absorption coefficient of 2.9×10−7  W cm−1  Hz−1/2.