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Cw-OPO-based photoacoustic spectrometer for highly sensitive detection of ethane and other volatile organic compounds

We present an all-solid-state, transportable photoacoustic spectrometer operated with a continuous-wave optical parametric oscillator. The PPLN-based OPO has a dual cavity configuration (pump-resonant singly-resonant) in order to combine low threshold with good tuning characteristics. A complete spectral coverage between 3.1 and 3.9 μm with a single-frequency output power of 2 x 100 mW is achieved. At 30 seconds lock-in time constant the noise level of the background corresponds to a minimum absorption coefficient of 7.2 x 10-10 cm-1, yielding an ethane detection limit of 25 ppt. The OPO based photoacoustic spectrometer including the wavemeter is installed on a 120 cm x 75 cm breadboard. The ethane concentration of ambient air is determined by a multigas analysis. Additionally an online-measurement of biogenic ethane emissions by a freeze-stressed lima bean leaf is presented. The results indicate a high potential of this transportable spectrometer for biological and medical applications.