Mid-infrared swept cavity-enhanced photoacoustic spectroscopy using a quartz tuning fork

We report the development of swept cavity-enhanced photoacoustic spectroscopy using a quartz tuning fork for ultra-sensitive and high-resolution molecular spectroscopy. By using a quantum cascade laser (QCL) as the mid-infrared light source, a dual-feedback Pound-Drever-Hall locking method is proposed to lock the QCL frequency to a continuously swept optical cavity. By placing an off-beam quartz-enhanced photoacoustic spectroscopy module in a 48-mm Fabry-Perot cavity, we are able to achieve ultra-sensitive gas detection based on the doubly resonant photoacoustic effect. As a proof-of-concept, we use a distributed feedback QCL to exploit the CO line at 2190.02 cm(-1), where the cavity-locked QCL is scanned over a spectral range of 10 GHz with a spectral resolution of & SIM;3 MHz. With the incident laser power of 7.3 mW, the optical cavity (finesse 1931) builds up the intracavity power beyond 3 W. Our photoacoustic spectrometer achieves the minimum detection limit of 375 part-per-trillion (ppt) at the averaging time of 150 s and the normalized noise equivalent absorption coefficient of 1.27 x 10(-9) Wcm(-1) Hz(-1/2).