Optical detection of molecular species at sub-ppt concentration levels

Optical detection of molecular species at sub-ppt concentration levels,10.1109/CLEOE.2011.5943069,D. Mazzotti,S. Bartalini,S. Borri,P. Cancio,I. Galli

Optical detection of molecular species at sub-ppt concentration levels  
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The fundamental (and strongest) ro-vibrational transitions of atmospheric molecules lie in the mid-IR region of the spectrum (the so-called “molecular fingerprint” region). Since trace-gas detection sensitivity scales inversely with line-strength, mid-IR is the best suited spectral region to perform high-sensitivity molecular spectroscopy. With this motivation, we have designed and realized a difference-frequency-generated coherent source, with 3850 4540 nm tuning range and TEM00 spatial mode [1]. Phase locking of the near-IR “pump” laser to the “signal” one involved in the non-linear mixing process, through a Ti:sapphire-based optical frequency comb, also gives the generated “idler” radiation narrow linewidth and Cs-standard traceability [2]. The relatively high power of this source (30 mW at 4510 nm), greatly enhanced by a high-finesse Fabry-Perot cavity (F > 11 000), enables saturation spectroscopy at pressures even higher than 10 mbar. By combining this source with our recently developed saturated-absorption cavity ring-down (SCAR) spectroscopic technique [3], we expect to achieve optical detection of rare molecular isotopologues with sub-ppt concentration. Indeed, the present spectrometer aims to detect partial pressures of the absorbing gas in the femtobar range per unitary noise bandwidth. More advantages of this setup are sub-Doppler resolution (at lower pressures), absolute frequency scale and absolute concentration measurement, with no need for calibrations. The setup may also be further simplified by replacing the DFG source with a quantum cascade laser (QCL), whose intrinsic linewidth was recently demonstrated to be much narrower than in conventional semiconductor lasers [4]. Nowadays, these compact, current driven mid-IR sources are commercially available with cw room-temperature operation. To achieve the frequency stability needed to be efficiently coupled to the high-finesse optical cavity employed in SCAR, a QCL can be easily frequency locked either to the cavity itself or a narrow molecular sub-Doppler line [5], by a current feedback loop. Trace gas detection of molecular species can be crucial to get knowledge about physical, chemical, and biological processes. Many possible applications of such a compact and sensitive spectrometer will be discussed, spanning different everyday-life fields: medicine (absorption of drugs, disease diagnosis through breath analysis), environment (monitoring of air pollution), security (detection of hazardous substances, e.g. toxics and explosives). References I. Galli, S. Bartalini, S. Borri, P. Cancio, G. Giusfredi, D. Mazzotti, and P. De Natale, “Ti:sapphire laser intracavity difference-frequency
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