1 H 2 S Trace Detection using Off-Axis Integrated Cavity Output Spectroscopy in the Near-Infrared Weidong Chen 1 , Anatoliy A. Kosterev 2 , Frank K. Tittel 2 1 Laboratoire de Physicochimie de l’Atmosphère, Université du Littoral Côte d’Opale 189A, Av. Maurice Schumann, 59140 Dunkerque, France 2 Rice Quantum Institute, MS 366, Rice University, 6100 Main St., Houston, TX 77005, USA * E-mail address: chen@univ-littoral.fr Abstract: H 2 S trace detection has been performed by means of DFB diode laser-based off-axis integrated cavity output spectroscopy (OA-ICOS) near 1571.6 nm. A minimum detectable concentration of 700 ppb (3σ) was obtained. ©2007 Optical Society of America OCIS codes: (300.6340) Spectroscopy, infrared; (300.6260) Spectroscopy, diode lasers; (280.3420) Laser sensors 1. Introduction Hydrogen sulfide (H 2 S) detection at the ppm level has been previously reported as for example performed by wavelength modulation spectroscopy [1], two-tone frequency modulation technique [2] and photoacoustic detection [3] using a telecommunication diode laser. In the present work, we demonstrate the feasibility of H 2 S trace detection by means of off-axis integrated cavity output spectroscopy (OA-ICOS) using a DFB diode laser near 1571.6 nm. A minimum detectable concentration of 700 ppb (signal-to-noise ratio=3) was obtained. 2. OA-ICOS experimental set-up The OA-ICOS approach is based on the use of a high finesse optical cavity to achieve an effective optical path lengths of hundreds of meters [4,5]. In the present work, the OA-ICOS cavity consisted of two 1" diameter spherical mirrors (1 m radius of curvature) separated by a 0.5 m long quartz coated stainless tube. The mirrors reflectivity was ~99.995% at 1560 nm as specified by the manufacturer (Los Gates Research). Based on the measured cavity ring down time with on-axis alignment, the effective optical path length of the cavity was ~ 10.2 km. In the off-axis configuration (OA-ICOS), the measured effective absorption path length was ~ 700 m. Three alignment screws located on one of the two cavity mirror mounts are piezo actuator-driven, which allows cavity length modulation. The diode laser source used was a GaInAsP DFB laser diode (JDS Uniphase) operating in the near infrared around 1571.8 nm (~ 6362 cm -1 ). The single mode diode laser was fiber pigtailed with a linewidth of <350 kHz and an output power of up to 63 mW. Frequency tuning of the diode laser is performed by scanning either the temperature (over 10 cm -1 with a tuning ratio of ~ 0.4 cm -1 /K) or the current (over more than 1 cm -1 ). An amplified, switchable- gain InGaAs detector (PDA10CS, Thorlabs) was used for the radiation detection after the optical cavity. 3. H 2 S trace concentration measurements Figure 1a shows a H 2 S FTIR spectrum acquired at 1 atm pressure (bottom line) from the Pacific Northwest National Laboratory (PNNL) database (absorption spectra of H 2 S in the near infrared are not available in HITRAN or GEISA.). The top line is a direct absorption spectrum of CO 2 from a reference cell in the same spectral region. The R20 CO 2 line of the 3ν 1 band at 6362.5038 cm -1 was used in this work as a frequency reference and for a determination of the effective optical path length of the OA-ICOS cavity using a calibrated CO 2 concentration. A gas standard generator (KIN-TEK Model 491 M) was used to provide a traceable calibration standard of H 2 S. The H 2 S concentrations range from 24 to 2.4 ppmv in a diluting gas (nitrogen). The ICOS cell pressure was maintained at 100 Torr with a pressure controller (MKS Instruments type 649). Absorption spectra of H 2 S were collected by scanning the DFB laser current at 10 Hz over ~ 1 cm -1 around 6362.6 cm -1 . Typically, 1000 spectral scans were averaged for each H 2 S concentration measurement. Figure 1b shows an OA-ICOS spectrum of 14.6 ppmv H 2 S. Based on the experimentally estimated SNR of the spectral signal, the corresponding minimum detectable concentration (MDC) was found to be 700 ppbv (SNR=3). a1260_1.pdf JThD78.pdf