We developed a compact trace gas sensor, which incorporates a cylindrical thin-disk multi-pass cell. This cell is useful because the optical path length can be selected easily in a small absorption volume across the beam path of 25 cm 3. We studied the optical characteristics of this cell and demonstrated some gas measurements with this system. This paper discusses the performance of the gas detection system which incorporates a cylindrical cell.

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A trace material detection system was developed on the basis of cavity-enhanced absorption spectroscopy (CEAS) using a fiber-coupled passively locked external cavity diode laser (PLEC-DL) in the near-infrared (NIR) wavelength region. The oscillation range of an antireflection-coated diode laser (AR-DL) coupled into an external cavity could be simply selected with a narrowband bandpass filter (1 nm), resulting in a stable wavelength oscillation in the wideband tunability between 1640 and 1680 nm. The external cavity acts as a trace material sensor that exhibits excellent flexibility because it is free from the DL source and is carefully designed with mirrors having reflectivities of ca. 99.995 and 99.99%. Trace-level detection was successfully demonstrated with the developed sensor having a minimum detectable absorption coefficient of 2.4 × 10−8 cm−1, which corresponds to 0.15 ppm for CH4 concentration without interference from H2O absorption lines under atmospheric pressure.

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