Abstract
A mid-infrared trace gas detection system based on off-axis integrated cavity output spectroscopy (OA-ICOS) is demonstrated for accurate and sensitive detection of N2O in combination with a continuous wave external-cavity quantum cascade laser (EC-QCL) working around 7.7 µm. A 13-times improvement in signal-to-noise ratio is achieved using a re-injection mirror and a minimum detection limit of 70 ppbv in less than 10 s averaging time is achieved, which yields a noise-equivalent absorption sensitivity (NEAS) of 6×10−9 cm−1 Hz−1/2. For comparison, a compact multipass cell is deployed to measure the same absorption line of N2O using wavelength modulation spectroscopy with second harmonic detection (WMS-2f). An enhancement factor of 20 in comparison to direct absorption spectroscopy (DAS) is achieved, yielding a minimum detection limit of 15 ppbv in less than 10 s averaging and a NEAS of 1×10−9 cm−1 Hz−1/2. A comprehensive comparison between the two systems is carried out in terms of residual amplitude noise (RAM), linearity, long-term stability, detection limit, spectral fitting, reproducibility, and background variations. The proposed sensor based on OA-ICOS is potentially advantageous for trace gas sensing in outdoor applications and harsh environments due to its robustness and flexibility of alignment.
Highlights
Recent advances in laser sources and optical detectors in the mid-infrared wavelength region have drastically enhanced the sensitivity of trace gas detection
The fundamental rovibrational transitions of most of the molecules are in the mid-IR wavelength region, which yields strong absorption line strengths resulting in low detection limits using absorption spectroscopy [1,2,3]
For ultrasensitive molecular absorption spectroscopy, it is best to perform the measurement in the mid-IR wavelength range, where most of the molecules have their strongest absorption features
Summary
Recent advances in laser sources and optical detectors in the mid-infrared wavelength region (midIR, 2-20 μm) have drastically enhanced the sensitivity of trace gas detection. Major research efforts have been focused towards simple, portable and low-cost optical gas sensors with trace gas sensitivities ranging from parts-per-billion (ppbv, 1:109) to parts-per-trillion (pptv, 1:1012) levels at sub-second time scale [4,5,6]. These highly sensitive sensors are utilized for many applications in different fields of research such as environmental monitoring [5,7] and medical breath analysis [3,8,9]. This can be performed by spectroscopy in the mid-IR region, where the absorption cross-section of N2O is more than four orders of magnitude higher compared to the near-infrared region (near-IR, 1-2 μm)
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