Abstract
Mid-infrared absorption spectroscopy for gas detection is reported in this study using a quantum cascade laser (QCL) coupled with a custom-made tellurite hollow-core antiresonant fiber (HC-ARF). The HC-ARF is fabricated from tellurite glass by extrusion and subsequent fiber drawing. The QCL emitting at 5.26 μm is coupled into the 21-cm long HC-ARF. As a proof-of-concept, this spectroscopic system is demonstrated for nitric oxide (NO) detection by exploiting its strong absorption line at 1900.08 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . By quickly filling gas mixtures into the HC-ARF, we first conduct direct absorption spectroscopy of NO and achieve a noise equivalent absorption (NEA) of 2.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Besides, we also conduct wavelength modulation spectroscopy to improve sensing performance. A minimum detection limit of 6 ppb NO is achieved at the integration time of 30 s, corresponding to 1.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in NEA. The HC-ARF is tested to show a response time of only 0.3 s when applying a pressure difference of 11 kPa between the two fiber ends. Such a tellurite HC-ARF-coupled QCL spectroscopic system makes it attractive for developing optical gas sensors with compact size, fast response, and high sensitivity.
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