Abstract
A mid-infrared quartz-enhanced photoacoustic sensor was developed using a multimode fiber (MMF)-coupled quantum cascade laser (QCL) and demonstrated for sensitive nitric oxide (NO) detection at a wavelength near 5.26μm. The QCL radiation was readily coupled into a solid-core InF3 MMF (100μm core) with 97% coupling efficiency using an aspheric lens. Despite the 25.5% transmission loss for the 1m long MMF, the Gaussian beam-like fiber output of 5.72° divergence was almost completely focused through the microresonator tube (length, 8.0mm; ID, 600μm) designed for off-beam quartz-enhanced photoacoustic spectroscopy. The sensor exploiting the R6.5 (Π21/2) doublet of NO at 1900.08 cm-1 demonstrated a minimum detection limit of 24 parts per billion by volume at an averaging time of 130s. The sensor was found to be insensitive to the fiber bending noise for a bending radius >5 cm.
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