Mid-infrared multimode fiber-coupled quantum cascade laser for off-beam quartz-enhanced photoacoustic detection.

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 5.26 μm. The QCL radiation was readily coupled into a solid-core InF MMF (100 μm core) with 97% coupling efficiency using an aspheric lens. Despite the 25.5% transmission loss for the 1 m long MMF, the Gaussian beam-like fiber output of 5.72° divergence was almost completely focused through the microresonator tube (length, 8.0 mm; ID, 600 μm) designed for off-beam quartz-enhanced photoacoustic spectroscopy. The sensor exploiting the R6.5 (Π2) doublet of NO at 1900.08  cm demonstrated a minimum detection limit of 24 parts per billion by volume at an averaging time of 130 s. The sensor was found to be insensitive to the fiber bending noise for a bending radius >5  cm.