Mid-Infrared Absorption Spectroscopy for NO and NO2 Detection in a Flexible Hollow Core Fiber(Invited)
In this study,we report the application of mid-infrared absorption spectroscopy for simultaneous gas sensing of NO and NO2 using a flexible hollow core fiber(HCF)and two quantum cascade lasers(QCLs).The utilization of QCLs with intermittent continuous wave(iCW)operations and a flexible hollow-core optical fiber can miniaturize sensor systems.The optimal absorption lines for NO and NO2 detection were selected at approximately 1929.03 cm-1 and 1599.91 cm-1,respectively.Both laser beams were simultaneously coupled into a 100 cm flexible HCF with an inner diameter of 530 μm.Initially,direct absorption spectroscopy was performed as an intuitive demonstration of the iCW operation of QCLs with a time-division multiplexing(TDM)strategy for dual-species detection.Following this,we applied wavelength modulation spectroscopy using the first harmonic normalized second harmonic(2f/1f)method to eliminate the influence of any nonabsorption intensity variation,thereby enabling us to thoroughly investigate the performance of the gas sensor.The precision of NO and NO2 detection was estimated by measuring the concentrations of 50×10-6 NO and 15×10-6 NO2.Results indicated a precision of 5.2%for NO and 4.1%for NO2.Furthermore,we achieved minimum detection limits of 39×10-9 NO with a 60 s integration time and 9.2×10-9 NO2 with a 50 s integration time.
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