Dual mid-infrared wavelength Faraday rotation spectroscopy NOx sensor based on NdFeB ring magnet array

Faraday rotation spectroscopy (FRS) exploits the magneto-optical Faraday rotation effect to achieve highly sensitive detection of paramagnetic molecules. Most of the currently reported FRS sensors suffer from high power consumption, while enabling detection of only a single component. In this work, a novel dual mid-infrared wavelength FRS sensor is proposed that enables the simultaneous detection of two paramagnetic molecules (nitrogen oxides, NOx = NO + NO2) in a single absorption cell. A neodymium-iron-boron (NdFeB) ring magnet array was designed to replace the solenoid coil to provide a stable, power-free magnetic field. The magnetic induction line distribution characteristics around the permanent magnets were analyzed based on the finite element method. Two mid-infrared quantum cascade lasers were used to detect the strongest fundamental & nu;2 rotational-vibrational band of NO (1875.81 cm-1) and the & nu;3 fundamental band of NO2 (1613.25 cm-1), respectively. A dual-wavelength Herriott cell (DWHC) is coaxially combined with a NdFeB ring magnet array to amplify the interaction of the laser beam with paramagnetic NOx molecules in an axial magnetic field. This low power FRS NOx sensor achieves minimum detection limits of 0.58 ppb and 0.95 ppb for NO2 and NO, respectively, over an optical length of 23.7 m.