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QCL based FRS for NO

Most of the QCL-based NO sensors employ sources emitting in the 5.2 μm region due to the presence of the strongest NO absorption band. However, the presence of water (H2O) and carbon dioxide (CO2) in this region can disturb the spectroscopic measurements. Therefore, it is essential to eliminate the influence of these interferences, especially when plant or breath samples are analysed. From this point of view, Faraday rotational spectroscopy (FRS) seems to be the most favourable among the spectroscopic techniques, because it is not sensitive to diamagnetic molecules such as H2O or CO2. The technique, first described in the 1980s [1], appears to be a powerful and versatile method for quantitative and selective detection of paramagnetic molecules, such NO, O2 or OH-.

The experimental set-up in FRS is shown in Fig. 1. The CW-DFB QCL is thermoelectrically cooled up to -25°C and generates about 1mW output power. The current of the QCL is modulated in order to scan the Q(5/2) NO transition at 5.33 μm (1875.73 cm-1). The light from the QCL is sent to the astigmatic multi-pass cell (AMAC-76, Aerodyne, USA) with a path length of 76 m and 80 mbar internal pressure. The cell is placed inside a solenoid producing a modulated magnetic field.

The QCL beam passes through two Rochon polarisers. The first polariser is placed before entering the multi-pass cell and filters out unwanted directions of light polarisation, while the second polariser (analyser) is placed after the cell almost perpendicular to the first one. The angle between the two polarisers is adjusted in order to optimize the signal-to-noise ratio of the signal. The beam is focused on a photovoltaic room temperature IR detector. This approach results in a sensitivity of 9 ppbv within 1 s averaging averaging (Fig 2), and the minimum detection limit of 2.3 ppbv for a 16 s averaging time [2].

QCL FRS NO setup

Fig 1. Schematic representation of the multi-pass-cell-based FRS spectrometer for NO detection at 1875.73 cm-1.

QCL FRS NO Allan variance

Fig 2. Allan variance for the FRS set up. The sensitivity of 9 ppbv is obtained within 1 s acquisition time. A minimum detection limit of 2.3 ppbv is achieved with 16 s integration time.

References:
[1] G. Litfin, C.R. Pollock, R.F. Curl, F.K. Tittel, "Sensitive Detection of Free-Radicals with Color-Center Lasers by Magnetic Rotation Spectroscopy", J. Chem. Phys. 72, 6602 (1980).

[2] S.M. Cristescu, D. Marchenko, J. Mandon, K. Hebelstrup, G.W. Griffith, L.A.J. Mur, F.J.M. Harren, "Spectroscopic monitoring of NO traces in plants and human breath: applications and perspectives", Appl Phys B: Lasers and Optics (2012).