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Quantum Cascade Lasers coupled to OF-CEAS spectrometer

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Recent developments on distributed feedback quantum cascade lasers (DFB-QCL) allow to extend the range of molecules that can be monitored with very high selectivity and sensitivity by Optical Feedback Cavity Enhanced Absorption Spectroscopy, OF-CEAS). By working in the mid-IR region (4-6µm) the strong absorption lines of the fundamental transitions in gas molecules are exploited in order to gain 2 to 3 orders of magnitude in sensitivity in comparison to the overtone transitions occurring in the near IR region.

QCL-OFCEAS systems performances:
The LAME group of the LIPhy demonstrated that QCL are well compatible with the OF-CEAS technique by developing different QCL-OFCEAS systems with typical sensitivity of about 10-9/cm-1 in absorption unit for an acquisition time of 1s. Together with the small volume sample (20 cm3) and the fast response time (1s), this high sensitivity is exploited in various field of applications illustrated by the recently developed OF-CEAS instruments that operate with a room temperature QCL:

- 4.30µm: CO2 isotopic ratio measurements of 13C and minor isotopes like 17O or doubly substituted isotopes (clumped isotopes 638) for measurements of gas bubbles trapped in ice cores in the framework of the Subglacior project. The isotopic ratio detection limit is about 0.1 ‰ in a few minutes.
- 4.46µm: N2O, environmental monitoring and isotopic ratio measurements 546 and 456 (first demonstration of QCL coupled to OF-CEAS [Maisons,2010]).
- 5.26µm: NO for breath analysis and environmental (especially outdoor) concentration monitoring. Detection limit of the mixing ratio: < 100pptv in 10s.
- 5.65µm: HCHO (formaldehyde) in plasma diagnostic and environmental (especially indoor) application. Detection limit of the mixing ratio: 60 pptv at 10 Hz , and should also be capable of detecting formic acid at the ppb level [Gorrotxategi-Carbajo, 2013].

OF-CEAS Spectra of indoor air OF-CEAS (black dots) together with a fit (grey line). It shows three water absorption lines, and a weak H2CO doublet at 1769.46 cm-1. The OF-CEAS spectrum was acquired in 100 ms. The lower part of the figure shows the residuals of the fit to the data. A HITRAN simulation is also plotted (shifted for clarity) [Gorrotxategi-Carbajo, 2013].

- Alcatel Thales III-V lab, Palaiseau, France;
- Laboratoire de Physique des Plasmas (LPP), Ecole Polytechnique, Palaiseau, France;
- Laboratoire de Glaciologie et Geophysique de l’Environnement (LGGE), Grenoble, France;
- Laboratoire TIMC-IMAG (CHU de Grenoble), équipe PRETA, Grenoble, France.

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