Frequency stabilization of quantum cascade laser for spectroscopic CO2 isotope analysis

Using off-axis integrated cavity output spectroscopy, named OA-ICOS, the absorption spectrum of C-O2 at 4.32 mu m is recorded by using a quantum cascade laser (QCL). The concentration of the three isotopo-logues (OCO)-O-16-C-12-O-16, (OCO)-O-16-C-13-O-16 and (OCO)-O-16-C-l2-O-18 is detected simultaneously. The isotope abundance ratio of C-13 and O-18 in CO2 gas can be obtained, which is most useful for ecological research. Since the ambient temperature has a serious influence on the output wavelength of the laser, even small temperature variations seriously affect the stability and sensitivity of the system. In this paper, a wavelength locking technique for QCL is proposed. The output of a digital potentiometer integrated in the laser current driver control is modified by software, resulting in a correction of the driving current of the laser and thus of its wavelength. This method strongly reduces the influence of external factors on the wavelength drift of lasers and thus substantially improves the stability and performance of OA-ICOS as is demonstrated with long-time measurements on CO2 in laboratory air. (C) 2018 Published by Elsevier B.V.