Hollow waveguide integrated laser spectrometer for ¹³CO₂/¹²CO₂ analysis

Abstract

Using hollow waveguide hybrid optical integration, a miniaturized mid-infrared laser absorption spectrometer for ¹³CO₂/¹²CO₂ isotopologue ratio analysis is presented. The laser analyzer described focuses on applications where samples contain a few percent of CO₂, such as breath analysis and characterization of geo-carbon fluxes, where miniaturization facilitates deployment. As part of the spectrometer design, hollow waveguide mode coupling and propagation is analyzed to inform the arrangement of the integrated optical system. The encapsulated optical system of the spectrometer occupies a volume of 158 × 60 × 30 mm³ and requires a low sample volume (56 µL) for analysis, while integrating a quantum cascade laser, coupling lens, hollow waveguide cell and optical detector into a single copper alloy substrate. The isotopic analyzer performance is characterized through robust error propagation analysis, from spectral inversion to calibration errors. The analyzer achieves a precision of 0.2‰ in 500 s integration. A stability time greater than 500 s was established to allow two-point calibration. The accuracy achieved is 1.5‰, including a contribution of 0.7‰ from calibrant gases that can be addressed with improved calibration mixtures.