VCSEL-based calibration-free carbon monoxide sensor at 2.3 μm with in-line reference cell

A compact and calibration-free carbon monoxide sensor approach utilizing the wide current-tunability of 2.3 μm VCSELs is reported. A separate reference cell is avoided by filling the reference gas (methane) in the photodetector housing. By applying bandwidth optimized wide/narrow wavelength scan concept, inherent wavelength scale calibration and self-monitoring of the sensor are realized, with which the laser aging process is also under control. An efficient linear least-squares curve fit using an analytical signal model for the narrow scan spectrum is done, utilizing the knowledge of the absolute wavelength scale and also the estimated WMS modulation amplitude obtained from the wide scan. The scan width of the narrow spectrum is optimized aiming at the maximum signal to noise ratio on the determined CO concentration. These concepts are universal and can be utilized for optical sensing of other gases as well and the sensor was tested under diverse applications e.g. fire detection and combustion optimization.