A portable low-power QEPAS-based CO2 isotope sensor using a fiber-coupled interband cascade laser

Current field and medical applications demand better quantification of CO2 isotopic ratios with high sensitivity, fast time-response, portable instrument size, and low power consumption. We report such a CO2 isotope sensor using quartz-enhanced photoacoustic spectroscopy (QEPAS) with a continuous-wave interband cascade laser (ICL). The ICL emitting at 4.35 μm with a low drive current (35-60 mA) and optical power (<2.6 mW) covered the P(43) and P(44) lines of 12CO2 and the R(18) line of 13CO2 with a single current scan. The ICL radiation was coupled into a solid-core (100-μm core diameter) indium fluoride optical fiber with a coupling efficiency of 88%. We successfully delivered the single-mode laser beam from the fiber output into the off-beam QEPAS spectrophone enclosed in a compact gas cell. The generated photoacoustic signal was acquired and analyzed by a digital lock-in amplifier to obtain its second harmonic (2f) component. Our sensor is capable of analyzing the CO2 isotopic ratio with an average precision of <1‰, which fulfills the precision requirements of most medical and field applications.