Optical-feedback cavity-enhanced absorption spectroscopy with an interband cascade laser: application to SO2 trace analysis

The combination of interband cascade lasers (ICL) with cavity-enhanced absorption spectroscopy (CEAS) offers new perspectives in trace analysis and isotope ratio measurements. ICLs cover a mid-infrared spectral window (3–4 µm), in between those covered by Ga(InAs)Sb diode lasers and quantum cascade lasers (QCL), where strong molecular transitions can be found. While ICLs have lower emission power than QCLs, their thermal dissipation is much closer to that of telecom diode lasers and their current tuning range larger, which are both major advantages for developing compact instruments. We present an OF-CEAS implementation with an ICL at 4.015 µm, in which optical feedback (OF) enables efficient injection into the high-finesse cavity. In this paper, we also discuss a procedure allowing to obtain an accurate measurement of the OF rate. With regard to performance, we obtain a rms noise-equivalent absorption of 7.7 × 10−9 cm−1 for one acquired spectrum (80 ms) with a cavity of finesse 3900, which translates to a normalized figure of merit of 2.2 × 10−9 cm−1/√Hz, allowing for SO2 trace analysis down to ppbv levels with a response time of seconds.