  Chip-scale optical frequency combs can provide broadband spectroscopy for
diagnosing complex organic molecules. They are also promising as miniaturized
laser spectrometers in applications ranging from atmospheric chemistry to
geological science and the search for extraterrestrial life. While optical
cavities are commonly used to boost sensitivity, it is challenging to realize a
compact cavity-enhanced comb-based spectrometer. Here, we apply the Vernier
technique to free-running operation of an interband cascade laser frequency
comb in a simple linear geometry that performs cavity-enhanced chemical
sensing. A centimeter-scale high-finesse cavity simultaneously provides
selective mode filtering and enhancement of the path length to 30 meters. As a
proof-of-concept, we sense transient open-path releases of ppm-level
difluoroethane with 2 ms temporal resolution over a 1 THz optical bandwidth
centered at 3.64 $\mu$m.
