  In recent years, microresonator-based optical frequency combs have created up
opportunities for developing a spectroscopy laboratory on a chip due to its
broadband emission and high comb power. However, with mode spacings typically
in the range of 10 - 1000 GHz, the realization of a chip-based high-resolution
spectrometer suitable for gas-phase spectroscopy has proven to be difficult.
Here, we show mode-hop-free tuning of a microresonator-based frequency comb
over 16 GHz by simultaneously tuning both the pump laser and the cavity
resonance. We illustrate the power of this scanning technique by demonstrating
gas-phase molecular fingerprinting of acetylene with a high-spectral-resolution
of < 80 MHz over a 45-THz optical bandwidth in the mid-IR. Our technique
represents a significant step towards on-chip gas sensing with an ultimate
spectral resolution given by the comb linewidth.
