High-resolution and gapless dual comb spectroscopy with current-tuned quantum cascade lasers

We present the first gapless, high-resolution absorption and dispersion spectra with quantum cascade laser frequency combs. In our phase-sensitive dual comb design, the comb lines are gradually swept over 10 GHz, corresponding to the free spectral range of the laser devices, by applying a current modulation. We show that this interleaving reduces the spectral point spacing by more than four orders of magnitude from 9.8 GHz down to 300 kHz over the full spectral span of the frequency comb. The potential of this technique for high-precision gas sensing is illustrated by measuring the low pressure (107 hPa) absorption and dispersion spectra of methane spanning the range of 1170 cm$^{-1}$ - 1225 cm$^{-1}$ with a resolution of 0.001 cm$^{-1}$.