Frequency stabilization and referencing of a mid-infrared dual-comb spectrometer

The compact footprint of all solid-state quantum cascade laser frequency combs (QCL-FCs) allows field-compatible applications of dual-comb spectrometers (DCS) such as continuous atmospheric multi-species monitoring, for which long-term frequency stability is critical. We show that by incorporating a stabilized single-mode quantum cascade laser as a frequency reference into one channel of a conventional QCL-DCS system, the combs can be actively stabilized to a molecular transition, which significantly reduces the uncertainties in the DCS frequency axis compared to a free-running system. With the stabilized DCS, the QCL-FCs’ offset frequencies are locked to the reference laser to within 0.3 MHz and the frequency instability for each comb line is estimated to be < 1 MHz for each 100 µs long spectral acquisitions with white noise limited averaging trend for up to 600 s. Additionally, we show that the comb’s optical frequencies can be inferred accurately during the multiheterodyne signal acquisition, as demonstrated through absorption DCS of N2O at low pressure.