Dual‐Comb Spectroscopy With Fully Controlled Repetition Rate and Offset Frequency of Quantum Cascade Laser Frequency Combs

Abstract

Quantum cascade laser frequency combs (QCL‐FCs) are promising light sources for sensing applications, thanks to their convenient electrical pumping, chip‐scale footprint, high optical power, and relatively broadband emission in the mid‐infrared molecular fingerprint spectral region. However, under free‐running operation, QCL‐FCs exhibit frequency instabilities that limit their use as metrological‐grade sources, primarily due to insufficient control of the repetition rate ( f rep ) and the carrier envelope offset frequency ( f ceo ), which are required for unambiguous calibration of the optical frequency axis. Here, precise and independent control of the two comb parameters is demonstrated by combining radio‐frequency (rf) injection for repetition rate control with thermal tuning of the offset frequency. While the thermal tuning of free‐running combs follows the conventional comb model with f ceo < f rep , the frequency tuning behavior under rf injection is fundamentally different and requires a modified comb model with an anchor frequency ( f 0 ) that is directly related to f ceo but is located at much higher optical frequency within the emission bandwidth of the comb. Full frequency control of both parameters enables gapless, high‐resolution dual‐comb spectroscopy of N 2 O over the entire 80 cm −1 comb bandwidth, and provides pathways toward fully controlled QCL‐FC systems that require minimal external hardware and enable precision mid‐infrared spectroscopy.