Spectral Interleaving with Quantum Cascade Laser Frequency Combs

Quantum cascade laser (QCL) based frequency combs are mid-infrared sources capable of producing hundreds of mW of optical power distributed across several hundred comb lines spanning tens of cm−1. In the dual comb (or multi-heterodyne) configuration, two nearly identical frequency combs with slightly different comb spacing are used as an interrogating and local oscillator comb, respectively, to probe the absorption or refractive index of a sample [1]. The multi-heterodyne beat note signal produced by overlapping the two beams on a fast photodetector allows simultaneous access to all optical frequencies of the interrogating comb, enabling fast (sub-μs) acquisition of time-resolved absorption and/or dispersion spectra [2]. The typical length of QCL devices of ca. 5 mm leads to comb spacings of the order of 10 GHz (0.33 cm−1). While suitable for spectroscopy in the condensed phase, gas-phase spectroscopy requires much finer spectral sampling (e.g. <10 MHz for Doppler-broadened transitions of small molecules). This can be achieved by spectral interleaving, i.e. by the continuous or step-wise shifting of the spectrum of the interrogating comb.