Dual-comb spectroscopy has been proven a powerful tool in molecular characterization, which remains challenging to implement in the mid-infrared (MIR) region due to difficulties in the realization of two mutually locked comb sources and efficient photodetection. An effective way to overcome those limitations is optical upconversion; however, previously reported configurations are either demanding or inefficient. Here we introduce and experimentally demonstrate a variant of dual-comb spectroscopy called cross-comb spectroscopy, in which a MIR comb is upconverted via sum-frequency generation (SFG) with a near-infrared (NIR) comb with a shifted repetition rate and then interfered with a spectral extension of the NIR comb. We experimentally demonstrate a proof-of-concept measurement of atmospheric CO2 around 4.25 micrometer, with a 350-nm instantaneous bandwidth and 25000 resolved comb lines. Cross-comb spectroscopy can be realized using up- or down-conversion and offers an adaptable and efficient alternative to dual-comb spectroscopy outside the well-developed near-IR region, where having two mutually coherent sources and efficient photodetection is challenging. Moreover, the nonlinear gating in cross-comb spectroscopy promises a superior dynamic range compared to dual-comb spectroscopy.
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