Broadband dual-comb spectroscopy with efficient baseline normalization enabled by frequency-domain multiplexing

Accurate baseline normalization is critical for reliable spectral retrieval in dual-comb spectroscopic gas sensing. To overcome the current limitations, such as poor real-time performance, computational complexity, user dependence, and reliance on prior absorption models, we propose a frequency-domain multiplexing approach. An acousto-optic modulator, inserted in the reference arm, introduces an overall frequency shift to the comb signal, leading to an adjustable separation of reference and sample signals. A transfer function then achieves efficient correlation and correction between the reference and sample signals. It enables rapid and robust normalization without time-domain apodization or reference spectrum reconstruction. Implemented in a mode-locked dual-comb spectrometer, this approach demonstrates robustness against source intensity fluctuations. Comparison with external-cavity diode laser scanning confirms the baseline normalization accuracy, with a maximum deviation of 2.9% and standard deviation of 0.35%. It enables us to retrieve the broad (∼10 THz ) absorption spectrum of mixing C 2 H 2 and CO 2 in a straightforward manner. With high robustness, low computational complexity, and minimal user dependence, it offers promising potential for high-precision, real-time dual-comb spectroscopic applications.