Dual-comb spectroscopy with tailored spectral broadening in Si₃N₄ nanophotonics

Si₃N₄ waveguides, pumped at 1550 nm, can provide spectrally smooth, broadband light for gas spectroscopy in the important 2 μm to 2.5 μm atmospheric water window, which is only partially accessible with silica-fiber based systems. By combining Er⁺ fiber frequency combs and supercontinuum generation in tailored Si₃N₄ waveguides, high signal-to-noise dual-comb spectroscopy spanning 2 μm to 2.5 μm is demonstrated. Acquired broadband dual-comb spectra of CO and CO₂ agree well with database line shape models and have a spectral-signal-to-noise as high as 48/√s, showing that the high coherence between the two combs is retained in the Si₃N₄ supercontinuum generation. The dual-comb spectroscopy figure of merit is 6 × 10⁶/√s, equivalent to that of all-fiber dual-comb spectroscopy systems in the 1.6 μm band. based on these results, future dual-comb spectroscopy can combine fiber comb technology with Si₃N₄ waveguides to access new spectral windows in a robust non-laboratory platform.