Fig. 1: Spectrally-tailored dual-comb spectroscopy with microring electro-optic (EO) frequency combs.

In this conceptual representation, three narrow continuous-wave (CW) laser lines of frequency f₁, f₂, and f₃ are used to excite the optical modes of a resonant EO modulator, driven at the microwave frequency f_RF1 that corresponds to the resonator free spectral range. The generated composite comb contains three non-adjacent combs, centered at f₁, f₂, and f₃, each having comb line spacing of f_RF1. Central frequency and span of each of the three combs are chosen to interrogate targeted features in an absorbing sample, while leaving out regions without absorber or with interfering species, and can be adjusted independently with agility. Three slightly frequency-shifted replica of the original laser lines are produced and injected into a second resonant EO modulator, driven at a slightly different repetition frequency f_RF2, providing a reference spectrum. The interrogating and reference comb beams interfere on a fast photodetector. In this process, pairs of comb lines, one from each beam, produce a composite radio-frequency (RF) comb of line spacing f_RF2-f_RF1, mapping the spectral information from the optical domain, centered at f₁, f₂, and f₃ to the RF domain, centered at δf₁, δf₂, δf₃, respectively. The frequency agility of the resonant EO modulator is a unique feature that enables on demand spectral tailoring and optimization of the signal-to-noise ratio.