Fig. 3.
Experimental dual-comb spectrum of ethylene sampled at exactly the comb-line spacing of 100 MHz in the region of 92 THz. (A) The amplitude of the FT provides the transmission spectrum. (B) Magnified representation of the transmittance (Top) and dispersion (Bottom) spectra around 91.25 THz. Note that the transmittance y axis does not go to zero. Some rotational assignments (39), in the P branch of the ν9 band, are given as an illustration. The strong lines are unresolved doublets of the (K′a = 5, ΔKa = K′a − K″a = −1, ΔKc = K′c − K″c = ±1) series, while the ΔKc = ±1 doublets are resolved for (K′a = 4, ΔKa = −1). Ka (respectively, Kc) is the quantum number for the projection of the rotational angular momentum (excluding electron and nuclear spin) onto the inertial axis of smallest moment (respectively, largest moment). Prime and double primes refer to upper and lower states, respectively. (C) Magnified representation of the transmittance (Top) and dispersion (Bottom) spectra around 94.56 THz. The intensity of the spectral envelope of our dual-comb system is significantly weaker here, in the region of Q branch of the (K′a = 7, K″a = 6, ΔKc = −1) lines of the ν9 band.