Dual-cavity dual-comb interferometric spectroscopy with incoherent light: Optics and Laser Technology

We show that high-resolution cavity-enhanced dual comb absorption spectroscopy can be performed by creating beat spectra with light transmitted by two high finesse optical cavities of similar lengths, which were neither actively stabilized nor locked to the excitation wavelengths. The incoherent light from a superluminescent light emitting diode (∼1535–1560 nm) was spectrally narrowed through filtering (∼50 GHz), amplified, and subsequently coupled into two quasi-confocal optical cavities. The light leaking from the two cavities contains all frequencies that belong to the cavities’ mode structures, which in the case of confocal cavity geometries resembles in essence a frequency comb. When overlapped and detected as a function of time an interferogram is observed whose Fourier transform yields a regular beat spectrum. The frequencies in the beat spectrum are determined by the difference of the free spectral ranges of the two cavities. The average free spectral range of the cavities determines the spectral resolution of this dual-cavity dual-comb interferometric method. The absolute frequency calibration can either be done through the known position of a target species absorption feature or by beating the comb with an appropriate single mode laser frequency. We demonstrate this approach through measurements of spectra of C2H2, CO2 and H2O and discuss its applicability, advantages, and limitations. © 2026 The Author(s)