In recent years, spectral measurement systems based on dual-comb interferometers have attracted more and more attention due to their characteristics of large bandwidth, high resolution, and high frame rate. However, the requirement to maintain strict coherence between light sources greatly hinders the development and application of dual-comb spectroscopy (DCS) systems. In this work, we use the phase-shifted fiber Bragg grating (PS-FBG) and the broadband Fabry-Perot cavity (F-P cavity) to calibrate the relative frequency jitter between two combs instead of introducing other complex feedback control loops. In this way, a DCS system operating at a free-running state was proposed and experimentally demonstrated. With the calibration of the PS-FBG, a 100-MHz resolution over the range of more than 100 nm was achieved without distortion. Benefitting from the excellent wavelength thermal stability, we subsequently corrected wavelength deviation through the optical frequency division process, and the equivalent locking accuracy improved by about 167 times compared with the traditional electrical reference scheme. Finally, the spectral absorption measurement of hydrogen cyanide gas molecules was carried out which was in good consistency with the HITRAN database to verify the potential in molecular spectroscopy. Our scheme is also compatible with other platforms and band ranges and will provide new solutions for free-running DCS systems.
Dual-comb spectroscopy has become a powerful tool for spectral analysis because of its ultra-high resolution and fast frame rate. To capture the stable spectrum in the frequency domain, it is essential to lock the repetition frequency and the carrier envelope offset frequency of the mode-lock laser. By introducing a phase-lock loop to a piezoelectric ceramic based cavity, the repetition frequency can be easily synchronized to an external clock. To lock the carrier envelope offset frequency, a phase-shifted fiber Bragg grating with 132-MHz narrow pass band is introduced, and it is capable of filtering out single comb line of each comb. Therefore, the post-calibration signal for carrier envelope offset frequency can be obtained from the beating signal of the filtered adjacent comb lines. Based on this post-calibration scheme, a stabilized dual-comb spectroscopy is proposed and demonstrated with 120-nm observation bandwidth, 1-pm spectral resolution, and 100-Hz acquisition frame rate.
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