Dual-comb interferometric systems with high time accuracy have been realized for various applications. The flourishing ultralow noise dual-comb system promotes the measurement and characterization of relative timing jitter, thus improving time accuracy. With optical solutions, introducing an optical reference enables 105 harmonics measurements, thereby breaking the limit set by electrical methods; nonlinear processes or spectral interference schemes were also employed to track the relative timing jitter. However, such approaches operating in the time domain either require additional continuous references or impose stringent requirements on the amount of timing jitter. We propose a scheme to correct the relative timing jitter of a free-running dual-comb interferometry assisted by a Fabry–Pérot (F–P) cavity in the frequency domain. With high wavelength thermal stability provided by the F–P cavity, the absolute wavelength deviation in the operating bandwidth is compressed to <0.4 pm, corresponding to a subpicosecond sensitivity of pulse-to-pulse relative timing jitter. Also, Allan deviation of 10−10 is obtained under multiple coherent averaging, which lays the foundation for mode-resolved molecular spectroscopic applications. The spectral absorption features of hydrogen cyanide gas molecules at ambient temperature were measured and matched to the HITRAN database. Our scheme promises to provide new ideas on sensitive measurements of relative timing jitter.
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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