Sub-Picometer Resolution Spectroscopy With Interleaved Recirculating Frequency-Shifting Loop Combs

We proposed and demonstrated a novel interleaved dual-comb spectroscopy technique based on a dual-recirculating frequency-shifting loop (RFSL), achieving sub-picometer resolution over an effective measurement bandwidth of more than 0.5 THz. The dual combs were generated by seeding a dual-RFSL with the same continuous-wave (CW) laser, eliminating the requirement for complex phase-locking systems. The dual-RFSL architecture produced flat-topped dual-comb spectra spanning over 5 nm, with line spacings of 16 GHz and 15.9995 GHz, respectively. A stepped-frequency tunable laser was demonstrated via modulation sideband optical injection locking, where the tuning frequency was precisely regulated by an RF signal applied to the electro-optic (EO) modulator, enabling arbitrary high-precision frequency stepping of the seed laser. By performing interleaved scanning of the dual-comb source with step sizes of 500 MHz (4 pm) and 100 MHz (0.8 pm) over 32 and 160 steps, respectively, we obtained the transmission spectrum of a fiber Bragg grating (FBG) over a bandwidth of 641.5 GHz and the absorption spectrum of the H¹³C¹⁴N gas over a bandwidth of 536.5 GHz. The results exhibited excellent agreement with the reference spectrum, demonstrating that this method provided a simple and effective spectroscopic approach for gas sensing and environmental monitoring.