All-fiber counterpropagating all-normal-dispersion laser for coherently averaged free-running dual-comb metrology

In this work, we demonstrate a compact all-fiber counterpropagating all-normal-dispersion (CANDi) laser architecture that brings novel phenomena and enables the long-sought turn-key operation and long-term stability for versatile and coherently averaged dual-comb metrology under free-running conditions. In addition to the traditional single-wavelength dual-comb state, a dual-wavelength bidirectional mode-locking state is realized in the CANDi laser for the first time, to the best of our knowledge, which shows 600-times higher repetition-rate difference and thus accommodates more versatile dual-comb application scenarios. Both states can be deterministically accessed and self-started with similar robustness by changing the center frequency of the intracavity Lyot filter through temperature regulation. The state-selecting mechanism is further verified through numerical simulation. Moreover, the noise characteristics of the two states are compared and analyzed, which reveals intriguing differences that shed lights on the puzzle of dual-comb cross talk. Finally, to verify the feasibility of the laser in practical applications, the transmission spectrum of a high- Q microcavity is measured through coherently averaged dual-comb spectroscopy under free-running conditions. A spectral resolution better than 3.4 GHz has been achieved, and the coherently averaging time exceeds 4 s. This work not only brings new insights into dual-comb laser physics, but also represents a critical step towards practical and versatile dual-comb applications.