Dual-comb spectroscopy techniques are superior to a conventional Fourier transform spectroscopy in terms of the acquisition time, sensitivity, and resolution. Owing to its excellent capabilities, it has been applied to several useful applications. The conventional dual-comb system employs two independent mode-locked lasers for the generation of two optical frequency combs (OFCs) with slightly different repetition rates (Δfrep), and a complex servo system for high mutual coherence were needed. Despite the capabilities of dual-comb spectroscopy, the use of the dual-comb technique is still mainly restricted to experts for OFC. In recent years, a dual-comb laser, which emits two OFCs from a single laser cavity with a small Δfrep, has attracted attention owing to its desirable properties such as passive mutual coherence. Recently, dual-wavelength , bidirectional [2, 3], and polarization-multiplexed  mode-locked Er-fiber lasers have demonstrated as dual-comb fiber lasers. In previous works [1, 2], the demonstrated spectral bandwidth of the dual-comb fiber laser was of the order of a few nanometers. Thus, it was difficult to expand the spectral bandwidth with high coherence by nonlinear spectral broadening.
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