Significance Optical frequency combs (OFCs) with ultrahigh stability have revolutionized metrology science and facilitated a series of new research disciplines and directions, such as optical clock, precision spectroscopy, absolute distance measurement, and astrophysics. Recently, dual-comb spectroscopy (DCS), as an emerging broadband spectroscopic technique, has attracted substantial attention owing to its outstanding advantages of ultrahigh-frequency resolution, accuracy, and sampling rate. DCS has significant applications in various areas, such as gas absorption spectroscopy, greenhouse gas monitoring, and nonlinear spectral imaging. To achieve DCS, two coherent OFCs with a small offset of comb spacing are demanded. Various approaches could be used to generate these two OFCs, including phase-locking two independent mode-locked lasers, modulating a continuous-wave (CW) laser using two electro-optic modulators, or by directionally pumping a microresonator using a CW laser ( Fig. 3). However, those dual-comb systems generally require high-cost and bulky electronic configurations, a limited number of comb lines, or sophisticated fabrication processes, which pose substantial challenges for their practical applications. Dual-comb lasing in a single fiber laser (known as single-cavity dual-comb mode-locked fiber laser) is a viable alternative for releasing DCS, owing to mode-locking of direction/wavelength/polarization/cavity multiplexing. Because both combs are created from a single cavity, a compact and low-complexity DCS is possible owing to intrinsic high phase coherence and the absence of complex servo locking systems. This article reviews the recent research progress of single-cavity dual combs, ranging from their generation approaches, soliton dynamics to applications.

Progress Four types of multiplexing methods to generate dual combs from a single cavity-directions ( Fig. 4), wavelengths ( Fig. 5), polarization ( Fig. 6), and cavity spaces are first summarized. Generally, the spectra of wavelength multiplexing dual combs do not overlapp, leading to the necessary implementations of amplification and spectral broadening for practical applications, which increases the complexity of the entire system. Polarization multiplexing and bidirectional mode-locking create dual combs with overlapped spectra, but their tunable range is a limitation. Although noncommon optical-path structures could flexibly tune the offset repetition rate in a larger range, most spatially optical delay lines cause the system to be complicated. Therefore, novel single-cavity dual-comb lasers with all fiber and flexible tunability of the offset repetition rate are vigorously explored to overcome those pitfalls.

Mode locking is a complicated process that locks a huge number of longitudinal modes with the same phase difference to form ultrashort pulses. Single-cavity dual-comb mode-locked lasers could produce two stable pulses with slightly varying repetition rates. These novel asynchronous have a range of spectral and temporal features as well as differing intracavity evolution tendencies. This distinctive behavior distributes different net gains on both pulses leading to their distinguishable buildup dynamics. The entire buildup processes of asynchronous vector solitons in a polarization-multiplexed dual-comb fiber laser include relaxation oscillation, quasi-mode locking, spectral beating dynamics, and stable mode-locking (Fig.7). Polarization hole burning in the erbium-doped fiber might facilitate the different buildup periods of both vector solitons. Besides, counter-propagating pulses in a bidirectional ultrafast fiber laser undergo splitting after beating dynamics caused by modulation instability and finally annihilate to a stable bidirectional pulse. Buildup dynamics of counter-propagating ultrashort pulses might also undergo a long-time Q-switched mode-locking, accompanied by the formation of multisoliton structures.

Various group-velocities of both pulses cause the inevitable soliton collisions in dual-comb fiber lasers. Strong soliton-soliton interactions during collisions lead to their fascinating transient spectral evolutions and rich nonlinear phenomena. We summarize the collision processes of asynchronous scalar and vector solitons in dual-wavelength and polarization-multiplexed mode-locked fiber lasers, respectively. Scalar solitons experience the collision-induced self-reshaping process, such as the central wavelength shifts, dynamic spectral fringes, and rebuilding process of the Kelly sidebands. Vector-soliton collisions, however, could yield substantial four-wave mixing sidebands owing to cross-polarization coupling and the formation of another subordinate pulse on each polarization component (Fig. 8).

Intrinsic mutual phase coherence of single-cavity dual combs ensures the simple configuration for practical applications. Both frequency combs’ heterodyne beats map the molecular fingerprint from optical domain to radio frequency domain, which could be reconstructed using the Fourier transform of the time-domain interferogram obtained by a photodetector. Thus, DCSs by free-running dual-comb mode-locked fiber lasers have been leveraged to measure the gas absorption spectrum of hydrogen cyanide, water vapor, and methane, the coherent anti-stokes Raman scattering spectroscopy, and especially the optical sensing with fiber Bragg gratings (Fig. 9), as well as the application domains in absolute distance measurement and fiber optic gyroscopes.

Conclusions and Prospects Single-cavity dual-comb mode-locked fiber lasers exhibit remarkable merits of simple configuration and low cost, presenting broadband applications in enormous disciplines. However, small repetition rates, weak tunability, difficult self-starting, limited operating wavebands, are some of its limitations. Thus, some works ought to be examined in the future.