Optical frequency comb (OFC), often dubbed as "the ruler in the frequency domain" and a "bridge" between microwave and optical frequencies, plays an important role in time and frequency metrologies. Among these OFC technologies, the one leveraging two OFCs with slightly different comb tooth spacings, that is, dual-comb technology, has been the most thoroughly studied one in recent years. It has been utilized in various applications such as absorption spectroscopy, absolute distance ranging, pump-probe measurement, and radio-frequency spectrum measurement, as it can realize high-resolution and broadband optical measurements. Based on the conventional optical frequency comb technology, dual optical combs are generated using two independent mode-locked lasers with slightly different frequencies. However, the need for complicated feedback control and laser systems to maintain mutual coherence between the dual combs could be a key bottleneck for this technology when moving towards on-site detection and a broader application range. The generation of high-quality dual-optical frequency combs with low complexity has become a popular topic in dual-comb technology research.

The single-cavity dual-comb technology that realizes high-coherence dual-optical frequency comb generation with one laser has become a prominent research direction in current optical frequency comb technology. This has significantly contributed to the advancement of low-complexity dual-comb technology. This study reviews the development of single-cavity dual-comb technology from a broad perspective, specifically focusing on the single-cavity dual-comb fiber lasers that have been extensively explored over the past decade. Various technical pathways to implement single-cavity dual-comb sources and their characteristics are summarized, as well as the new directions for further developing this technology.

Studies on ultrafast lasers have traditionally focused on achieving shorter pulse width, higher power, and lower noise. Ensuring the generation of a single pulse train in the laser cavity is the preferred choice for realizing high-quality mode-locked lasers. However, for dual-comb sources, the mutual stability between two pulse sequences, instead of the stability of a single optical comb, is the key performance target. Over the past decade, to develop dual-comb lasers with good mutual coherence, researchers worldwide have designed various lasers based on the concept of multiplexed mode-locked laser (M²L²), an idea introduced by our group. Because both combs share a laser cavity, M²L² can realize good passive mutual coherence of the output combs without active stabilization. Thus far, four types of multiplexing methods, namely, wavelength-, directional-, polarization-, and pulse shape multiplexing, corresponding to different physical dimensions of optical pulses, have been studied to generate dual combs from a single cavity. These are illustrated in Fig. 1, and their corresponding performances are summarized in Table 1.

The advantages of the single-cavity dual-comb (SCDC) technique pave the way for realizing low-complexity dual-comb systems. Dual-comb measurement techniques can be divided into two categories: time-domain and frequency-domain measurements, which impose different requirements on the coherence and frequency stability of dual combs. Because the single-cavity dual-comb laser is significantly different from the traditional fully referenced dual-comb source in terms of power, spectral width, repetition rate difference, coherence, and stability, it is necessary to validate the applicability of SCDC lasers to existing dual-comb applications. All major dual-comb applications, such as optical spectroscopy, terahertz spectroscopy, ranging, and pump-probe measurements, have been demonstrated using SCDC lasers (Fig. 6).

Although the shared cavity design of SCDC sources yields considerable advantages in terms of overall system complexity and cost, there could be inevitable periodic collisions between two ultrashort pulses in such a laser cavity. This is fundamentally different from undisturbed pulse propagation in conventional single-pulse-train mode-locked lasers. Thus, this could be an interesting research subject, as well as a potential engineering challenge. Therefore, several studies have recently been conducted on pulse interaction in SCDC lasers with different multiplexing methods (Fig. 11).

Moreover, novel measurement schemes leveraging more than two combs have shown new capabilities required by certain applications. However, such light sources based on frequency-stabilized combs are considerably complex and expensive. Therefore, single-cavity multi-comb technology could be an attractive alternative solution. The concept of a multidimensional multiplexed mode-locked laser (M³L²) has been proposed to realize single-cavity triple-comb and multi-comb generation. Several demonstrations, such as dead-band-free high-resolution microwave frequency measurement, real-time absolute distance measurement with increased ambiguity range, and dynamic spectroscopic characterization for fast spectral variations based on the single-cavity tri-comb laser and quad-comb laser, have been demonstrated (Figs. 12 and 13).

CDC technology, owing to its unique advantages in system complexity, power consumption, and cost, has grown extensively over the past decades. From an often-overlooked phenomenon in the labs, it has been transformed into a major driving force with the potential to propel dual-comb technology into out-of-lab applications. Owing to the unique multiplexed optical cavity structure and intracavity dual-comb pulse dynamics, related studies span various topics, from fundamental soliton physics to engineering solutions. However, some technical challenges remain to be overcome before it can become truly successful in real-world applications. The trade-off and balance between these performance parameters in SCDC systems, such as the tunability and stability of the repetition frequency difference, pulse energy, and mutual interaction, and the associated intriguing physics behind them, could further motivate the academia to conduct innovative investigations.