Signature For the past two decades, ultrafast fiber lasers have become fundamental building blocks in many applications, such as optical communications, biomedical imaging, and industrial processing. Passive mode-locking techniques have been investigated. The nonlinear saturable absorption (SA) effect is the core of the passive mode-locking technology of fiber lasers. Passive mode-locking techniques can be categorized into real and artificial saturable absorbers. The real saturable absorbers consist of semiconductor saturable absorption mirrors (SESAM) and nanomaterials. The artificial saturable absorbers consist of the nonlinear polarization rotation evolution (NPE), nonlinear optical loop mirror (NOLM), nonlinear multimode interference (NLMMI), and Mamyshev regenerator (Mamyshev). The abovementioned passive mode-locking technologies have their advantages and disadvantages. In this study, we briefly illustrate their pros and cons and review their recent development in various types of saturable absorption effects in the application of ultrafast pulse generation.

For real saturable absorbers, rising from the extraordinary physical, optical, and electronic properties of graphene in 2004, layered-dependent nanomaterials have attracted significant attention because of their advantages of cost-effectiveness, broadband optical response, high nonlinear, fast relaxation, and flexible compatibility with other photonics structures. The optical modulation effect of nanomaterials provides a pulse shaping mechanism (i.e., reduced absorption with increasing optical intensity); thus, it can support stable pulse generation and operation in a laser system. Most optical modulators are based on the third-order nonlinear optical response of nanomaterials, such as saturable absorption and optical Kerr effects. SA is a process originating from valance band depletion, conduction band filling, and ultrafast intra-band carrier relaxation exhibited by the input power-dependent optical absorption. Various nanomaterial-based SAs have been demonstrated for pulse generation in fiber lasers operating from the visible to mid-infrared regions. For the artificial saturable absorbers, these passive mode-locking technologies including NPE, NOLM, NLMMI, and Mamyshev could generate ultrashort, high repetition ratio, and high peak power pulses with demand. The optical modulation effect of artificial saturable absorbers originates from the operating optical properties: NPE technique using the optical properties of polarization, NOLM technique using optical nonlinear interference, NLMMI technique utilizing nonlinear multimode interference, and Mamyshev mode-locking technique using nonlinear self-phase modulation. These artificial SAs could largely tune optical modulation depths and enable some types of pulse generation.

The above-mentioned passive mode-locking techniques have progressed considerably. NOLM mode-locking fiber laser has been used in frequency comb applications due to its high repetition ratio feature. Mamyshev mode-locking fiber laser-generated ultra-high peak power pulses could be compared to commercial Ti: Sapphire lasers. NPE mode-locking fiber lasers could largely tune operating pulse states, acting as an ideal seed source for laser amplifier systems. Mode-locking fiber lasers made of nanomaterials have wide application prospects due to their flexible features. With increasing demand for fiber lasers, the above-mentioned traditional passive mode-locking techniques need to improve their performances. Thus, we review the recent advancement of these techniques to illustrate how they overcome their disadvantages.

Progress There are two solutions to improve the stability of the NPE passive mode-locking technique. The first one is to replace traditional fiber with polarization maintaining (PM) fiber (Fig. 3), and the second one is to use an external intelligent algorithm to stabilize laser performance (Fig. 4). All-PM fiber laser cavity construction is not easy since the orthogonal polarization states will induce a walkaway effect when propagating. Szczepanek et al. used the PM fiber segments cross-fused method to solve the intracavity orthogonal polarization walkaway effect. They obtain ultrashort pulses under the condition of all-PM fiber construction. An external intelligent algorithm is a method to actively control fiber laser performance and successfully realize an intelligently controlled fiber laser system. Its advantages include not only stabilizing the laser performance but also switching the pulsed laser operation states. A major advantage of the NOLM mode-locking technique is phase stability. Therefore, a high repetition ratio fiber laser for frequency comb application always employs the NOLM technique (Fig. 7). Besides, NOLM can tolerate larger cavity loss than other mode-locking techniques due to its nonlinear interference mode-locking mechanism. Thus, the NOLM mode-locking technique was the first reported visible waveband mode-locking fiber laser where the cavity loss is very large (Fig. 8). The nanomaterial-based saturable absorption (SA) mode-locking technique has a flexible feature that could extend its application region. However, its mode-locking mechanism is laser intensity-dependent absorption rising from nanomaterials. The mode-locking laser performance mainly depends on the optical properties of nanomaterials. The optical properties of integrated nanomaterials SA cannot be operated. To solve this issue, an external gate-controlled fiber laser can be used to tune the optical properties of nanomaterials, thereby controlling the mode-locking performance (Fig. 11). NLMMI is a novel mode-locking technique inspired by the Kerr effect mode-locking mechanism of a solid-state mode-locking laser. Its advantages are its ultrafast response time and all-fiber structure (Fig. 14). Mamyshev regenerator with self-phase modulation mechanism can generate ultra-high peak power. However, it has certain disadvantages such as the Mamyshev regenerator requires an external laser to excite Mamyshev regenerator operation (Fig. 16). With the above mode-locking techniques, ultrafast fiber lasers can satisfy end-users.

Conclusions and Prospects As the mechanism of saturable absorber effects for mode-locked fiber lasers are clear, researchers will be able to choose appropriate mode-locking mechanisms to satisfy the specific demands of end-users. Finally, the recent progress of ultrafast fiber lasers poses a new challenge; thus, more investigation is required.