Fiber-mode-locked lasers play an important role in generating ultrashort pulses of picosecond or even femtosecond durations, featuring high peak power and broad spectral characteristics. These pulses have important applications in precision machining, spectroscopic measurements, high-capacity optical communications, terahertz technology, and nonlinear optical imaging. Ultrashort pulses generated by fiber-mode-locked lasers result from a double balance between dispersion and nonlinearity, as well as between gain and loss. Grelu et al. extended the concept of dissipative solitons to include ultrashort pulses generated in nonconservative systems such as fiber-mode-locked lasers. These stable dissipative solitons exist in a continuous exchange of energy with the environment and a dynamic energy redistribution between the components of the soliton. In a mode-locked fiber laser, the process of mode phase-locking to produce periodic short pulses is known as mode locking, and the resulting pulses are generally termed optical dissipation solitons. A comprehensive understanding of the mechanisms underlying dissipative soliton generation holds great promise for advancing mode-locked fiber lasers in both scientific and practical applications, offering greater innovation and possibilities across a wider range of fields.
The peak pulse power generated by single-mode fiber mode-locked lasers approaches its limitations in the megawatt (MW) order, which are limited by the number of modes and the mode-field area of the single-mode fibers. To further enhance the performance of mode-locked fiber lasers, it is essential to consider higher dimensions (i.e., introducing spatial dimensions) and explore the impact of increasing the spatial modes (i.e., transverse modes) on soliton mode-locking in multimode fiber lasers. Consequently, mode-locked fiber lasers have evolved from traditional single-mode to multi-mode configurations, and mode-locking mechanisms have transitioned from one-dimensional (1D) temporal dissipative soliton mode-locking to (3+1)D spatiotemporal dissipative soliton mode-locking. The expansion of spatial dimensions results in complex nonlinear spatiotemporal interactions and rich physical spatiotemporal phenomena. Spatiotemporal dissipative solitons not only exhibit periodic pulse output in the time domain but also show the distribution characteristics of multiple transverse modes in the space domain. Spatiotemporal dissipative solitons achieved using multimode fiber lasers have potential applications in precision ranging, laser processing, nonlinear spectroscopy, optical tweezers, and scattering medium imaging, offering new possibilities in information transmission and imaging.
In this review, we focus on the study of dissipative soliton generation mechanisms in fiber mode-locked lasers, trace the development of fiber mode-locked lasers, and review the principles of generating one-dimensional temporal dissipative solitons in single-mode fiber lasers to generate three-dimensional spatiotemporal dissipative solitons in multimode fiber lasers. First, we explore the generation mechanisms of temporally dissipative solitons in single-mode fiber lasers with different chromatic dispersions. Temporal dissipative solitons can form when the positive chirp (owing to self-phase modulation) balances the negative chirp (owing to an anomalous even-order dispersion). Early studies generally considered the case in which self-phase modulation balances second-order dispersion, resulting in the formation of a second-order dispersion soliton (Fig.2). Higher-order even-order dispersive solitons can also be levelled with self-phase modulation and form the corresponding solitons, which are referred to as higher-even-order dispersive solitons (Fig.3). Notably, stable temporally dissipative solitons can be generated even in the absence of dispersion, and this type of soliton is referred to as a dispersionless soliton (Fig.4). The different types of solitons have different properties, and their formation involves various physical processes.
Subsequently, we delved into the latest achievements in spatiotemporal dissipative soliton mode-locking in multimode fiber lasers. In contrast to temporal dissipative solitons in single-mode fiber lasers, spatiotemporal dissipative solitons in multimode fiber lasers add spatial dimensions by incorporating multiple transverse modes. In this case, the dispersion consists of both chromatic (intramode) and intermode (modal) dispersions. Therefore, balancing the modal dispersion is important to generate spatiotemporal dissipative solitons in a multimode fiber laser. We discuss compensation methods for modal dispersion (Fig.5) and reveal their rich spatiotemporal mode-locking mechanisms and potential application scenarios. Finally, we provide an outlook on the research prospects for mode-locked fiber lasers.
In this paper, we present the research history from the traditional single-mode temporal dissipative soliton to a more complex multimode spatiotemporal dissipative soliton and summarize the generation mechanisms of various dissipative solitons in fiber lasers. Through this development, we aim to summarize the differences between temporal and spatiotemporal dissipative solitons, emphasize the significance of understanding the mode-locking mechanism for conducting research and the application of fiber-mode-locked lasers, and show the potential application scenarios of fiber-mode-locked lasers in the future. In summary, focusing on temporal/spatiotemporal dissipative solitons in fiber lasers not only enhances our understanding of the principle of fiber mode-locking lasers but also opens up more opportunities for fiber laser applications. With the continuous progress of technology and theoretical improvements, we believe that fiber-mode-locked lasers will continue to play an important role in the future and provide more possibilities for applications in optical frequency combs, material processing, medical diagnostics, and other fields.
