Perspectives on ultrafast fiber lasers: Multidimensional modulation and intelligent control

Abstract

Ultrafast fiber lasers, capable of generating picosecond and femtosecond pulses, have emerged as an indispensable platform for investigating nonlinear optical phenomena and a versatile tool for diverse applications. The past few years have witnessed transformative advances in several aspects: the extension of soliton physics from one-dimensional to three-dimensional spatiotemporal regimes via multimode-fiber mode-locking; the unprecedented control over soliton properties through programmable phase and intensity modulation; the revolution of cavity design and intelligent control; and the probing of soliton transient dynamics via advanced measurement techniques. This perspective synthesizes these interconnected frontiers, spotlighting key breakthroughs and charting future research directions with the potential to reshape the landscape of ultrafast fiber lasers.

Graphical abstract

Keywords

spatiotemporal mode-locking / phase and intensity modulation / cavity design and intelligent control / soliton transient dynamics

1 Introduction

Ultrafast lasers have driven transformative advances in modern science and technology, exemplified by breakthroughs such as optical frequency combs, chirped pulse amplification, and attosecond pulse generation [1−4]. As typical ultrafast light sources, fiber lasers feature excellent beam quality, high heat dissipation capability, compact structure, and cost-effectiveness, rendering them indispensable in applications such as material processing, precision measurement, and nonlinear optics [5−7].

In the past several decades, researchers seeking to boost the pulse energy and stability of ultrafast fiber lasers have chiefly concentrated on two areas: cavity design optimization and the management of intracavity dispersion and nonlinearity. The former has spurred the evolution of various cavity configurations, including linear [8], figure-of-eight [9], sigma [10], ring [11], and Mamyshev cavities [12]. The latter, in turn, has led to dispersion management spanning from the anomalous to the normal regime, giving rise to conventional soliton [13, 14], stretched-pulse [15], self-similar pulse [16], and dissipative soliton [17]. Recently, the research focus in this field has gradually shifted toward four directions: spatiotemporal mode-locking [18, 19], intracavity phase and intensity modulation [20, 21], cavity design with intelligent control [22, 23], and soliton transient dynamics [24, 25].

This perspective surveys four emerging aspects that collectively chart the course for ultrafast fiber lasers: spatiotemporal mode locking, which broadens soliton physics to three-dimensional spatiotemporal regimes; phase and intensity modulation, which affords programmable tailoring of soliton characteristics; cavity design and intelligent control, which enhances the tunability of fiber lasers; and soliton transient dynamics, which enables real-time observation of pulse evolution and interactions. We discuss recent progress and outline perspective for each of these directions.

2 Spatiotemporal mode-locking: Broadening soliton to three-dimensional domains

Conventional mode-locking in single-mode fibers synchronizes only longitudinal modes. In 2017, Wright et al. [18] overcame this limitation by incorporating a multimode graded-index fiber into the laser cavity, thereby achieving simultaneous locking of longitudinal and transverse modes — a regime known as spatiotemporal mode-locking. This landmark achievement elevated mode-locking from one-dimensional longitudinal confinement to full three-dimensional spatiotemporal regimes, yielding coherent pulses that comprise roughly 100 transverse modes with picosecond-to-femtosecond durations and energies up to 150 nJ. Such spatiotemporal mode-locking was realized in multimode fiber lasers through the counteraction of intermodal and chromatic dispersions, enabled by strong spatial and spectral filtering. The “attractor dissection” theory rationalized this phenomenon as the cavity’s self-organized evolution toward a minimum-loss state [19]. Recently, spatiotemporal mode-locking has also been realized in large-dispersion step-index multimode fibers [26] and in carbon-nanotube-based all-fiber configurations [27]. Multimode fibers, with their large mode area, enable high-energy pulse generation, while the output’s adjustable spatial coherence opens up new opportunities in precision measurement and nonlinear microscopy.

3 Phase and intensity modulation: Programmable tailoring of soliton properties

Programmable pulse shapers enable independent phase and intensity control of each spectral component, offering considerable flexibility for both dispersion and spectral management in fiber lasers. Runge et al. [20] realized a pure quartic soliton laser using an intracavity spectral pulse shaper to introduce quartic dispersion. The resulting energy scaling E_PQS∝τ⁻³ significantly outperforms that of conventional solitons, underscoring that higher-order dispersion is a useful tool, not an unavoidable drawback. More recently, the experimental realization of bound soliton complexes across pure-quartic, -sextic, -octic, and -decic dispersions has significantly broadened the dispersion-engineering landscape, opening up new avenues for exploring higher-order soliton dynamics and their potential applications in ultrafast photonics [28].

Parallel efforts have also focused on synchronized multi-wavelength mode-locked solitons, also known as multi-wavelength soliton molecules. Mao et al. [21] first demonstrated the synchronization of multi-wavelength solitons in a fiber laser using programmable group-delay engineering, generating up to five wavelengths with terahertz-level sub-pulse repetition rates. They subsequently achieved switchable breathing and stable Talbot solitons in a triple-color mode-locked fiber laser by manipulating the frequency spacing between neighboring spectra [29], and further demonstrated that spectral phase programming enables on-demand soliton formation with separations following constant, geometric, or arithmetic sequences [30]. It is demonstrated that phase and intensity modulation not only enables on-demand control of soliton properties — covering pulse profiles, energy, and spacing — but also opens avenues for terahertz difference-frequency generation.

4 Cavity design and intelligent control: Pushing performance boundaries

Advanced cavity designs and intelligent control strategies have dramatically boosted the performance of ultrafast fiber lasers. A Mamyshev oscillator based on single-polarization large-mode-area photonic crystal fiber delivers 9 W average power at an 8 MHz repetition rate, with pulse energy exceeding 1 μJ and peak power reaching 13 MW after compression [31]. Introducing strong spectral filtering in all-anomalous-dispersion fibers and extracting >90% of the pulse energy after amplification enhances the key dissipative effects with controlled frequency chirping, generating high-energy pulses in the few-picosecond regime [32]. Incorporating solid-core photonic crystal fiber in laser, optomechanically bound soliton states enable programmable generation and long-term storage of pulse for over 100 hours [33]. By managing birefringence in normal-dispersion hybrid-structure fiber lasers containing a few meters of polarization-maintaining fiber, near-chirp-free soliton states are achieved [22].

Meanwhile, in the realm of intelligent control, Pu et al. [23] presented an intelligent programmable mode-locked fiber laser based on a human-like algorithm, which enables automatic multi-regime locking while ensuring fast switching and stable operation. Real-time spectral shaping has been achieved by combining dispersive Fourier transform with intelligent polarization search [34]. Optimization algorithms have also enabled intelligent control of soliton-molecule dynamics and inter-soliton separation [35, 36]. More broadly, equation-free machine learning and deep reinforcement learning have facilitated autonomous stabilization, self-tuning, and reconfiguration of mode-locked lasers and frequency combs [37, 38]. Advanced cavity designs will likely extend the performance envelope of ultrafast fiber lasers, whereas intelligent control algorithms are poised to enable fully self-optimizing laser systems.

5 Soliton transient dynamics: Real-time probing of pulse formation and evolution

The dispersive Fourier transform technique serves as a powerful diagnostic tool for soliton transient dynamics, which maps the optical spectrum into the time domain and permits rapid, continuous single-shot measurements [39]. Various ultrafast dynamics have been revealed in mode-locked lasers, such as soliton explosions, soliton molecules, noise-like pulses, rogue waves, breathing solitons, and mode-locking buildup processes [24, 40]. In particular, the evolution of femtosecond soliton molecules has been observed [41], and their internal separation and relative phase difference provide additional encoding dimensions, holding promise for applications in optical communications and information encoding [25, 42]. Extending real-time measurement to the spatiotemporal domain via speckle-resolved spectral-temporal techniques is crucial for understanding spatiotemporal mode-locking dynamics, enabling observation of 3D soliton molecules and spatiotemporal dynamics [43].

6 Outlook

In summary, ultrafast fiber lasers are transitioning from conventional performance optimization toward multidimensional, intelligent, and real-time capabilities. This Perspective reviews recent breakthroughs across four interwoven frontiers — spatiotemporal mode locking, phase and intensity modulation, cavity design and intelligent control, and soliton transient dynamics — that together will shape the field. We also address key challenges and opportunities, aiming to provide researchers with a clear roadmap for future work.

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