The invention of laser technology has had a transformative impact on society. Mode-locked fiber lasers have been widely used in research and industry, and they play an important role in basic science as a convenient nonlinear system. A mode-locked fiber laser is a complex nonlinear dissipative system with a large number of internal nonlinear dynamical phenomena that, in addition to outputting stable femtosecond pulses, exhibits a series of complex mode-locked states, including the breather locked mode, strange waves, noise-like locked modes, soliton explosions, and self-organized modes arising from soliton interactions, such as soliton molecules, soliton crystals, soliton complexes, and supramolecular structures. Even chaotic states have recently been discovered in mode-locked lasers. The study of these mode-locked states helps to understand the nonlinear dynamical properties of femtosecond fiber lasers. Additionally, because the femtosecond fiber laser is a universal nonlinear dissipative system, studying its dynamics can clarify the complex dynamics in related fields, such as Bose-Einstein condensation, microcavities, and oceanography. The intrinsic dynamics of these systems and the mode-locked laser are described uniformly by the nonlinear Schr?dinger equation and thus have similarities.
Owing to the presence of numerous mode-locked regions in mode-locked lasers, it has long been a challenging problem to control the parameters of the laser and thus access specific mode-locked states. For example, the most commonly used femtosecond fiber laser based on the nonlinear polarization rotational mode-locking technique is mathematically a multidimensional parametric space and experimentally requires tuning of at least seven parameters (pump, loss, dispersion, nonlinearity, and angles of the three waveplates) to traverse the entire parametric space. Because of the lack of a definite functional relationship between the mode-locking state and these parameters, a long trial-and-error process is needed to obtain the desired mode-locking state. In addition, even if the target locked mode is obtained, its repeatability is a problem.
Recently, a major breakthrough was made in intelligent mode-locked lasers, which can resolve the difficulty of precise control of mode-locked states. In 2015, Prof. Grelu’s group in France applied a genetic algorithm to the intelligent control of mode-locked lasers for the first time and realized the intelligent control of soliton pulses and noise-like pulses. Subsequently, the development of intelligent mode-locked lasers has accelerated. Hence, it is necessary to summarize the existing studies to rationally guide subsequent research in this area.
The principle of the commonly used smart locking algorithms and recent scientific research results are summarized. First, the principles of the genetic algorithm, human-like algorithm, and artificial neural network are explained, and a schematic (Fig.1) and architecture diagram (Fig.2) are presented. Then, recent scientific achievements in smart mode-locked lasers are described, including the first implementation of a soliton-locked mode in smart lasers by Andral et al. at the Université de Bourgogne, France (Fig.3); the development of genetic algorithms for soliton-locked mode recovery by Winters et al. at Kapteyn-Murnane Laboratories, USA (Fig.4); the development of the first smart programmable mode-locked laser by Pu et al. at Shanghai Jiao Tong University (Fig.5); and the development of the first smart programmable mode-locked laser using deep learning for intelligent mode-locking recovery by Yan et al. at the National University of Defense Technology (Fig.6). Subsequently, the realization of programmable control of the spectral width and spectral shape by Pu et al. of Shanghai Jiao Tong University (Fig.7) and the intelligent control of spatiotemporal mode-locking by Wei et al. of South China University of Technology (Fig.8) are elaborated. The intelligent regulation of the breather ultrafast laser is summarized, starting with the design of an adaptation function based on the radiofrequency signal of the breather locked mode (Fig.9), in which the relaxation oscillation dynamics and noise-like pulse dynamics in the laser are excluded (Fig.10). Then, experimental results of the genetic algorithm (Fig.11) are discussed, along with the control of the breather breathing ratio, the breathing period, and the number of pulses (Figs.12-14). Finally, the work related to the intelligent control of fractal respiratory subsets is briefly described. The differences in the spectra and stability of frequency-locked and non-frequency-locked breathers are examined (Figs.15 and 16), the evolutionary dynamics of fractal breathers are specified (Fig.17), and the intelligent search for fractal breathers is implemented using a smart laser based on a liquid-crystal phase delay (Figs.18 and 19).
This paper reviews the application of intelligent-control technology in passively mode-locked fiber lasers. Using intelligent-control technology, the automatic generation and control of the mode-locked state can be realized without manual tuning, which reduces the tuning time of the laser and improves the tuning accuracy as well as the repeatability of the mode-locked state. This self-optimizing ultrashort pulse laser has promising applications in certain environments. Although the passive mode-locked fiber laser is a complex dynamical system, the successful achievement of accurate tuning of multiple mode-locked states by genetic algorithms indicates the universality of these algorithms. A series of intelligent algorithms, including genetic algorithms, are expected to be applied to the intelligent control of more complex mode-locked states. The current intelligent-control technology focuses on controlling lasers and achieving automatic laser tuning. Whether intelligent-control techniques can have an impact on laser physics remains an open question.