Significance Since the invention of the fiber laser is more than half a century ago, the performance index of fiber beam output has continuously improved, which is mainly reflected in the improvement of output pulse power, shortening of output pulse width, improvement of beam quality, and shaping of the laser spectrum. Femtosecond fiber laser, as an important branch of the fiber laser field, has been widely used in the industry, mainly because of its advantages over solid-state femtosecond lasers in the following three aspects.1) All-optical slimming and high optical efficiency. Because of the pump light in the optical fiber waveguide transmission, the effective action distance is longer, which improves the conversion efficiency. Additionally, the fiber has the advantages of soft and easy-to-coil integration, so it can be connected through the welding process.2) Close to the diffraction limit of the beam quality and good heat dissipation. As the signal light is confined to the optical fiber waveguide in the single-mode form, the output from the photonic crystal fiber with a large mode field diameter and chirality coupling optical fiber can help maintain a good speckled pattern. Additionally, the large cooling area of the heating effect of the solid gain medium can help overcome the inherent effect. It is an important way to realize high-power laser.3) The ion emission spectrum in the fiber is wider, so that the fiber laser can achieve the output pulse with a narrower pulse width and a wider spectral width. Thanks to the ultra-short pulse width, ultra-wide spectrum, and ultra-high peak power of the fiber laser, it has wide application prospects in industrial processing, communication detection, biological medicine, high-energy physics, material preparation, and chemistry, and other fields.
High energy, narrow pulse width, high stability of ultra-short pulses are also being researched throughout the optical fiber mode-locked oscillator development. Therefore, new principles, new technologies, and a variety of new laser materials have been revealed at historic moments, from active to passive mode-locking, from small energy width with a broad pulse width to high energy with a narrow pulse width, from a space structure to an all-fiber structure, from theoretical research to practical applications. In the past several decades, the fiber mode-locked oscillator has undergone rapid and comprehensive development, which has greatly promoted the development of new research fields and industrial markets. Now, the fiber mode-locked oscillator has become one of the core members of the laser family.
Progress Development of a fiber mode-locked oscillator revolves mainly around two themes. One theme involves a narrow pulse width and large energy in two main directions. One direction is based on theoretical study and is given priority to explore a new type of mode-locking mechanism. In this direction, it has experienced the traditional soliton, dispersion-management soliton, self-similar soliton, dissipative soliton resonance, and Mamyshev mode-locked soliton. The other direction is based on the use of new laser materials, including a new optical fiber, pump source, saturable absorption materials, and so on. To realize the all-optical fiber integration of the oscillator, adopting a special fiber structure is generally not suitable and realizing self-starting using the Mamyshev mode-locking is difficult; therefore, the nonlinear polarization evolution (NPE)-based all normal dispersion mode-locked oscillator is still the mainstream trend. NPE mode-locking can achieve a high single-pulse energy and a narrow pulse width. However, a polarization controller is necessary to adjust the cavity polarization state. Introducing a mechanical structure makes this type of oscillator sensitive to the environment, and ensuring the long-term stable mode-locking state is difficult. The mode-locked laser based on nonlinear amplified loop mirror (NALM) is the best choice for all-fiber integration. Such an oscillator can be used for all polarization-maintaining devices, which effectively avoid mode-locked dependence on the mechanical structure and have high environmental stability and reliability. Additionally, its output is linearly polarized—used as a CPA system of the seed source—and it can achieve high compression efficiency using a grating pair. However, when the femtosecond pulse is directly output by the oscillator, the dispersion management in the cavity becomes critical, especially in the 1-μm band. Because of a lack of negative dispersion devices, construction of such lasers becomes more difficult. Recently, the microfiber has injected new vitality in femtosecond fiber lasers. Therefore, we will review the development of these two types of mode-locked fiber oscillators herein.
Conclusion and Prospect Mode-locked fiber lasers are gradually becoming a powerful tool in many fields. High-energy and highly stable oscillators based on all-fiber structures are more attractive; among these, NALM and microfiber laser have undergone rapid development recently. However, the stability, reliability, and startup performance of these lasers need more detailed research to ensure their wider commercial use.
