Significance In 1960, after the invention of the first ruby laser, fast-developed solid-state, fiber, gas, and semiconductor lasers provided great support for the research and development of multiple applications, such as optical communication, industrial processing and manufacturing, military and national defense, and state-of-the-art scientific research. Fiber lasers with good heat dissipation characteristics, excellent transverse mode, high amplification efficiency, compact laser construction, and less costs have become the first choice in developing next generation high-power ultrafast lasers. Fiber lasers can achieve long-term operation stability with good beam quality under above-average power because of their waveguide characteristics and large specific gain fiber surface area. High-power ultrafast fiber lasers usually contain four modules, ultrafast fiber oscillators, optical parameters management, ultrafast fiber amplifiers, and nonlinear compression. Ultrafast fiber oscillators provide seed lasers to achieve high-power ultrafast fiber lasers. A qualified mode-locked fiber oscillator has long-term stability and a proportional repetition shared rate corresponding to the requirements of high-power fiber amplifications. Optical parameters management plays a key role in inhibiting uncompensated nonlinear effects and enabling high-energy pulse output with good pulse quality after optical pulse stretching, high power fiber amplification, and optical pulse compression. The ultrafast fiber amplifiers are key modules to scale up the average power of the stretched-signal pulses. Unfortunately, the uncompensated nonlinear phase introduced by the high-peak power of the signal pulse distorts the pulse profile during its propagation in the fiber system. Based on the well-managed optical parameters of fiber lasers, the well-known fiber amplification methods, such as chirped-pulse, divided-pulse, and pre-chirp managed amplifications are making a significant breakthrough in achieving high-power ultrafast fiber lasers. The pulse duration after high-power fiber amplification is hundreds of femtoseconds limited by the gain-narrowing effect. Therefore, a further cascaded nonlinear compression stage is needed for shortening the amplified pulses, which can realize single/few optical cycle pulse duration to fulfill the requirements of the state-of-the-art physical experiments. With their excellent optical characteristics, the fast-developing high-power fiber lasers can play an increasingly important role in multiple applications.
Progress Progress in developing ultrafast fiber oscillators, optical parameters management, ultrafast fiber amplifiers, and nonlinear compression are summarized in this paper, and latest published results are discussed by illustrating the advantages and disadvantages of different methods. The highest repetition rate of fiber oscillators reported using the method of nonlinear polarization rotation is 1 GHz provided to be useful in astronomical optical frequency comb, pulse stacking, and the cavity-enhanced high harmonic generation. The highest average output power and pulse energies are 1.98 W and 684 nJ, which are achieved with the nonlinear loop mirror mode-locking scheme, respectively. Applying a semiconductor saturable absorber mirror to the mode-locked fiber laser can generate an output mode-locked laser with the repetition rate range of 10 kHz--1 GHz and sub-μJ pulse energy. As a newly invented mode-locked method, Mamyshev mode-locked fiber laser has attracted attention for its broadband optical spectrum, high-pulse energy output, and high-peak power. As the seeder for a high-power ultrafast fiber laser system, further efforts need to be taken in developing a more stable fiber oscillator with better parameters.
Relying on optical parameter management, current ultrafast fiber amplifiers are realized with different amplification methods, such as chirped-pulse, divided-pulse, and pre-chirp managed amplifications. The highest average output power of 830 W at 1 μm was reported by applying the chirped-pulse amplification. Limited by the transverse mode instability and thermal damage threshold, there is one research direction for further improvement that can be realized by searching for new gain materials with better optical performances. Combining the chirped-pulse and multi-channel divided-pulse amplifications, the highest average output power of 10.4 kW was obtained in a 12-channel fiber laser amplifier. 36 fs mode-locked pulses with 100 W average power were achieved with the method of pre-chirp managed amplification, avoiding adding a cascaded nonlinear compression stage. Apart from the aforementioned amplification methods, coherent pulse stacking method is also an efficient way in realizing ultrafast fiber laser with high-pulse energy. Pulse energy of 10 mJ was achieved with the coherent pulse stacking based on the high-power ultrafast fiber laser source.
It is difficult to realize sub-100 fs or even shorter pulse durations in a high-power fiber chirped pulse amplification system due to the gain-narrowing effect. Therefore, a further nonlinear compression stage is necessary to satisfying the state-of-the-art applications, requiring short pulse duration. Multipass cells with quartz sheet/noble gas and noble-gas-filled hollow-core fibers are two common constructions in building the nonlinear compression stage, which are illustrated in the nonlinear compression section of this paper. The pulse duration can be compressed to 22 fs, and a pulse energy of 15.6 μJ was realized in the multipass cell construction. Using the noble-gas-filled hollow-core fibers, pulse duration was shortened to approximately 4.3 fs corresponding to a 1.6 optical cycle with a pulse energy of 1 mJ.
Conclusions and Prospect In this paper, the high-power ultrafast fiber laser systems are introduced. Research and development status of high-power ultrafast fiber lasers are illustrated along with introducing principles and internal relations of four fundamental modules of ultrafast fiber oscillators, optical parameters management, ultrafast fiber amplifiers, and nonlinear compression. Depending on the fast-developing requirements from multiple state-of-the-art applications, more efforts need to be taken. Further research directions in developing high-power ultrafast fiber lasers have prospected. One promising way is investigating new fiber materials with promising better optical parameters compared to fused silica. Further, making contributions in developing the aforementioned fiber amplification methods is also an efficient way in developing fiber lasers with above-average power, higher-pulse energy, and shorter pulse duration. Newly designed optical fiber amplification methods still need to be invented by carefully considering the optical characteristics of fiber gain material and theoretical nonlinear optical conditions. High-power ultrafast fiber lasers can play a key role in multiple state-of-the-art applications relying on the development of searching for more functional fiber gain materials, optimizing aforementioned amplification techniques, and inventing new methods in amplifying high-power ultrafast fiber lasers.