Significance In 2018, Gerard Mourou and Donna Strickland were awarded the Nobel Prize in Physics for their work on chirped pulse amplification (CPA) technology, which provides a reliable concept for improvement in femtosecond-laser energy. Ti∶sapphire femtosecond amplifiers have been developed using CPA technology and are widely used in the fields of attosecond science, and strong-field physics. Due to the limitations of pump power and thermal management, the repetition rate of Ti:sapphire amplifiers is typically less than 100 kHz. However, high-repetition-rate amplifiers at several hundred kilohertz and megahertz are essential for some scientific and industrial applications, such as XUV optical-frequency combing, high-flux high-harmonic generation, angle-resolved photo-emission spectroscopy, and micromachining. Under a fixed pulse energy, work at a high repetition rate means work at a high average power, which causes severe thermal issues.

With the rapid development of diode-pumping technology since the 1990, all-solid-state Yb femtosecond lasers have opened up a new path for the generation of high-power ultrashort laser pulses. Yb-doped lasers are very promising for high-power ultrashort-pulse generation due to their ability to be pumped by readily available high-power diode lasers, their intrinsically high efficiency and narrow pulse width made possible by their simple energy-level diagram, and their broad emission bandwidth and low quantum defect. Moreover, Yb-doped laser materials possess higher gain and thermal conductivity at cryogenic temperatures than at room temperature, enabling much higher output powers. Some Yb-doped materials with emission cross sections broad enough for femtosecond-pulse amplification include Yb∶YAG, Yb∶KGW, Yb∶KYW, Yb∶CaF₂, and Yb∶CGA; these were evaluated and found to have potential for use in high-repetition-rate amplifiers (Table 1). In combination with cryogenic refrigeration and traveling-wave amplification (Fig.5), the average power can reach several hundred watts.

Progress The key technical bottlenecks arise from thermal management of gain media and the gain-narrowing effects that accompany high-repetition-rate femtosecond amplification. Thus, the technical routes and research progress on amplifiers with different gain media are comprehensively summarized with reference to previous research on regenerative and traveling-wave amplifiers. For regenerative amplifiers (Fig. 1), Caracciolo's research group at the University of Pavia, Italy, has made pioneering contributions using crystals of Yb∶Lu₂O₃, Yb∶CGA, and Yb∶CaF₂. Pouysegur's research group from University of Paris-Sud reported on the first sub-100-fs regenerative amplifier based on an Yb-doped bulk gain medium (Fig. 3). For hundred kilohertz and above, traveling-wave amplification is one of the most effective ways to further enhance the power to mJ level and even higher. Thin-disk and slab technologies have proven to be very efficient and enable high output powers in the ultrashort regime with up to several kW of output power because of their thermal-management advantages. However, their inherently complex amplifier setups undermine their stability and wider industrial applicability. Single-crystal fiber is a promising alternative with a compact size, greater simplicity, and lower cost compared with other such technologies for obtaining several hundred watts of output power. In addition, most lasers are dominated by Yb∶YAG crystals; other Yb materials are also used in traditional rod amplifiers. Thermal effect and amplified efficiency of rod amplifiers are outstanding difficulties that need to be solved urgently. Among currently available technologies, diode-pumped cryogenically cooled solid-state amplifiers have emerged as the most promising alternative for achieving an output power of hundreds of watts with a rod amplifier. Zapata at the Center for Free-Electron Laser Science, Deutsches Elektronen Synchrotron increased the output power of the Yb∶YAG rod amplifier to 250 W (Fig.9). Due to the gain bandwidth of Yb∶YAG at low temperature, a femtosecond pulse is still difficult to achieve. Therefore, research on simple and versatile high-power amplifiers for femtosecond-pulsed operation is still ongoing. The problems faced in this field and our ongoing research are discussed.

Conclusions and Prospects Ultrafast pulse-laser amplifiers based on an Yb-doped laser medium can reach new heights of amplification efficiency and output power due to the development of efficient diode-pumping technology and the emergence of various new laser media. In the near future, we expect that the amplification system will serve in both scientific and industrial applications. Combining the current development trend of an all-solid-state ultrashort laser with the research basis of our group, we recognize the prospect of achieving a femtosecond laser with an output power of >100 W using new Yb-doped laser crystals and cryogenic technology.