High-average-power and high-energy femtosecond lasers are urgently required in both fundamental science and industrial applications. The exceptional combination of these parameters allows the exploration of extreme nonlinearities of matter and the extension of ultrashort pulse generation, heralding a new revolution in ultrafast science. Owing to small quantum defects and the availability of high-power laser diodes (LDs) as a pump source, substantial progress has been made in enhancing pulse energy and peak power of Yb-doped solid-state lasers. However, scaling pump power in solid-state laser amplifiers introduces severe thermal lensing effects in the gain medium, leading to cavity mode mismatch between the pump and signal beams. This degradation in stability poses a fundamental limitation on further power and energy scaling.

In this study, we overcome the critical challenge of thermal lensing-induced cavity instability under high-power pumping. A Yb:CALGO (Yb:CaGdAlO₄) crystal based regenerative cavity with robust thermal stability and dynamic mode matching at pump power levels approaching 400 W is realized. This cavity design enables high-performance femtosecond laser operation, delivering simultaneous high average power and high pulse energy with approximately 130-fs pulse durations. The research published in High Power Laser Science and Engineering, Volume 13, Issue 2, 2025 (Zhengru Guo, Jiandong Liu, Tingting Liu, Tianjun Yao, Qiang Hao, Heping Zeng, Edgar Kaksis, "High-power high-energy Yb-doped CaGdAlO4 regenerative amplifier with approximately 130 fs pulses," High Power Laser Sci. Eng. 13, 02000e17)

The regenerative amplifier (RA) system relies on the CPA technique and consists of four blocks (see Figure 1), namely, a broadband fiber seeder, stretcher, RA and compressor.

Figure 1. Layout of the Yb:CALGO regenerative amplifier.

The system incorporates a Yb-doped fiber seeder generating sub-100-fs ultrashort pulses to serve as the front end. Broadband spectrum ranging from 1000-1060 nm is achieved through nonlinear amplification techniques, ensuring complete overlap with the Yb:CALGO crystal's emission bandwidth (Δλ≈60 nm) to effectively mitigate gain narrowing effects in subsequent regenerative amplification stages. Then, the seed pulses are temporally stretched to 600 ps using an Offner-type stretcher configuration prior to injecting into the RA. When the PC is added with an opening time of 1050 ns (equivalent to 54 round trips), the amplified pulse extracted from the RA reaches an average power of approximately 53 W at a repetition rate of 50 kHz, corresponding to a pulse energy exceeding 1 mJ. At lower repetition rate of 5 kHz, the obtained pulse energy exceeds 7 mJ, corresponding to an average power greater than 35 W. After compression, the maximum average power and pulse energy are 55.0 W at 200-kHz repetition rate and 6.6 mJ at 5-kHz repetition rate, respectively. The compressed pulse has a duration of 130 fs, corresbonding to a peak power of approximately 45 GW.

Figure 2. Measured output spectra (a), Frog trace (b) and power stability (c).

Looking forward, by improving the RA design, such as incorporating Brewster-angle cleaved crystal, utilizing stronger pumping, implementing longer seed pulse stretching and employing post-compression techniques, we anticipate achieving 10-mJ level, over 100 W and sub-terawatt level output pulses.