State-of-the-art Yb-based fiber, Innoslab and thin-disk architectures facilitate efficient and power scalable femtosecond lasers, which, however, do not exhibit sufficient gain bandwidth for pulses shorter than 300 fs. Due to gain narrowing, workhorse Yb-doped crystals such as Yb:YAG and Yb:Tungstate, typically generate pulses of 400 fs to 1 ps pulse duration after chirped pulse amplification. To generate even shorter pulses, other strategies have proven to be viable, such as adoption of disordered media with broader gain bandwidth, combinations of gain media with slightly shifted gain spectra or spectrally coherent synthesis. Nonetheless, the disordered media displays poor thermal conductivity while the other two schemes suffer from complexity. Nonlinear spectral broadening by self-phase modulation (SPM) during propagations in regenerative and multipass amplifiers are impressive alternatives, which, however, demand elaborated control.

In addition to the above techniques implemented during construction of ultrafast lasers, another route to achieve shorter pulses is through the post-compression techniques. The basic process incorporates nonlinear spectral broadening and chirp removal with dispersive elements. No alterations to the laser system are required and a high efficiency can be guaranteed if implemented properly. Among the various techniques based on the optical Kerr-effect or photoionization for spectral broadening, the gas-filled multi-pass cell spectral broadening (MPCSB) and the multiple-thin-solid-plate (MTSP) schemes are now widely adopted with impressive results. The MPCSB method is characterized with almost unperturbed beam quality and spectral homogeneity across the beam profile, but suffers from pointing fluctuations and delays. The MTSP technique avoids the catastrophic collapse caused by the self-focusing effect in bulk medium with strategically arranged thin plates. It is more compact, flexible and economical, applicable for peak power well above the medium's self-focusing critical power of the medium and for pulse energies up to the millijoule level with robustness and reproducible performance. Early multiple-thin-solid-plate configurations were typically organized empirically, where substantial losses caused by conical emission usually arose. The conical emission also led to degradation of quality in time or space (or even both). In some cases, requirement for special designed chirped mirrors or pulse shapers is indispensable. As a relatively systematic study, the concept of quasi-stationary spatial solitons generation in periodic layered Kerr media (PLKM) stands out as a practical strategy. As the term periodicity indicates, the distances between adjacent plates are the same in this configuration. Specifically, a layer of solid thin plate with thickness of l and a layer of free space of length L are treated as a period. The repetitive propagation of an intense beam in PLKM was regarded as a resonator with intensity-dependent non-spherical mirrors. Thus, the Fresnel-Kirchhoff diffraction (FKD) integral was introduced into this theory to identify the self-consistent stationary modes. The PLKM arrangement improved the spatial quality and supported nonlinear light-matter interaction during a rather long distance even under tight focusing conditions. The integration of PLKM device and Yb-based high power chirped pulse amplifiers will favour an ultrafast source with high efficiency and great beam quality.

The research group from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences demonstrated an economical combination to mitigate the bandwidth limitations of Yb-based high power chirped pulse amplifiers through post compression technique. The research results are published on High Power Laser Science and Engineering, Volume 10, Issue 2 (Jie Guo, Zichen Gao, Di Sun, Xiao Du, Yongxi Gao, Xiaoyan Liang. An efficient high-power femtosecond laser based on periodic-layered-Kerr media nonlinear compression and a Yb:YAG regenerative amplifier[J]. High Power Laser Science and Engineering, 2022, 10).

Figure Caption: (a) Schematic of the ultrafast source: ①frontend; ②mode matching optics and isolators; ③regenerative amplifier; ④grating compressor; ⑤PLKM nonlinear compression stage. L1, L2, lenses of the telescope for mode matching; FI, Faraday isolator; TFP, thin-film polarizer; M1-M11, highly reflective mirror; HWP, half-wave plate; QWP, quarter-wave plate; PC, Pockels cell; HRM, horizontal roof mirror; VRM, vertical roof mirror; TG, transmission grating; CM1-CM4, Chirped mirrors. (b) Detailed periodic layered Kerr media (PLKM) configuration.

In this work, a compact and efficient ultrashort laser source was presented, which comprised a Yb:YAG regenerative amplifier and a subsequent close-to lossless periodic-layered-Kerr-media-based nonlinear pulse compression stage. The nonlinear pulse compression stage featured a transmission of 96%, excellent beam quality and spectral homogeneity across the beam profile. Compared with the setup described in most other references, there's no need to filter out the conical emission or apply custom-tailored chirped mirrors in our work. The absence of conical emission intrinsically ensured the high efficiency and excellent beam quality simultaneously. To the best our knowledge, for pulse energy over 200 μJ, this is the highest output power reported for the multiple-thin-solid-plate schemes. This configuration successfully compensated the gain bandwidth limitation of Yb:YAG regenerative amplifier. The final output pulse duration was 195 fs with an average power of 54 W at 200 kHz repetition rate, while the pulse duration directly from the grating-based compressor of the chirped pulse amplifier (CPA) system was 534 fs. These results underline the benefits of this combination. The demonstrated source is promising for further power scaling and compression to sub-50 fs to drive high field physical processes and bright secondary radiation at high average power.