In recent years, with significant development in ultrashort pulse laser technology, femtosecond pulse lasers occupy an increasingly critical role in scientific research and industry. The overall tendency of femtosecond lasers is the higher peak power density, which is manifested by increasing pulse energy and reducing pulse duration. Multiple methods have been proposed to obtain femtosecond lasers with a pulse duration of <100 fs and a pulse energy of >10 μJ. In addition to Ti∶sapphire femtosecond lasers, which directly output femtosecond laser pulses with a high energy and short pulse duration, the compression of high-energy laser pulse output from Yb³⁺-doped femtosecond lasers is another pertinent method. Yb³⁺-doped fiber and solid-state femtosecond lasers output femtosecond laser pulses with higher average power, and therefore, show further application potential. However, owing to the gain bandwidth limitation, it is difficult to obtain laser pulses shorter than 300 fs. However, it is important to obtain femtosecond laser pulses with >100 μJ pulse energy and <100 fs pulse duration for many applications. To address this challenge, the nonlinear compression of femtosecond laser pulses is proposed, which improves the peak power density of laser pulses from Yb³⁺-doped femtosecond lasers. Thus, the proposed method enables further applications in industrial processing among other fields. However, some harmful effects exist which reduce nonlinear compression efficiency, such as conical emission. On the contrary, to increase nonlinear compression efficiency, it is important to suppress the conical emission and avoid nonlinear medium damage.

In this study, periodic layered Kerr media (PLKM) nonlinear compression principles are analyzed and experiments are conducted. For the conical emission effect in the nonlinear compression experiments, the harmful effect cause and its influence on nonlinear compression are experimentally studied. The spectrum broadening of pulse output from the Yb³⁺-doped femtosecond fiber laser is measured and analyzed. To weaken its influence on spectral broadening and nonlinear compression efficiency, phase shift distribution optimization during spectral broadening is proposed to suppress the obvious conical emission. The proposed method avoids the conical emission caused by the spectral broadening process in nonlinear compression. Subsequently, a nonlinear compression system based on PLKM is developed, and the output pulses from the Yb³⁺-doped fiber and Yb³⁺-doped solid-state femtosecond lasers are nonlinearly compressed.

With this two-stage nonlinear compression of laser pulse output from the Yb³⁺-doped femtosecond fiber laser, laser pulses can be obtained with 64 μJ pulse energy and 42 fs pulse duration. Also with the two-stage nonlinear compression of the laser pulse output from the Yb³⁺-doped solid-state femtosecond laser, laser pulses can be obtained with 315 μJ pulse energy and 79 fs pulse duration. Details of the experiment results are shown in Figs. 3 and 4. During the nonlinear compression of laser pulses output from the Yb³⁺-doped solid-state femtosecond laser, the spectral evolution is shown in Fig. 5. Owing to the optimizing nonlinear effect in each thin plate, the peak power density of femtosecond pulses on the thin plates is reduced, the obvious conical emission effect on each thin plate is avoided, and finally, the homogeneous broadening of the pulse spectrum is realized. With the increase in plate numbers, additional spectral broadening is obtained. By analyzing the compression results of laser pulses from the Yb³⁺-doped femtosecond fiber and Yb³⁺-doped solid-state femtosecond lasers, this nonlinear compression method is proved. Optimizing the distribution of the phase shift during spectral broadening effectively improves PLKM nonlinear compression efficiency and enables further applications for the nonlinear compression system in industry.

In nonlinear compression experiments, the Kerr lens caused by the self-focusing effect, intensifies imaging spherical aberration and produces obvious conical emission. The conical emission effect will affect beam quality and reduce compression efficiency, whose intensity is closely related pulse peak power density. During the nonlinear compression of femtosecond pulses with different energies using PLKM, the arrangement of thin plates in spectral broadening should be adjusted according to the level of pulse energy. To maintain a high compression ratio and compression efficiency, conical emission can be suppressed by increasing the numbers of medium plates and lowering the nonlinear phase shift of laser pulses in each thin plate, to avoid extremely high peak power density on plates, and improve nonlinear compression efficiency.