Significance High-power all-solid-state ultra-fast lasers are highly efficient, stable, compact, and cost-effective. They have several scientific and industrial applications that benefit from short pulse width and high power. Laser systems based on Ti:Sapphire deliver sub 100 fs pulses easily, but their average power is limited to a few watts. Yb-based laser systems can reach a few hundred watts, even more than a kilowatt, but their pulse widths are restricted to 100 fs due to the gain bandwidths of the laser medium. In order to meet the increasingly demanding requirements of lasers in various fields, it is necessary to develop nonlinear pulse compression technology to obtain higher peak power.

According to the theory of the time-width product of a Fourier-transform limited pulse, spectral broadening is an inevitable step before nonlinear pulse compression. When propagating through nonlinear medium, the spectrum of high energy laser pulses is broadened due to self-phase modulation based on the optical Kerr effect. After subsequent chirp removal of the spectrally broadened pulse, a temporally compressed pulse can be obtained. As a result, the pulse compression relies on nonlinear spectral broadening. Nonlinear pulse compression methods can be distinguished according to the nonlinear medium used for spectral broadening. Different methods are suitable for different pulse energies and peak powers; their compression effects also differ.

In recent years, various methods have been proposed to achieve higher peak power, enhance compression ratio, and improve efficiency. However, these methods still face a series of challenges. Therefore, it is necessary to summarize the current research progress and future prospects of nonlinear pulse compression to guide future development of this field.

Progress The method based on bulk dielectrics is the earliest compression approach for high energy pulses. Rolland et al used bulk silica as the nonlinear medium to broaden the spectrum. They compressed a 100-μJ pulse from 100 fs to 20 fs, but space chirp of the broadened spectrum was severe and the loss of pulse energy was too high. When propagating through a bulk dielectric, an ultrafast laser with high peak power can cause catastrophic self-focusing due to the low critical power of bulk media. The self-focusing critical power of noble gases is much higher than that of dielectrics because of the smaller nonlinearity of gases. Hence, hollow-core fibers with noble gases are effective in the compression of high peak power ultrafast lasers. This approach was proven in 1996 and the disadvantages of compression based on a dielectric nonlinear medium were overcome. Nonlinear pulse compression from 740 fs to 88 fs was achieved using the gas-filled hollow-core fiber approach in 2014 (Fig.3). Pulse energy is limited in this scheme due to the waveguide effect.

Therefore, a different approach for higher peak power, broadening the spectrum with multiple fused silica plates, was developed in 2014 (Fig.4). This approach overcame the limitation of pulse energy and can be applied to higher peak power ultrafast lasers. However, the broadened spectrum of this method is inhomogeneous, thereby limiting the compression efficiency. In order to solve this problem, a new technique using a multi-pass cell (MPC) was presented by Schult et al. in 2016. The MPC technique depends on repeated propagation through a nonlinear medium with a small nonlinear phase for each pass. This ensures homogeneous spectral broadening because the nonlinear phase per pass is chosen to be so small that the impact of propagation is negligible. A compressed pulse with 6.5-MW peak power and 330 W average power from 860 fs to 115 fs was achieved by using fused silica as the nonlinear medium; the conversion efficiency of this method is more than 90% (Fig.6). A noble gas was used as the nonlinear medium, due to the larger self-focusing critical power of gas compared with dielectric material, and they achieved compressed pulses with 530-W average power from 590 fs to 26.5 fs with 58 passes and 3.5 bar Argon (Fig.8). The highest compressed pulse energy achieved by MPC is 18 mJ with a 39 fs pulse duration.

Furthermore, when a certain compression method is insufficient, two-stage nonlinear compression is another effective method that can obtain laser pulses with shorter pulse duration and higher peak power. A compressed average power of 98 W and 166 MW peak power with 27 fs was demonstrated in 2019 by combining a MPC with multiple plates (Fig.10).

Conclusions and Prospects Nonlinear pulse compression is a current research focus in the field of ultra-fast lasers. The approach for pulse compression based on gas-filled hollow-core fibers is still the method primarily adopted to obtain few-cycle pulses with high energy. However, the pulse energy obtained by this method is limited due to catastrophic self-focusing. Multiple fused silica plates is an approach that omits the waveguide, so it can be applied to laser pulses with high peak power. Additionally, the approach based on a MPC ensures homogeneous spectral broadening, reduces energy loss, and improves compression efficiency. Compared with a dielectric medium, such as fused silica, a noble gas has a lower nonlinear index of refraction, therefore a gas-filled MPC is suitable for higher peak power. Furthermore, two-stage nonlinear pulse compression is also an effective option. Nonlinear pulse compression technology enhances the applications of Yb-based and even Ti:Sapphire laser systems. New and efficient methods applicable for higher energy pulses or capable of achieving a higher compression ratio are expected to emerge.