Divided pulse (divided pulse) combined with optical amplification has produced divided pulse amplification (DPA) technology. In recent years, many groups have applied divided pulse to various nonlinear pulse compression schemes and developed divided pulse nonlinear compression techniques to enhance pulse energy.
The first work presented in this issue combines split-pulse with multi-pass cavity compression for the first time. Multipass cavity compression has many advantages, such as higher transmittance and the possibility of safe and reliable nonlinear compression of kilowatt and millijoule pulses, however, it is also limited by cavity optical damage thresholds and gas ionization. To overcome these limitations, the Jena Limpert group in Germany used the split-pulse technique in a multipass cavity compression experiment. The front end is a 16-way chirped pulse amplification (CPA)-based fiber laser with a synthesized beam diameter of 8 mm and two-stage CPA compression, which can output a laser with an average power of 200 W, pulse energy of 4 mJ, and pulse width of 175 fs. The output laser is divided into four pulses by two BBO crystals and then enters the multi-pass cavity to broaden the spectrum, after which the pulse synthesis is realized by the BBO crystals, and finally the synthesized pulses are compressed using multiple chirped mirrors.
The beam passes through the multipass cavity 26 times with 350 mbar argon gas, and the overall output efficiency is 84% with a final output power of 169 W. The spectrum before and after compression is about 120 nm at 20 dB bandwidth of the compressed spectrum. When the pulse approaches the pulse transformation limit width of 32 fs, negligible small pulses can be seen at 800 fs, and no small pulses at all can be seen at 1600 fs, indicating a very good pulse time domain contrast and synthesis.
The second work presented in this issue combines split-pulse and hollow-core fiber compression to compress high-energy pulses. To avoid self-focusing effects and gas ionization by compressing pulses with inert gas-filled hollow-core fibers, options include changing the pulse polarization state to circular polarization, introducing a gas pressure gradient, and using the higher-order modes of the hollow-core fiber, but these methods still cannot raise the compressible pulse energy above the gas ionization threshold. They used calcite, half-wave plates and polarizers to split-pulse and synthesize the pulses, used a hollow-core fiber filled with Xe gas as the spectral broadening medium, and then used a chirped mirror to compress the synthesized single pulse.
The experimentally broadened spectrum has a typical parabolic structure broadened by the self-phase modulation effect, where the modulation stripe spacing is 0.5 nm, matching the 7.2 ps delay introduced by calcite. Finally, the chirped mirror is used to introduce a dispersion of -18000 fs2, compressing the pulse to 89 fs with a peak power of 91% of the peak power of the transform-limited pulse, which has an energy of 5.0 mJ.
The first successful application of split-pulsing to a multipass cavity nonlinear pulse compression scheme using four pulses for spectral broadening increased the total output pulse energy of the existing multipass cavity compression to 3.4 mJ with an average power of 169 W. G. W. Jenkins et al. used split-pulsing to overcome ionization limitations in hollow-core fiber compression, and for a single pulse, ionization limited the output pulse energy to 2.7 mJ. By splitting the pulse into four low-energy pulses, the group obtained a compressed pulse of 5.0 mJ.
