# Multistage Smoothing Compressor for Multistep Pulse Compressors
## 4 Sample Design for a 100 PW Laser  
To date, the central wavelengths of the proposed 100 s PW femtosecond laser facilities have mainly been located at approximately 925 nm. The optical schematic of a sample design for a 100 PW laser is also shown in Figure 5. Since the PS_I is not indispensable in this MS-MPC, it will not be discussed in detail here. The laser beam after the main amplifier is assumed to be a 370 \(\times\) 370 mm\({}^{2}\) beam with a 10 th-order super-Gaussian profiles in both the X and Y directions. The laser pulse is positively chirped to approximately 4 ns, of which the transform limited pulse duration is approximately 14.3 fs. The central wavelength is 925 nm, and the spectral bandwidth is about 200 nm. A spatial chirp width of approximately 5 mm is applied to the laser beam using two perpendicular prism pairs (PS_II) with a 15\({}^{\circ}\) apex angle and a separation distance of approximately 6 m. Then, a smoothed laser beam (-375 \(\times\) 375 mm\({}^{2}\)) is obtained, where the center of the 365 \(\times\) 365 mm\({}^{2}\) region has full spectral bandwidth. A beam-expander and relay-imaging system BE_2 is used to expand the laser beam by 1.73 times (approximately) on both sides to -650 \(\times\) 650 mm\({}^{2}\), where the center 633 \(\times\) 633 mm\({}^{2}\) region possesses full spectral bandwidth. Since the first grating G1 has a relatively high damage threshold owing to the nanosecond chirped pulse, which is approximately 2.7-folds higher than that of the last grating G4 with a femtosecond pulse, the bearable LSIM of G1 should be 1.6-folds higher than that on the last grating G4, considering 60% total compression efficiency. Then, the reserved intensity space for G1 is -2.0 times (1.6 \(\times\) 1.3) that of its damages threshold, which is a typical design value for a safe operation of most PW laser systems, if 1.3 times of the damage threshold fluence is reserved for the last grating. This means that G1 should usually be safe if we only consider the near-field affect. The PS_II can also remove the far-field affect from the laser beam before PS_II. Except for beam smoothing, the two prism pairs with a small apex angle may also be used to precisely correct the pulse front tilt and angular dispersion of the whole PW laser system. This is because all the transmitted plane optics in PW laser system, such as crystals in the amplifier, intentionally induced a wedge with a small angle to avoid the temporal contrast of the output laser pulse decreasing [28]. These wedges induce angular dispersion and a pulse front tilt to the output laser beam, which affect the final focal intensity. The weak reflected beam from the surface of the prisms  
Figure 5: Optical schematic of a typical MS-MPC. PS_I and PS_II: prism-pair system for beam smoothing; DM_1, DM_2, and DM_3: reflective deformable mirror for wavefront compensation at different stages; BE_1 and BE_2: 4f-system-based beam expander; asymmetric four-grating compressor: asymmetric four-grating compressor. M1-M5: plane-reflective mirrors; CP: compensation plate; PM: parabolic reflective mirror; FP: focal point. can also be used to monitor the spectrum, pointing stability, beam profile, time jitter, and wavefront of the laser beam. After the pre-compressor, the spatiotemporal modified laser beam is guided into an asymmetric four-grating compressor with four identical 1400 lines/mm gold-coated gratings. The incident angle on the grating is set to approximately 61\({}^{\circ}\), and this allows the beam size on the grating in the diffraction direction to be about 650/cos(61\({}^{\circ}\)) = 1340 mm. The perpendicular distances of the first and second grating pairs are set to 1215 mm and 1285 mm, respectively. As a result, there is a 70 mm length difference between the two grating pairs, and this induces a spatial chirp width of approximately 25 mm on the last grating.