## 2 Experimental Section  
The experimental setup of the hundred picoseconds laser system based on SBS pulse compression is shown in Figure 3. The laser system is composed of a SLM oscillator, an optical isolator, a beam expander and a SBS pulse compressor. Figure 3: Schematic layout of the hundred picoseconds laser system. SLM: single longitudinal mode; SBS-PCM: Stimulated Brillouin scattering phase conjugation mirror; FR: Faraday rotator; YAG: Y\({}_{3}\)Al\({}_{5}\)O\({}_{12}\). Figure 2: Three kinds of pulse compression device based on SBS technology (**a**) single-cell structure; (**b**) two-cell structure; (**c**) compact single-cell structure. Beam splitter: polarizing plate or polarizing prism; \(\lambda/4\): quarter wave plate. Part 1 is the flash-lamp-pumped SLM oscillator based on three-plan resonant reflector. The Nd\({}^{3+}\):\(Y_{3}\)Al\({}_{5}\)O\({}_{12}\) (Nd:YAG) rod has a length of 120 mm and a diameter of 3 mm with a 1.0% Nd\({}^{3+}\) concentration. The oscillator is passively Q-switched by a Cr\({}^{4+}\):YAG crystal with initial transmittance 7.5%. Lower transmittance of the saturable absorber can guarantee the Q-switched output with narrower linewidth and pulse width. The laser cavity is a compact plane-plane configuration with cavity length of 480 mm. M\({}_{1}\) is a 0\({}^{\circ}\) concave high reflectivity (HR) mirror at 1064 nm. M\({}_{2}\) is a flat mirror with reflectivity of 50% at 1064 nm as an output coupler (OC). A Fabry-Perot (F-P) etalon E with the thickness of 2 mm is inserted into the cavity to restrict the SLM output through the interaction with the output mirror M\({}_{2}\), which is equivalent to a composite etalon. P\({}_{1}\) is a Brewster angle polarizer. An aperture diaphragm (AD) is used to filter high-order mode. Part 2 is the optical isolator, consisting a polarizer P\({}_{2}\), a 1/2 wave-plate and a Faraday rotator (FR), used to guarantee the stability of the system by preventing backward Stokes returning to the oscillator. Part 3 is the beam expander system with a factor of 2 expands the diameter of the laser beam from oscillator. Part 4 is the SBS pulse compressor consists of a flash-lamp-pumped Nd:YAG amplifier and a SBS cell. 1.0% doped Nd:YAG crystal with a diameter of 7 mm and a length of 140 mm is used as gain medium. FC-770 is chosen as the SBS medium in the SBS cell, which is a multi-constituent perfluorinated compound with molecular formula of C\({}_{7}\)F\({}_{15}\)NO. FC-770 has the characteristics of low phonon lifetime (0.57 ns), small Brillouin frequency shift (1081 MHz) and moderate gain coefficient (3.5 cm/GW), which is a suitable medium for SBS pulse compression [22; 23]. To reduce the optical breakdown caused by impurities, we filter the FC-770 medium in a vacuum using two layers Millipore filter with pore size 0.05 \(\upmu\)m. Stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) and pulse compressor are two primary functions of the SBS cell in our experiment. As a PCM, it can compensate the phase distortion caused by thermal effect and inhomogeneity of the gain crystal [24]. As the illustration of SBS-PCM distortion compensation shows, optical distortion occurred when the seed light (red line) first passes through the laser medium. Due to phase reversal property of the SBS-PCM, after double passing through the optical component, distorted light can be recovered to diffraction-limited (green line) as the original. Meanwhile, the reflected light path from SBS-PCM is strictly same as the incident path, no matter what angle of the SBS cell placed or changed. These remarkable characteristic is helpful for us to realize high beam quality amplified output with high stability. As a pulse compressor, the SBS cell can realize the high efficiency pulse compression of the high energy pulses. The adopted structure of the SBS compressor in our design is a compact single-cell. The length of the SBS cell is 600 mm.