## 1 Introduction  
Ultrashort lasers with high energy and high power intensity have wide application prospects, such as laser induced plasma (LIP), space debris detection, laser peening, nonlinear optics, spectroscopy, and laser processing [1; 2; 3; 4; 5]. Generally, the so-called ultrashort pulse refers to the lasers with pulse width between nanosecond (\(\sim\)ns) and femtosecond (\(\sim\)fs). In terms of output characteristics, the picosecond laser has shorter pulse width and high peak power compared with nanosecond laser; on the other hand, compared with the femtosecond laser, the picosecond laser also has higher stability and easily achieves high energy. These characteristics make picosecond lasers more attractive for many applications. At present, mode-locking technique is the main method for obtaining picosecond pulses with pulse energies a few nano-joule and repetition rate up to hundreds MHz [6; 7; 8]. It is necessary to achieve significant higher pulse energies while keeping the pulse duration amplifiers. In this case, regenerative amplifiers are commonly used to strongly amplify selected individual optical pulses from a mode-locked train emitted by oscillators and allow multi-pass through the gain medium placed in an optical resonator. Although the amplification gain is about \(10^{5}\) or \(10^{6}\) of the injected energy, the maximum energy obtained after the regenerative amplifier is just in the order of few milli-joules as reported [9; 10; 11; 12]. In addition, the structure of the regenerative amplifier is very complicated with poor stability. Such a low single pulse energy is still unable to meet the needs of a lot of applications. Moreover, in some specific areas, ultra-short pulse lasers with single longitudinal mode (SLM) are often required such as optical sensing, laser radar and optical frequency standard. That means that the mode-locked picosecond laser cannot meet the application requirements in those fields. The stimulated Brillouin scattering (SBS) phenomenon was discovered decades ago and has been widely used in phase stabilization, pulse compression, beam combination and ultrafast pulse shaping [13]. Among them, in the field of laser pulse compression, SBS is an effective method to compress a long pulse for its high energy reflectivity and high compression ratio. In particular, SLM output can be obtained by SBS pulse compression due to the Stokes spectrum is the result of convolution of the pump spectrum and the SBS medium gain spectrum. The temporal schematic diagram of SBS pulse compression process is shown in Figure 1. The medium of SBS cell can be liquid, gas or solid. By SBS pulse compression, Stokes light with pulse width of few and sub- nanoseconds can be obtained [14; 15; 16]. In order to achieve pulse compression, different types of SBS compressor were designed including taped waveguide, one-cell geometry, oscillator amplifier, compact two cell, scalable two cell, and so on. Three kinds of the most widely used structures are schematically depicted in Figure 2. Figure 2a shows the original single-cell pulse compressor with long focal-length lens. This structure suffers disadvantages of easily occuring optical breakdown and low compression ratio that make it unsuitable for practical application [17; 18]. Figure 2b is the basic two-cell structure with two separate cells: one relatively short cell as a generator, and the other as an amplifier. A short focal lens is adopted to obtain higher reflectivity with less sensitivity to injected energy. In order to ensure the amplification effect, length of the SBS cell in Figure 2a,b should be no less than shortest theoretical length \(L\):  
\[L=\frac{\tau\cdot c}{2n} \tag{1}\]  
where \(\tau\) is the pump pulse duration at full width at half maximum (FWHM), \(c\) is the light speed, \(n\) is the refractive index of SBS medium. Figure 2c is the compact single-cell structure involving a concave mirror coated for high reflectivity at pump wavelength that redirects the generated Stokes light through the same cell for amplification.