1 Introduction

Inertial confinement fusion (ICF) has drawn the attention of governments and academies because it’s the candidate to solve the energy crisis and to defend the national security. To reduce the laser driver scale, Tabak et al. [1] proposed the fast ignition scheme, but the time jitter between the ignition pulse and the compression pulse must be less than 10 ps. Moreover, it is difficult to manufacture the target of the fast ignition scheme. In 2007, Betti et al. [2] proposed the shock ignition scheme for ICF, which pointed out that the ignitor shock wave would be launched if a laser pulse of 100 ps with an energy of about 100 kJ can be generated. And the shock ignition scheme has made a great influence on the laser ignition dream which human has striven for nearly 40 years [3–5]. However, one of the key technologies to realize the shock ignition is the generation of a drive laser pulse. The drive laser pulse requires a width of about 200 ps, an energy of several kilojoules and a pulse peak power of ten terawatts, which presents a new challenge for amplification technology of the high-power laser.

Stimulated Brillouin scattering (SBS) provides a simple and effective method to compress pulses for high-power and high-energy lasers [6–15]. Also, a pulse of 100 ps can be amplified to high intensity by a nanosecond pulse using SBS [16–18]. In order to attain the shock ignition laser driver, SBS can be used to transfer energy from a nanosecond laser to a 100-ps laser. To achieve this idea, the proper Brillouin medium must be found.

In this paper, we studied two new SBS medium, FC-43 and FC-70. They are the candidate mediums for SBS pulse compression and high-energy Brillouin amplification because that the liquid nature is lower absorption, higher laser intensity load and shorter phonon lifetime. In the pulse compression experiment, the shortest compression pulse is 235 ps of FC-43 and 175 ps of FC-70. The reflectivity is more than 80 %. When the input laser pulse is about 300 mJ, the compressed pulse width is between 235 and 355 ps for FC-43, with a fluctuation range of 100 ps and the average width of 280 ps. And for FC-70, the compressed pulse width is between 175 and 235 ps, with a fluctuation range of 60 ps and the average width of 200 ps. The compression efficiency and stability of FC-70 is better than FC-43, and FC-70 is very suitable to be chosen for generating 100-ps pulses.

2 New SBS medium

An excellent SBS medium for generating 100-ps pulse and amplifying high-intensity laser pulse must fulfill three requirements: low absorption, high-energy load and short phonon lifetime. Firstly, from the molecular structure view, the medium absorption of light in the near IR region is dominated by chemical bonds of X–H in stretching modes (X refers to carbon, nitrogen, oxygen, sulfur and phosphorus; H refers to hydrogen) [19]. So the low absorption medium should include the least amount of these chemical bonds. Secondly, the outer atoms of the medium molecules must protect the inner chemical bond. Otherwise, optical breakdown emerges easily in the medium interaction with high-intensity laser. So the radium of the outer atoms must be small [20]. Finally, the phonon lifetime is dominated by the kinematic viscosity. And they have an inverse relationship. So the medium should have a big viscosity.

We find two new SBS medium, FC-43 and FC-70 which belong to perfluorinated amines [21]. The perfluorinated amines have the chemical structure (C n F2n+1)3N. FC-43 is composed of (C3F7)3N and FC-70 is composed of (C5F11)3N, as shown in Fig. 1.

Fig. 1
figure 1

Molecular structure. a FC-43, b FC-70

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First, the perfluorinated amines are composed of the chemical bonds C–C, C–F and C–N and without X–H so their absorption coefficient is small. Secondly, the radius of fluorine atom is smallest (Van der Waals radius is 0.135 nm), and they can protect the C–C bond efficiently [20], so the optical breakdown threshold (OBT) is high. Finally, the viscosities of FC-43 and FC-70 are bigger than those of other mediums so their phonon lifetimes are shorter, especially for FC-70. The physicochemical properties and SBS parameters of liquid SBS media FC-43 and FC-70 are shown in Table 1. The absorption coefficient and the OBT are measured directly [22], phonon lifetime, gain coefficient and Brillouin shift are calculated according to the equations given by Ref. [23]. As shown in Table 1, the phonon lifetime of FC-70 is only 0.031 ns, which is the recorded shortest in current literatures. FC-70 also has the prosperity of low absorption and high load. It is better than other short phonon lifetime mediums such as HT-230, HS-260 and HT-270 [22].

Table 1 Physicochemical properties and SBS parameters of FC-43 and FC-70
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3 Experiment study

The experiment setup is shown in Fig. 2. The laser source was a single mode injection seeded Q-switched continuum Nd:YAG laser with a TEM00 mode at a fundamental wavelength 1.064 μm. The optical isolator system consisted of Faraday rotator, half-wave plate, polarizer and 1/4 plate. The SBS pulse compression system consisted of two SBS cells, convex lens F1, F2. The input laser energy was adjusted by rotating the half-wave plate. The energy of the input laser and the compressed laser was detected by MIN-E1000 energy meter. The pulse shapes were detected by a new focus PIN photodiode (18.5-ps rise time) and recorded by a digital oscilloscope with a bandwidth of 12.5 GHz, a sample rate of 100 Gs S−1, a rise time of 32 ps.

Fig. 2
figure 2

Schematic of the experiment setup

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In the experiment, the input pulse width was 12.6 ns with the divergence angle of 0.45 mrad. The compact double-cell system was chosen as the compressing structure. The generating cell L2 was 100 cm long, and the amplifying cell L1 was 80 cm long. To improve the laser intensity in L2, the focal length of the convex lens F1 was 150 cm long. And to optimize the generating process, the focal length of F2 was 60 cm and the distance between F2 and the generating cell was 10 cm. FC-43 and FC-70 were chosen as the SBS medium.

Figure 3 shows the relationship between Stokes pulse width and input energy in experiment. It can be seen that the Stokes pulse (the compressed pulse) width of both FC-43 and FC-70 narrows down rapidly while improving the input laser energy from 100 to 150 mJ and then changes slowly. This is because that the overall gain of the compression system grows fast and the leading edge of the Stokes pulse which was amplified in the amplifying cell grows faster than the other part. But when the Stokes pulse width was close to the medium phonon lifetime, increasing the input energy contributes to the compression process little [24]. For comparison, the compressed pulse of FC-70 was narrower than the compressed pulse of FC-43. Using FC-70, a laser pulse with 200 ps or shorter can be achieved. As shown in Table 1, the phonon lifetime of FC-70 is shorter than FC-43. It is shown that the phonon lifetime of the SBS medium is shorter, the compressed pulse can be got narrower.

Fig. 3
figure 3

The experimental curve of the Stokes pulse width with different pump energy

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Figure 4 shows the narrowest pulse we got in this experiment, using FC-43 and FC-70, respectively. The narrowest pulse with FC-43 was 235 ps, and the narrowest pulse with FC-70 was 175 ps. The statistic distribution of 80 Stokes pulses is shown in Fig. 5, when the input laser was 300 mJ. The compressed laser pulse width of FC-43 was between 225 and 335 ps, with a fluctuation range of 100 ps and an average width of 280 ps. And for FC-70, the compressed pulse width was between 175 and 235 ps, with a fluctuation range of 60 ps and an average width of 200 ps.

Fig. 4
figure 4

The shortest compression pulse of FC-43 and FC-70

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Fig. 5
figure 5

Statistical pulse width of Stokes when input energy is 300 mJ

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The energy reflectivity of the FC-43 and FC-70 under different input energies is shown in Fig. 6. When the input energy was equal, the energy reflectivity of FC-43 and FC-70 tends to be close, and the highest energy reflectivity is more than 80 % for both of them. The energy reflectivity is related to not only the gain coefficient but also the absorption coefficient, optical breakdown threshold and other nonlinear effects.

Fig. 6
figure 6

SBS energy reflectivity with different pump energy

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4 Conclusion

In this paper, we find two new SBS mediums FC-43 and FC-70, from the relationship between the medium molecular structure and SBS parameters. The SBS parameters of these two mediums are measured and calculated. The absorption coefficient of FC-43 and FC-70 is 0.0012 cm−1, the OBT is 178 and 180 GW cm−2, and the phonon lifetime is 0.2 and 0.031 ns, respectively. They are both candidates for generating 100-ps pulse and high-intensity Brillouin amplification. The shortest pulse width achieved by SBS compressor was 235 ps for FC-43 and 175 ps for FC-70, and the highest energy reflectivity was more than 80 % in both mediums. When the input laser pulse was 300 mJ, the compressed pulse width was between 235 and 355 ps for FC-43, with a fluctuation range of 100 ps and an average width of 280 ps. And for FC-70, the compressed pulse width was between 175 and 235 ps, with a fluctuation range of 60 ps and an average width of 200 ps. The compression efficiency and stability of FC-70 is better than FC-43, and FC-70 is very suitable to be chosen for generating 100-ps pulses.