Signatures of the Self-Similar Regime of Strongly Coupled Stimulated Brillouin Scattering for Efficient Short Laser Pulse Amplification

Plasma-based laser amplification is considered as a possible way to overcome the technological limits of present day laser systems and achieve exawatt laser pulses. Efficient amplification of a picosecond laser pulse by stimulated Brillouin scattering (SBS) of a pump pulse in a plasma requires to reach the self-similar regime of the strongly coupled (SC) SBS. In this Letter, we report on the first observation of the signatures of the transition from linear to self-similar regimes of SC-SBS, so far only predicted by theory and simulations. With a new fully head-on collision geometry, subpicosecond pulses are amplified by a factor of 5 with energy transfers of few tens of mJ. We observe pulse shortening, frequency spectrum broadening, and down-shifting for increasing gain, signatures of SC-SBS amplification entering the self-similar regime. This is also confirmed by the power law dependence of the gain on the amplification length: doubling the interaction length increases the gain by a factor 1.4. Pump backward Raman scattering (BRS) on SC-SBS amplification has been measured for the first time, showing a strong decrease of the BRS amplitude and frequency bandwidth when SBS seed amplification occurs.

Figure 1

Evolution of the growth rate (solid curves, left scale) ${\gamma }_{\mathrm{SC}}=\Im ({\omega }_{\mathrm{SC}})$ seen by the seed along its propagation (left to right), and for different delays $\mathrm{\Delta }t$ with the counterpropagating pump (dotted red curve, right scale). At $\mathrm{\Delta }t=0$ the pulses cross at $z=0$, in the middle of the plasma profile (blue area with dash-dotted curve, right scale). Calculated from Eq. (1) with experimental parameters (${\mathrm{H}}_2$, $I_{15}=1$, $n_e=0.1n_c$).

Figure 2

Energy gain $E_{\text{out}}/E_{\mathrm{in}}$ of the seed (left scale, blue circles) and pump Raman backscattered signal (right scale, green squares), as a function of relative delay between pump and seed arrival time at the plasma center. Full (empty) circles are the 2D (1D) calorimetry. Plasma density is 0.1 $n_c$. $\text{Gain}=1$; dashed line is the level of incident light. The level of seed transmission without pump is represented by red triangles. Raman signal is normalized to its value at the $\mathrm{\Delta }t=0\mathrm{ps}$ delay. The insets illustrate typical raw data for each diagnostics: 2D calorimetry (a), 1D calorimetry, i.e., seed spectrum (b-1 and b-2), Raman spectrum (c-1 and c-2).

Figure 3

Duration (FWHM, left axis, triangles) and energy gain (right axis, circles) of the transmitted light evaluated from a Gaussian fit of the autocorrelation trace. Same data set of Fig. 2. The width of the red band shows the amplitude of shot to shot fluctuations on the incident seed duration. The empty triangle (circle) is the seed duration (gain) without pump. The insets illustrate typical raw data.

Figure 4

(a) Transmitted spectra, expressed in $\mathrm{mJ}/\mathrm{nm}$, as a function of the delay. Two reference spectra in vacuum propagation are shown. Same data set of Figs. 2 and 3. (b) 1D-PIC simulated spectra: optimal case (blue solid line) and delayed (green). The peak at the initial pump and seed wavelength (1058 nm) is masked to show only the spectra related to the amplification process.