Laser plasma amplification of sub-picosecond pulses above the Joule level is demonstrated, a major milestone for this scheme to become a solution for the next-generation of ultra-high intensity lasers. By exploring over 6 orders of magnitude the influence of the incident seed intensity on Brillouin laser amplification, we reveal the importance of a minimum intensity to ensure an early onset of the self-similar regime, and a large energy transfer with a very high efficiency, up to 20%. Evidence of energy losses of the seed by spontaneous backward Raman is found at high amplification. The first three-dimensional envelope simulations of the sub-picosecond amplification were performed, supplemented by one-dimensional PIC simulations. Comparisons with the experimental results demonstrate the capability of quantitative predictions on the transferred energy. The global behavior of the amplification process, is reproduced, including the evolution of the spatial profile of the amplified seed.
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