Amplification of Ultrashort Laser Pulses by Brillouin Backscattering in Plasmas

Abstract

Plasma media, by exciting Raman (electron) or Brillouin (ion) waves, have been used to transfer energy from moderately long, high-energy light pulses to short ones. Using multidimensional kinetic simulations, we define here the optimum window in which a Brillouin scheme can be exploited for amplification and compression of short laser pulses over short distances to very high power. We also show that shaping the plasma allows for increasing the efficiency of the process while minimizing other unwanted plasma processes. Moreover, we show that, contrary to what was traditionally thought (i.e., using Brillouin in gases for nanosecond pulse compression), this scheme is able to amplify pulses of extremely short duration.

Short-pulse amplification by strongly coupled stimulated Brillouin scattering

“…a greater degree of pump depletion is obtained [45,46,57], (3) SBS is more robust than SRS to plasma inhomogeneities in density or temperature [3,39,45,51], (4) SBS is better suited for producing pulses with high total power or energy, in part because the lower sensitivity to inhomogeneity allows larger diameter plasmas to be used [51], (5) only SBS may be used in the regime 0.25 < N < 1 [52], (6) the duration of a Brillouin-amplified pulse can be shortened to within a factor of 8 of that for a Raman compressed pulse [3], suggesting that the two methods are capable of comparable pulse-compression, (7) a shorter interaction length is required for SBS because the energy transfer is fast [3,45,57], which is sometimes quantified as SRS requiring mm to cm scale plasmas whereas SBS can be conducted in 100 µm [46], and (8) SBS may be viable in regimes where SRS is limited by particle trapping and wavebreaking [3] and can therefore support pump amplitudes several orders of magnitude higher than SRS [59]. Additionally, we make the argument (9) that SBS may be appropriate in regimes where Langmuir waves would be collisionally damped.…”

“…In plasmas, this may result from reduction in the plasma density by heating or ponderomotive expulsion of electrons and ions, or a reduction in the plasma frequency due to relativistic mass increase of electrons in intense fields. Difficulty associated with filamentation instabilities at high plasma densities has been a key motivation for studying the utility of SBS at N < 0.25 [45,51].…”

“…The differences between SRS and SBS have been articulated with varying degrees of rigor, though direct comparisons appear mostly in literature on SBS, where the following advantages for Brillouin amplification over Raman amplification have been presented: (1) the pump and seed lasers may have almost the same frequency [3,39,45,48,49,57,59], (2) energy loss to the plasma wave, which results from conservation of energy and is described by the Manley-Rowe relations, may be lower for SBS than for SRS, [3,39,46,52], i.e. a greater degree of pump depletion is obtained [45,46,57], (3) SBS is more robust than SRS to plasma inhomogeneities in density or temperature [3,39,45,51], (4) SBS is better suited for producing pulses with high total power or energy, in part because the lower sensitivity to inhomogeneity allows larger diameter plasmas to be used [51], (5) only SBS may be used in the regime 0.25 < N < 1 [52], (6) the duration of a Brillouin-amplified pulse can be shortened to within a factor of 8 of that for a Raman compressed pulse [3], suggesting that the two methods are capable of comparable pulse-compression, (7) a shorter interaction length is required for SBS because the energy transfer is fast [3,45,57], which is sometimes quantified as SRS requiring mm to cm scale plasmas whereas SBS can be conducted in 100 µm [46], and (8) SBS may be viable in regimes where SRS is limited by particle trapping and wavebreaking [3] and can therefore support pump amplitudes several orders of magnitude higher than SRS [59].…”

“…approximately six times longer than the inverse frequency, so the actual limit on pulse duration may be somewhat longer than the inverse frequencies in Fig. 4. B. Brillouin Scattering Growth Rate (7) The assertion that amplification by stimulated Brillouin scattering requires a plasma of shorter length than that necessary for SRS because the energy transfer is faster [3,45,57] demands some care. In the linear regime, the degree of amplification is set by the growth rate and the amplification distance, so this is fundamentally a statement about the respective growth rates of Raman and Brillouin scattering.…”

“…Initial study of amplification by SC-SBS considered plasma densities above one quarter of the critical density, where SRS is suppressed, but in an effort to avoid deleterious instabilities, particularly filamentation, study has expanded to include lower plasma densities, where SBS is in direct competition with SRS [43,45,46,51]. In this regime, the dominant amplification mechanism may not be obvious [62], and some care must be taken to determine the underlying process.…”