Plasma Wave Seed for Raman Amplifiers

It is proposed to replace the traditional counterpropagating laser seed in backward Raman amplifiers with a plasma wave seed. In the linear regime, namely, for a constant pump amplitude, a plasma wave seed may be found by construction that strictly produces the same output pulse as does a counterpropagating laser seed. In the nonlinear regime, or pump-depletion regime, the plasma-wave-initiated output pulse can be shown numerically to approach the same self-similar attractor solution for the corresponding laser seed. In addition, chirping the plasma wave wavelength can produce the same beneficial effects as chirping the seed wave frequency. This methodology is attractive because it avoids issues in preparing and synchronizing a frequency-shifted laser seed.

Figure 1

Seeding of the amplified output pulse. The pump beam is injected at the right. (a) Triggered by a counterpropagating laser seed. (b) Triggered by a stationary plasma wave seed. (c) In the case of an advantageous result, the amplified output pulse assumes a $\pi$-pulse shape, accompanied by depletion of the pump laser.

Figure 2

(a) Envelopes of plasma seed (blue solid line) and laser seed (red dashed line). (b)–(e) Amplified probe pulse at times ${\gamma }_{0}t=1$, 5, 15, and 40. Blue solid lines correspond to plasma seed; red dashed lines to laser seed.

Figure 3

Amplified probe pulses using a plasma seed (blue solid line) and a laser seed (red dashed line) at interaction time $t=50\mathrm{fs}$ (a) and $t=8.5\mathrm{ps}$ (b). (c) Comparison of the amplified probe pulse peak intensity. The thick solid and dashed lines are PIC simulations; and the thin dotted-dash lines are fluid-model simulations using Eqs. (1).

Figure 4

(a) Amplified probe beams triggered by chirped (blue solid) and nonchirped (red dashed) plasma seeds with a 600 fs pump pulse; (b) growth of the peak intensities. The seed amplitudes and shapes are identical.