Compression of Laser Radiation in Plasmas Using Electromagnetic Cascading

Compressing high-power laser beams in plasmas via generation of a coherent cascade of electromagnetic sidebands is described. The technique requires two copropagating beams detuned by a near-resonant frequency $\Omega \lesssim {\omega }_p$. The ponderomotive force of the laser beat wave drives an electron plasma wave which modifies the refractive index of plasma so as to produce a periodic phase modulation of the laser field with the beat period ${\tau }_b=2\pi /\Omega$. A train of chirped laser beat notes (each of duration ${\tau }_b$) is thus created. The group velocity dispersion of radiation in plasma can then compress each beat note to a few-laser-cycle duration. As a result, a train of sharp electromagnetic spikes separated in time by ${\tau }_b$ is formed. Depending on the plasma and laser parameters, chirping and compression can be implemented either concurrently in the same plasma or sequentially in different plasmas.

Figure 1

Schematic of a two-stage EM cascade compressor. The frequency modulation (FM) occurs in a rarefied plasma. Denser plasma is used for the compression.

Figure 2

The two-stage compression of a chirped beat note. (a) The beat note intensity at the entrance ($z=0$, dashed line) and exit of the modulator ($z=z_8$, dash-dotted line) and after the compressor ($z=z_8+z_C$, solid line). (b) The laser spectra near $\xi =0$ shown after the modulator ($z=z_8$, stairs) and after the compressor ($z=z_8+z_C$, bars). The nonlinearities and GVD in both plasmas are included.

Figure 3

The single-stage compressor with concurrent phase modulation (chirping) and compression. (a) Initial ($z=0$, dashed line) and final ($z=z_8$, solid line) intensity profiles of one beat note. (b) Laser spectrum near the pulse maximum, $\xi \approx 0$, at the plasma border $z=z_8$, with (bars) and without (stairs) GVD and all the nonlinear frequency shifts.