Reaching the Nonlinear Regime of Raman Amplification of Ultrashort Laser Pulses

The intensity of a subpicosecond laser pulse was amplified by a factor of up to 1000 using the Raman backscatter interaction in a 2 mm long gas jet plasma. The process of Raman amplification reached the nonlinear regime, with the intensity of the amplified pulse exceeding that of the pump pulse by more than an order of magnitude. Features unique to the nonlinear regime such as gain saturation, bandwidth broadening, and pulse shortening were observed. Simulation and theory are in qualitative agreement with the measurements.

Figure 1

Schematic of the experimental setup. Plasma is formed by the Nd:YAG laser. Part of the output from the Ti:sapphire laser is compressed by C1 to 10 ps and serves as the pump. The other part passes through a Raman shifter, is compressed by C2 to 550 fs, and serves as the seed pulse. The two pulses are focused to the plasma in the vacuum chamber.

Figure 2

Output energy versus input seed energy and pump energy. (a) Output energy versus input seed energy when the pump energy was maintained at 40 mJ. Inset shows the output when the input was low and has the same scale as the main figure. (b) Output energy versus pump energy when the seed energy was maintained at $7.5\mu \mathrm{J}$. In both cases, the dotted line denotes the thermal Raman.

Figure 3

Normalized (to 1) spectra of (a) pump pulse and (b) input seed pulse and amplified seed pulse for amplification of 40.

Figure 4

Output seed pulse profiles (a) without amplification (no plasma) and (b) after amplification (image obtained through a 2.5% transmission filter).

Figure 5

The pulse duration versus energy of the amplified seed pulse. Dotted line denotes the initial pulse duration.

Figure 6

The 1D, time dependent computer simulation of energy amplification (solid line) and amplified pulse duration at its FWHM (dotted line) versus plasma length for 10 ps pump.