The 10 fs pulses from a mode locked Ti:sapphire laser were used to seed a three-stage optical parametric amplifier scheme. In the first stage BBO crystal, the stretched (5 ps) seed pulses at 800 nm were synchronously pumped by the 20 mJ pulses of the second harmonic of the stretched, amplified, and compressed signal from a synchronously seeded (from the oscillator) femtosecond supercontinuum source, used to control the temporal contrast. Output pulse energies of 2 mJ were obtained at a 0.2 Hz repetition rate. These were subsequently amplified again in BBO pumped by a frequency doubled Yb fiber laser seeded fiber and bulk amplifier line providing up to 0.7 J in a 3 ns pulse at a repetition rate of 0.025 Hz. A saturated gain of 103 generated 200 mJ pulses for launch into the third-stage amplifier that utilized dual-stage LBO crystals. These were also pumped by the frequency doubled output from a fiber and bulk amplifier line seeded by the CW fiber laser oscillator, generating in excess of 1 kJ in a 3 ns pulse, with the amplifier operating at a pulse repetition rate of about once every two hours. Following multistage grating compression, at maximum 91.1 J was achieved in a pulse of 18.6 fs with third order autocorrelation measurement indicating that a pulse to peak to background of 1010 was achieved for times up to 20 ps to the front of the pulse, although no information was provided about that following the pulse.
Clearly the versatility and potential of the technique has been demonstrated by this record reporting experiment, while it is proposed that with increased aperture LBO crystals output energies exceeding several hundreds of Joules and peak powers in excess of 15 PW will be obtained.
An interesting related work was also reported by Sung et al. In solid-state petawatt CPA systems, gain-narrowing limits minimum pulse widths achievable and hence peak power, while gain-depletion which also limits bandwidth can be partially compensated for by spectral shaping. As has been shown, gain narrowing can be negated by replacing high-gain preamps with an optical parametric chirped pulse amplifier (OPCPA). In the upgrade of the petawatt Ti:sapphire laser facility at the Center for Relativistic Laser Science in Gwanju, Korea, the front end of the assembly has been significantly modified and the final stage amplification scheme upgraded to permit multi-petawatt outputs at 0.1 Hz repetition rates.
Contributing significantly to the upgrade was the application of the cross-polarized wave generation technique to broaden the spectrum and enhance the temporal contrast, improving the latter by four orders of magnitude, by deploying a 20 cm long hollow-core fiber and a 3 mm thick BaF3 crystal. To maintain the broad spectral operation in the preamplifiers, an OPCPA based upon two BBO crystals operating at 5 Hz repetition rate was inserted before the first power amplifier. A 75 MHz, 10 fs Ti:sapphire oscillator seeded the amplifier chain, which was preamplified at 1 kHz before passing the cross-polarized wave stage and a stretcher prior to entering the OPCPA, where it was amplified to pulse energies of 50 mJ. Following two stages of power amplification and two booster amplifiers, with the final stage consisting of a double passed 100 mm diameter Ti:sapphire crystal 30 mm thick, pumped by 12 beams at 527 nm delivering a total of 170 J in a 15 ns pulse and giving an output final amplified pulse energy of 112 J in a beam of 300 mm diameter. These stretched pulses were compressed by four gold-coated diffraction gratings to 19.4 fs with an energy of 83 J, corresponding to a peak power of 4.2 PW at 0.1 Hz repetition rate. Third-order correlation indicated the ASE background to be 3 × 10-12 of that relative to the pulse peak. Although prepulses were recorded in the correlation, they were identified as ghosts arising from correlations between the main pulse and postpulses; however, physical prepulses were recorded about 40 fs and 20 fs from the main pulse.
Both reports show the enormous potential of the OPCPA scheme in the power scaling and in the enhancement of pulse contrast ratios, along with other techniques to approach the multi-tens of petawatt power levels.
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