Experiment and modeling show that the refractive index nonlinearity can significantly degrade the contrast of a chirped-pulse amplifier seeded with a pulse and a single postpulse. Multiple powerful non-equidistant pre- and postpulses are generated. For a Gaussian pulse and a hat-top beam, an incident postpulse of energy W results in a prepulse of energy 0.58B(2)W, where B is the nonlinear phase (B-integral) of the main pulse. Calculations show that level of satellites due to gain saturation is negligibly small. Experimental results for Ti:Sapphire regenerative and multipass amplifiers and prepulse generation in fused silica agree well with the theory.

The first stage of the petawatt excimer laser project started at the P.N. Lebedev Physical Institute, implements a development of multiterawatt hybrid GARPUN-MTW laser facility for generation of ultra-high intensity subpicosecond ultraviolet (UV) laser pulses. Under this project, a multi-stage e-beam-pumped 100-J, 100-ns GARPUN KrF laser was upgraded with a femtosecond Ti:Sapphire front-end, to produce combined subpicosecond/nanosecond laser pulses with variable time delay. Attractive possibility to amplify simultaneously short and long pulses in the same large-scale KrF amplifiers is analyzed with regard to the fast-ignition, inertial confinement fusion problem. Detailed description of hybrid laser system is presented with synchronized KrF and Ti:Sapphire master oscillators. Based on gain and absorption measurements at GARPUN amplifier and numerical simulations with a quasi-stationary code, we are predicting that 1.6 J can be obtained in a short pulse at hybrid GARPUN-MTW Ti:Sapphire/KrF laser facility, combined with several tens of joules in nanosecond pulse. Amplified spontaneous emission, which is responsible for the pre-pulse formation on a target, was also investigated: its acceptable level can be provided by properly choosing staged gain or loading the amplifiers by quasi-steady laser radiation. Fluorescence and transient absorption spectra of Ar/Kr/F 2 mixtures conventionally used in KrF amplifiers were recorded to find out the possibility for femtosecond pulse amplification at the broadband Kr 2 F (4 2 G ! 1,2 2 G) transition, which benefits in 100 times higher saturation energy density than for KrF (B ! X) transition.

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The technique of frequency shifting of sub-100 fs laser pulses was developed. It is based on the stimulated Raman scattering pair of chirped laser pulses with orthogonal polarization. The 50 fs laser pulse at the wavelength of 810 nm was converted to 68 fs Stokes pulse at the wavelength of 1060 nm with energy conversion efficiency of 20%.

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