**2. MELS-4 k laser system**

of up to 3.7 kJ (KrF molecule) at a pulse duration of about 240 ns. The Super-Ashura system is used for experiments on the generation of high-power nanosecond and picosecond pulses and the interaction of these pulses with matter. The Titania laser system, created in England, employs KrF molecules and generates pulses with an energy of about 1 kJ and a pulse duration of about 150 ns [3]. The aperture size of the output amplifier is 42 cm. This system is used for experiments on the generation of high-power picosecond and femtosecond pulses and their interaction with matter. Russian institutions also develop and create the powerful excimer lasers and laser systems [4–12]. In particular, a KrF laser system with a Garpun output unit (Institute of Physics, Russian Academy of Sciences) has an output aperture with a size of 16 × 18 cm and one generates radiation pulses

The excimer lasers developed at the High-Current Electronics Institute (HCEI), Siberian Division, Russian Academy of Sciences, generate pulses with an energy of greater than 100 J [4–6, 8–11]. The two most powerful XeCl lasers (308 nm) with apertures of 40 [8] and 60 cm [9–11] generate pulses with energies of 660 J and 1.9 kJ and pulse durations of about 350 and

At the end of the nineteenth century, the hybrid approach to femtosecond pulse amplification was developed. To amplify femtosecond pulses, the rare-gas-halide excimer media excited by a high-voltage discharge or an electron beam were used. In these media, the laser transitions between the excited B–state and ground weakly bound or weakly repulsive X-state of ArF, XeCl, XeF, or KrF excimer molecules were used [13]. However, B-X transitions have rather narrow gain bandwidths (broadest bandwidth of Δλ ≈ 2 nm) and rather small saturation flu-

are required. Furthermore, a high gain of the amplifiers limits the temporal contrast of output

А photodissociative-driven XeF(С-А) medium has a wide amplification band (~ 60 nm) in

cules on the B-X transition [16]. At present, the development of ultra–high-power laser systems with a pulse duration of 10–100 fs is based mainly on near-infrared solid-state Ti:sapphire or parametric amplifiers. In these systems, positively chirped pulses, i.e., stretched in time (up to 0.5–1 ns) by linear frequency modulation [17], are amplified, and following its temporal recompression, the initial duration is realized. A pulse stretching allows avoiding nonlinear

recompression, a vacuum compressor based on diffraction gold-coated gratings is used.

An alternative approach to the design of multiterawatt and petawatt femtosecond laser systems has been developed at the Lebedev Physical Institute (Moscow, Russia) [18, 19], LP3 Laboratory of the Marseille University (Marseille, France) [20–22], and Institute of High Current Electronics (Tomsk, Russia) [23–25]. This approach is based on a solid-state femtosecond front-end and a photochemical XeF(C-A) boosting amplifier with a gaseous active medium [26, 27]. The advantage of this hybrid (solid/gas) design is that due to the much lower optical nonlinearity of gas compared to solids, the admissible factor of stretching femtosecond pulse is three orders of magnitude smaller than for solid-state systems. This allows the amplification of picosecond chirped pulses and their subsequent compression by

). To realize high-output peak powers, (more than 1 TW) large apertures

[14]. The highest peak power of output radiation reached in

unlike the traditional excimer mole-

times, and for pulse

with energy of 80 J and pulse duration of 100 ns [12].

–103

475 nm range and a high saturation fluence of ~ 0.05 J/cm<sup>2</sup>

traditional rare-gas-halide excimer amplifiers does not exceed ~ 4 TW [15].

effects related to self-focusing. The pulses are normally stretched ~10<sup>4</sup>

250 ns, respectively.

2 High Power Laser Systems

ence (*εsat* ≈ 1 mJ/cm<sup>2</sup>

radiation at the level of 10<sup>2</sup>
