Progress made in generation and amplification of femtosecond optical pulses resulted in creation of the first petawatt laser system in the USA in 1998. It is a unique tool for investigation of properties of a substance in fields with giant radiation intensity amounting to 10²¹ -10²² W/cm². Programs aimed at designing petawatt lasers are also under way in France , Great Britain and Japan . Petawatt power is attained by amplification of frequency-modulated laser pulses stretched out in time in conventional wide-aperture amplifiers and by their subsequent compression down to duration of several hundred femtoseconds in a system of diffraction gratings. This process is called chirped pulse amplification. Further advance towards power enhancement on this way is restricted by a relatively narrow band of light amplification in neodymium-doped glass. This initiated active discussions and investigation of alternative resources to overcome the petawatt barrier using more broadband amplifier systems.

One of the most interesting schemes in terms of physics uses optical parametric amplifiers instead of conventional laser amplifiers. By choosing appropriate nonlinear crystal, propagation directions, as well as signal wave and pump frequencies it is possible to realize conditions of broadband phase-matching at parametric amplification and, at the same time, to use the principle of successive stretching, multicascade amplification and recompression of the amplified pulses that is traditional for generation of super-high fields. Parametric amplification has a number of advantages over traditional amplification:

• broad amplification band (up to 1000 cm^-1 and larger), if the necessary phase-matching conditions for the available wide-aperture nonlinear crystals (e.g., DKDP) are fulfilled, which ensures generation of amplified pulses with duration up to 10-30 fs;
• substantial reduction of thermal load on the amplifying element with a possibility to work with high repetition rate;
• a strongly reduced level of amplified spontaneous radiation and the resulting high contrast of petawatt pulses on the target.

The Project is carried out by IAP RAS in collaboration with the Russian Federal Nuclear Center – All-Russian Research Institute for Experimental Physics (VNIIEF, Sarov) and includes 3 stages.

At the first stage (completed in 2003) a parametric terawatt amplification complex was designed at IAP RAS that generated pulses with duration of about 70 fs, energy higher than 30 mJ and repetition rate 2 Hz. An analogous complex was supplied to VNIIEF in the first half of 2004.

The main merit of the scheme of parametric amplification is a possibility to scale it to multiterawatt and petawatt power using unique wide-aperture nonlinear DKDP crystals

grown in IAP RAS. The second stage of works done in IAP RAS on creation of a 200 TW laser complex based on nonlinear crystals with the aperture of 10 х 10 cm² and a new Nd-doped glass pump laser with pulse energy up to 75 J at the second harmonic was completed in 2005. An analogous amplification cascade will be supplied to VNIIEF.

The third stage of work aimed at creating a terminal multipetawatt amplifying cascade will be accomplished in VNIIEF with participation of IAP RAS specialists in 2006. To generate a laser pulse of record power a unique wide-aperture (30 х 30 cm² ) DKDP crystal will be used. Radiation from one of the channels of “Luch” facility with energy up to 1 kJ at the second harmonic and pulse duration of about 1 ns will pump parametric amplification. We expect to obtain output pulse of laser radiation with power more than 100 J and duration about 50 fs. When such a pulse in a vacuum chamber is focused on a target, the intensity is expected to reach 10 22 W/cm².

The OPCPA terawatt laser system based on a DKDP crystal comprises three basic units: a pumping system, a chirped pulse injection system, and a three-cascade parametric amplifier with diagnostic system. Besides, it has an electronic system for laser synchronization that ensures simultaneous passage of pump pulses and amplified radiation in nonlinear crystals.

The injection system includes a femtosecond source, a stretcher, matching telescopes, and a dispersive element (prism) that induces the desired angular dispersion of the injected radiation. A femtosecond Cr:Forsterite-laser with average power of ~ 0.25 W generating pulses with a duration of ~ 40 fs and spectrum width of ~ 400 см^-1 (FWHM) is used as a source of injected radiation. The stretcher having 1000 сm -1 transmission band ensures stretching of 40-fs pulse of injected radiation up to durations of 0.5 ns.

Radiation at the second harmonic of a single-mode single-frequency Nd:YLF laser operating at the wavelength of 527 nm with the energy up to 1 J and pulse duration of 1.5–1.7 ns pumps the parametric amplifier. The pulse repetition rate is 2 Hz. The intensity of the pump beam with the radius of 5 mm at the input to the parametric amplifier has an almost homogeneous transverse distribution of about 1 GW/cm² .

A three-cascade parametric amplifier consists of one double-pass amplifier and one single-pass amplifier. A two-cascade parametric amplifier realizes broadband transformation of chirped pulses at the conjugated wavelength ₂₀ = 1250 nm into signal radiation pulses ( ₁₀= 911 nm) and their amplification in the intense pumping field. To reduce the influence of pump radiation divergence on the transformation, the radius of the injected beam was specified to be ~ 1 mm. In the second cascade of the first amplifier broad-band amplification of collimated signal radiation occurs at the same synchronism parameters as in the first cascade thanks to the angular mirror reflecting the signal and the retroprism reflecting the pumping, which makes alignment easier.

When the signal pulse was compressed without fine alignment of the stretcher-compressor system, pulses with duration ~ 70 fs were obtained. Pulse duration was measured by means of a SPIDER system; temporal and spectral profiles of the reconstructed pulse are plotted in Fig. 2.

Pump laser of parametric neodymium glass amplifier III operates with a period of one burst per 30 minutes and gives the energy of 130 J at the fundamental and 75 J at the second harmonic with pulse duration of 1-1.5 ns at the output of a five-cascade neodymium-doped phosphate glass. A multistage diaphragm-line filter gives the aperture duty factor of 0.65 at the laser output (Fig. 3). This allows us to effectively extract the stored energy and reduce the number of cascades. In addition, the glass amplifier can be accommodated on one optic table (see the photo). Divergence of the output radiation was three diffraction limits, thus meeting the requirements to pump radiation of the third parametric amplifier.

Тhe third parametric amplifier (80 mm long uncoated DKDP crystal with a net aperture of 100 mm) had a 1600-fold weak-signal amplification factor at pump energy 65 J. For the input signal of several tens of millijoules, this ensured deep saturation of parametric amplification. Saturation and high quality of pump beam enabled us to attain 14.5 J of chirped pulse energy at the compressor input (fig. 4) with good beam quality (fig. 5). Maximal physical energy efficiency of the parametric amplifier was 25%. No radiation spectrum narrowing was observed.

The pulse was compressed by means of a vacuum compressor consisting of two diffraction gratings and one angle reflector with clear aperture of 110 mm. The compressor transmission coefficient was 66%. A remote alignment system ensured alignment of all optical elements of the compressor to an accuracy of 5 angular seconds. Neither spectrum narrowing nor angular chirp was observed in the output radiation. Maximal energy of the compressed pulses was 9 J. The autocorrelation function is plotted in fig.6. It corresponds to a gaussian pulse with FWHM duration of 45 fs. Thus, the peak power at the output of the femtosecond laser complex with parametric amplification of chirped pulses amounted to 200 TW, which is 12 times the record level reached in lasers with parametric amplification of chirped pulses. The near and far field distribution of output radiation are demonstrated in fig. 7.

Thus, 200 terawatt peak power was attained in experiments using the high-power femtosecond laser architecture proposed earlier. Calculations demonstrate that for achieving multipetawatt power one more parametric amplifier is needed. It must have an aperture of 200-300 mm and pump pulse energy of 1-2 kJ at the wavelength of 527 nm. A DKDP crystal for such an amplifier has been grown in IAP RAS; a pump laser is available in VNIIEF in Sarov, that is one of the channels of the powerful neodymium-doped phosphate glass facility “Luch”. Works on creating a multipetawatt laser source are now under way.

References

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Katin E.V., Lozhkarev V.V., Palashov О .V., Khazanov E.A. Synchronization of a femtosecond laser to a laser with Q-factor modulation to an accuracy of 50 ps, Quantum Electronics, v.33, №9, p.836-840, 2003.

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Lozhkarev V.V., Freidman G.I., Ginzburg V.N., Khazanov E.A., Palashov О .V., Sergeev A.M., Yakovlev I.V. Study of broadband optical parametric chirped pulse amplification in DKDP crystal pumped by the second harmonic of a Nd:YLF laser, Laser Physics, v.15, №9, 2005.