Post-compression of powerful femtosecond pulses after second harmonic generation.

A proof-of-principle experiment of highly efficient (38%) second harmonic generation was implemented at a ∼1T W/c m 2 intensity of a 45 fs pulse in a composite nonlinear sample-a 1 mm KDP crystal glued onto a 1 mm fused silica substrate. The attenuated replica of the second harmonic pulse (455 nm) was compressed down to 28.6 fs by means of dispersive mirrors, with a significant reduction in both the wings and the far temporal contrast. The peak power of the second harmonic was ∼74% of the power of the fundamental harmonic, which ensured a three-fold increase in the focal intensity.

Upgrading the front end of the petawatt-class PEARL laser facility.

A new front-end laser system with optical synchronization of chirped femtosecond and pump pulses for the petawatt laser complex PEtawatt pARametric Laser (PEARL) has been developed. The new front-end system provides a broader femtosecond pulse spectrum, temporal shaping of the pump pulse, and a significant increase in the stability of the parametric amplification stages of the PEARL.

Experimental evidence for short-pulse laser heating of solid-density target to high bulk temperatures.

Heating efficiently solid-density, or even compressed, matter has been a long-sought goal in order to allow investigation of the properties of such state of matter of interest for various domains, e.g. astrophysics. High-power lasers, pinches, and more recently Free-Electron-Lasers (FELs) have been used in this respect. Here we show that by using the high-power, high-contrast "PEARL" laser (Institute of Applied Physics-Russian Academy of Science, Nizhny Novgorod, Russia) delivering 7.5 J in a 60 fs laser pulse, such coupling can be efficiently obtained, resulting in heating of a slab of solid-density Al of 0.8 µm thickness at a temperature of 300 eV, and with minimal density gradients. The characterization of the target heating is achieved combining X-ray spectrometry and measurement of the protons accelerated from the Al slab. The measured heating conditions are consistent with a three-temperatures model that simulates resistive and collisional heating of the bulk induced by the hot electrons. Such effective laser energy deposition is achieved owing to the intrinsic high contrast of the laser which results from the Optical Parametric Chirped Pulse Amplification technology it is based on, allowing to attain high target temperatures in a very compact manner, e.g. in comparison with large-scale FEL facilities.

Neodymium glass laser with a phase conjugate mirror producing 220 J pulses at 0.02 Hz repetition rate.

For pumping multipetawatt Ti:sapphire laser facilities we developed a compact repetitively pulsed laser based on neodymium phosphate glass with pulse energy of 220 J, pulse repetition rate of 0.02 Hz, beam diameter of 43 mm, aperture fill factor of 0.8, and FWHM pulse duration of 30 ns. The phase distortions of laser radiation were compensated by optical phase conjugation via stimulated Brillouin scattering. The depolarization was reduced to 0.4% using linear compensation methods. The beam quality was 2.5 x diffraction limit (150 µrad).

Influence of cubic nonlinearity on accuracy of polarization transformation by means of a quarter-wave plate.

We consider the propagation of powerful laser radiation in an anisotropic medium with natural birefringence and cubic nonlinearity. By the example of a quarter-wave plate, we show theoretically and experimentally that, under the simultaneous influence of linear birefringence and nonlinearity, the accuracy of polarization transformation decreases in proportion to squared В-integral.

Theoretical and experimental study of laser radiation propagating in a medium with thermally induced birefringence and cubic nonlinearity.

We consider a problem of laser radiation propagating in a medium with birefringence of two types: linear birefringence independent of intensity and polarization, and intensity and polarization dependent circular birefringence caused by cubic nonlinearity. It is shown theoretically and experimentally that the efficiency of the broadly employed method of linear depolarization compensation by means of a 90° polarization rotator decreases with increasing В-integral (nonlinear phase incursion induced by cubic nonlinearity). The accuracy of polarization transformation by means of a half-wave and a quarter-wave plate also decreases if В > 1. By the example of a λ/4 plate it is shown that this parasitic effect may be suppressed considerably by choosing an optimal angle of inclination of the optical axis of the plate.

Large-aperture Nd:glass laser amplifiers with high pulse repetition rate.

Nd:glass amplifiers are used in most of the existed petawatt laser facilities. A typical repetition rate of such lasers is 1 shot per 30 minutes or less. Limitations are thermally induced distortions of radiation and tensile stresses in Nd:glass. An increase of the repetition rate is an urgent problem. We have investigated thermally induced depolarization and thermal lens effects in Nd:glass rods up to 10 cm in diameter at a pump pulse repetition period of 3 minutes. It is shown that the rods have a safety factor of at least 5 before thermal stress induced damage would occur, and despite of their size phase and polarization distortions could be compensated.

Two-screen single-shot electron spectrometer for laser wakefield accelerated electron beams.

The laser wakefield acceleration electron beams can essentially deviate from the axis of the system, which distinguishes them greatly from beams of conventional accelerators. In case of energy measurements by means of a permanent-magnet electron spectrometer, the deviation angle can affect accuracy, especially for high energies. A two-screen single-shot electron spectrometer that correctly allows for variations of the angle of entry is considered. The spectrometer design enables enhancing accuracy of measuring narrow electron beams significantly as compared to a one-screen spectrometer with analogous magnetic field, size, and angular acceptance.

Spatial filters for high-peak-power multistage laser amplifiers.

We describe spatial filters used in a Nd:glass laser with an output pulse energy up to 300 J and a pulse duration of 1 ns. This laser is designed for pumping of a chirped-pulse optical parametric amplifier. We present data required to choose the shape and diameter of a spatial filter lens, taking into account aberrations caused by spherical surfaces. Calculation of the optimal pinhole diameter is presented. Design features of the spatial filters and the procedure of their alignment are discussed in detail.

200 TW 45 fs laser based on optical parametric chirped pulse amplification.

200 TW peak power has been achieved experimentally using a Cr:forsterite master oscillator at 1250 nm, a stretcher, three optical parametrical amplifiers based on KD*P (DKDP) crystals providing 14.5 J energy in the chirped pulse at 910 nm central wavelength, and a vacuum compressor. The final parametrical amplifier and the compressor are described in detail. Scaling of such architecture to multipetawatt power is discussed.