We propose a new compressor geometry - a full-aperture two-grating slanted-groove compressor (F2SC), consisting of two parallel Littrow-mounted gratings with slanted grooves. The angle of the slanting is chosen so as to ensure decoupling of the input and output beams. The gratings and the beam on the first grating have the same size. Optimal parameters of this and other previously proposed compressor geometries are found analytically, considering restrictions not only on the grating length but also on its height. It is shown that, similar to a full-aperture two-grating Treacy compressor (F2TC), the F2SC provides maximum power and focal intensity. The advantages of these geometries are the absence of hot spots on the second (last) grating due to beam smoothing. Power above 160 PW and focal intensity up to 5.5 & times; 1024 W/cm2 at the F/2 parabola seem quite achievable with the grating sizes of 150 cm & times; 122 cm for the F2SC and 162 cm & times; 108 cm for the F2TC. Another advantage of the F2SC is the potential possibility of using multilayer dielectric gratings.

The results of numerical simulation of spatial noise filtering prior to the stage of nonlinear temporal compression of PW laser pulses are presented. It is shown that small-scale self-focusing (filamentation instability) can be substantially suppressed in a diffraction-grating optical compressor, since the spatial frequencies in the instability region lag behind or overtake the main pulse. This effect in an asymmetric compressor is weaker than in a symmetric one, despite a more efficient smoothing of spatial fluence fluctuations.

The generation of exawatt-class lasers, as well as high focal intensities, is an important goal in ultra-intense laser physics, and grating compressors are the key devices for achieving this goal. Since large-size diffraction gratings are not perfectly plane, any grating compressor inevitably introduces spatiotemporal coupling phase distortions, which reduce the focal intensity of an ultra-intense laser. In this paper, we show that the dependence of the focal intensity on the root mean square (RMS) grating surface roughness is Gaussian and that for RMS roughness below 10 nm, the focal intensity decrease is negligible, giving an RMS requirement of gratings for ultra-intense lasers. In a two-grating compressor, the impact of the large-scale part of the grating surface profiles may be negligible, because it can be completely eliminated by use of two adaptive mirrors. However, in a four-grating compressor, such elimination is almost impossible. On the basis of a complete analytical model and an improved numerical code, we study the grating compressors of two sub-exawatt laser projects (XCELS and SEL-100PW), and the results show that a two-grating compressor is the best choice for exawatt-class ultra-intense lasers such as XCELS, SEL-100PW, and OPAL. This work also provides a basis for compressor grating fabrication as part of the further development of ultra-intense lasers.

We measured the groove pattern of the diffraction gratings with an aperture of 690 x 420 mm by three different interferometers at two wavelengths with spatial resolution 0.3 mm. The measured 2D power spectrum density of both the groove pattern and the substrate surface profile were power dependent with exponents 3.5 and 2.5 in the wavevector range from 0.003 to 1 mm(-1) . To improve pulse contrast, it is necessary to improve substrate polishing, and to increase focal intensity, it is necessary to improve the quality of the optics used to record the gratings or (preferably) to polish the grating substrates, striving for an optimal surface that compensates for the imperfection of the groove pattern rather than a perfectly flat one. Using the measured groove pattern we numerically studied focal intensity reduction versus a beam aperture for the different compressor designs.

The quintic nonlinear response of fused silica was investigated using a modified z-scan technique and a cubic-quintic propagation model at femtosecond pulse intensities up to 10 TW/cm^2 . A statistically significant deviation from the purely cubic Kerr response was observed only at the highest intensities. Interpreting this deviation within the perturbative cubic-quintic model yields effective quintic nonlinear refractive indices n_4=-(4...7)· 10^-6 cm^4/TW^2 at a wavelength of 1033 nm and n_4=-(8...16)· 10^-6 cm^4/TW^2 at 517 nm. Since plasma-induced refractive effects may contribute to the observed higher-order response, these values should be regarded as the upper bounds of the negative quintic nonlinear refractive index of fused silica. The obtained bounds are approximately 1–2 orders of magnitude lower than the previously reported values.

Manufacturing imperfections in large-aperture holographic gratings critically limit multi-petawatt Chirped Pulse Amplification systems. This study investigates their impact on wavefront quality using Zemax ray-tracing simulations based on real interferometric measurements. The results confirm that distortions are cumulative, with imperfections in the second and third gratings primarily driving beam degradation. Significantly, we demonstrate that rotating these critical gratings by 180° alters the residual wavefront structure. The study concludes that optimizing grating orientation, combined with adaptive optics, is an effective strategy to mitigate aberrations and restore peak intensity, offering vital guidelines for next-generation high-field laser facilities.

This paper presents a four-channel prototype system for the geometric combining and coherent addition of tightly focused femtosecond laser radiation into a standing-wave field configuration. A stabilization system for beam pointing and relative phase of the four optical channels has been implemented, and its performance has been experimentally demonstrated. To characterize the standing-wave electromagnetic field distribution at the main focus of the system, an original measurement technique based on a fiber subwavelength optical probe has been employed. This work has been conducted in support of the exawatt-scale XCELS project.

Spatial intensity modulation in amplified laser beams, particularly hot spots, critically constrains attainable pulse peak power due to the damage threshold limitations of four-grating compressors. This study demonstrates that the double-smoothing grating compressor (DSGC) configuration effectively suppresses modulation through directional beam smoothing. Our systematic investigation validated the double-smoothing effect through numerical simulations and experimental measurements, with comprehensive spatiotemporal analysis revealing excellent agreement between numerical and practical pulse characteristics. Crucially, the DSGC enables a 1.74 times energy output boost compared to conventional compressors. These findings establish the DSGC as a pivotal advancement for next-generation ultrahigh-power laser systems, providing a viable pathway toward hundreds of PW output through optimized spatial energy redistribution.

A programming library was developed, based on Stratton-Chu diffraction integrals for calculating reflected optical fields. Dipole-type focusing schemes with a tunable number of beams and mirror placements were studied, considering the influence of phase distortion and aberrations. The intensity above 3×1026 W/c m 2 was found theoretically attainable in a system of 12 beams of 50 PW each, with about 90% of that value realistically achievable.

Analytical expressions for the focal intensity (Strehl ratio) were obtained for both small-scale and large-scale variations in the surface profile of the compressor diffraction grating substrate. The validity of the expressions is confirmed by numerical simulations. It is shown that for small (much less than the beam diameter)-scale variations with rms of less than 10 nm of their influence can be neglected. Numerical simulations confirmed that quadratic aberrations (defocus and astigmatism) can be suppressed by rotating the fourth grating; and therefore, grating manufacturers should ignore these aberrations to minimize higher orders.

The impact of compressor gratings and transport optics imperfections on the power contrast ratio (PCR) is considered analytically, taking into account diffraction and all dispersion orders. All types of imperfections, including surface roughness, reflectivity fluctuations and surface dirt/damage/obscuration as well as the roughness and obscuration on the optics used to write holographic gratings are allowed for. For the same roughness and obscuration, the contribution to the PCR of the latter is significantly greater than the contribution of the gratings. Comparison of the PCR caused by obscuration and by roughness showed that at short times the latter prevails, whereas at long times the obscuration is dominant. The radiation scattered by the second and third gratings arrives at the target before the main pulse in the form of a vertical strip near the beam axis. Then this strip moves uniformly towards the axis, reaching it simultaneously with the main pulse.

Using mathematical modeling, we studied the influence of defects in the Treacy compressor diffraction gratings on wavefront quality and compared the results with an analytical model. We obtained good qualitative agreement: distortions from substrate defects and writing-system aberrations add and lead to a noticeable reduction of the Strehl ratio in the focal plane. Modeling showed that the second and third compressor gratings contribute most to the degradation, and rotating these gratings by 180 degrees about the normal can markedly improve or worsen the resulting wavefront. We demonstrate that an optimal grating orientation combined with adaptive optics substantially reduces unwanted effects and restores the beam's high peak intensity. The practical significance of this work is the recommendation to account for and compensate large-scale defects when designing and operating large apertures in chirped pulse amplification systems to ensure maximum peak intensity and stable compression in multi-petawatt lasers.

Non-equidistant and non-parallel grooves of a diffraction grating distort the diffracted wavefront. For a fixed wavelength, a grating with an arbitrary groove pattern is equivalent to a grating with perfect grooves having an effective surface profile different from the geometric one. It is shown that the previously proposed method for measuring non-equidistance can be used for simultaneous measurement of non-parallelism, and the measurement error is determined without any a priori information and fitting parameters. The demonstrated error (RMS) for the diffraction gratings with an aperture of 320 x 230 mm manufactured by us was 2 nm for the effective profile, 4 10-7for non-equidistance, and 4.5 10-7 for the groove inclination angle when measuring with a spatial resolution of 0.27 mm. Rms of the wavefront distortions of the beam reflected from these gratings was 10 +/- 2 nm. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

An analytical expression for intensity at the focal point for a full-aperture compressor of arbitrary configuration has been derived. At small times, the intensity contrast ratio (ICR)-the focal intensity normalized to its maximum- does not depend on clipping and is determined exclusively by the shape of an undistorted pulse spectrum. At large times, contrariwise, the ICR does not depend on the spectrum shape and is determined by clipping. In the latter case, the ICR is proportional to the second power of the group velocity dispersion of the compressor and is inversely proportional to the second power of the grating length and the pulse bandwidth as well as the fourth power of time. For the compressors proposed for the Exawatt Center for Extreme Light Studies (XCELS) and Station of Extreme Light (SEL-100 PW) projects, the ICR is 10-10 ... 10-9 at t approximate to 0.7 ps, which is sufficient for most experiments. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

A modified Z-scan technique was used to measure the polyethylene terephthalate (PET) cubic nonlinearity coefficient n2 at two different wavelengths (1030 and 515 nm). The proposed technique was benchmarked with the well-studied fused silica (FS) sample. The obtained n2 values for fused silica n2(FS, 1030 nm) = 1.72, n2(FS, 515 nm) = 3.1 and for PET n2(PET, 1030 nm) = 11.5 in the units of 10-7cm2/GW agree well with the previously published data. The nonlinear coefficient for PET at the wavelength of 515 nm n2(PET, 515 nm) = 26.5 10-7cm2/GW was obtained in this work, presumably, for the first time, and demonstrates high nonlinearity of PET in the visible band. The proposed modified Z-scan technique exhibits a factor of three better sensitivity with respect to the traditional one, which was crucial for PET measurements due to its low damage threshold. (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

The power contrast ratio (PCR) always measured in experiments is the same in the near and far fields. We have obtained expressions for the PCR as a function of the power spectral density of the compressor/stretcher grating surface and transport optics for 2D diffraction, taking into account all dispersion orders. The only approximation is that the spatial scale of the surface profile is much smaller than the beam diameter. It is shown that PCR is independent of the dispersion of all orders except the first one. Allowance for diffraction leads to the violation of the parity of the PCR(t) function and to the appearance of the cutoff time below which PCR = 0. For all optical elements except the second and third gratings, the cutoff time is zero, and they contribute to PCR only at t > 0. This explains the fact that the measured pre-pulse is always smaller than the postpulse. Exceptions are the elements (if any) whose images are related to a plane located farther than the PCR measurement plane; they contribute to PCR only at t < 0. Although the scattered radiation can overtake the main pulse, this time is insufficient for the plasma to enter the focal region of the main pulse before its arrival.

An analytical expression for focal intensity is derived for arbitrary surface profiles and arbitrary groove patterns of compressor gratings. The expression is valid for different compressor designs: plane and out-of-plane compressors, symmetric and asymmetric compressors (compressors composed by two not-identical pairs of gratings) and a two-grating compressor. It is shown that the quality requirements for the optics used to write a grating are higher than for the grating. The focal intensity can be maximized by rotating each grating around its normal by 180 degrees. Moreover, it may be increased to maximum by interchanging any two gratings in the compressor, because imperfections of an individual grating do not additively affect the focal intensity. The intensity decrease is proportional to the squared pulse spectrum width and the squared total distortions of the second and third gratings of the four-grating compressor and the total distortions of two gratings of the two-grating compressor.

When two identical counter-propagating laser pulses sharply focused by parabolic mirrors overlap, the electric field amplitude is optimized based on vector calculations. An optimal aspect ratio of a rectangular laser beam and f-number of a parabolic mirror that maximizes the field during focusing, as well as an optimal frequency spectrum of the pulse at the compressor output within a specified spectral range, is determined. The potential for further increasing the field in focus by optimizing the spatial intensity distribution in the near field is explored. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

Spatial noise inevitably arising when femtosecond laser pulses are reflected from imperfect surfaces leads to fluctuations of beam fluence (energy density), which are the main cause of optical breakdown in high-power lasers. These fluctuations are evolving significantly when the beam passes through optical systems with spatio-temporal mixing, such as free space or a femtosecond pulse compressor with diffraction gratings. An analytical expression for the spatial spectrum of beam fluence fluctuations is obtained in a general form and verified by numerical simulation. The fluctuation smoothing efficiencies of symmetric and asymmetric compressors are compared. It is shown that the asymmetric compressor is advantageous only if the asymmetry is pronounced, while a slight violation of compressor symmetry worsens beam fluctuation smoothing. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

Femtosecond pulse shaping is a variety of experimental methods for manipulating temporal profile of ultrashort laser pulses. One of common tools for pulse shaping is acousto-optic programmable Bragg gratings. We describe the Dispersive Fourier Synthesis algorithm for generating arbitrary transmission functions of acousto-optic programmable dispersive filters and delay lines used therein. The algorithm performs computation of radio-frequency waveforms providing arbitrary complex-valued transmission functions for the said devices. An open-source MATLAB script was developed based on this algorithm taking into account the ease of use of a commercial radio-frequency driver. The experiments include full validation of the algorithm: calibration of frequency-to-wavelength mapping, determination of optimal driving power, and closed-loop femtosecond pulse compression in the front-end of the PEARL (PEtawatt pARametric Laser) facility.