This PDF file contains the front matter associated with SPIE Proceedings Volume 11034, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Novel architectures of Petawatt-class, high peak power laser systems that allow operating at high repetition rates are opening a new arena of commercial applications of secondary sources and discovery science. The natural path to higher average power is the reduction of the total heat load induced and generated in the laser gain medium and eliminating other inefficiencies with the goal to turn more energy into laser photons while maintaining good beam quality. However, the laser architecture must be tailored to the specific application and laser parameters such as wavelength, peak power and intensity, pulse length, and shot rate must be optimized. We have developed a number of different concepts tailored to secondary source generation that minimize inefficiencies and maximize the average power. The Scalable Highaverage- power Advanced Radiographic Capability (SHARC) and the Big Aperture Thulium (BAT) laser are examples of two such high average power laser concepts; SHARC is designed for production of ion beams and x-rays, and exploration of high energy density physics at 1.5 kW average power, and BAT is envisioned for driving laser-based electron accelerators at 300 kW average power.

A technique to measure the intensity profile of a focused laser pulse at full power is a long-standing desire. High power lasers allow experiments at relativistic intensities (1018 W/cm2 and beyond). At those photon densities all atoms are ionized and therefore it is very difficult to measure directly the peak intensity or the 3D-profile of the focal spot intensity. We would like to present a way to measure it, based on residual atoms in the experimental chamber. A low-density gas will imply a number of atoms at the laser focal volume. Those atoms will be instantaneously ionized, and the released electrons will move at relativistic speeds driven by the laser field. Plasma effects, at low density, can be neglected, and electrons move independently driven only by the laser field. Nearly 50 years ago, an approach was suggested that is based on relativistic Thomson scattering, which consists of a rich spectrum of Doppler shifted radiation of the laser light, and its harmonics [1]. This reference provides very simple expressions for the scattered Doppler shift vs. intensity. Therefore, such scattered photons give very valuable information about the intensity profile. We propose to measure the Doppler shift of the low order harmonics as an in-situ direct measure of the intensity. In particular, we will present the first preliminary experimental observation of such a shift of the second harmonic as a non-destructive way to measure the intensity profile of the Salamanca VEGA-2 laser focal profile. The spectrum is consistent with a peak intensity beyond 1018 W/cm2, which correlates well with the expected intensity. This promising result is the theme of this presentation. Details of the experiment, numerical simulations, related experiments and prospects for exploiting relativistic Thomson scattering to develop an in situ intensity profiler will be discussed. [1] E. S. Sarachik and G. T. Schappert, Phys. Rev. D 1, 2738 (1970).

The limited aperture and damage threshold of the compressor gratings remains one of the bottlenecks in reaching higher peak powers for the current state-of-the-art laser systems. Object-image-grating self-tiling method provides a way how to double the effective aperture of compressor gratings by phasing them with perpendicularly positioned mirrors. This method is planned to be used in the main compressor for the L4 beamline in ELI Beamlines. A subaperture version of the main compressor was designed to test the feasibility of the objectimage- grating self-tiling method and to measure the temporal profile of the pulse throughout the amplification stages during the operation. The subaperture compressor was successfully implemented and temporal profile of the amplified pulse close to its transform limit was retrieved. The grating-mirror alignment was secured through the online measurement using an in-house developed Fizeau interferometer.

We present a complete modeling of the PETAL [1] laser chain, including associated diagnostics with the Miró code. We show that the software is able to model most relevant physical aspects of the petawatt PETAL laser chain in the subpicosecond regime, from the front-end to the focal spot in a broadband regime. Precise gain and loss values of the amplifier section have been obtained with experimental measurement of the amplifier both in monochromatic and broadband regime in the energy range 200J-4.8kJ. For the compression stages, segmentation, propagation and recombination of the four beams are considered with respect to the phase delay between the gratings. The reconstructed pulse from experimental data’s was obtained by taking into account the transfer functions of each diagnostics, including non-linear effects, and confirmed the calculated compressed pulse properties.

We report on the status of the re-commissioning of a high energy OPCPA laser system with programmable spectrum that serves as a frontend for a 10 PW laser at ELI-Beamlines. The OPCPA chain was developed by a consortium of National Energetics and Ekspla along with scientists of ELI-Beamlines.1 The laser system, consisting of three picosecond OPCPA stages, pulse cleaner, Offner stretcher, and 5 nanosecond OPCPA stages pumped by Nd:YAG lasers with programmable pulse shape (NL944, Ekspla), allows for precise spectral shaping while achieving high nonlinear conversion efficiency. Employing a subsequent Nd:glass power amplifiers (PA), the system was demonstrated to yield>1 kJ of energy, while maintaining broad spectrum of > 13 nm (FWHM). After recommissioning the OPCPA frontend in Dolní Břežany, an output energy of 4.3 J, flat beam-profile and good far-field quality has been demonstrated. The spectral shape has been optimized to support > 15 nm bandwidth and >1.5 kJ, consistent with 10 PW operation of the fully integrated laser system after compression.

Plasma mirror is an effective approach to improve the temporal contrast of high power ultra-short laser system, while it might deteriorate the focal spot, which is reported in some experiments using plasma mirror. In order to investigate such far-field degradation by plasma mirror, we established a spatiotemporal multi-step focusing propagation algorithm based on the formula of plasma expansion and wave-front modulation model. The influence of plasma expansion time, amplitude and spatial frequency of wave-front error on focal spot degradation are quantitatively analyzed. The simulation results reveal that the far-field focal spot degradation by plasma mirror is caused by the non-uniform plasma expansion due to the wave-front error and the wave-front error with higher amplitude and lower spatial frequency has relatively greater effect on the focusing ability. From the perspective of high-contrast ultra-intense output capability, the requirement on the spatiotemporal quality of the pulse is put forward to avoid the far-field focal spot degradation when using plasma mirror in high power ultra-short laser system.

We are currently focusing on the improvement of contrast pedestal (CP) in the compressed laser pulse of PW Ti:Sapphire lasers. In our previous studies, we have identified the stretcher in our laser system as the source of CP. In order to underpin the true origins of CP, we have quantitatively characterised the surface quality of large optics used in the Gemini laser stretcher, where the laser beam is spatially dispersed and the spectral phase noise is induced by the optical surface roughness. We have measured the surface profiles of 2 different gold gratings, the new and old grating, and back mirror to a very high precision (~ a fraction of nm) by using ZYGO Dynafiz, with a spatial resolution of ~50µm over a width up to ~320mm, an unprecedented combination of very high spatial resolution with a very wide field of view. The surface roughness of the large curved mirror was determined experimentally. We have developed a simple physical model to deal with the influence of the surface roughness on the contrast pedestal. Based on the measured surface profiles and by taking the actual laser beam size into account, we are able to determine the spectral phase noise induced by the optical surface roughness in the stretcher. Consequently, we are able to accurately evaluate the impact of individual large optics in the stretcher and an overall impact of the stretcher on the contrast pedestal. The calculated contrast induced by both stretches with the new and old gratings are in an excellent agreement with the experimental results measured by the Sequoia scan. For the stretcher with the old grating, the grating is the dominant impact factor on the contrast. However, for the stretcher with the new gold grating of higher quality, the impact of the curved mirror on the contrast is comparable to that of grating. This implies that the influence of curved mirror on the contrast pedestal becomes more significant when the surface quality of grating is further improved. It is clearly observed that the impact of back mirror on the contrast is more than one order of magnitude lower than that of gratings and also much lower than that of curved mirror. In conclusion, we have demonstrated a novel method to evaluate the impact of large optics in the stretcher on the contrast pedestal by precisely quantitative characterization of optical surface quality. It is possible to accurately predict the contrast pedestal based on the stretcher configuration and precise characterisation of the optical surface in the stretcher prior to the construction of actual CPA high power laser system.

We present a study of the temporal pre-pulse contrast degradation of high focused intensity pulses produced in CPA laser systems due to imperfections in amplifier design, alignment of amplifier components, and crystal inhomogenity. Using a measurement technique we have developed, we demonstrate the presence of multiple crystal domains inside Ti:sapphire slabs with ≈10 cm diameter. The results of our numeric calculations show that crystalline c-axis orientation inhomogenity caused by these crystal domains can lead to generation of pre-pulses with relative contrast >10^-10 within several picoseconds before the main pulse. In a multiple-slab amplifier head configuration sometimes used in high repetition rate systems, the misalignment of the amplifier slabs crystalline c-axes with respect to each other can lead to the generation of pre-pulses with relative contrast as high as 10^-6, depending on the magnitude of misalignment.

A passive peak intensity stabilization method based on optimal combination of Kerr nonlinearity and linear dispersion is presented. By analyzing the Kerr nonlinearity with a 3D numerical model it was found that if a Gaussian laser pulse acquires a certain amount of nonlinear phase and is consequently over-compressed to have sight negative chirp, the uctuations in peak intensity caused by energy uctuations are reduced. Thus, even if the energy of laser pulses uctuates the peak intensity can be stabilized. The simulations for realistic pulse parameters show an increase in peak intensity stability by well-over an order of magnitude. We demonstrate this process experimentally using a thin disk pump laser at ELI{Beamlines

To ensure a high signal to noise contrast ratio, lots of challengeable work must be done during the construction of a petawatt level laser system. In this report, we analyse the effects on the contrast ratio by the optical element manufacture errors expressed as the peak-valley value (PV value) and the PV gradient value, the chromatic aberration and group delay in system design. Using the Fourier transformation method with the random phase attached on the laser beam in frequency domain, it is proved that for manufacture errors, PV gradient value is more tolerable than that of PV value. At the terminal end of a petawatt level laser system, there exist, in pulse compressor, spectral clip, grating manufacture errors and non-uniformity of the diffraction efficiency that will affect the final SN contrast ratio of the laser system. Since the spectral clip here is soft that can benefit the promotion of the contrast ratio. But for manufacture errors of the large size grating, when PV = 1/5 wavelength, and PV differential gradient about 1/75 wavelength per centimeter. The terminal SN contrast ratio is restrained. When focused on the target, simulation for SN contrast ratio near the focal region caused by the residual distortion is taken. Calculation shows that, for a 20 microns focal spot, to maintain the 10⁸:1 contrast ratio across the whole focal spot, residual wavefront distortion should be compensated to PV value less than 0.2 wavelength.

A high power, high repetition rate optical parametric chirped pulse amplification (OPCPA) system is presented. The OPCPA delivers Carrier-Envelope Phase (CEP)-stable 7 fs pulses, with up to 0.19mJ of energy, with a total compressed average power of 19 W. The system is being currently integrated in an attosecond pumpprobe beamline capable of combining attosecond pulses in the extreme ultraviolet (XUV) with synchronized few-cycle pulses in the near infrared (NIR). The main motivation for this development is to perform pump-probe experiments with attosecond time resolution, in which electrons and ions produced during photo-ionization are detected in coincidence utilizing a reaction microscope.

Optical parametric chirped-pulse amplification (OPCPA) implemented using multikilojoule Nd:glass pump lasers is a promising approach to produce ultra-intense pulses (>10^23 W/cm^2) [1]. Systems using deuterated potassium dihydrogen phosphate (DKDP) for high-energy amplifiers are being developed by a number of institutions [2–4]. Noncollinear optical parametric amplifiers (NOPA’s) made of DKDP produce broadband gain for supporting pulses as short as 10 fs centered near 920 nm. Large-aperture DKDP crystals (>400 mm) make it possible to use Nd:glass lasers as kilojoule pump sources. Although OPCPA is now routinely used as a broadband front-end technology for many hybrid systems, scaling OPCPA to energies >100 J is still an active area of laser research and development. This paper reports on a technology development program at the Laboratory for Laser Energetics where progress is being made toward the long-term goal of a femtosecond-kilojoule system pumped by the OMEGA EP laser. The goal is to pump an optical parametric amplifier line (EP OPAL) with two of the OMEGA EP beamlines. The resulting ultra-intense pulses (1.5 kJ, 20 fs, 10^24 W/cm^2) would be used jointly with picosecond and nanosecond pulses produced by the other two beamlines. A midscale all-OPCPA laser is being designed and constructed to address the technical challenges of the full-scale system. The mid-scale OPAL pumped by the Multi-Terawatt (MTW) laser will produce 7.5-J, 15-fs pulses and demonstrate scalable technologies suitable for the upgrade. MTW OPAL will share a target area with the MTW laser (50 J, 1 to 100 ps), enabling several joint-shot configurations. We report on the status of the MTW OPAL system, and the technology development required for this class of all OPCPA laser system for ultra-intense pulses. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856, the University of Rochester, and the New York State Energy Research and Development Authority. References 1. Ross, I. N., Matousek, P., Towrie, M., Langley, A. J. and Collier, J. L., “The prospects for ultrashort pulse duration and ultrahigh intensity using optical parametric chirped pulse amplifiers,” Opt. Commun. 144(1-3), 125-133 (1997). 2. Lozhkarev, V. V., Freidman, G. I., Ginzburg, V. N., Khazanov, E. A., Palashov, O. V., Sergeev, A. M. and Yakovlev, I. V., “Study of broadband optical parametric chirped pulse amplification in a DKDP crystal pumped by the second harmonic of a Nd:YLF laser,” Laser Phys. 15(9), 1319-1333 (2005). 3. Tang, Y., Ross, I. N., Hernandez-Gomez, C., New, G. H. C., Musgrave, I., Chekhlov, O. V., Matousek, P. and Collier, J. L., “Optical parametric chirped-pulse amplification source suitable for seeding high-energy systems,” Opt. Lett. 33(20), 2386-2388 (2008). 4. Cartlidge, E., “The light fantastic,” Science 359(6374), 382-385 (2018).

The Petawatt beamline at the Vulcan laser facility is capable of delivering pulses with 500J of energy in <500fs, and has been operational as a user facility since 2003; being used to study laser matter interactions under extreme conditions. In addition to this short-pulse beamline there is a single long pulse beamline capable of 250J with durations from 0.5 to 6ns. In this paper we present our plans to add an auxiliary beamline to this facility based on Optical Parametric Chirped Pulse Amplification (OPCPA) using LBO as the non-linear crystal. This new beamline will have a dedicated laser area where the seed will be generated, stretched and amplified before being transported to the target area for compression and delivery to target. The beamline will be implemented in 2 phases the first phase will see the development of a 7J <30fs capability with the second phase increasing the delivered energy to 30J. This additional beamline will open up the potential for novel pump probe experiments when operated with the existing PW and long pulse beamlines.

Most of the laser sources of ELI-ALPS are operating in the 100 W average power regime, while the peak power and the repetition rate range from 0.1 TW at 100 kHz up to PW at 10 Hz. The pulse duration lasts typically a few optical cycle, feeding six attosecond beamlines, two particle and two THz beamlines, as well as many end stations associated with the beamlines [1]. The first 100 kHz repetition rate lasers have been installed by end 2017, and became available for experiments from the beginning of 2018. The Mid-InfraRed laser (MIR) provides 150 µJ, 40 fs pulses at 3.1µm [2]. The first High-Repetition rate laser (HR1) has been operating in long pulse mode (1.5 mJ, 40 fs pulses at 1030 nm). The short pulse operation mode (1mJ, <7fs) will be completed by March 2019. The second High-repetition Rate laser (HR2) offering 5 mJ and sub-7 fs pulses, will be commissioned by mid 2019. The kHz repetition rate SYLOS laser [3] has been completed its second development phase for providing >5 TW peak power pulses at sub-7 fs pulse duration. The system is available to run four beamlines from April 2019. In the meanwhile, parallel to its operation, the final upgrade stage will be developed, reaching an energy of >100 mJ by mid 2020. In order to serve the four beamlines efficiently, a so called SYLOS experimental alignment laser has been implemented. The laser “mimicking” the SYLOS laser at lower repetition rate, providing the similar beam size, peak intensity, and duration (3.8TW, 12 fs) at 10Hz repetition rate is available from end 2018. The High Field Petawatt (HF PW) laser of ALPS has been designed to operate at 10 Hz repetition rate with a pulse duration shorter than 17 fs. The system is under installation, 10 Hz operation at 0.4PW level will be reached by mid 2019, while the full 10Hz operation will be demonstrated by Fall 2020. We also carry on experiments towards the elimination of bottlenecks of high repetition rate sub-10fs operation of the 100Hz branch of the HF laser. The MIR and HR1 lasers have successfully served almost ten commissioning user experiments with external collaborators, investigating phenomena like electron migration in water, K-shell excited elections, etc., altogether for 51 operational weeks in 2018. Next year we expect to extend the commissioning experiments for the SYLOS laser as well as at least two attosecond and THz beamlines. The challenge of the laser R&D to design and implement such lasers was taken by the industry (EKSPLA, Light Conversion (both Lithuania), Fastlite, Amplitude Technologies (both France), AFS GmbH (Germany)), academia, and ELI-ALPS, resulting in a change of paradigm in laser engineering, leading to novel state of the art research grade lasers just in a few years. [1] S. Kuhn et al., Mol. Opt. Phys. 50 (2017) 132002 [2] R.Budrinas et al., Opt.Exp. 25 (2017) 5797 [3] N.Thiere et al., Opt.Exp. 26 (2018) 26907

The Allegra femtosecond laser system is the main driver for high harmonic and plasma x-ray secondary sources at ELI-Beamlines operating at a 1 kHz rep rate. The system is based on OPCPA technology and consists of seven amplification stages pumped by thin-disk picosecond lasers. It is designed to reach 30 mJ output in the first phase of operation and to be ramped up to 50 mJ by engaging an additional pump laser. The amplified pulse is compressed to sub-20fs by an array of chirped mirrors and higher order dispersion is pre-compensated for by a Dazzler AOPDF in the front-end. In this paper we present the overview of Allegra system and the current status of deployment with a special focus on the high average power OPCPA in vacuum.

TRUMPF Scientific Lasers develops and provides customized ultrafast amplifiers based on thin-disk technology. Due to its efficient one-dimensional heat removal and the small longitudinal extension of the gain medium, the thin-disk geometry offers exceptional scaling performance both in terms of energy and average power. All systems are based on the industrialized TRUMPF thin-disk laser technology [1]. Regenerative amplifiers systems with multi-millijoule pulses, kilohertz repetition rates and picosecond pulse durations are currently available. Record pulse energies of 220 mJ at 1 kHz could be demonstrated originally developed for pumping optical parametric amplifiers [2-4]. In this contribution, we present different commercial ultrafast solutions based on regenerative amplifiers with up to 200 mJ of pulse energies and more than 1 kW of average power [3-5]. New developments with thin-disk based multipass amplifier cells led to multikilowatt average output powers [6-9]. First measures to scale the energy with multipass thin-disk amplifiers towards 1 J will be presented. In addition, concepts for nonlinear compression to reach pulse durations below 50 fs will be discussed.

Gratings for high peak and high average power lasers

The Extreme Light Infrastructure (ELI)-Beamlines facility is host to high energy, ultrashort high repetition rate lasers with state-of-the-art parameters. The design peak powers range from 10’s of TW/1kHz to 1PW/10Hz or even a 10PW class shot every minute. Those chains contain broadband systems as well as many ns or ps high power pump lasers.

Apart from the high damage resist optical components, the high threshold beam dump materials are also of paramount importance for developing the high power ultrafast lasers, particularly, for the high repetition rate laser systems. The high power beam dumps requires the highly absorptive and high damage threshold optical materials which can prevent the back reflection and overheating. In order to evaluate the optimal performance of the various beam dump materials, we characterized the optical properties of these materials and examined the damage threshold measurements. We developed a LIDT test station to measure the damage threshold of various kinds of optical components for ELI Beamlines facility. The developed LIDT test station can offer the measurements at various laser parameters and environmental conditions that are similar to usage conditions of the ELI beamlines lasers.

The damage resistance of various beam dump materials was tested using LIDT test station at ELI Beamlines. The various beam dump materials include highly absorptive (>99.8%) vantablack coatings and different absorptive glasses. The LIDT tests were performed using 800nm, ultrafast laser beams with pulse duration of 40fs at 1 kHz repetition rate. The LIDT test methods such as S-on-1 and R-on-1 were used for determining the damage threshold. From the presented results, we discuss the best beam dump materials with high damage resistance for high power laser systems.

Limited by the volume and power of lasers, nanosecond short-pulse lasers are commonly used in laser communication, laser range finder, laser guidance, pulse laser Radar, high-speed photography and 3D imaging systems. Therefore, as one of the key laser indicators, the nanosecond short-pulse laser peak power measurement is essential. In this paper, a new method for measuring peak power of nanosecond short-pulse laser is proposed. The peak power can be measured directly by extending the pulse width without using super high speed ADC chip. Research is made from two parts: detector and control host. As for the detector, a new attenuation unit is designed, which can well suppress the peak power decrease caused by the traditional detector broadening the short pulse and realize the original waveform output (non-distortion). In the control host, a new pulse broadening circuit is proposed, which realized the peak signal acquisition of 20ns pulse width (5ns rising edge) laser without the use of ultra-high speed ADC acquisition chip. The measurement system can achieve technical indicators as follows: the measurable pulse width is 20ns～150us, Pulse rising edge is 5ns～10us, the repetition frequency is 100Hz～500kHz, and the measurement range of laser peak power is 1W～500W.

Results of a comparative study on single-shot surface ablation of commercial optical glasses together with the transient reflectivity enhancement during the process are reported. Three types of optical glasses: Schott’s BOROFLOAT®, BK7 and B270 are ablated by single pulses of 34 fs duration at 800 nm central wavelength of the TeWaTi laser systems at University of Szeged, varying systematically both the pulse energy and the beam diameter on the surface, while recording the reflected signal. The depth and diameter of the ablated holes are characterized ex-situ by a DEKTAK profilometer. Very similar ablation characteristics have been determined: Above the ablation thresholds at 5.84±0.21 Jcm^-2 (1.72±0.06*1014 Wcm^-2), 6.43±0.56 Jcm^-2 (1.89±0.16*10¹⁴ Wcm-2) and 5.86±0.31 Jcm^-2 (1.75±0.09*10¹⁴ Wcm^-2) for BOROFLOAT®, BK7 and B270, respectively, both the diameter and the depth of the holes produced show logarithmic increase as a function of pulse energy/fluence until saturating above ~18 Jcm^-2. On the contrary, significant differences have been obtained in the time integrated transient reflectivities, with the highest absolute values measured for the BOROFLOAT® glass. Strong spot size dependence has been revealed: The reflectivity increases monotonously with increasing pulse energy for all spot sizes, with decreasing absolute values/slopes with decreasing spot areas. Different reflectivities belong to the same fluence/intensity depending on the actual spot size, consequently the fluence/intensity alone does not define unambigously the characteristics of the plasma. The correct description of the changes in reflectivity requires the specification of the spot size together with the pulse energy/fluence/intensity.

High order harmonics is generated in a modulated slab waveguide which is filled with helium when a femtosecond laser is focused into this kind of waveguide. The modulated slab waveguide is used to implement quasi-phase matching of high order harmonics so that an obvious increase of the high harmonics yield at wavelengths close to the cutoff region was observed. High order harmonics generated in modulated waveguides with a period of 0.8mm and a period of 0.5mm respectively were compared. The results show that the shorter the period of the modulated slab waveguide is, the higher order high harmonics with phase mismatch we can compensate.