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A single-shot measuring apparatus with optical limiting for temporal pulse contrast of kJ-class petawatt lasers in the nanosecond range is proposed. A temporal linear filter comprising an electro-optical switch, a polarizer, a temporal nonlinear filter composed of cascaded SHG crystals, and a dichromatic mirror are, respectively, used as an optical limiting apparatus for contrast measurement of nanosecond and picosecond pulses to improve dynamic range and temporal resolution. The apparatus has been applied to pulse contrast measurements at the SG-II petawatt facility, achieving a high dynamic range of 1 0 10 and a fast time resolution of 107 ps in the 350 ns range. This technique can also be universally applied to the limiting of the main pulse of varying pulse widths to diagnose pre-pulses during generation and transmission.

An iterative computation method was proposed to accurately reconstruct the optical transfer function (OTF) of an imaging lens from a series of recorded images of different patterns displayed on a digital micromirror device (DMD). Since several tens of recorded images are adopted in combination, the measurement noise is remarkably reduced in comparison with the conventional measurement method using a point object, and quite high measurement accuracy is reached by taking the known shape of each tiny mirror of the DMD into computation. While theoretical analysis is demonstrated, the validity of this proposed method is verified both numerically and experimentally.

We focused on a single-shot method for directly measuring the temporal contrast enhancement of a single plasma mirror by analyzing the spectrum of a chirped pulse spatiotemporally overlapped with the igniting laser used for generating a plasma mirror. Experimentally, temporal contrast enhancement of 102 by one plasma mirror was successfully measured in a hundred picosecond timescale and was consistent with the theory. This single-shot measurement method caused no degradation on the performance of the plasma mirror, which was proved by monitoring the efficiency and far-field pattern of the igniting laser after the plasma mirror. Combined with calorimeters and CCD cameras, this method is expected to realize the single-shot online diagnosis of plasma mirrors. This method is expected to be an efficient approach for measuring the temporal contrast enhancement of the plasma mirrors.

Here, we have studied the nonthermal acceleration of energetic electrons/protons under the near-QED regime by extending the laser intensity beyond 1023 W/cm2 based on a two-dimensional particle-in-cell simulation. The radiation-reaction (RR) effect plays a critical role and brings a quantum stochastic effect to the charged-particle acceleration process. Background electrons in plasma are accelerated in an intense laser field to several GeVs with strong oscillations and thus radiate γ-ray photons. The emitting γ-photons have a broad energy spectrum with maximal energy up to 3 GeV and result in radiation-reaction trapping of the electrons, forming a relativistic plasma bunch in the plasma channel. The accumulation of electrons and protons produces a charge-separation field for the acceleration/deceleration of charged particles. The accelerated electrons have a nonthermal spectrum with a power-law index of 1.5 with a laser intensity 1023 W/cm2 lower than that in the non-QED regime. As the laser intensity further increases over 1024 W/cm2, the power-law index further drops to 1.2. Moreover, the energy spectrum of accelerated protons has a nonthermal distribution with a power-law index of 0.7, which is much lower than that of electrons in the near-QED regime.

In order to break through the limitation of off-axis holography in the field of measuring rough or strong scattering objects, a new iterative algorithm based on the concept of wavefront-coding was proposed. The reference wave is regarded as a wave modulator and it starts with random guess freed from the result of traditional off-axis holography. The full frequency spectrum could be retrieved iteratively after taking full advantage of the space-bandwidth production of the detector. As one form of coherent diffractive imaging, the theoretical resolution is diffraction limitation. According to the simulations and experiments with random phase plate, when the object fails to be reconstructed by traditional off-axis holography and other iterative off-axis holography algorithm due to the frequency spectrum of object is too wide, the proposed algorithm works well. It could be a general algorithm to prominently improve the capability of off-axis holography to measure rough or strong scattering objects.

Laser induced damage in the final optics system is one of the bottleneck problems in the high-power laser system. For almost all the eight beams of final optics assembly (FOA), there are usually damages in the middle areas of the focusing lens in SG-II laser facility. Through detailed analysis, we find a correlation between the damage in the focusing lens and defect in the continuous phase plate (CPP). The main mechanism for downstream damage is regarded as the defect induced light intensification. The phase distribution of the CPP is characterized by coherent diffraction imaging method. Through simulation, we can clearly see a much stronger light intensification in the middle of the beam caused by the real CPP than the theoretical designed CPP. Compared with the designed CPP, there are defects on the CPP causing the downstream intensification. Meanwhile, there are unexpected periodic modulation on the surface of CPP. We assume the central defect is a kind of laser induced defect because the defects caused by the optical processing are randomly distributed. Through ray tracing analysis, we find a ghost image near the center of the CPP position. So the CPP is slightly damaged or modified in the middle area of the ghost image ray, thus forming a defect with strong modulation. A stray light management is proposed base on ground glass to mitigate the ghost image problem. The periodic modulation is possibly formed by the manufacturing process of CPP. Small-period modulation can cause greater downstream modulation. It should be controlled with power spectral density specification in the manufacturing process. Once the laser induced defect problem is solved, the laser induced damage in the middle of focusing lens is greatly mitigated.

Target alignment technology is one of the most critical technologies in high-energy physics experiments, and is an important technology related to the success of the laser fusion experiment. This paper describes the error in the basic target alignment scheme, and designs a series of alignment fiducials around the target to achieve the improvement in target alignment precision. This paper analyzes the influence of fiducials-group’s forms on the target alignment error. According to their respective properties, under the constraints of the experimental conditions, and combined with the linear and nonlinear fiducials-group’s design, improvement of target alignment precision and error transfer are achieved. Under the experimental requirements of the SG-II-U facility, combined with the working conditions, a surrogate target with alignment fiducials-group is designed, and through the interaction of the fiducials-group and the alignment mark in viewers, the error of target recognition can be greatly optimized and the uncertainty of target alignment can be reduced.

Phase-shifting image-plane digital holography is considered as a widely-used measuring method for observation of micro-scale phase objects and for nondestructive testing of phase morphology. The elliptical vortex sieve is a kind of amplitude-only diffractive lenses which can be used for focusing and imaging from coherent X-rays to terahertz regime. Here an elliptical vortex lens is introduced into phase-shifting image-plane digital holography, and verifies the validity of phase shift by measuring a test target and the multiplanar focal spots of a Taiji lens. The experimental results agree well with the theoretical analysis. Owing to amplitude-only diffractive lenses, they will have great potential in X-rays holography and biochemical microscopy for the next generation of synchrotron radiation and free electron laser in the future.

We report on comprehensive research regarding the potential of phase-conjugate wave (PCW) generation via backward degenerate four-wave mixing process as a pulse cleaner in femtosecond high-power laser systems. We analyzed the performance of this technique by investigating the contrast enhancement capability, gain bandwidth, and energy conversion efficiency. All of these parameters are important in pulse cleaning. The numerical calculations verified that a considerably large efficiency bandwidth product and an excellent contrast enhancement capability can be realized by finding a balance between each physical effect. The simulation results based on this study provide guidance for the application of this technique in laser systems with different bandwidths. The method, which is based on PCW generation, provides new insights for femtosecond petawatt laser systems as pulse cleaners.

Point spread function defines the imaging property of a linear optical imaging system. Here Fermat spiral is introduced into Greek-ladder sieves to generate multiple different point spread functions along light propagation and further to realize multiplanar anisotropic imaging. Theoretical analysis and experimental results show that this kind of diffractive lenses has trifocal spots on the optical axis. The first and third foci are asymmetrical, meaning that they can provide different spatial resolution in different directions. The second focus is a vertex focal spot, which can be used for phase-contrast imaging. Due to amplitude-only diffractive lenses, they will have great potential in X-ray holography, X-ray microscopy, phase-contrast bio-imaging and optical trapping for the next generation of synchrotron radiation and free electron laser in the future. © 2018 Elsevier Ltd. All rights reserved.

To improve the beam pointing accuracy of PW laser facility and reduce the optical axis deviation caused by the deflection amplitude response deviation of the confocal lens of spatial filter for microvibration action, an Optomechanical coupling vibration active control theory is proposed to make the peak value of output optical angle response lower than the pointing threshold value by 0.2 µrad. To establish an Optomechanical coupling vibration active control system, the active control parameters are introduced into the beam transmission matrix of the Optomechanical coupling system. The active control parameters and the peak value of the output light angle response are linked point to point. The algorithm flow of the active control system is designed, the control rules are established, and the control effect is verified. The results show that the peak value of the output optical angle response of the spatial filter decreases by 98.13%, and the attenuation is nearly two orders of magnitude after the active control, which effectively improves the convergence accuracy of the beam pointing of the spatial filter of the PW laser facility, and realizes the beam pointing control under the broadband excitation, and the control result is consistent with the expectation.

We report on the design and first results from experiments looking at the formation of radiative shocks on the Shenguang-II (SG-II) laser at the Shanghai Institute of Optics and Fine Mechanics in China. Laser-heating of a two-layer CH/CH-Br foil drives a $\sim$40 km/s shock inside a gas-cell filled with argon at an initial pressure of 1 bar. The use of gas-cell targets with large (several mm) lateral and axial extent allows the shock to propagate freely without any wall interactions, and permits a large field of view to image single and colliding counter-propagating shocks with time resolved, point-projection X-ray backlighting ($\sim20$ $\mu$m source size, 4.3 keV photon energy). Single shocks were imaged up to 100 ns after the onset of the laser drive allowing to probe the growth of spatial non-uniformities in the shock apex. These results are compared with experiments looking at counter-propagating shocks, showing a symmetric drive which leads to a collision and stagnation from $\sim$40 ns onward. We present a preliminary comparison with numerical simulations with the radiation hydrodynamics code ARWEN, which provides expected plasma parameters for the design of future experiments in this facility., Comment: 16 pages, 5 figures, accepted for publication in High Power Laser Science and Engineering (25 March 2021)

We proposed a single-shot diagnostic for spatiotemporal laser-plasma evolution by the multi-dimensional encoding (MuDE) holography. It can achieves high and adjustable temporal resolution measurement of three-dimensional plasma distribution without delay scanning. Experimentally, we verified the feasibility of this technique, and the retrieved results of laser-plasma evolution agreed well with the direct shadow measurement. This technique is expected to provide a helpful tool for the complex spatiotemporal evolution of plasma in ICF and high energy density physics.

The current developments of laser facilities at the NLHPLP are presented and the prospect of using low spatial incoherence beams to improve laser irradiation uniformity for direct-drive and indirect drive laser fusion is introduced.

The characterization of energetic protons generated in the ShenGuang-II UP petawatt laser interactions with foil targets has been systematically studied. The proton energy spectra and angular distributions are measured with a radiochromic film stack. It shows that the proton energy spectra have a Boltzmann distribution with temperature of about 2.8 MeV and cutoff energy of about 20 MeV. The divergence angles of protons vary from 10 degrees to 60 degrees, dependent on the proton energy. The proton source size and location are investigated via the proton point-projection mesh imaging. The proton virtual sources are found to locate tens to hundreds of microns in front of the foil target, depending on the proton energies. A Monte Carlo simulation estimates the diameter of the virtual proton source to be about 12 mu m for the protons with energy of 16.8 MeV, which is much smaller than the laser focus size of about 50 mu m. The spatial resolution of the 16.8 MeV proton imaging is quantified with the point spread function to be about 15 mu m, which is consistent with the proton virtual source size. These results will be important for the users conducting experiments with the protons as a backlighting source on the ShenGuang-II UP petawatt laser.

As a key part in high-power laser facility, the spatial filter is used to determine the optical axis of the entire laser system, which consists of two conjugate lenses at both ends and filter hole in the middle. The lenses usually require precise adjustment, and the adjustment mechanism is integrated to both ends of the filter currently. To this end, a novel parallel adjusting machine for the filter lenses was proposed as an auxiliary tool in the spatial filter process. The machine was designed to realize the five-dimensional adjustment functions of the filter lenses with X-Y-Z axial translation, pitch, and yaw motion. The first-order ghost point of the filter lens was used as the feedback reference for the parallel machine to adjust Z-position of the lens. A more comprehensive optimization-oriented framework was established to improve the sensitivity of the lens to the posture error and to optimize the positioning speed based on the steepest descent method (SDM). The experimental results verified the superiority of the proposed adjustment machine and feedback reference point.

Relativistic electrons generated by the interaction of petawatt-class short laser pulses with solid targets can be used to generate bright x-rays via bremsstrahlung. The efficiency of laser energy transfer into these electrons depends on multiple parameters including the focused intensity and pre-plasma level. This paper reports experimental results from the interaction of a high intensity petawatt-class glass laser pulses with solid targets at a maximum intensity of 1019 W cm−2. In-situ measurements of specularly reflected light are used to provide an upper bound of laser absorption and to characterize focused laser intensity, the pre-plasma level and the generation mechanism of second harmonic light. The measured spectrum of electrons and bremsstrahlung radiation provide information about the efficiency of laser energy transfer.

Nonlinear hot image is one of the key elements that limit the output performance of high-power laser systems. In most hot-image researches, only one hot image peak is observed in the conjugate position for a single defect. Generally, multiple hot image peaks occur for multiple defects or cascaded nonlinear media. However, a new phenomenon is found by numerical simulation in our work: one defect can also afford two hot-image peaks near the conjugate position when considering the defect edge steepness. The super-Gaussian defect model is employed to mimic the defect edge steepness. When the super-Gaussian order is higher than one, there could be two hot image peaks under certain conditions. The formation of the double hot image peaks is primarily due to the co-effect of the hard-edge diffraction and the self-focusing effect. The influence of different factors, including the super-Gaussian order, defect size, modulation depth, and Kerr medium thickness, on the double hot image peaks intensity and location is systematically investigated. The results show that with the increase in the super-Gaussian order, the intensity of the double hot image peaks increases gradually. The defect size has a great influence on the position of the two hot image peaks. The modulation depth and thickness of the Kerr medium influence the intensity of the two hot image peaks; however, they have less impact on the peak location. Importantly, the defect edge steepness and size dependences of multiple nonlinear hot-image formation from a single-phase defect are further discussed in this paper. The two hot image peaks are fatal to optical components in high-power laser systems; in particular, the hot image peak behind the conjugate position is totally unexpected for a single defect. This research provides insights into basic physical images and hot-image formation laws. It also provides important guidance for optical defect specification evaluation and optical component layout design, as well as for beam quality control, in high-power laser systems.

A type of plasma-based optical modulator is proposed for the generation of broadband high-power laser pulses. Compared with normal optical components, plasma-based optical components can sustain much higher laser intensities. Here we illustrate via theory and simulation that a high-power sub-relativistic laser pulse can be self-modulated to a broad bandwidth over 100% after it passes through a tenuous plasma. In this scheme, the self-modulation of the incident picoseconds sub-relativistic pulse is realized via stimulated Raman forward rescattering in the quasi-linear regime, where the stimulated Raman backscattering is heavily dampened. The optimal laser and plasma parameters for this self-modulation have been identified. For a laser with asub-relativistic intensity of I ∼ 1017W/cm2, the time scale for the development of self-modulation is around 103 light periods when stimulated Raman forward scattering has been fully developed. Consequently, the spatial scale required for such a self-modulation is in the order of millimeters. For a tenuous plasma, the energy conversion efficiency of this self-modulation is around 90%. Theoretical predictions are verified by both one-dimensional and two-dimensional particle-in-cell simulations.

Stimulated Raman scattering (SRS) in plasma in a non-eigenmode regime is studied theoretically and numerically. Different from normal SRS with the eigen electrostatic mode excited, the non-eigenmode SRS is developed at plasma density $n_{e}>0.25n_{c}$ when the laser amplitude is larger than a certain threshold. To satisfy the phase-matching conditions of frequency and wavenumber, the excited electrostatic mode has a constant frequency around half of the incident light frequency $\unicode[STIX]{x1D714}_{0}/2$ , which is no longer the eigenmode of electron plasma wave $\unicode[STIX]{x1D714}_{pe}$ . Both the scattered light and the electrostatic wave are trapped in plasma with their group velocities being zero. Super-hot electrons are produced by the non-eigen electrostatic wave. Our theoretical model is validated by particle-in-cell simulations. The SRS driven in this non-eigenmode regime is an important laser energy loss mechanism in the laser plasma interactions as long as the laser intensity is higher than $10^{15}~\text{W}/\text{cm}^{2}$ .

Phase defect detection with micrometer scale on large aperture optical elements is one of the challenges in precision optical systems. An efficient scheme is proposed to detect phase defects. First, the defects are positioned in a large aperture by dark-field imaging based on large aperture photon sieves to improve the detection efficiency with a relatively low cost. Second, static multiplanar coherent diffraction imaging is used to retrieve the phase of the positioned defects in a small field of view. Here, a spatial light modulator is used as a multifocal negative lens to eliminate the mechanical errors in multiplanar imaging. The use of a negative lens instead of a positive lens has the advantage of a larger imaging space for the system configuration. Compared to the traditional interferometry system, this diffraction detection system has a simpler optical path and doesn’t require sparse distribution of the defects. Experiment results demonstrate the success of the proposed scheme with a detection resolution better than 50 µm. We believe this work provides an effective method to rapidly detect phase defects on large aperture optics with high accuracy and high resolution.

The spatiotemporal plasma-lens filter proposed here enhances the temporal contrast of the ultrashort pulse laser by combining plasma optics and spatial filtering. Experimentally, the spatiotemporal plasma-lens filter has improved the temporal contrast by 2 orders of magnitude with 80% laser transmission efficiency under a 1 Hz repetitive laser operation. Not only were the pre-pulse and the pedestal cleaned out, but also the rising edge of the main pulse was sharpened.

In a recent article by Li et al (2019 Sci. Adv. 5. eaav7940), experimental results from a dual-color laser wakefield acceleration (LWFA) were presented. In the present paper we, primarily, focus on detailed simulation studies of such a scheme in the self-injection and ionization injection regimes, respectively. The spatiotemporally-overlapped 30 fs dual-color laser pulses are at fundamental (FL, 800 nm, ‘red’) and second-harmonic (SH, 400 nm, ‘blue’) wavelengths. They are (a) co-propagating in an under-dense plasma, (b) relativistically intense (I > 1018 W cm−2) and (c) having relatively high-energy (multi-Joule, loose focusing) and low-energy (sub-Joule, tight focusing), respectively. The basic concept of the scheme is the fact that the depletion length (L pd) for a relativistic laser pulse in an under-dense plasma has an inverse quadratic dependence on the laser wavelength (∝1/λ 2). Here, first by using a single FL 77 TW/30 fs laser pulse to drive a LWFA, an electron beam was accelerated up to ∼400 MeV from a background plasma having an electron density of 1019 cm−3. Then, by driving the same LWFA by co-propagating ‘blue’ 7 TW/30 fs and ‘red’ 70 TW/30 fs laser pulses, the electron energy reached ∼700–800 MeV (maximum). The simulations confirm that in such a dual-color LWFA scheme, the role of the SH laser pulse is post-accelerating electrons after a rapid depletion of the FL laser pulse in the plasma. Furthermore, the SH pulse assists the ionization-injection of the electrons which is an additional benefit of the dual-color LWFA scheme.

As the key part for energy amplification of high-power laser systems, disk amplifiers must work in an extremely clean environment. Different from the traditional cleanliness control scheme of active intake and passive exhaust (AIPE), a new method of active exhaust and passive intake (AEPI) is proposed in this paper. Combined with computational fluid dynamics (CFD) technology, through the optimization design of the sizes, shapes, and locations of different outlets and inlets, the turbulence that is unfavorable to cleanliness control is effectively avoided in the disk amplifier cavity during the process of AEPI. Finally, the cleanliness control of the cavity of the disk amplifier can be realized just by once exhaust. Meanwhile, the micro negative pressure environment in the amplifier cavity produced during the exhaust process reduces the requirement for sealing. This method is simple, time saving, gas saving, efficient, and safe. It is also suitable for the cleanliness control of similar amplifiers.

We have used the Shenguang II laser in third harmonic (351 nm) to investigate the emission of L-shell radiation in the 3.3 to 4.4 keV range generated using thin foils of Sn coated onto a parylene substrate with irradiation of order 1015 Wcm−2 and nanosecond pulse duration. In our experiment, we have concentrated on assessing the emission on the non-laser irradiated side as this allows an experimental geometry relevant to experiments on photo-ionised plasmas where a secondary target must be placed close to the source, to achieve X-ray fluxes appropriate to astrophysical objects. Overall L-shell conversion efficiencies are estimated to be of order 1%, with little dependence on Sn thickness between 400 and 800 nm.

To satisfy the beam pointing and centering requirements of a sophisticated sampling beam path for petawatt picosecond diagnostic systems, a laser-beam alignment system is applied to this diagnostic platform. A mathematical model of a 4D linear matrix is applied to achieve rapid and accurate beam alignment. This alignment system can also switch quickly between two channels sampling beams without disturbing the beam’s pointing and centering. Experimental results indicate that the near-field centering accuracy is 0.3 mm and the far-field pointing accuracy is 0.16 mrad. With this alignment system, the picosecond diagnostic system can realize stable and reliable pulse width diagnostic between 1 and 10 ps, and obtains high dynamic range signal-to-noise ratio diagnostic results.

Owing to the novel physicochemical properties of graphene, materials such as graphene oxide (GO) are being developed for applications in various fields such as biomedicine. Nonetheless, considerable knowledge gaps still exist regarding the impact of GOs on environmental health and safety (EHS). Thus far, its secondary toxicity, synergistic effects, and mal-adaption have not been focused much upon. Here, we show that at low concentrations (that did not directly result in significant cytotoxicity), GO could greatly enhance metal toxicity in macrophages by altering their cellular priming state. Specifically, GO caused impairments to the cellular morphology and membrane integrity of macrophages, and remarkably enhanced the cellular uptake of Cd and other non-essential metal ions (such as Hg and Gd). Furthermore, upon low-dose GO pre-treatment, the uptake of Cd at a non-toxic concentration brought about a remarkable amount of oxidative stress in macrophages, and ultimately resulted in increased cell death. Mechanistic investigations illustrated that GO pre-treatment triggered cell death through apoptosis because of Cd exposure. Overall, this study’s results reveal a new path for understanding the impact of GOs on EHS through the perspective of its synergistic and secondary effects, previously unidentified mechanisms via which nanomaterials might pose detrimental effects on organisms.

We proposed a technique of tunable plasma narrow-band filter (PNBF) to generate high-power narrow-band pulses based on a plasma mirror and pulse chirp. PNBFs have an ultra-wide free spectral range, high damage threshold, and accurate wavelength-tunable function, and tunable sub-nanometer pulses could be achieved by controlling the pulse chirp. The feasibility of the proposed technique was verified by both simulation and experiment. Three ~0.6 nm bandwidth pulses centered at different wavelengths were obtained respectively, maintaining good near and far-field beam quality. PNBF is expected to be an effective technique for high-power tunable narrow-band pulse generation with high-power ultrafast lasers.

The target area of a 288-beam inertial confinement fusion laser driver was designed to allow lasers for direct-drive illumination, spherical hohlraum with six laser entrance holes (6LEHs), and baseline cylindrical hohlraums. The suggested radius of a target chamber was 6.0 m based on the ratio of the total port area to chamber area. Beam port distribution on the chamber was calculated if the direct propagation of laser beams into opposing beam ports was avoided, and this distribution was compatible with spherical hohlraums with 6LEHs without additional ports opened. According to the symmetry of the beam port distribution, an X-shaped beam-guiding system (BGS) in the switchyard was proposed and arranged within a baseline algorithm. The switch between direct- and indirect-drive modes was easy to operate using this BGS concept.

To obtain seed pulses of large energy, high contrast and broad spectral bandwidth for the high power laser system, a new cross-polarized wave (XPW) generator based on the gadolinium gallium garnet (GGG) crystal is designed. For the injected fundamental wave (FW) pulses exceeding tens of millijoule, the output XPW characteristics of conversion efficiency, spectral broadening and contrast improvement are theoretically analyzed. We also investigated the influence of spatial distribution characteristics of the injected chirped FW pulses on the output XPW. With incident FW pulses of 8 order super gaussian spatial profile for instance, conversion efficiency of ~73%, spectral broadening ratio (SBR) of ~2.7 (from 40nm to 108nm) and contrast boost by >7 orders of magnitude can be obtained. The impact of the FW spatial modulation on XPW was sensitive to the modulation depth, which is significant for spatial quality control of the laser beam in high power laser systems.

Laser induced damage in the final optics assembly (FOA) is one of the bottleneck problems in high power laser systems for the inertial confinement fusion. In the online experiment, a correlation between the transport mirror defects and the optics damage in the final optics assembly has been found. A physical model is built to analyze the influence of defect size and modulation depth on the light intensification in the FOA. Basically, small size defect on the mirror has a tiny influence on the downstream modulation in the FOA. Optical propagation simulation is also carried out with the real defect phase information got from the interferometer. Results show that there could be strong light intensification caused by the upstream mirror defect. So the local high enough intensity is the main reason for the optics damage. The abnormal intensification is mainly caused by the irregular defect contour, which is produced in the laser induced damage process on the mirror. It is best to eliminate this kind of coating defect on the transport mirrors. When this kind of damaged defect on the transport mirror is inevitable, a mitigation strategy for the optics damage in the FOA is proposed. An inverse design of the best distance between the transport mirror and the FOA can be used to reduce the influence of the transport mirror defects on the downstream FOA. The study can provide some reference for the improvement of the damage resistance of final optics assembly in the high power laser facility.

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.

In high power laser system, the wavefront quality of large optic elements in the mid-frequency region plays a critical role in the system performance and safe operation. A simple and efficient measurement method for mid-frequency wavefront error is used, which employs the extended ptychographical iterative engine algorithm and has simple structure, low environment requirement and flexible adjustable frequency ranges. This method has been successfully implemented for the wedge focused lens to achieve accurate mid-frequency measurement. Further it can be extended to a wide range of large optical components, especially for which the wavefronts are not easy to be measured using interferometers.

3ω laser damage of fused silica optics is the bottleneck of high power laser systems for ICF. Excellent beam quality plays an important role in improving the anti-damage capability of final optics system. We have developed a new optical field measurement technology based on computational optical imaging. With the high power laser prototype of SGII-UP facility, damage resistance of final optics was experimentally studied. The near filed of laser beam was measured with a high resolution to study the effects of modulation and propagation on laser damage. The near field improvement of high power laser beam are reported and the influence of near filed quality on damage performance of final optics are discussed. The development of the defect detection techniques of final optics are introduced. Finally, we present the development perspective of final optics system for ICF laser driver. At present, the damage resistance capability of final optics assembly is 6J/cm2 at normal operation, we will continue to improve the ability in the next step of work.

A three-wavelength coherent-modulation-imaging (CMI) technique is proposed to simultaneously measure the fundamental, second and third harmonics of a laser driver in one snapshot. Laser beams at three wavelengths (1053 nm, 526.5 nm and 351 nm) were simultaneously incident on a random phase plate to generate hybrid diffraction patterns, and a modified CMI algorithm was adopted to reconstruct the complex amplitude of each wavelength from one diffraction intensity frame. The validity of this proposed technique was verified using both numerical simulation and experimental analyses. Compared to commonly used measurement methods, this proposed method has several advantages, including a compact structure, convenient operation and high accuracy.

High power laser science and engineering 7, e54 (2019). doi:10.1017/hpl.2019.36, Published by Cambridge Univ. Press, Cambridge

This paper presents a laser diode (LD) end-pumped square-rod Nd:glass laser system capable of achieving joule-level output energy at 1-Hz repetition rate. The laser system generated 966 mJ of energy in a 5-ns pulse for a pumping energy of 9.66 J at 802 nm, corresponding to an optical-to-optical conversion efficiency of 10%, which is higher than those of similar existing systems. A single-pass small-signal gain of 2.54 and pulse energy stabilities of 2.5% (peak-valley) and 0.37% (root-mean-square) were achieved by eight-pass amplification. The output energy of 966 mJ was generated, and the near-field modulation of 90% beam aperture was 1.37; further, the 82.5% far-field energy concentration was 2.5 times higher than the diffraction limit. The measured single-pass thermally induced wavefront aberration (1.15 λ ) was similar to that calculated using the thermal model. The beam quality could be further improved using adaptive phase corrector technology.

The temporal contrast of ultrashort and ultraintense laser pulses can significantly influence the laser–plasma interactions. As the optical parametric chirped pulse amplification (OPCPA) has been widely adopted in these laser facilities, the inherent optical parametric fluorescence, exhibiting a pedestal in the time window of the pump pulse, has become a significant problem. In this paper, we investigated experimentally its influence on the contrast of the compressed pulses at 808 nm using a multistage OPCPA amplifier. Compared to the highest value of 1 0 8 ever reported in the literature for such a type of regime, by adjusting the pump energy allocation between OPCPA stages and controlling the gain of small signal regime, we for the first time, to the best of our knowledge, realized a 1 0 11 temporal contrast in a pure nanosecond OPCPA design at a gain exceeding 7 × 1 0 8 , without adopting any other noise cleaning methods, such as picosecond OPCPA, cross-polarized wave generation, etc. This indicates that the OPCPA has a very significant potential for contrast improvement and to become a candidate for the future high-energy amplifiers in ultrashort high-power laser facilities.

We present a novel method utilizing the χ(2) nonlinear optical technology, which can realize high precision measurement of linear electro-optic (EO) coefficients of nonlinear materials. By applying the linear EO effect to the nonlinear optical process, the theoretical model of this measurement method was established, and the calculation formula of the linear EO coefficient was given. In the proof-of-principle experiment, by introducing an external electric field into the fourth harmonic generation (FHG) process, we comprehensively obtained the linear EO coefficients of K(H1−xD x )2PO4 crystals and revealed the relationship between deuterium content (x) and EO coefficient (γ63): γ63 = −9.789 − 16.53x. Meanwhile, the stability of FHG was greatly improved, and the angular range of efficiency stability was increased to 4.4 times in maximum. This work not only systematically demonstrates the FHG characteristics of KDP-family crystals, which provides a good reference for the deep ultraviolet laser generation, but also offers a new way to measure the basic parameters of nonlinear optical materials.

We experimentally investigated the morphology and mechanism of laser-induced damage in the ablation cutting of thin glass sheets with picosecond pulsed lasers and we compared the experimental results to our models. After several passes of laser ablation, we observed two different kinds of damage morphologies on the cross-section of the cut channel. They are distinguished to be the damage region caused by high-density free-electrons and the heat-affected zone due to the heat accumulation, respectively. Furthermore, micro-cracks can be observed on the top surface of the workpiece near the cut edge. The nano-cracks could be generated by high energy free-electrons but opened and developed to be visible micro-cracks by thermal stress generated in the heat-affected zone. The crack length was proportional to the volume of heat-affected zone. Heat-affected-zone and visible-cracks free conditions of glass cutting were achieved by controlling the repetition rate and spatial overlap of laser pulses.

A model to calculate the optical field distribution of quadruplet beams on a hohlraum target wall is presented. This model combines geometrical ray tracing, coordinate transformation, and Fresnel diffraction integral to capture the quadruplet beams propagating in four different directions and the typically non-planar geometry of the hohlraum wall. The results demonstrate that the optical field distribution arises mainly from individual beam diffraction, and the interference with other beams in the quadruplet hardly devotes to the distribution. A movie is also produced to interpret the spatio and temporal evolution of the optical field on a cylindrical hohlraum wall.

The evolution of hot-image formation in consideration of focusing lens effect is theoretically and numerically investigated. Through analysis of linear propagation theory with lens, it is shown that the location of hot image in focusing systems can be predicated through definite functional relation to hot image position in non-focusing systems, on condition that the distance between the hot image and the lens is relatively small. In addition through numerical calculation, specific variation characteristics of maximum intensity with the propagation distance representing hot-image formation process are presented, it is obtained that perturbing scatter size directly affects the location and corresponding peak intensity of the hot image. The lens influences the value of scatter size where minimum distance between hot-image plane and focus lens is generated. Finally, the effects of phase and amplitude modulation of scatter on hot image are also given. Results in this paper are of fair referenced value for controlling perturbing scatter and restraining the damage risk of optical components in focusing systems.

Optimizing and measuring the femtosecond pulse duration of compression are very difficult in the case of online and vacuum in a multi-petawatt laser facility. Therefore, a method is presented in this study to determine the optimum point of pulse duration by simulation calculation and experimental measurements. Using the method in the multi-petawatt laser facility, the residual dispersion is completely eliminated, and a near Fourier-transform-limited pulse duration can be attained. These results are beneficial in finding an optimized pulse duration and guarantying the repeatability of the laser physical experiment results in multi-petawatt femtosecond laser facility.

A novel stress vector measurement method combining ptychography and polarization technologies is proposed. By placing the stress sample in the probe-forming path and combining two circular polariscopes with a beamsplitter, the complex amplitudes of two orthogonal probe components are reconstructed under a single ptychographic scan. Three parameters are extracted with a simple and effective approach, where the isopachics and isoclinics are directly obtained from the phase information and the solution of isochromatics is easily realized by the combination of amplitude and phase. The feasibility of stress vector measurement is successfully verified through a classic diametrically compressed disc and a ring under diametric compression. This method avoids multiple scans, and has short data acquisition and reconstruction time and good adaptability for large samples. It can be further combined with single-shot ptychographical method to realize the dynamic measurement, which has great significance for online engineering applications.

Accurate temporal characterization both in intensity and phase distribution is important in the diagnosis of the petawatt (PW) class. We present a single-shot picosecond frequency-resolved optical gating (ps-FROG) setup based on an autocorrelator with ps measurement range that is spectrally resolved through a fine grating. The modified ptychographic-based algorithm with a changing update coefficient was used for the reconstruction of the pulse distribution; it can better adapt to the reconstruction of pulse with a large time–bandwidth product. We calibrated and verified the homemade ps-FROG in a 100-μJ ps laser system and used it to characterize the pulse distribution generated by the PW laser system of the Shen Guang II facility. The system shows good performance and high accuracy in reconstructing the intensity and phase distributions of a ps pulse, which provides reference for accurately adjusting the grating pair to acquire the pulse width as a preset.

The uniformity of temperature distribution of an end-pumped laser can be improved simply and effectively by introducing the correct amount of spherical aberration into the pump system. An iterative model of optical-thermal coupling is proposed to calculate the temperature distribution of the end-pumped laser amplifier. The results showed that, the thermal effect of the laser gain medium in the coupling system composed of spherical lenses decreased significantly compared with the coupling system composed of aspherical lenses. This is proved by a neodymium-doped yttrium orthovanadate (Nd:YVO4) amplification experimental system, which was end-pumped by a fiber-coupled laser diode (LD) with the optimized wavelength of 878 nm. A comparative study of the two pump systems showed that the decrease in the gain medium thermal wavefront peak-to-valley (PV), the thermal lens effect, and an improvement in beam quality are still observed in the pump amplification experiment with spherical aberration. At last, the relationship between the spherical aberration and the output brightness of the amplifier was established, which provides guidance for further optimization of the spherical aberration of coupling pump systems and improving the beam quality.

A holographic and deep learning-based method is presented for three-dimensional laser damage location. The axial damage position is obtained by numerically focusing the diffraction ring into the conjugate position. A neural network Diffraction-Net is proposed to distinguish the diffraction ring from different surfaces and positions and obtain the lateral position. Diffraction-Net, which is completely trained by simulative data, can distinguish the diffraction rings with an overlap rate greater than 61% which is the best of results reported. In experiments, the proposed method first achieves the damage pointing on each surface of cascade slabs using diffraction rings, and the smallest inspect damage size is 8µm. A high precision result with the lateral positioning error less than 38.5µm and axial positioning error less than 2.85mm illustrates the practicability for locating the damage sites at online damage inspection.