Significance The proposal of chirped pulse amplification (CPA) technology marked a breakthrough in the advancement of high-power laser, successfully resolving the fundamental challenge between scaling up the peak power of ultrashort pulses and avoiding optical damage. Prior to CPA, laser amplification faced a critical bottleneck: direct amplification of ultrashort pulses resulted in extremely high peak power densities, which readily induced nonlinear self-focusing within the gain medium. This not only caused optical damage but also severely limited further increases in output peak power. In 1985, Mourou and Strickland first introduced CPA, whose core principle involves dividing the amplification process into three stages-pulse stretching, amplification, and compression-thereby significantly reducing the instantaneous peak power density during amplification and effectively mitigating the risk of optical damage. With an output of 660 J in 440 fs, NOVA-PW, the world's first petawatt laser system based on CPA, was developed at the Lawrence Livermore National Laboratory (LLNL) in the United States in 1996. The system employed a large-aperture neodymium-doped glass (Nd & ratio;glass) amplifier chain, establishing the foundational architecture for picosecond petawatt laser systems. With a gain bandwidth of hundreds of nanometers, titanium-doped sapphire (Ti & ratio;sapphire) emerged as a primary gain medium in CPA for generating tens-of-femtosecond pulses, enabling the realization of femtosecond petawatt laser systems. Since the late 1990s, the integration of CPA with optical parametric amplification (OPA) has led to the development of optical parametric chirped pulse amplification (OPCPA). This approach offers advantages such as high gain, broad spectral bandwidth, and low thermal load, effectively alleviating the gain narrowing limitations inherent in conventional CPA, thus facilitating the advancement of high-power laser systems toward broader spectral bandwidth and higher peak power. Progress In recent years, the development of ultrashort pulse lasers has been driven by three primary technical approaches. The first is CPA based on Nd & ratio;glass systems, exemplified by facilities such as NIF-ARC and OMEGA-EP in the United States, LMJ-PETAL in France, Vulcan in the United Kingdom, LFEX in Japan, PHELIX in Germany, and SG-II-UP PW in China. The second is CPA employing Ti & ratio;sapphire systems, including ELI in the European Union, BELLA in the United States, CORELS in South Korea, J-KAREN-P in Japan, and SULF in China. The third is OPCPA based on nonlinear crystal systems, such as PEARL in Russia, CAEP-PW and SG-II fs Multi-PW in China. The National Laboratory on High Power Laser and Physics (NLHPLP) at the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has successively developed three ultrashort pulse laser facilities based on the SG-II platform. The sub-picosecond system (SPS), completed in 2003, was the first Nd & ratio;glass-based ultrashort pulse laser facility in China, providing critical experience for the subsequent design, construction, and operation of domestic high-power picosecond laser systems. The SG-II-UP PW facility, completed in 2014, marked the first picosecond petawatt laser system in China. Several significant physics experiments were conducted by this facility, including the world's first indirect drive fast ignition and the acceleration of 70 MeV protons. Following the expansion of the SG-II-UP facility, the capability for dual high-energy picosecond petawatt lasers to perform synchronized target irradiation was successfully established in 2024. The SG-II fs Multi-PW, developed in 2017, incorporated an amplification chain with fully non-collinear OPCPA architecture, realizing the target shooting with femtosecond petawatt laser and high-energy nanosecond laser for the first time. Conclusions and Prospects NLHPLP has taken a leading role in the research, development and application of high-power ultrashort pulse laser facilities based on CPA and OPCPA technologies in China. It has established the SG-II platform and formed a comprehensive research system of "technological breakthrough-facility development-physical application". The completion of three major facilities-SPS, SG-II-UP PW, and SG-II fs Multi-PW-has filled domestic technological gaps in high-power laser. Significant technological advances have been made in areas such as high signal-to-noise ratio (SNR) front-ends, broadband pulse amplification, laser pulse compression, and advanced ultrafast diagnostics. In physical applications, the SG-II platform has achieved multiple milestones, including the world's first indirect drive fast ignition experiment, a new national record in proton acceleration, and innovative plasma diagnostic techniques. Significant potential remains for advancing both the output capability and spatiotemporal precision control of high-power ultrashort pulse lasers. Efforts are urgently required to overcome key technical bottlenecks, including ultrashort pulse laser-induced damage, small-focal-spot beam combining for target irradiation, and ultra-high SNR control. Addressing these challenges will enable laser-target interactions characterized by higher intensities, improved coupling precision, and enhanced energy transport efficiency. Sustained research is essential in areas such as optimization of pulse compression architectures, beam quality control, and increasing the damage thresholds of optics-all of which are critical to further enhancing the performance of picosecond petawatt laser systems. To achieve high-precision beam combining for target irradiation, two core requirements must be met: first, each beam must support tight focusing with microradian level pointing accuracy and precise target positioning; second, sub-picosecond beam synchronization must be achieved. Furthermore, improving the efficiency of laser energy delivery to the target necessitates further enhancement of the SNR in picosecond lasers, as well as a critical measure to suppress target pre-ionization and the reflective effects of pre-plasma on the main pulse. The development of ultrashort pulse lasers is undergoing a shift, from a primary focus on fundamental scientific research driven by peak power as a singular metric toward meeting the demands of engineering applications that require simultaneous high peak power and high average power. This transition is driving the evolution of laser systems from traditional single-shot operation to stable, high-repetition-rate operational regimes. Petawatt lasers featuring high efficiency, high energy output, and high repetition rates not only represent a major leap in laser performance but also catalyze systematic innovation across enabling technologies, including new laser gain materials, high-quality optics and new-generation amplification system. Meanwhile, high-repetition-rate petawatt lasers are progressively transforming high-energy-density physics research, providing new momentum for disciplinary advancement through the deep integration of cutting-edge technologies such as artificial intelligence.

The pulse duration is a critical parameter of picosecond-petawatt laser systems because it directly affects the results of high-energy-density physics experiments. This study systematically investigated the effects of the spectral width, central wavelength and beam-pointing deviations on pulse duration stability at the SG-II facility. A theoretical analysis of the relationship between spectra and pulse duration is conducted to quantify the impact on pulse duration stability, and the results are further validated through experimental measurements. In addition, beam-pointing deviations at the stretcher significantly affect the pulse duration. For example, a 27 mu rad deviation can induce a 30% pulse duration variation. In contrast, the compressor exhibits greater robustness. Based on simulation and experimental results, we identify operational tolerance ranges for spectral width and beam-pointing deviation to maintain pulse duration stability within 5% at the SG-II facility. These findings provide critical guidance for optimizing the performance and reliability of chirped-pulse amplification/optical parametric chirped-pulse amplification-based high-power laser systems.

Pulse contrast is a crucial parameter for evaluating the performance of petawatt laser systems. On the nanosecond timescale, pre-pulse-induced plasma expansion can significantly influence the interaction between the main pulse and the target, potentially degrading experimental outcomes. To enhance the reliability of pulse contrast measurement, we propose a novel method based on laser filamentation in water, which effectively protects the photodiodes while maintaining high measurement fidelity. Experimental results demonstrate that this method enables a main pulse attenuation factor of up to 40, while pre-pulses with relative intensity below 10-5 are not attenuated. After seven thousand laser shots, the photodiode retains approximately 60 % of the initial sensitivity. Furthermore, a Michelson-type optical platform was developed to enable high-precision pulse contrast calibration. Based on the measurement and calibration platforms, pulse contrast was measured on SG-II. The results reveal significant variations in pulse contrast under different optical path configurations, in agreement with theoretical predictions. In addition, independent validation using a commercial cross-correlator confirms the reliability of the results. The setup achieves a dynamic range of 109 at 2 mJ, potentially extending to 1010 at 20 mJ, providing a robust solution for nanosecond-scale contrast diagnostics in high-energy laser systems.

The ptychographic reconstruction algorithm is a commonly utilized method for pulse phase retrieval due to its super-resolution and robustness, allowing it to retrieve pulses from incomplete traces. However, the algorithm's performance can be hindered by occasional convergence stagnation caused by local minima in the gradient descent strategy. To address this issue, we propose a pulse reconstruction approach for frequency-resolved optical gating (FROG), which employs a multi-grid flexible sampling and parallel extended ptychographic iterative engine (ePIE), ultimately converging to the global FROG trace. The approach can effectively escape from the local minima and demonstrates extremely stable convergence without any prior information. We demonstrate, numerically and experimentally, that this approach can converge well to the correct pulse, especially for complex pulse reconstruction, even in cases of high noise and highly incomplete traces.

Significance Laser driver is known to be the most mature tool for inertial confinement fusion (ICF) research. The high - power Nd glass laser driver is one of the most representative large - scale optical engineering projects, and its scale and overall performance represent the highest level of a nation's science and technology. Many countries worldwide have built laser drivers to conduct ICF research, and some drivers have gone through several generations. The scale of the driver is continuously growing, and the laser performance is constantly improving. China is also an important player in the international stage of laser fusion research. The Shanghai Institute of Optics and Mechanics (SIOM) is not only the cradle of China's laser fusion, but also the birthplace of the "Shenguang" device, simplified as SG . The SIOM developed a relatively complete support system with unit technologies such as large - aperture Nd glass, pulse xenon flash lamps, thin films, and optical processing. Early research and development included the creation of a 108 W laser facility and a six - beam laser system. To further develop laser fusion research in China, a joint laboratory named National Laboratory on High Power Laser and Physics (NLHPLP) was established at the SIOM, thus ushering a new era of laser fusion research in China. Subsequently, the joint laboratory set up a series of laser facilities (Fig. 1), including the No. 12 laser facility (known as the SG I facility), SG II eight - beam facility, multifunctional SG II 9th laser facility, SG II UP facility (including the SG 9th high - energy PW system), and SG II fs 5 PW laser facility. The No. 12 laser facility played a decisive role in China's laser fusion research, demonstrating that China had become one of the few countries in the international high - power laser field with comprehensive research and development capabilities at that time. After combining the series of facilities, a multifunctional comprehensive platform, with the SG II facility as the core, has been formed. It has supported many physical experiments and maintained efficient operation for many years. This platform has made important contributions to the research and development of fusion physics in China, and it has an important international status. On the occasion of the 60th anniversary of the establishment of the SIOM, a brief review of the series of high - power laser facilities built and the related technology development are presented. Progress The SG II comprehensive platform is the crystallization of the collective wisdom of several generations, reflecting the persistent efforts of hundreds of scientists and engineering technicians. Each facility has its own unique features. During the development of the SG II eight - beam (Fig. 5), a series of technical difficulties were independently addressed, and 15 new unit technologies were innovatively integrated. The innovative design and development of a switchless coaxial double - pass main amplification were successfully explored for the first time internationally. The SG II 9th system is not only the second successful high - energy probe laser system after that built in the United States but also has the significant characteristics of high energy output, multi - functionality, and high - performance operation. As the first physical experimental platform in China to support fast ignition ICF research, the SG II UP facility consists of eight nanosecond laser beams and one kilojoule - level picosecond laser beam. The entire amplification chain of the nanosecond laser beam adopts a multi - pass amplification optical structure of "four pass cavity amplification + two pass booster amplification + large aperture PEPC". The high - energy picosecond pulsed laser system adopts the overall technical route of high - power optical parametric chirped pulse amplification combined with Nd glass chirped pulse amplification. The SG II fs 5 PW facility is entirely based on three - level non collinear OPCPA to achieve a 150 J/30 fs laser pulse output at the target wavelength of 808 nm. Conclusions and Prospects Aiming at the major strategic needs of the country, the NLHPLP has two main development lines: research and development of high - power laser technology and facilities and efficient operation of the facilities. In addition, the laboratory puts special emphasis on international cooperation and exchanges, providing hundreds of experiments for international users. The NLHPLP will continue to play a role in the future development of laser ICF projects, opening up new content and creating its own unique core technology according to future needs.

High-energy-synthesized laser pulses through a nonlinear frequency-conversion process with different characteristics, such as polarization, central wavelength, and pulse duration, play important roles in materials science, high-energy physics, and ultrafast optics. In this study, we present an improved transient-grating frequency-resolved optical gating based on a self-referenced and reflective structure, which enables the single-shot complete measurement of complex high-power synthesized laser pulses in the broadband range and analysis of the nonlinear frequency-conversion process of ultrashort pulses. The waveform/spectrum evolution of both the fundamental and second harmonic pulses in a nonlinear frequency-conversion process with different injected energies was studied for the first time using this method. Moreover, the method was numerically and experimentally verified to be able to completely characterize double pulses with spectral and temporal separation, including the relative phase between the two components. This method has considerable potential for studying the complex physical processes of high-power synthesized laser fields.

Abstract The target backsheath field acceleration mechanism is one of the main mechanisms of laser-driven proton acceleration (LDPA) and strongly depends on the comprehensive performance of the ultrashort ultra-intense lasers used as the driving sources. The successful use of the SG-II Peta-watt (SG-II PW) laser facility for LDPA and its applications in radiographic diagnoses have been manifested by the good performance of the SG-II PW facility. Recently, the SG-II PW laser facility has undergone extensive maintenance and a comprehensive technical upgrade in terms of the seed source, laser contrast and terminal focus. LDPA experiments were performed using the maintained SG-II PW laser beam, and the highest cutoff energy of the proton beam was obviously increased. Accordingly, a double-film target structure was used, and the maximum cutoff energy of the proton beam was up to 70 MeV. These results demonstrate that the comprehensive performance of the SG-II PW laser facility was improved significantly.

Objective The chirped pulse amplification technology improves the peak energy of the pulse and greatly promotes the development of the ultrafast laser. However, high-order dispersion will be introduced in chirped pulse amplification technology, which leads to the oscillation of pulse waveform in the petawatt laser system and affects the signal-to- noise ratio (SNR) of the petawatt laser. To optimize the SNR characteristics of the petawatt laser and improve the efficiency of laser accelerating electrons, protons, and other particles, a new third-order dispersion control method based on birefringent crystal for the active control of SNR is proposed. The needs of petawatt laser cannot be well satisfied by conventional highorder dispersion compensation methods including grating pairs, prism pairs, and acousto-optic programmable dispersion filters due to their complex optical paths or limited dispersion adjustment. The active control method of the third-order dispersion based on the birefringent crystal is simple to operate. On the basis of the original optical path, the residual thirdorder dispersion in the system can be changed only by rotating the in-plane rotation angle of the birefringent crystal to realize the active control of SNR. Methods When an incident beam with a certain spectral width passes through a birefringent crystal, it will follow different optical paths due to the inconsistent principal refractive indices of wavelengths for the crystal, introducing a specific frequency- domain spectral phase. In polarized optics, The Jones matrix is often employed to describe birefringent crystals. In front of and behind the birefringent crystal, polarizers are placed to control the polarization state of incident light and outgoing light and thus select the matrix elements of the Jones matrix. The complex amplitude of the outgoing light field in a specific polarization state can be obtained by calculation, and then the spectral phase expression introduced by the birefringent crystal and the high- order dispersion expansion are obtained. For the laser system with determined central wavelength, the high-order dispersion introduced by the birefringent crystal is a function of the crystal thickness and the in-plane rotation angle of the crystal. Therefore, the key parameters such as crystal thickness and crystal in-plane rotation angle in high- order dispersion introduced by crystal are simulated respectively. The results show that the crystal thickness affects the magnitude and the spectral width of the flat change of third-order dispersion, and the in-plane rotation angle of the crystal affects the specific value. When the crystal thickness is determined, the required third-order dispersion can be introduced by changing the in-plane rotation angle of the crystal. The possible additional group delay dispersion introduced by the birefringent crystal is also analyzed in this paper. It is shown that as the scheme is designed for the picosecond laser system, the introduced group delay dispersion has little effect on the pulse width which can be ignored. Additionally, Dazzler in optical paths can be adopted to compensate for the group delay dispersion according to the change in pulse width, which is monitored by the autocorrelation instrument after the dispersion control module. Results and Discussions Firstly, the theoretical model for describing the dispersion control of birefringent crystal is built (Fig. 1), and an optical axis parallel to the crystal surface is considered. On this basis, with the TM polarization of the incident beam and the outgoing beam as an example, the expression of the spectral phase introduced by the birefringent crystal is obtained, the Taylor expansion of which is the expression of the high-order dispersion. The effects of several critical parameters on the introduction of high-order dispersion into birefringent crystal are analyzed, including the central wavelength of the incident beam, crystal thickness, and in-plane rotation angle of the crystal. In this paper, the influence of the residual third-order dispersion on SNR is analyzed for the picosecond petawatt laser system (central wavelength of 1053 nm and spectral width of 3. 4 nm). It is concluded that the SNR can be changed with different values of the residual third-order dispersion. According to the fitting results of the OPCPA pre-compression SNR state curve of the SHENGUANG. ninth picosecond petawatt laser system (Fig. 7), a birefringent crystal with a thickness of 2. 35 mm can be selected to compensate for the residual third-order dispersion. At the same time, according to the simulation results of the in- plane rotation angle and third-order dispersion (Fig. 6), the angle can be rotated to around 23 degrees or 32 degrees to compensate for the residual third- order dispersion of the system. Then, the dispersion modulated beam is imported into Sequoia to measure the SNR, and the control effect of third-order dispersion is judged according to the measured results. Conclusions In this paper, the models for analyzing second- order and third- order dispersion of the birefringent effect are built. According to the central wavelength and spectral width of the picosecond petawatt laser system, the special crystal thickness and in-plane rotation angle are designed, which introduce the third-order dispersion with sufficient magnitude and adjustable positive and negative. In addition, a small group delay dispersion is also ensured to avoid the influence on pulse width. On this basis, combined with the SNR measurement data of the ninth picosecond petawatt laser of SHENGUANG., the influence of third-order dispersion on the SNR of the petawatt laser pulse is simulated and analyzed, and an active SNR control scheme based on birefringent effect is proposed. Employing birefringent crystals to change the residual thirdorder dispersion of the petawatt laser system is of great significance to realize the numerical simulation analysis of SNR control. The results can provide a theoretical basis for the optimization of the SNR of laser systems.

With the development of ultrafast laser technology, the peak energy of ultrashort pulses continues to increase. In addition to the demand for energy enhancement, many frontier physical experiments also put forward more stringent requirements for signal-to-noise ratio (SNR) of lasers. When the petawatt-level laser interacts with the target for physical experiments, the pre-pulses interact with the target in advance, affecting the density scale length of the pre-plasma and changing the spectral distribution of the generated electrons. In order to meet the requirements of pre-pulse control schem, we developed a SNR active control module based on isolated pre-pulses, which generates an isolated pre-pulse with adjustable time delay and relative intensity. The time delay of the isolated pre-pulse can be continuously adjusted in the range of -1300 ps to 0 ps. At the same time, by controlling the number of attenuators in the pre-pulse optical path, the relative intensity of the pre-pulse can be adjusted in the range of 10-8 to 10-3. We placed the module in front of the main amplification chain of the ShenguangⅡ ninth picosecond petawatt laser, adjusted the time delay and relative intensity of the pre-pulse, and measured the SNR at the terminal. The results verified the feasibility of the SNR active control scheme based on isolated pre-pulse.

A spectrum series learning-based model is presented for mode-locked fiber laser state searching and switching. The mode-locked operation search policy is obtained by our proposed algorithm that combines deep reinforcement learning and long short-term memory networks. Numerical simulations show that the dynamic features of the laser cavity can be obtained from spectrum series. Compared with the traditional evolutionary search algorithm that only uses the current state, this model greatly improves the efficiency of the mode-locked search. The switch of the mode-locked state is realized by a predictive neural network that controls the pump power. In the experiments, the proposed algorithm uses an average of only 690 ms to obtain a stable mode-locked state, which is one order of magnitude less than that of the traditional method. The maximum number of search steps in the algorithm is 47 in the 16°C–30°C temperature environment. The pump power prediction error is less than 2 mW, which ensures precise laser locking on multiple operating states. This proposed technique paves the way for a variety of optical systems that require fast and robust control.

A half-trace retrieval algorithm based on an extended-ptychographical iterative engine algorithm is proposed to reconstruct the temporal structure of pulse from a polluted and recorded frequency-resolved optical gating (FROG) trace that was modulated by poor spatial profile of output pulses, stray light, or misalignment of the measurement setup. In the proposed algorithm, the probe pulse and the gated pulse were retrieved simultaneously from a recorded FROG trace with a half-delay range, and the measured pulse was obtained by combining the different edges of the probe pulse and the gated pulse. Numerical simulations were carried out to verify the feasibility of the proposed algorithm. A single-shot picoseconds (ps)−THG−FROG setup with a 100-μJ ps laser system and an online ps−SHG−FROG setup in PW laser system were built to test the proposed algorithm experimentally. The results show that the temporal structure of pulses retrieved by the half-trace retrieval algorithm is closer to the real temporal structure than that retrieved by the conventional ptychographical algorithm when the recorded FROG trace is badly polluted.

In strong-field physics experiments with ultraintense lasers, a single-shot cross-correlator (SSCC) is essential for fast optimization of the pulse contrast and meaningful comparison with theory for each pulse shot. To simultaneously characterize an ultrashort pulse and its long pedestal, the SSCC device must have both a high resolution and a large temporal window. However, the resolution and window in all kinds of single-shot measurement contradict each other in principle. Here we propose and demonstrate a novel SSCC device with two separate measurement channels: channel-1 for the large-window pedestal measurement has a moderate resolution but a large window, while channel-2 for the ultrashort pulse measurement has a small window but a high resolution; this allows the accurate characterization of the pulse contrast in a single shot. A two-channel SSCC device with a 200-fs resolution and 114-ps window has been developed and tested for its application in ultraintense lasers at 800 nm.

Objective Ultrashort laser pulses have become an important tool for studying the interaction between lasers and matter and have important application values in the fields of biomedicine, high-energy physics research, and communications. The pulse width is an important parameter of the time characteristics of ultrashort laser pulses. For picosecond and femtosecond laser pulses, the pulse width is often measured by an autocorrelator. The time resolution of the autocorrelator must be accurately calibrated before the measurement. For traditional calibration schemes such as the mobile optical path retarder, although their calibration results are accurate, it cannot be calibrated in a single time. On the contrary, the discriminant rate board method can be calibrated in a single time; however, the accuracy of the calibration result is poor, and the accuracy is not high. This study proposes a new method for calibrating the time resolution of the autocorrelator. A flat crystal that can produce a specific time delay is designed, manufactured, and placed in the optical path during calibration. The time resolution can be obtained through a single calibration. Moreover, the measurement results are accurate and reliable. Vlcthods The designed and manufactured flat crystal with a specific time delay generates double pulse with a fixed time interval T (ignoring high-order reflections), when the pulse to be measured passes through the flat crystal. When the double pulses met in the autocorrelation crystal, the generated autocorrelation signal was a three-peak structure, i. e., a weaker secondary peak signal appeared at equal distances on the left and right sides of the primary peak signal. The time interval between the main peak and the secondary peak signal was denoted as T. The time resolution of the autocorrelator could be obtained by calculating the number of pixels between the primary and secondary peaks. The influence of the thickness, refractive index, and angle of the flat crystal on the calibration result was then analyzed. The time resolution and the relative expanded uncertainty of the autocorrelator were calculated. Finally, the autocorrelator was calibrated using two other calibration methods. The measurement results of the time resolution and the relative expanded uncertainty were given. Moreover, the advantages and the disadvantages of the three schemes were compared. Results and Discussions After the flat crystal is placed in the optical path, the placement angle, thickness, and refractive index will affect the calibration accuracy of the time resolution. Figure 4 shows the deviation caused by the placement angle of the flat crystal. The deflection angle deviation only slightly affects the time resolution. A resolution error of 2% requires a deflection angle of 16. 6. The measurement error primarily comes from the deviation in the thickness h of the flat crystal and the reading. Figure 5 ( a) depicts the autocorrelation signal collected on the CCD. Obvious secondary peak signals can be found at both ends of the main peak signal, which is consistent with the theoretical analysis results. In this experiment, the thickness h= 1.02 mm and refractive index n = 1.450 of the plane flat crystal were maintained. The time interval between the primary and secondary peaks is 9.86 ps. The time resolution of the autocorrelator is 217. 88 fs/pixel. The relatively extended uncertainty is 1.50%. Table 3 shows the results of the time resolution and the expanded uncertainty obtained by the three calibration methods. The results of the time resolution calibration using a flat crystal are accurate and reliable. Conclusions This study proposes a new method for calibrating the time resolution of an ultrashort pulse measuring device based on a flat crystal. By placing a special flat crystal in front of the ultrashort pulse measuring device, double pulse with a time interval T is generated after the pulse to be measured passes through. The generated autocorrelation signal exhibits weaker sub -peaks on both sides of the primary peak. As shown in Fig. 2, the time interval of the sub-peak 2 T can be obtained from the refractive index and the thickness of the flat crystal. The pixel value between the sub -peaks can ascertain the time resolution of a single picosecond autocorrelator and calculate the pulse width at the same time. Subsequently, experiments are performed on a femtosecond laser with a pulse width of 180 fs. Figure 5 illustrates the autocorrelation signal collected by the CCD in the experiment, which is consistent with the theoretical prediction. Using this method to calibrate the time resolution of the autocorrelator yields 217.88 fs/pixel. Compared with the calibration result of 214.27 fs/pixel obtained by the moving optical path retarder method, the relative error is only 1. 68%. Compared with the discrimination rate board method, the relative expanded uncertainty of the calibration result using this method is 1. 50 Yo, which is far better than the discrimination rate board method of 6. 96%. A single calibration of the autocorrelator is realized. In conclusion, the calibration result is accurate and reliable.

A high-energy, high-beam-quality, high-contrast picosecond optical parametric chirped-pulse amplification (ps-OPCPA) laser system was demonstrated. The pulse from a femtosecond oscillator was stretched to 4 ps, after which it was amplified from 140 pJ to 600 µJ by an 8 ps/6 mJ pump laser in two non-collinear OPCPA stages. The total gain was >10 6 , and the root mean square of the energy stability of the laser system was 1.6% in 10 h. The contrasts of the solid and fiber mode-locked femtosecond oscillator-seeded ps-OPCPA systems were compared, and a signal-to-noise ratio of >10 11 was achieved. Using this system, the contrast of the front end in high-power picosecond petawatt laser facility was improved by ∼40 dB to >10 11 , beyond ∼200 ps ahead of the main pulse with an output level of 60 mJ.

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 10(19) 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.

In strong-field physics experiments with intense lasers, it is of paramount importance to single-shot diagnose the temporal contrast between laser pulse peak and its noise pedestal. This allows fast optimization of pulse contrast and meaningful comparison with theory for each pulse shot, and it can help new outcomes from clean laser-plasma interactions. Thus far, high contrast ratios up to ~10^10, required by present petawatt (PW) class lasers, have been accessible in both generation and single-shot characterization. However, ultrahigh contrast ~10^13, required by the planned 200-PW lasers, challenges intense laser technology and remains an open question. This paper reports on the first demonstration of such an ultrahigh-contrast measurement by adapting single-shot cross-correlator (SSCC). We introduce an ultrafast method that enables to determine the SSCC detection limit. Our strategy mimics the test laser having known ultrahigh contrast in the measurement frame of time-to-space mapping. The ultimate contrast-measurement limit of 10^13 is achieved, which corresponds to the highest pulse intensity set by SSCC damage threshold and the lowest noise pedestal set by single-photon detection. As a consequence, photon noise in the detection is observed and increases as the noise pedestal reduces. The demonstrated measurement ability at the photon noise limit is applied to a high-contrast laser system based on second-harmonic generation and optical parametric chirped-pulse amplification, suggesting accessible of ultrahigh contrast pulses.

Objective In the past 20 years, ultra-short ultra-intense laser technology has experienced rapid development. However, the maximum output power of these lasers is limited by nonlinear effects, large diameter compression grating technology, gain bandwidth limitations, and other factors. One of the most promising technologies to further enhance output ability is coherent beam combining. Effective coherent beam combining requires strict inter-beam synchronization. In recent years, many attempts have been made to improve synchronous measurement and control. The research progress of most implementations has been solely based on photoelectric detection, optical balanced cross-correlation, and temporal and spatial interferences. Nevertheless, these methods need to maintain the time interval of the two beams in coherent time, limiting the femtosecond pulse synchronous measurement range within 1 ps. The ability of an electronic oscilloscope to achieve a time resolution less than 10 ps is difficult; therefore, it is more difficult to accurately measure the pulse delay within 1-10 ps. In addition, for online synchronous measurement of a multichannel ultra-short pulse coherent beam combining system, the abovementioned methods are more complicated to implement and cannot achieve a single-shot measurement. In this paper, a single-shot measurement method for a multichannel ultra-short pulse with large dynamic range time synchronization based on all-fiber spectral interference is proposed. This method has a wider measurement range to measure synchronization than the nonlinear correlation method and a larger measurement accuracy than an oscilloscope. Our method improves efficiency in multichannel laser synchronous measurements for engineering applications and has important application potential for multichannel ultra-short pulse laser coherent beam combining systems. Methods First, theoretical and simulation analyses based on multichannel optical fiber array spectral interferometry were carried out. Predictions of tau and tau(max) for the designated measurement range were made according to Equation (6). Considering the purpose of synchronous measurements, this study created the concept of fixed time offset. The beneficial effect of this concept is that through the comparison of measured values and fixed offset time, we can determine the absolute time difference between the referenced light and the light to be measured. Moreover, with a fixed offset time, when the measured values were equal to the fixed offset times introduced by optical delay lines on the referenced light fiber paths, the two pulses reached a zero-synchronization state. In our experiment, the feasibility of the single-shot multichannel synchronous measurement method was verified. The experimental optical path was built using the path of a four-channel pulse synchronous measurement as an example (Fig. 3). The three formed interference signals and one beam of reference light were input to the imaging spectrometer using a multipath fiber buncher. Results and Discussions The spectrogram in the experiments is recorded by an imaging spectrometer, which indicates that the spectrometer has the ability to record 20 signals (Fig. 4). The delay, r, between the reference and measured beams is obtained through the data processing method described in Section 2. 1. This method illustrates that rmax is equal to 14. 751 ps and r is equal to 1.055 ps, which determine the measurable range (Fig. 5). From experimental results, the range that can be measured is slightly less than the theoretical interval, mainly due to airflow disturbances, mechanical vibration, and dark current noise from the spectrometer. For measurement precision of different offset points, the deviation of the statistical mean value of multiple measurement results is obtained from the present value. In Figure 6, it is shown that with the increase of temporal spacing (TS) between the two pulses, the beta value decreases. When TS reaches 6.139 ps, the beta value is at its minimum. When TS is greater than 6.139 ps, the p value increases continuously. The measured jitter, gamma, is shown on the right vertical coordinate of Figure 6 and it shows the same trend as the beta value (Fig. 6). Measurement error is because of uncertainty of the wavelength or frequency spacing of the interference fringe in the spectrogram caused by noise. However, the degree of response of different fixed offset times to noise is different. Therefore, the measurement accuracy is varied at different fixed offset times. Conclusions This paper demonstrates that the single-shot synchronous measurement technique for a multichannel ultra -short pulse laser based on all-fiber spectral interference is feasible through simulation and experiment. The measuring range is determined by the spectral interference fringe spacing, and the theoretical simulation results show that a fixed time offset is beneficial for the realization of a zero-synchronization state measurement. The optimal solution of the offset time is obtained using experimental statistical results. Experimental data prove that setting the fixed time offset in the center of the measurable range area can improve measurement accuracy. The minimum time synchronization accuracy is 5.3 fs and the measurement range is 1.055-14.751 ps, which are in good agreement with results of the theoretical analysis. The all-fiber spectral interference synchronization measurement method combines the characteristics of spectral interference and optical fiber array in design. The advantages of the method are easy integration of an optical fiber path, fast processing speed of spectral interference data, and lowenergy demand of signals. Our method can satisfy the ultra-short ultra-intense laser facility real-time and multichannel measurement diagnosis requirements. The method also makes up for a small measurement range and poor temporal resolution when measuring the synchronization state using the nonlinear correlation method and an oscilloscope, respectively. The complexity of the configuration and difficulty of a single-shot measurement in multichannel synchronous measurements are solved. Therefore, our method has important application prospects in multichannel ultra-short pulse laser coherent beam combining systems.

In strong‐field physics experiments with high‐intensity lasers, single‐shot characterization of the temporal contrast between the laser pulse peak and its temporal pedestal is important; this allows fast optimization of the pulse contrast and meaningful comparison with theory for each pulse shot. To date, high contrast ratios of 10 10 have been demonstrated in single‐shot measurements for petawatt (PW) lasers. However, ultrahigh contrast ratios of ≈10 13 , as required for the planned 200 PW lasers, pose challenges to high‐intensity laser technologies and have thus far remained open for investigation. This article reports a pilot demonstration of ultrahigh‐contrast measurements by adapting a single‐shot cross‐correlator (SSCC). An evaluation method for the SSCC detection limit is introduced. The strategy mimics the test beam with known spatial contrast, whose cross‐correlation is equivalent to that of a test pulse with ultrahigh temporal contrast. The ultimate contrast measurement limit of 10 13 is achieved, which corresponds to the highest pulse intensity by optical damage and the lowest temporal pedestal by single‐photon detection. The photon noise of the detector is observed and becomes dominant as the temporal pedestal of the optical pulse decreases. The demonstrated detection ability is applied to a high‐contrast laser system, suggesting the accessibility of ultrahigh‐contrast measurements.