To achieve accurate temporal characterization of broadband laser pulses, a sufficient phase-matching bandwidth is required. The problem is mainly solved by using third-order nonlinear effects or few-micrometer-thin secondorder nonlinear crystals. However, difficulties persist in enhancing signal conversion efficiency while ensuring sufficient bandwidth. In this work, we propose a bandwidth-extending method based on spectral filtering function and ptychographic algorithm, termed Spectral Correction and Trace Truncation (SCTT), for characterizing broadband pulses. In experiments, we achieved the characterization of 720-880 nm broadband pulses using a beta-barium borate (BBO) crystal with a thickness of up to 100 mu m-far beyond the scale of a few microns-under phase-matching configuration. The reconstructed pulses are consistent with the standard results obtained using a 5-mu m-thin BBO crystal, but the measurement sensitivity and signal-to-noise ratio are increased by a factor of similar to 18, and the minimum measurable pulse energy is less than 10 pJ.

Laser-driven particle acceleration and related laser-matter interaction experiments require an ultrashort pulse laser with high temporal contrast. Here, we presented a plasma mirror (PM) temporal contrast enhancement system implemented at the SG-II 5PW laser facility, with a comprehensive investigation of spatiotemporal properties and physical applications. Key performance parameters of a PM were successfully obtained through single-shot online measurement by combining a spatiotemporally overlapped chirped pulse method. At a 45° incidence angle, the plasma reflectivity reached 84% for S-polarization and 48% for P-polarization, while the focal spot maintained excellent quality and the temporal contrast was improved by two orders of magnitude. The PM system was further applied in proton acceleration experiments under both polarization configurations. Supported by corresponding physical diagnostics, a significant reduction in optimum target thickness from 8 to 0.8 μm was achieved-clear evidence of effective pre-pulse suppression. Additionally, the PM and target installation were evaluated using a triple laser-damaged imaging method, based on the analysis of the three PM damage spots.

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.

Laser-driven inertial confinement fusion (ICF) diagnostics play a crucial role in understanding the complex physical processes governing ICF and enabling ignition. During the ICF process, the interaction between the high-power laser and ablation material leads to the formation of a plasma critical surface, which reflects a significant portion of the driving laser, reducing the efficiency of laser energy conversion into implosive kinetic energy. Effective diagnostic methods for the critical surface remain elusive. In this work, we propose a novel optical diagnostic approach to investigate the plasma critical surface. This method has been experimentally validated, providing new insights into the critical surface morphology and dynamics. This advancement represents a significant step forward in ICF diagnostic capabilities,with the potential to inform strategies for enhancing the uniformity of the driving laser and target surface, ultimately improving the efficiency of converting laser energy into implosion kinetic energy and enabling ignition.

For accurately characterizing broadband laser pulses, an ample phase-matching bandwidth is required. This is primarily addressed through the employment of third-order nonlinear effect or few-micrometer-thin second-order nonlinear crystals. However, there are still challenges in enhancing signal conversion efficiency while ensuring sufficient bandwidth. In this study, we introduced a bandwidth-extending method utilizing spectral filtering function and ptychographic algorithm, called Spectral Correction and Trace Truncation (SCTT), for broadband pulse measurement. Experimentally, we successfully characterized 160-nm-wide broadband pulses at a center wavelength of 800 nm through Second Harmonic Generation-Frequency Resolved Optical Gating (SHG-FROG) with a 100-mu m-thick beta-barium borate (BBO) crystal in phase-matching configuration. Its reconstructed pulses are in line with the standard results obtained using a 5-mu m-thin BBO crystal, but the measurement sensitivity and signal-to-noise ratio (SNR) are enhanced by a factor of 18.

Objective Chirp rate measurement is essential for pulse compression with chirped pulse amplification (CPA) and for chirped pulse utilization in physical experiments. Methods such as frequency-resolved optical gating (FROG), cross correlation frequency resolved optical gating (X-FROG), time-domain ptychography (TDP), two-dimensional spectral shearing interferometry (2DSI), and spectral phase interferometry for direct electric field reconstruction (SPIDER) are widely adopted for measuring pulses with sub-picosecond durations. Moreover, although a two-dimensional extension of the stationary phase point (SPP) method can be applied to the dispersion measurement of long pulses with durations exceeding tens of picoseconds, it achieves this by analyzing the phase behavior near the SPP in the interference fringes. In this paper, we propose a concept for chirp rate measurement based on a temporal-spectral scanning parametric process (TSSP). Methods The chirped pulse to be measured is beam split, with one branch providing the narrow bandwidth probe beam and the other maintaining a high fidelity with incidence. They are injected into a nonlinear crystal to realize noncollinear sum-frequency generation (NSFG). By precisely introducing a temporal delay into the probe beam, a temporal-spectral scanning between the probe beam and chirped pulse is achieved. Simultaneously, the sum-frequency generation (SFG) at different temporal delays is recorded. The chirped rate can be achieved by numerical calculation on the temporal delay and NSFG spectrum. The TSSP only requires recording the central wavelength of the SFG, rather than capturing intensity or interference fringes. Results and Discussions The time delay corresponding to different SFG spectra measured using the TSSP is shown as the dashed curve in Fig. 5(a). Correspondingly, the theoretical time delay calculated based on Eq. (5) with a preset grating separation of b(0)=34.3 mm is depicted as the solid curve in Fig. 5(a). A comparison of these two curves reveals that the deviation between the experimental measurements and theoretical calculations is minimal. When selecting a probe wavelength of 807.73 nm, the spectral component of the SFG signal is in one-to-one correspondence with the spectral component of the injected chirped pulse. Within a spectral width of 51.42 nm, ranging from 777 nm to 829 nm, the experimentally measured time delay is 20.63 ps, which results in a difference of 0.43 ps and a total deviation rate of 2.04% compared with the theoretical value of 21.06 ps. This demonstrates that the TSSP method achieves high accuracy in chirp rate measurements for pulses with durations on the order of tens of picoseconds. In this experiment, the primary source of deviation is the limited optical resolution of the spectrometer. Employing a spectrometer with higher optical resolution can effectively enhance the accuracy of chirp rate measurements. For instance, the spectrometer used in this study has an optical resolution of 0.5 nm, leading to a theoretical maximum measurement deviation of 2.99%. Commercially available spectrometers can achieve optical resolutions as high as 0.01 nm, theoretically reducing the deviation to 0.04%. Furthermore, if the TSSP method is applied to the measurement of chirped pulses with broader pulse durations, the measurement deviation will further decrease. In the field of picosecond pulse width measurements, the TSSP method exhibits significantly higher measurement accuracy than that based on the autocorrelation principle of large-aperture nonlinear crystals, highlighting its substantial application potential in picosecond pulse characterization. Figure 5(b) presents a comparison between the chirp rate measured using the TSSP method and the preset chirp rate. The solid curve represents the experimental measurements, calculated by substituting the SFG wavelength data and corresponding time delays obtained by the TSSP method into Eq. (4). The dashed curve is a fitted curve based on the experimental data, whereas the dotted curve represents the theoretical chirp rate derived from Eq. (6) using the preset grating separation. The fluctuations observed in the solid curve mainly originate from the derivative operation on the SFG wavelength in Eq. (4), which makes it highly sensitive to the recorded central wavelength data in the experiment. If a spectrometer with extremely high optical resolution is used for SFG spectrum measurements, ensuring more accurate recording of the central wavelength data, these fluctuations will be significantly reduced. The deviation between the theoretically preset chirp rate and the experimentally fitted chirp rate is nearly zero around 812 nm, with maximum deviations of approximately 5.9% and 4.0% in the shorter and longer wavelength regions, respectively, both significantly smaller than the fluctuations observed in the solid curve. This is because the SFG signal is independently measured and recorded during scanning, ensuring non-divergence of the measurement data in the TSSP method. Consequently, the fitted curve closely matches the theoretical curve, further validating the accuracy and reliability of the TSSP method. Conclusions This paper proposes an absolute chirp rate measurement method based on a time-frequency scanning nonlinear process. A TSSP-based measurement is implemented and experimentally studied using the front end of the SG-II 5 PW laser system. For a spectral range of 777?829 nm (pulse width Delta lambda=51.42 nm), the experimentally measured relative time delay is 20.63 ps, which compared with the preset value of 21.06 ps, yields a relative deviation of 2.04%. Furthermore, the chirp rate obtained from fitting the experimental data shows minimal deviation from the theoretical preset value near the central wavelength, demonstrating the high accuracy of TSSP measurements. The TSSP method is a simple and direct measurement technique that does not require algorithmic reconstruction, intensity distribution measurement, or interference fringe analysis. When combined with a high-optical-resolution spectrometer, it not only achieves higher measurement accuracy but is also applicable to chirp rate measurements for pulses with durations ranging from tens of picoseconds to nanoseconds. Additionally, by incorporating intensity information and phase reconstruction algorithms, the TSSP method holds significant potential for applications in the spectral phase and pulse width measurements.

Optical parametric amplification (OPA) is a promising method of producing extremely intense light. A new OPA scheme with comprehensively high performance is urgently required for future development. In this study, an amplification scheme known as crossed-Fabry-Perot-cavity OPA (XOPA) is proposed. It is based on the principle of periodic idler elimination, which prevents energy back-conversion among the three coupling waves, resulting in a monotonically increasing overall conversion efficiency. Using a signal at 808 nm and a pump at 532 nm, a chirped pulse XOPA is experimentally demonstrated with a conversion efficiency of 56.28% and a gain bandwidth of 120 nm. The measured pulse duration after compression is 19.2 fs, which is comparable to the Fourier-transform-limited 16.8 fs. Further investigations revealed several advantages. Stable pulse shaping in spatial, temporal, and frequency domains is realized by a spatiotemporally modulated pump. Pulse contrast adjustability on the front edge of the signal is verified in the XOPA of different Fabry-Perot cavity lengths. These results indicate astringency and precise regulation of output in nonlinear processes. Considering numerous crystals suitable for noncollinear configurations from the near-infrared to mid-infrared regions, XOPA has a universal potential application in laser systems with extreme intensity, few-cycle duration, and internal confinement fusion drivers. A Crossed-Fabry-Perot-Cavity OPA (XOPA) scheme is proposed and experimentally demonstrated. The pump-to-signal conversion efficiency of up to 56.28% and gain bandwidth of 120 nm are simultaneously achieved with signal pulses at 808 nm and pump 532 nm. The excellent performance makes XOPA universal potential utilization in laser systems of extreme intensity, few-cycle duration, and internal confinement fusion drivers. image

We present an optical parametric chirped-pulse amplification (OPCPA) based on mixed cascaded crystals, taking advantage of the unique parametric phase-matching of lithium triborate (LiB 3 O 5 , LBO) and yttrium calcium oxyborate ((YCa 4 O(BO 3 ) 3 , YCOB) crystals. The OPCPA properties of LBO at 880 nm and YCOB at 750 nm are studied respectively. After amplification by two LBO and two YCOB crystals, a total signal gain of 10 8 and spectral bandwidth close to 400 nm is obtained. After accurate dispersion compensation with a grating-pair compressor and chirped mirror compensator, a pulse duration of 9.4 fs is obtained by a SHG-frequency-resolved optical grating (FROG). This approach will be of great significance in high energy amplifier for high peak power few-cycle laser sources.

Single-shot ultrafast imaging reveals the transient dynamics of ultrafast phenomena. Herein, we introduce a new approach for capturing ultrafast micron-scale dynamics using an arbitrary time-wavelength-encoded biprism interferometer (TWEBI). In TWEBI, a time-wavelength-encoded pulse train is generated using a set of cascaded frequency-doubling crystals with slightly different phase-matching angles and independent delay lines, which have the advantages of independent temporal resolution, time window, and frame interval parameters. Phase measurement of TWEBI is achieved using a plug-and-play biprism interferometer. In the experiment to capture the transient dynamic of femtosecond laser-induced plasma filament, we demonstrated that the TWEBI setup can capture a total of 12 frames in a single shot, and its effective frame rate is 5 trillion frames per second (Tfps), corresponding to a temporal resolution of 200 f s and a spatial resolution of 4 mu m. Moreover, the time window of the TWEBI setup can be adjusted from sub-picosecond to nanosecond timescales. Thus, TWEBI has the advantages of high-precision temporal-spatial resolution, high frame rate, adjustable time window, and no reference arm, thus providing a practical and feasible diagnostic scheme for complex transient dynamics.

As optical parametric chirped pulse amplification has been widely adopted for the generation of extreme intensity laser sources, nonlinear crystals of large aperture are demanded for high-energy amplifiers. Yttrium calcium oxyborate (YCa4O(BO3)(3), YCOB) is capable of being grown with apertures exceeding 100 mm, which makes it possible for application in systems of petawatt scale. In this paper, we experimentally demonstrated for the first time to our knowledge, an ultra-broadband non-collinear optical parametric amplifier with YCOB for petawatt-scale compressed pulse generation at 800 nm. Based on the SG-II 5 PW facility, amplified signal energy of approximately 40 J was achieved and pump-to-signal conversion efficiency was up to 42.3%. A gain bandwidth of 87 nm was realized and supported a compressed pulse duration of 22.3 fs. The near-field and wavefront aberration represented excellent characteristics, which were comparable with those achieved in lithium triborate-based amplifiers. These results verified the great potential for YCOB utilization in the future.

The laser probe method is one of the main techniques for capturing ultrafast dynamic processes and has extensive applications in fields such as plasma physics, photochemistry, and biomedical science. In this paper, a time-wavelength encoding optical probe generation scheme is proposed, which uses cascaded frequency doubling crystals with different phasematching angles and independent delay lines to achieve time-wavelength encoding. This method offers single-shot high spatiotemporal resolution, high frame rate, a wide range of adjustable time windows. The temporal resolution of the optical probe depends on the pulse width of the second harmonic, which can be adjusted by changing the phase-matching angle of the frequency doubling crystal. The time window of the optical probe is only related to the change in the delay line, which can be adjusted by changing the length of the delay line. Therefore, the time resolution and time window of the optical probe are independent of each other. An optical probe generation system was constructed with 247 fs temporal resolution, 4 μm spatial resolution, 4.05 THz maximal frame rate, and an adjustable time window from sub-picosecond to 3 ns. The threedimensional spatiotemporal evolution process of plasma filaments was captured within a single shot using the optical probe. The experimental results showed that the ionization front of the plasma propagated forward at a velocity of (2.963 ± 0.024) × 108 m/s,which was consistent with the theoretical prediction. This demonstrated the feasibility of using the probe for capturing ultrafast events. In the discussion, we analyzed that the key parameters of the optical probe can reach a maximum frame rate of 35.7 THz, a maximum time resolution of 28 fs, and a time window range that can be adjusted from hundreds of femtoseconds to tens of nanoseconds. Finally, the optimal design parameters of the optical probe are given for different application scenarios. The optical probe generation scheme has good scalability and versatility, and can be combined with any wavelength decoding device, diffraction imaging, holographic imaging, tomography scanning, and other technologies. The high spatiotemporal resolution of the optical probe and the independent adjustability of its parameters provide a feasible solution for single-shot high spatiotemporal resolution captures of ultrafast dynamic processes at multiple time scales.

The laser probe is one of the main techniques for capturing ultrafast dynamic processes and has extensive applications in fields such as plasma physics, photochemistry, and biomedical science. In this work, a time wavelength encoded optical probe generation scheme is proposed, which uses cascaded frequency doubling crystals with different phase-matching angles and independent delay lines to achieve time-wavelength encoding. This method offers single-shot high-spatiotemporal resolution, high frame rate, and a wide range of adjustable time windows. The temporal resolution of the optical probe depends on the pulse width of the second harmonic, which can be adjusted by changing the phase-matching angle of the frequency-doubling crystal. The time window of the optical probe is only related to the change in the delay line, which can be adjusted by changing the length of the delay line. Therefore, the time resolution and time window of the optical probe are independent of each other. An optical probe generation system is constructed with 247 fs temporal resolution, 4 mu m spatial resolution, 4.05 THz maximal frame rate, and an adjustable time window from sub-picosecond to 3 ns. The three-dimensional spatiotemporal evolution process of plasma filaments is captured within a single shot by using the optical probe. The experimental results show that the ionization front of the plasma propagates forward at a velocity of (2.963 +/- 0.024) x 108 m/s , which is consistent with the theoretical prediction. This demonstrates the feasibility of using the probe for capturing ultrafast events. In the part of discussion, we analyze that the key parameters of the optical probe can reach a maximum frame rate of 35.7 THz, a maximum time resolution of 28 fs, and a time window range that can be adjusted from hundreds of femtoseconds to tens of nanoseconds. Finally, the optimal design parameters of the optical probe are given for different application scenarios. The optical probe generation scheme has good scalability and versatility, and can be combined with any wavelength decoding device, diffraction imaging, holographic imaging, tomography scanning, and other technologies. The high spatiotemporal resolution of the optical probe and the independent adjustability of its parameters provide a feasible solution for single-shot high spatiotemporal resolution captures of ultrafast dynamic processes on a multiple time scale.

Laser wakefield acceleration (LWFA) using PW-class laser pulses generally requires cm-scale laser-plasma interaction Rayleigh length, which can be realized by focusing such pulses inside a long underdense plasma with a large f-number focusing optic. Here, we present a new PW-based LWFA instrument at the SG-II 5 PW laser facility, which employs f/23 focusing. The setup also adapted an online probing of the plasma density via Nomarski interferometry using a probe laser beam having 30 fs pulse duration. By focusing 1-PW, 30-fs laser pulses down to a focal spot of 230 µm, the peak laser intensity reached a mild-relativistic level of 2.6 × 1018 W/cm2, a level modest for standard LWFA experiments. Despite the large aspect ratio of >25:1 (transverse to longitudinal dimensions) of the laser pulse, electron beams were observed in our experiment only when the laser pulse experienced relativistic self-focusing at high gas-pressure thresholds, corresponding to plasma densities higher than 3 × 1018 cm-3.

The continuous-wave (CW) linear accelerator, SSC-Linac, has been put into operation successfully as the new injector of Separate Sector Cyclotron (SSC) for Heavy Ion Research Facility in Lanzhou (HIRFL). The SSC-Linac is operated at the frequency of 53.667 MHz, which is the four times of the SSC operating frequency. In order to improve the longitudinal capture efficiency and increase the beam current extracted from the SSC, an independent multi-harmonics buncher (MHB) operating at a fundamental frequency of 13.417 MHz had been designed and manufactured. This buncher adopts the dual-gap structure and sawtooth waveform is generated by multi-harmonics synthetized technology. Beam dynamics simulation with MHB was carried out and discussed in this paper. Furthermore, the beam commissioning using $$^{84}$$ Kr $$^{14+}$$ had been performed on SSC-Linac. The measured maximum capture efficiency 86.5% was obtained, which was well agreed with the simulation results. Due to the successful commissioning of MHB and the outstanding-performance of SSC-Linac, the extracted beam current from SSC has been improved one order of magnitude higher than before.

We present a new scheme of picosecond optical parametric chirped pulse amplification (OPCPA) in which a Fourier-transform-limit 5.0 ps pulse is optically sheared from a single-longitudinal-mode 1064 nm CW laser. The pulse is amplified and frequency-doubled as the pump in order to maintain the pump narrow bandwidth and picosecond duration simultaneously, which is very important to ensure the high temporal contrast for an OPCPA amplifier. Combined with the cross-polarized wave generation (XPW), a compound frontend for the high-power femtosecond laser system that delivers a 1 Hz chirped pulse train is established. The experiments provide an output pulse energy of 17.1 mJ, a spectrum bandwidth 71 nm (FWHM), and a pulse duration 16.4 fs. The pulse contrast reaches 1:10−12 several picoseconds before the peak of the main pulse, which is the best value of the available measuring instruments.

The study of the origin of asymmetries in mirror β decay is extremely important to understand the fundamental nuclear force and the nuclear structure. The experiment was performed at the National Laboratory of Heavy Ion Research Facility in Lanzhou (HIRFL) to measure the β-delayed γ rays of 26P by silicon array and Clover-type high-purity Germanium (HPGe) detectors. Combining with results from the β decay of 26P and its mirror nucleus 26Na, the mirror asymmetry parameter δ ( ≡ft+/ft−− 1) was determined to be 46(13)% for the transition feeding the first excited state in the daughter nucleus. Our independent results support the conclusion that the large mirror asymmetry is close to the proton halo structure in 26P.

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 10(2) 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.

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.

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.