SignificanceLaser 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.ProgressThe 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 ProspectsAiming 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.

SignificanceThe rapid emergence and development of ultra-intense and ultra-fast lasers have created unprecedented physical conditions and novel experimental methods, enhancing and organizing human comprehension of natural laws while significantly advancing innovation in basic and interdisciplinary research, as well as strategic high-tech areas. Ultraintense and ultrafast laser devices have been pivotal in advancing human comprehension of fundamental laws governing the natural world and have contributed to the advancement of cutting-edge scientific research fields.In terms of ultra-intensity, using chirped pulse amplification (CPA) technology, laser systems can achieve peak powers surpassing the petawatt (PW) level, reaching up to 10 PW, with focal intensities of 1022‒1023 W/cm². This enables easy tearing of electrons and atomic nuclei by ultraintense laser fields, enabling exploration at the subatomic level of microscopic structures. For an ultrafast laser, ultrafast pulses can achieve pulse widths in the femtosecond range, enabling the measurement of femtosecond-scale physical changes. The high temporal resolution properties of lasers are crucial for investigating molecular dissociation processes, which are essential in chemical research. Additionally, the use of ultra-intense ultrafast lasers to drive nonlinear processes, such as high-harmonic generation, can produce attosecond (10-18 s) pulses, enabling the measurement of attosecond-scale electron dynamics.An article in the “Opinion” column of the January 2010 issue of Nature, titled “2020 Visions,” has analyzed the development directions of key scientific fields for the next decade and presented prospects for 2020. This study predicted five major breakthroughs that may occur in the laser field over the next 10 years, with four of them directly related to ultra-intense ultra-fast lasers. The four breakthroughs include: the precise measurement of cosmic constants using ultra-precise laser clocks; the generation of new states of matter and the provision of carbon-free and infinite clean energy using next-generation lasers; tracking extreme ultrafast electron motions in chemical reactions with attosecond pulses; and the acceleration of electrons and protons to near-light speeds to achieve low-cost, desktop high-energy particle accelerators. An article in Science magazine has stated that the development of ultra-intense ultrafast lasers “will affect research on everything from fusion to astrophysics,” with significant applications in laser acceleration, laser fusion, attosecond science, atomic and molecular physics, materials science, nuclear physics, plasma physics, high-energy-density physics, astrophysics, high-energy physics, nuclear medicine, and other fields, rendering it one of the major frontiers in international scientific competition.ProgressIn recent years, the State Key Laboratory for High Field Laser Physics at the Shanghai Institute of Optics and Fine Mechanics (SIOM) has developed multiple sets of ultra-intense ultra-fast laser devices, including a new generation of ultra-intense ultra-fast laser comprehensive experimental facilities, the Shanghai super-intense ultra-fast laser facility (SULF), and the construction of stations of extreme light (SEL) for Shanghai high-repetition-rate X-ray free electron laser (XFEL) and extreme light facility (SHINE), forming a group of devices represented by SULF.The new generation of ultra-intense ultra-fast laser comprehensive experimental facilities aim to develop strategic advanced technologies such as ultra-intense ultra-fast laser-driven desktops and short-pulse XFELs, explore high-intensity attosecond coherent X-ray science and new fields of strong-field physics in the mid-infrared region, and address major scientific and technological issues in high-density high-energy electron laser acceleration in phase space. This novel generation consists of four systems: a high-performance multi-terawatt ultra-intense ultra-fast laser system with a high repetition rate, mid-infrared tunable ultra-intense ultra-fast laser system, laser wakefield electron acceleration and desktop XFEL system, and high-harmonic extreme ultra-violet (XUV) coherent light source system. These facilities can output multi-terawatt ultra-intense ultra-fast laser pulses at a wavelength of 800 nm with high repetition rates, mJ-level mid-infrared-tunable ultra-intense ultra-fast laser pulses, and high harmonic coherent radiation with photon energies ranging from 30.0 eV to 5.5 keV, as well as GeV-level high-quality electron beams and short-pulse XFELs.The SULF mainly includes a 10 PW ultra-intense ultra-fast laser system with a high-repetition-rate output beamline at the 1-PW level. This laser system is used to drive the generation of high-brightness ultra-short pulse high-energy photons and particle beams, establishing three user experimental terminals: the dynamics of materials under extreme conditions (DMEC), ultrafast sub-atomic physics (USAP), and big molecule dynamics and extreme-fast chemistry (MODEC) platforms. This project aims to build the world’s first 10 PW ultra-intense ultra-fast laser system. The laser system achieves a peak power of 10 PW with a pulse duration of 30 fs, center wavelength of 800 nm, and maximum laser focusing intensity exceeding 1022 W/cm² while also providing a high repetition rate (0.1 Hz) 1 PW laser pulse output.Using the research foundation of the SULF device, SIOM has innovatively proposed the establishment of an SEL device centered around a 100 PW ultra-intense ultra-fast laser on SHINE. Currently, the SEL-100 PW laser system has completed the front-end system of PW-level high-repetition-rate optical parametric chirped pulse amplification (OPCPA), breaking through key laser technologies such as generating ultra-wideband seed lasers, high-fidelity OPCPA amplification, and managing ultra-wideband laser dispersion. The independently developed PW-level high-repetition-rate OPCPA prototype realized the research and validation of key technologies for PW-level new-bandwidth OPCPA, laying the technical foundation for the construction of a 100 PW laser system. This facility is currently the first and only initiated 100 PW laser project worldwide. Leveraging XFELs and 100 PW ultra-intense ultra-fast lasers can pave the way for new frontiers in strong relativistic physics research.Conclusions and ProspectsThe development and application of ultra-intense ultrafast lasers represent the latest frontier and a key competitive area in international laser technology. This holds significant value and is one of the major frontiers in international scientific competition. The development trend of large-scale ultra-intense ultra-fast laser facilities includes further increasing peak power and focusing peak intensity, continuous enhancement of repetition rate and average power, further compression of laser pulse width, expansion of laser wavelengths, and transition from laboratory-based platforms to mobile platforms and even space-based and airborne platforms.The construction of SULF is currently a focal point of international competition and represents an organic combination of basic research and engineering implementation. This facility can provide unprecedented research conditions for investigating material structures, motions, and interactions under extreme physical conditions, thereby deepening and systematizing the understanding of laws governing the objective world. This drives the exploration and development of interdisciplinary basic and frontier sciences and promotes innovation in related strategic high-tech fields, sparking technological transformations and creating new industries, which have social and economic benefits for outstanding project outcomes.

SignificanceThe appearance and rapid development of superintense and ultrafast laser have opened up many frontier areas in subjects such as atomic and molecular physics, strong field physics, and plasma physics, and superintense and ultrafast laser itself has become a powerful tool in numerous applications.The extreme nonlinear interaction between superintense and ultrafast lasers and atomic or molecular systems induces strong field ionization, attosecond radiation, femtosecond laser filamentation, and air lasing. Attosecond physics may develop new technological means for the study of electronic dynamics in complex systems that are relevant to physics, chemistry, biomedicine and other subjects. Laser filamentation and air lasing have brought new opportunities for applications based on laser plasma associated radiation, laser atmospheric remote sensing and material analysis, weather modification, and laser material processing.With the increase of laser intensity, physical processes enter the relativistic plasma regime, where many applications have been developed in laser-driven particle acceleration, laser-driven high-energy radiation sources, ultrafast electronic dynamics, laser nuclear physics, and laboratory astrophysics. Laser-driven ultra-high-gradient particle accelerators and high-energy ultrafast radiation are advantageous in producing compact particle and radiation sources with ultra-high peak brightness and ultrafast time duration, thus are unique in several key applications. Especially, laser-driven plasma wakefield acceleration is promising in generating high energy electrons to develop table-top X-ray free electron lasers. On the other hand, laser-driven ion acceleration is believed to bring new opportunities in tumor therapy, proton imaging, and fast ignition fusion. As the laser intensity further increases, the extreme strong field effect gradually becomes apparent and it enters the strong field quantum electrodynamics (QED) regime, resulting in a series of new phenomena such as quantum gamma radiation, generation of electron-positron pairs, vacuum polarization, QED cascades and so on. These studies would extend boundaries of laser-matter interaction. New properties such as spin and orbital angular momentum also emerge as new degree of freedom in laser-plasma interaction.The interaction between superintense laser and plasma is an important means of generating radiation at different wavelengths. The study of terahertz radiation sources driven by superintense lasers can further improve the laser energy conversion efficiency. The free electron and quasi-particle radiation driven by superintense laser is of great significance for the development of miniaturized and integrated new coherent light sources and terahertz-driven electron sources. Ultrafast lasers provide processing accuracy beyond the limits of optical diffraction, enabling high-quality micro-nano processing. In addition, micro-nano lasers are important for promoting the miniaturization and integration of optoelectronic devices.ProgressThis review first introduces nonlinear atomic and molecular physics driven by ultrafast lasers, followed by superintense lasers and relativistic plasma physics at higher intensities, and finally reviews the cross-disciplinary frontier applications of superintense ultrafast laser. This review will highlight the progresses and achievements made by the State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, in the long-term dedicated research on superintense ultrafast laser physics and its frontier applications.The laboratory has made significant progresses in the generation and control of high-order harmonics and single attosecond pulse, high brightness high-order harmonic coherent light sources, water window or keV high-order harmonic generation driven by mid-infrared wavelength lasers, and high-order harmonic generation in condensed matter. Important progresses in femtosecond laser filamentation measurement and control, femtosecond laser weather modification, and air lasing are also elaborated.In recent years, the laboratory has developed stable and usable high-quality laser-driven electron sources. The laser energy conversion efficiency and beam quality of high-energy and ultrafast radiation sources have also been improved. Compact free electron laser in the extreme ultraviolet band has been demonstrated based on laser-driven wakefield electrons for the first time in the world. Laser-driven ion acceleration has seen rapid development in the past decades, where new acceleration mechanisms have been proposed, and the laser energy conversion efficiency and the cut-off energies of protons are now sitting in the first class in the world. In the strong field QED regime, the radiation-reaction trapping of electrons and the upper limit of laser intensity caused by non-ideal vacuum are discovered. The exploits of vortex laser on particle acceleration and secondary radiations are developed in the laboratory. The laboratory has also successfully generated positrons using superintense laser and proposed new schemes for the detection of dark matter particles such as axion.Significant progress in the generation of intense terahertz radiation has been made. The quasi-particle radiation amplification mechanism and electron acceleration driving the terahertz radiation have been explored, which provides a unique platform for surface light sources and applications. In addition, a number of research achievements have been made in ultrafast laser micro-nano processing and micro-nano lasers.Conclusions and ProspectsThe development and application of superintense ultrafast laser technology have significant impacts on disciplines such as physics, chemistry, and biology. It is a highly competitive area of research worldwide. The progresses highlighted in this review will further guide the development of advanced laser technology and light sources, compact particle accelerators, and extend our knowledge on light-matter interaction. It is now an important stage to bring the research in laboratories to real world applications, where key aspects of the interaction should be controllable and the whole system must be stable or even cost efficient. On the other hand, the extreme field provided by 10‒100 PW lasers shows unparalleled capacities in fundamental research, such as strong field QED, high energy density physics or even dark matter search. It relies on joint efforts among multiple disciplines such as plasma physics, theoretical physics, particle and nuclear physics and so on.

SignificanceThe inertial confinement fusion (ICF) high-power laser facility is an important driver of laser fusion ignition and high-energy-density physics. To achieve fusion ignition, the energy control accuracy, time-power curve controllability, spectral control, and control of near-and far-field beam quality directly affect the results of fusion-ignition physics experiments. High-power laser devices have been developed in China for more than 60 years, and the front-end and pre-amplification technologies of high-power laser facilities have evolved from single-time waveform control and synchronous static control to the high stability and global active controllability of laser beams. This paper describes the development of precision control technology for front-end and pre-amplification laser systems in SG-II series facilities.ProgressA high-power laser driver is primarily composed of a front-end system, pre-amplification system, main amplification system, and terminal system. The front-end and preamplification systems serve as the driver source, which is typically referred to as the laser injection system. The front-end system serves three major functions: (i) provide seed pulses to the device to satisfy various requirements with the corresponding functions and accurate synchronization; (ii) control the laser time domain, frequency domain, and signal-to-noise ratio (SNR); (iii) perform detection and abnormal feedback control. Similarly, the pre-amplification system serves three major functions: (i) pre-amplify the pulse energy to realize the J-level energy output of the shaping pulse; (ii) precisely control the near-field distribution of the beam and gain spectral pre-compensation; (iii) control the service capabilities of the device, which provides output laser pulses with a certain repetition rate and energy for the subsequent auto-collimation system of the optical path, wave-front correction, and system-operation monitoring.The ICF high-power laser device includes multiple main nanosecond laser pulses and short picosecond pulses. The former is intended primarily for achieving high-density compression of the target sphere, which requires high spatial uniformity and time symmetry. The latter is primarily used in diagnostic lasers or ignition heating pulses. Synchronization between nanosecond main lasers and picosecond short-pulse lasers is the core requirement for the active diagnosis of the evolution of the high-energy-density physical state as well as for investigating rapid ignition and high-efficiency implosion. The National Laboratory on High Power Laser and Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (hereinafter referred to as the front-end research group) has developed a high-precision synchronization scheme with a homologous clock-lock, (a phase-locked frequency technology), which achieved a long-short pulse synchronization accuracy of <20 ps (PV) and 3 ps (RMS) (2 h test results). Moreover, the electric-scale signals and high-precision synchronous trigger signal systems required for multichannel physical test diagnosis have been established. The time jitter between the signal and main laser is <3 ps (RMS) and 20 ps (PV), and a certain amount of delayed tuning is satisfied.A high-power laser system is used to drive the ignition of the ICF, for which precise control of the time–power curve is the main requirement. To improve the accuracy of pulse shaping and the system efficiency, a feedback closed-loop control system is typically adopted to realize the closed-loop control of the pulse-time waveform. The front-end research group utilized an AWG and closed-loop feedback control, in addition to neural-network algorithms, to realize an accurate online calibration of the transfer function of the front-end pulse-shaping system (Fig. 1). Consequently, a 1 Hz real-time noise reduction of shaping signals, as well as various types of isentropy-like double-slope pulse operations are realized.Meanwhile, transverse stimulated Brillouin scattering must be suppressed (TSBS) to protect large-aperture optics and ensure the uniformity of the focal spot; hence, spectral-broadening technology is adopted in high-power laser systems. However, after spectral broadening, the discrepancy of the transmission rate of the spectral phase or amplitude modulates the time waveform, which is an FM-to-AM effect. To improve the temporal-power curve, a full single-polarization front-end system based on a single-polarization transmission fiber and a group-velocity dispersion (GVD) compensation unit were utilized in the front-end system to solve the FM-to-AM conversion caused by the polarization mode dispersion (PMD) and GVD (Fig. 4). Real-time monitoring of the FM-to-AM conversion was performed to observe the modulation depth caused by different modulation frequencies to provide feedback and control the FM-to-AM conversion caused by the GVD. Additionally, fidelity-amplification technology was developed for the full spectral range, thus solving the gain narrowing of the amplification system and resulting in a fundamental-frequency time-domain modulation degree of less than 5%@0.3 nm (3 GHz+20 GHz).Subsequently, the FM-to-AM degree of the nanosecond terminal and target surface was evaluated. The results show that the combination of the dispersion grating used in smoothing via spectral dispersion (SSD) and the focusing system is equivalent to an 8 GHz (3 dB bandwidth) Gaussian low-pass filter (Fig. 7). The modulation degree on the incident surface of the wedge focusing lens (WFL) is 19%, whereas that on the target is 4.9%, and the high-frequency components can be effectively filtered out.For near-field control, the front-end research group developed a near-field intensity control scheme (Fig. 10) and a series of light-field control devices, such as passive-dielectric-film binary optical elements (aperture: 50 mm×50 mm; damage threshold: 7 J/cm2)(Fig. 12), active optically addressed spatial light modulators (aperture: 300 mm×300 mm; damage threshold: 300 mJ/cm2) (Fig. 13), and optically oriented liquid-crystal shaping devices (aperture: 62 mm×62 mm; damage threshold: 2 J/cm2) to control the near-field intensity distribution of high-power laser devices. To increase the irradiation energy density to 1 J/cm2, spatial-light modulator technology based on high-damage-threshold photoconductive materials is being developed to adapt to more application scenarios of high-power lasers. To remain relevant with the development of ICF laser drivers and the continuous expansion of the functions of the preamplifier system, the front-end research group has developed a repetition-frequency preamplifier system (Fig. 18) based on LD pumping, which can achieve an output energy of 3 J/Hz, an energy stability level of 0.5% (RMS), and an output beam near-field modulation of ≤1.21.Conclusions and ProspectsSince the invention of lasers, laser-fusion drive technology has been developed for more than 60 years. To satisfy the application requirements of laser fusion ignition, precision control technology for lasers has been developed and has progressed from the initial time-domain smoothness and coarse synchronization technology to high-precision, high-stability, long-range centralization, thus providing controllability in time, space, and frequency domains, as well as in polarization. Additionally, the development of precision control technology for laser fusion drivers necessitates the development of a laser injection system for high-power laser devices.Owing to the success of ignition in the United States, laser fusion has evolved from exploratory technology research to the realization of laser fusion energy. Regarding the challenges of laser technology, in addition to the high repetition frequency of laser technology, one should improve the beam-target coupling efficiency of lasers and the key control device to achieve an efficient repetition-frequency operation.

SignificanceWith a high peak power and high pulse energy, the physical effect of the interaction between picosecond petawatt laser and matter is related to key factors such as pulse signal-to-noise ratio, focusing power density, and spatio-temporal control accuracy. This paper presents a comprehensive study of these three factors, as well as the associated limiting mechanism, key technologies, and latest development of the SG-II-UP picosecond petawatt laser. This provides has a guiding role in improving the performance of petawatt lasers. For the load limitation of picosecond petawatt lasers, the damage threshold of optics, optical field degradation, and cleanliness control are analyzed, and the new trend of pulse amplification and compression technology based on plasma optics in exploring the load bottleneck of the high-power laser is reviewed. In addition, advancing the high-efficiency, high-energy and high-repetition-rate picosecond petawatt laser technology research is crucial to the transition of picosecond petawatt lasers from laboratory to a wider range of applications.ProgressThis study aimed to improve the overall performance of the SG-II-UP picosecond petawatt laser. The intricate challenge of signal-to-noise ratio in the pulse generation system of the picosecond petawatt laser was systematically addressed. Through the engineering application of picosecond optical parametric amplification technology, a significant reduction in parametric fluorescence was achieved, amounting to four orders of magnitude. Concurrently, two distinct groups of pre-pulses, attributed to the non-linear process between the primary pulse and post-pulses associated with the B-integral in a nanosecond optical parametric amplification, were identified and eliminated. This resulted in a three-orders-of-magnitude reduction in the pre-pulse amplitude. The optimization of key parameters of the adaptive optics system and the implementation of a comprehensive closed-loop control strategy for the entire laser beamline significantly improved the typical focal spot of the SG-II-UP picosecond petawatt laser to 17.0 μm×20.8 μm, corresponding to a peak focusing power density of 1.4×1020 W/cm2. During the laser-driven proton acceleration experiments conducted in 2021, the maximum cutoff energy of the proton beam reached up to 70 MeV. These results illustrate a significant enhancement in the overall performance of the SG-II PW laser facility.Additionally, a verification experiment was conducted on chirped pulse amplification utilizing a multi-pass amplification configuration. This research comprehensively investigated challenges associated with pencil-beam pre-pulses, amplified spontaneous emission, focusing capabilities, and anti-laser isolation during the commissioning of a picosecond-petawatt laser integration beamline (PPLIB) at the SG-II-UP facility. The suppression of the pre-pulses from pencil-beams in the subsequent multi-pass chirped-pulse amplification (CPA) was achieved by optimizing the timing sequence of the Plasma electrode pockels cell (PEPC). This optimization is anticipated to boost the contrast by over 20 dB. The focusing laser power density of this multi-pass chirped-pulse amplification configuration achieved an improvement of approximately sevenfold through a novel wavefront control system. The experiment demonstrated an output capability of 4126 J/3.3 nm via chirped pulse amplification in a single beam, thereby establishing a technical foundation for the development of the second picosecond petawatt laser at SG-II-UP.Moreover, a series of advanced laser technologies were developed at SG-II-UP. A closed-loop control method for the laser pointing stability by adopting high-speed camera combined with fast-steering mirror in a high-power laser facility was proposed and demonstrated using the SG-II-UP picosecond petawatt laser. The implementation of this approach resulted in an improvement in beam pointing stability of more than three times. Correspondingly, the root-mean-square value of the pointing uncertainty decreased from 2.80 to 0.63 μrad. Further enhancements in the focusing power density of the petawatt laser systems were achieved through the study of dynamic chromatic aberration pre-compensation schemes and curved plasma mirrors.Conclusions and ProspectsHigh-power laser facilities, capable of providing extremely high temperature and pressure conditions, serve as a critical scientific infrastructure for fundamental research, advanced science and technology, and national defense and security. To satisfy the demand in cutting-edge fields such as precision backlight probes, fast ignition and proton imaging, and to improve the brightness and quality of laser generated secondary rays and particle sources (X-rays, electrons, protons and etc.), the SG-II-UP PW, a representative of picosecond petawatt lasers, has developed a series of advanced laser technologies in the aspects of pulse signal-to-noise ratio, focusing power density and spatio-temporal coupling accuracy. Owing to the limited load capacity of the picosecond petawatt laser, several strategies such as improving component damage thresholds, effectively suppressing optical field degradation, and implementing closed-loop control of cleanliness can significantly enhance its output capacity. The anticipated advancements in new technologies such as pulse amplification and compression based on plasma optics can potentially overcome the current bottlenecks in the development of high-power lasers, including picosecond petawatt lasers.In the future, the trajectory of picosecond petawatt laser research is expected to evolve from a singular emphasis on achieving peak power in discovery science to a dual focus on both high average and high peak powers in applied science. The advent of high-efficiency, high-energy, and high-repetition rate picosecond petawatt laser technology is set to revolutionize the developmental paradigm for high power laser systems. This transformation will influence all aspects of high-power laser systems, including laser materials, optical components, and laser configurations.

ObjectiveUltra-short ultra-intense lasers can provide unprecedented extreme physical conditions and new experimental methods, leading to significant applications in laser acceleration, plasma physics, strong field physics, and high-energy density physics. OPCPA technology is an important method for realizing ultrashort laser amplification. Currently, the highest output peak power based on OPCPA technology is up to 4.9 PW. To further increase the peak power, larger-size crystals are required, and the gain spectrum width can be compressed to tens of femtoseconds. The extreme light physical line station (SEL), an integral part of the hard X-ray free electron laser (SHINE) project, a significant national science and technology infrastructure in China, is undertaking the construction of an ultra-short and ultra-intense laser (SEL-100 PW), aiming for a remarkable peak power of 100 PW. The interaction between the ultra-high focused intensity produced by a 100 PW laser and hard X-ray free electron laser is used to explore the strong field quantum electrodynamics (QED) and other frontier science and technology fields. For ultra-intense and ultra-short laser-matter interaction experiments, the focused peak power density (focused intensity) of the laser is one of the most important technical indexes, which requires ultra-high peak laser power and a focused focal spot close to the diffraction limit level. The ability of a laser to focus near the diffraction limit is primarily limited by its spatial phase (wavefront) distortion. In addition to the static wavefront aberration introduced by optical component processing and mounting errors, dynamic wavefront aberration during the laser amplification process is also an important influencing factor. Additionally, the near-field spatial distribution of a laser beam determines the safety of high-energy lasers during transmission. To study the dynamic phase evolution of the 100 PW laser OPCPA high-energy main amplifier, this study focuses on the three-wave beam quality evolution in the process of OPCPA amplification. Furthermore, the influence of the intensity distribution and wavefront distribution of different pump light on the wavefront distortion of the amplified signal is studied.MethodsSolving the three-wave coupling equation in the spatial domain is important for studying the spatial phase evolution characteristics in high-energy, ultra-wideband OPCPA processes. The split-step Fourier method is important for numerically solving the three-wave coupling equations. The basic idea was to divide the nonlinear crystal into multiple parts, in which the free propagation of light is considered first, and then the nonlinear effects were considered. Once a reasonable mathematical model was developed, the characteristics of the signal light under the experimental conditions of a 100 PW laser device were first calculated. Subsequently, the phase mismatch and intensity distribution of the pump light were considered. Furthermore, the effect of the spatial phase distribution and saturation effect of the pump light on the energy and spatial phase of the amplified signal was considered.Results and DiscussionsThe energy of the amplified signal light can be obtained by simulating the experimental conditions of the 100 PW laser, and the spectrum and intensity distribution can satisfy the requirements of subsequent compression (energy >300 J, spectral range is 925 nm±100 nm)(Figs.2, 3). First, the phase mismatch is analyzed, and it can be concluded that the phase mismatch has a significant influence on the conversion efficiency and PV value of the amplified signal light phase; therefore, phase matching is an important condition for determining the output performance of OPCPA. Subsequently, the influence of the spatial phase distribution of the pump light on the amplified signal light is analyzed, which is proportional to the spatial derivative of the defocus of the pump light and occurs only in one dimension of the nonlinear angle (Fig.6). The influence of the intensity distribution of the pump light on the amplified signal light is analyzed. The spatial distribution of the pump light is imprinted on the phase of the amplified signal light, which is evident in the single-directional linear modulation and central-region protrusions (Figs.9, 10, 11, 12). Finally, the effect of the saturation effect on the amplified signal light is considered, and with a further increase in the pump energy, the signal light reaches saturation and the conversion efficiency decreases. The PV value of the amplified signal light phase also increases with the increase in energy, although the growth gradually decelerates (Fig.13).ConclusionsIt can be concluded that under the input condition of the amplifier stage, the output of ultrashort and ultra strong pulses on the order of 100 PW can be realized. The pump light phase experiences only a small overall change gradient due to the large spatial size of the pump light. Therefore, the phase distortion of the pump light has slight effect on the phase of the amplified signal, and the effect of the phase distortion of the pump light at the wavelength level of 10-4 can be neglected in the design process. However, in a phase mismatch, the intensity distortion of the pump light has a significant impact on the intensity and phase of the signal light. Hence, the intensity distribution and phase matching of the pump light are necessary to realize a high-quality ultrashort pulse output in the design process. In subsequent studies, we will also conduct validation experiments to verify the simulation results.

ObjectiveCurrently, the development of high-power laser drivers is limited by the damage to terminal optical system components. Furthermore, self-focusing is an important nonlinear propagation effect that causes damage to components. As a special self-focusing phenomenon, a hot image, with extremely high intensity of light field, typically appears in the downstream conjugate position of the upstream defect relative to the nonlinear medium. Therefore, in the design of system, the nonlinear propagation effect is usually controlled by regulating the B integral and arranging the optical elements to avoid the conjugate plane of the hot image. This in turn reduces the damage threat to the components. However, in the operation experiment of the laser drivers, it is determined that the position and intensity of certain light field strength areas deviate from the existing theory. Hence, unexpected damage often occurs in the components. Therefore, laws and mechanisms relating to nonlinear propagation effects of high-power lasers should be further examined.MethodsThe formation of hot image can be divided into three processes. First, the beam modulated by defects reaches the nonlinear medium via linear propagation. Second, when the beam propagates in the nonlinear medium, the third-order nonlinear polarization of the medium is induced, and the beam gradually converges. Third, the beam propagates linearly after ejecting from the medium, and it finally converges to form hot images. Based on the thin slice approximation, the analytical expressions of the position and intensity of the hot image can be obtained by solving the nonlinear Schr?dinger equation. However, in practical high-power laser systems, nonlinear media usually do not satisfy the condition of slice approximation. In this study, the angular spectrum method for the linear diffraction propagation and the split-step Fourier algorithm for the nonlinear propagation were used to explore the evolution of the hot image characteristics.Results and DiscussionsFor the single defect, when considering the edge steepness, a single hot image caused by a single defect is divided into double-peak hot images located in front of and behind the conjugate plane (Fig.2). This breaks the longitudinal correspondence between the defect and hot image and can cause accidental damage to the optical elements downstream of the conjugate plane. The modulation degrees of double-peak hot images exceed that of single hot image, and the positions are affected by the size of defects (Fig.4). For double defects, when they are very close to each other in the same plane, two additional hot image peaks appear before and after the double-peak hot images. Specifically, both peaks are located in the central axis direction of the double defects (Fig.7). The positions and modulation degrees of these two peaks are affected by size, center spacing, and modulation depth (Fig.9). The existence of the new peaks indicates that defects and hot images no longer correspond in the transverse direction. When double defects are located in different planes, the coaxial double defects can produce an emphasis peak formed by the superposition of double-peak hot images (Fig.12). Furthermore, when the transverse distance of defects increases, the downstream modulation decreases rapidly (Fig.14).ConclusionsA systematic study is conducted on the deviation between the observed positions and intensities of hot images in high-power laser devices when compared to existing theories. The results show that fine morphology differences and mutual interference of the upstream defects can further enhance the optical field’s intensity area and intensity downstream. This disrupts the correspondence between the defects and hot images in the horizontal and vertical directions, undermining the effectiveness of nonlinear control strategies for laser drivers that rely the original B-integral and hot image cognition. The results of this study deepen the understanding of the propagation law of complex nonlinear optical field. This understanding not only aids in optimizing the arrangement of terminal components in the system, but also highlights new requirements for the precise detection and evaluation of defects of large-aperture optical components in the high-power laser devices. Furthermore, this provides an important technical reference for improving the load capacity of the final optics system in the high-power laser drivers.

SignatureSince the invention of laser in 1960, laser technology has been widely used in many fields, such as scientific research, industrial processing, medical treatment, military confrontation, space exploration, and daily life. In the continuous development of laser technology, laser materials are key as core gain media. Laser glass is an important component of solid laser media and is an optical glass composed of a glass matrix and activated ions. The physicochemical and process characteristics of laser glass depend on the matrix glass, the spectral performance depends primarily on the activated ions, and the final laser properties are determined by both the glass composition and preparation process. To date, rare-earth ions, which are an important class of activated ions, have afforded multiwavelength laser outputs in a bulk glass matrix. Compared with other rare earth ion-doped glasses, Nd3+ ions offer advantages such as multiple absorption lines in the UV and visible regions, large stimulated emission cross sections, and low thresholds for ~1 μm wavelength lasers. Therefore, neodymium-doped laser glass is the preferred gain medium for xenon lamp-pumped high-energy laser devices.Since the introduction of ruby laser in the early 1960s, glass science and technology workers in China have begun to analyze the possibility of generating lasers in a glassy state. In 1961, Snitzer of the American Optical Company publicly reported the results of generating laser using crown optical glass doped with neodymium oxide. On April 27, 1963, Gan et al. generated laser using rod-shaped neodymium glass measuring Φ3 mm×5 cm, which propelled scientific investigations into laser neodymium glass in China. In 1964, the Shanghai Institute of Optics and Fine Mechanics (SIOM) was established, and laser glass research was transferred from Changchun to Shanghai. Laser neodymium glass has been investigated for 60 years at the SIOM. The latter is one of the few facilities in the world that can master the entire technology of laser neodymium glass, from glass composition design to batch preparation. Laser neodymium glass products developed at the SIOM have been widely used in China’s Shenguang series facility, the Shanghai Superintense Ultrafast Laser Facility (SULF), and high-energy lasers for laser peening. Over the recent 60 years, different types of laser glass have been launched in China, and the preparation technology has progressed significantly, thus facilitating the development of laser technology in different periods. The related technologies are being investigated worldwide.ProgressSince its establishment 60 years ago, the SIOM has continuously developed laser glass technology to satisfy the application demands of high-power lasers and large-scale laser-facility constructions. Two types of laser glasses, i.e., silicate neodymium glass and phosphate neodymium glass, have been successively developed. The key issues in preparing laser neodymium glass, i.e., impurity removal, hydroxyl removal, stripe removal, platinum particle removal, and edge wrapping, have been overcome. Self-developed crucible melting and continuous melting technologies for laser glass have been used in large-aperture laser glass manufacturing. Compared with crucible melting, continuous melting offers significantly better manufacturing efficiency and consistency.Conclusions and ProspectsOver the recent 60 years, different types of laser glass have been launched in China, and the associated preparation technology has been developed continuously to satisfy the development demands of laser technology in different periods.

SignificanceLarge-size, high-power laser coatings are key components in inertial confinement fusion (ICF) laser facilities such as Shenguang (SG) series laser facilities in China, National Ignition Facility (NIF) in the USA, and the Laser Megajoule (LMJ) facility in France. The performance of coatings directly affects the beam quality and output power of the laser facilities.The large-size, high-power laser coatings required for large-scale laser facilities mainly include antireflection coatings, high reflection coatings, polarizer coatings, and beam splitter coatings. Typical high-power laser coatings should exhibit specific spectral characteristics to satisfy beam transmission requirements, low stress to achieve excellent wavefront quality, and high laser-induced damage threshold (LIDT) to ensure the safe and stable operation of these high-power laser facilities. Among the available coating deposition technologies, electron-beam evaporation deposition is the most commonly used method to prepare coatings for nanosecond laser applications. It offers the advantages of high LIDT, good thickness uniformity, and easy production of large-size optics.ProgressThis paper briefly introduces the progress of work related to large-size, high-power laser coatings. Furthermore, the main progress of our research group in recent years with respect to key properties, including thickness control, stress control, laser-induced damage mechanism, and methods to improve the LIDT, is presented.In terms of coating thickness control, an improved optical monitoring strategy using multiple pieces of witness glass was proposed. To reduce thickness errors, some thick layers were split into two layers and monitored with different witness glasses. Each witness glass was monitored via a wavelength selected based on the required thickness tolerance. The proposed monitoring strategy is suitable for quarter-wavelength and non-quarter-wavelength multilayer coatings and can obtain spectral performance close to the theoretical value. A model of coating thickness distribution correction was established, and the uniformity of large-size laser coatings was improved based on shutters correction technology (Fig.2).In terms of coating stress control, the influence of the deposition process on coating stress was studied. By optimizing the deposition process, the wavefront quality of the coating was improved and the crack problem of thick coatings was solved. Dense SiO2 layer, prepared via an ion-assisted deposition, could isolate the electron-beam coatings from water vapor in the air. This decreased the compression stress and improved the environmental stability of HfO2/SiO2 multilayer coatings.To improve the LIDT value, the laser-induced damage mechanism was investigated. Experimental results show that under nanosecond laser pulse irradiation, the laser damage is closely related to the electric field distribution inside the coating and different types of defects located at the substrate, coating, and layer interface. Subsequently, our research group focused on coating design, defect suppression, and repair technologies.A “reflectivity and laser resistance in one” design was proposed by using tunable nanolaminate layers that served as an effective layer with a high refractive index and large optical bandgap. Al2O3-HfO2 nanolaminate-based mirror coatings for ultraviolet laser applications were experimentally demonstrated, with simultaneously improved high-reflectivity bandwidth and LIDT (Fig.17).To solve the problem of high defect density due to the alternations of two materials at the coating interface, co-evaporated interfaces (CEIs) were proposed to reduce the interfacial defect density and improve the interface adhesion. Coatings with CEIs exhibit better damage performance with respect to high-power lasers than coatings without CEIs (Fig.19).For laser damage due to nodules and pits, a nodule dome removal strategy (Fig.22) and a pit suturing strategy (Fig.23) were proposed to eliminate the unwanted local electric field enhancement caused by nodules and pits, respectively. Experimental results showed that the proposed strategy can effectively improve the LIDT of laser coatings.Conclusions and ProspectsBased on the aforementioned research, large-size laser coatings, such as high-reflection coatings, polarizer coatings, and harmonic beam splitter coatings, have been successfully produced and effectively used in large laser facilities, including SG series laser facilities and the Shanghai Superintense Ultrafast Laser Facility (SULF).In recent years, deposition technologies, such as atomic layer deposition, glancing angle deposition, and water bath treatment, have also received increasing attention in the research of high-power laser coatings, especially antireflection coatings. These technologies are expected to further enhance the performance of laser coatings in the future.

SignificanceThe main methods of controlled fusion are magnetic and inertial confinement. Between them, inertial-confinement fusion (ICF) uses a higher-power laser beam or higher-energy particle-beam irradiation to focus energy on a deuterium-tritium fuel target pellet with a diameter of only a few millimeters, which rapidly yields a temperature and pressure similar to the core of a star or nuclear explosion, followed by nuclear fusion. Basov. N. G. first proposed the concept of ICF in 1963, and then Chinese nuclear physicist Wang Ganchang proposed laser-driven ICF, which propelled investigations into ICF in China. The emergence and development of high-power lasers have enabled ICF. Currently, the worlds major stakeholders are investing significant resources into developing high-power laser devices, e.g., the worlds largest laser device NOVA developed in the United States in 1985, the construction of a megajoule laser device at the United States National Ignition Facility (NIF), France, and the development of a series laser device by Shenguang, China. Short-wavelength laser beams have greater energy and couple more efficiently to the target pellet compared with long-wavelength laser beams. Therefore, for large ICF devices, high-quality electro-optical and nonlinear optical materials are required to convert the 1064 nm laser output wavelength of neodymium glass to 355 nm. Additionally, the materials must exhibit a large aperture, a high laser-damage threshold, large nonlinear optical and electro-optical coefficients, a wide transmission band, and low refractive-index inhomogeneity. KDP-class crystals are excellent nonlinear optical crystal materials with high resistance to laser damage, wide transmittance bands, high electro-optical coefficients, good optical uniformity, and the ability to grow into large crystals. Therefore, large KDP-class crystals are the only type of crystal that can be used for electro-optical switches and frequency-conversion devices in large-caliber high-power laser-driven devices.ProgressTo address the low efficiency of the conventional rapid growth of point-seed crystals at the cylindrical cone interface, an innovative process for the rapid growth of long-seeded crystals was proposed (Fig. 2). Compared with the conventional growth method, the rapid-growth method reduces the crystal growth time from 3 years to half a year. Additionally, the cutting efficiency is doubled and the column-cone interface is absent. The rapid-growth method revamps the entire process route from batching, point crystal, growth, cutting, to annealing. Additionally, a full set of production equipment and processes required for crystal growth, cutting, rough grinding, and annealing is independently developed, thus providing a solid foundation for the entire chain of DKDP components. Dynamic-light-scattering technology was developed to characterize the particle size of solutions prepared using DKDP crystals, which supports the rapid increase in the probability of the laser-damage threshold and provides technical support for obtaining high-quality DKDP crystals (Fig. 3). This technology involves the independent development of large-caliber DKDP crystal wire-cutting equipment, requires fewer cutting process parameters, and achieves large-caliber crystal cutting flatness (0.2 mm), thus providing technical support for large-sized crystal cutting (Fig. 4). A precision annealing equipment for large-diameter DKDP crystals was independently developed (Fig. 5), and the damage threshold of 1‒2 J/cm2 increased after annealing (Fig. 6). Based on the free-growth technology of long-seeded crystals, 320 mm long-seeded crystals were used to rapidly achieve 520 mm×521 mm×540 mm large-diameter DKDP crystals at 120 d of growth. The internal transparency of the crystal blank was favorable and satisfied the cutting requirements of large-diameter second-type mixing elements and did not present a conical column interface. The successful growth of the crystals validates the rapid-growth technique for long-seeded DKDP crystals. Owing to continuous research efforts, the growth success rate of DKDP crystals has exceeded 80%, and their optical performance has improved continuously, among which the core index of the crystal anti-laser damage ability has increased significantly and the anti-laser damage ability of the fundamental-frequency KDP component has reached 30 J/cm2 (1ω, 3 ns). The zero-probability laser-damage resistance of triple-frequency small-aperture DKDP crystals has reached 18 J/cm2 (3ω, 3 ns), and after subnanosecond pretreatment, it exceeded 20 J/cm2 (3ω, 3 ns), which is the highest level for DKDP crystals with 70% deuteration rate in China.Conclusions and ProspectsTo develop high-power laser drivers, four requirements are to be fulfilled: technology, quality, production capacity, and target cost, Additionally, two goals are to be achieved. First, the technology should be improved continuously and the quantitative and deterministic control of crystal growth should be realized. Second, the development of a full-link device for the growth of DKDP crystals should be promoted; the extremely limited growth, cutting, and annealing capabilities should be addressed; and large-caliber DKDP components should be mass produced.

SignificanceSpaceborne laser remote-sensing technology has the advantages of high vertical resolution, high accuracy, all-day measurements, and the ability to obtain information that cannot be obtained via traditional passive optical or microwave sensors. With the progress of laser and detection technology, spaceborne laser remote sensing technology with different measurement principles has been developed, such as laser altimeters, aerosol backscattering LiDAR, Doppler wind LiDAR, and differential absorption LiDAR. Spaceborne LiDAR is utilized in many fields, such as deep space exploration as well as Earth, land, and atmosphere observation. The main applications of spaceborne LiDAR include three-dimensional elevation measurements of planets and moons, profiles of atmospheric clouds and aerosols, 3D wind speeds of the atmosphere, concentrations of greenhouse gases, and profiles of the ocean subsurface. Spaceborne LiDAR technology plays an important role in surveying, climate and meteorological research, and environmental monitoring.The first Laser ALTimeter (LALT) was launched by Japan in 2007 with the lunar exploration satellite SELenological and ENgineering Explorer (SELENE), and it obtained three-dimensional elevation measurements of the lunar orbit for nearly a year. In 2008, India’s first lunar mission, namely, Chandrayaan-1, was equipped with a lunar laser ranging instrument (LLRI) and operated in orbit for ten months to obtain three-dimensional elevation measurements.In October 2007, China launched its first lunar exploration satellite with the Chang’e-1 Laser Altimeter, which effectively obtained elevation data, including the north and south poles of the moon, for the first time. Moreover, it provided important three-dimensional images of the lunar surface. The Chang’e-2 satellite launched in October 2010 was equipped with the same laser altimeter. The Chang’e-3 lunar mission lander was launched in 2013, and it was equipped with a laser range finder and a laser 3D imaging system to assist the lander in finding the best point on the lunar surface.In the 21st century, NASA launched several spaceborne LiDAR payloads to measure the global vertical profiles of aerosols and clouds as well as polar ice cover and land elevations. The first Geoscience Laser Altimeter System (GLAS) was launched in 2003. In 2006, the first Cloud Aerosol LiDAR with Orthogonal Polarization (CALIOP) was launched. In 2018, the first six-beam Earth measurement laser altimeter (ATLAS) was launched, and it applied single-photon sensing technology to obtain ice, land, and forest elevation measurements. The European Space Agency (ESA) conducted several spaceborne laser radar missions and launched the first wind LiDAR Atmospheric Laser Doppler Instrument (ALADIN) in 2018. The high-spectrum-resolution LiDAR (HSRL) ATmospheric LIDar (ATLID) was launched in 2024 at 355 nm to measure clouds and aerosols.In the field of land surveying and mapping, since 2019, China has launched several LiDAR payloads, including the ZY3-02 satellite laser altimeter, GF-7 laser altimeter, and terrestrial ecosystem carbon monitoring satellite (TECIS) multibeam LiDAR. The GF-7 laser altimeter is China’s first long-life laser altimeter with full-waveform sampling. In the field of atmospheric environmental monitoring, China launched the Atmospheric and Carbon Dioxide Detection LiDAR (ACDL) satellite in 2022 for atmospheric carbon dioxide and aerosol monitoring. The ACDL is the first spaceborne carbon dioxide detection LiDAR and the first aerosol-measured HSRL, worldwide. In 2025, a high-precision greenhouse gas comprehensive monitoring satellite (DQ-2) with carbon dioxide measured optical passive and active optical instruments will be installed on the same platform.ProgressThe first spaceborne laser altimeter using a xenon lamp-pumped mechanical Q-switched ruby laser was launched by NASA in 1971 during the Apollo 15 mission. Such altimeters have also been applied in the Apollo 16 and 17 lunar missions. The Mars Orbiter Laser Altimeter (MOLA) was launched by NASA in 1996, and it has been in operation for five years. All-solid-state lasers were first used to obtain three-dimensional elevations of the Mars surface. Subsequently, NASA launched the Mercury Laser Altimeter (MLA) in 2004 and the first multibeam Lunar Orbiter Laser Altimeter (LOLA) in 2009.Conclusions and ProspectsThe long life and high measurement accuracy of spaceborne LiDAR have been verified in orbit, and spaceborne laser remote-sensing technology has gradually realized commercial operations from space demonstrations and will play an important role in future land surveying and mapping, climate and meteorological research, and environmental monitoring. In the past 20 years, China's spaceborne laser remote sensing technology has developed rapidly and achieved world-leading innovation achievements in several fields, thereby serving major national requirements and making important contributions to space-active optical sensors worldwide. In the future, methane-measured LiDAR, multi-beam LiDAR with more than 100 beams, and dedicated ocean-sounding LiDAR will operate in space.

SignificanceSpace cold atomic clocks, which are high-precision atomic clocks operating in space, have shown great potential for application in navigation positioning, deep space exploration, and fundamental physics research. Since the 1990s, with the development of laser cooling technology, atomic fountain clocks have been realized and have improved the frequency stability and accuracy from the 10-14 level of cesium beam atomic clocks to the 10-16 level. France, the United States, and China have all proposed plans to operate high-precision cold atomic clocks in microgravity environments. With the continuous development of space optical clock technologies, the United States, the European Union, and China have all presented different experimental project proposals based on space optical clocks.ProgressWith the implementation of atomic fountain clocks, French scientists proposed the concept of space cold atomic clocks, abbreviated as PHARAO, which utilize the advantages of microgravity to improve the accuracy of cold atomic clocks. On-ground results indicate that the frequency stability of PHARAO is 3.0×10-13τ-1/2, and the frequency accuracy is 2.3×10-15. The frequency stability is expected to reach 1.1×10-13τ-1/2 when operating under microgravity. According to the latest report, PHARAO will be launched to the International Space Station in 2025. The United States almost simultaneously proposed the space cold atomic clock program with France, the cesium space atomic clock abbreviated as PARCS, and the rubidium space atomic clock abbreviated as RACE.With the support of Chinas manned space program, the Shanghai Institute of Optics and Fine Mechanics (SIOM) began the engineering development of the space cold atomic clock in 2010, which achieved its first international in-orbit operation on the Tiangong-2 Space Lab in 2016. With appropriate parameter settings, an estimated short-term frequency stability of approximately 3.0×10-13τ-1/2 was attained. The demonstration of the long-term operation of cold atom clocks in orbit opens the possibility of applying space-based cold atom sensors. A comparison of the in-orbit and on-ground results indicates that a higher cooling efficiency exists under microgravity, including a smaller loss rate during the trapping and cooling process and a lower ultimate temperature of laser-cooled atoms. The China Space Station provides a better platform for a high-precision time-frequency experimental system. The cold atom microwave clock abbreviated as CAMiCS is one of three atomic clocks in the high-precision time-frequency experimental system of the China Space Station. CAMiCS provides a more stable and accurate frequency signal, evaluates strontium optical lattice clocks, and controls the hydrogen maser over the long term.Currently, the frequency uncertainty and stability of optical clocks in the laboratory have reached the level of 10-18. The European Unions space optical clock (SOC) project was funded by the European Space Agency in 2007, the goal of which was to install and operate an optical lattice clock on the International Space Station, and it is currently in the testing stage of a ground-based prototype. In 2018, Heinrich-Heine-Universität Düsseldorf, the University of Birmingham, and Physikalisch-Technische Bundesanstalt jointly reported the research results of a compact and high-performance 88Sr optical lattice clock as an optical clock for space. A fractional uncertainty of 3×10-17 was achieved. The volume of the device excluding the clock laser was 60 cm×163 cm×99 cm, which was significantly lower than the volume of the laboratory optical clock. Moreover, a stability of 4.1×10-16τ-1/2 was achieved under the condition of a spectral line width of 220 mHz.The Mengtian experimental module of the China Space Station is equipped with a high precision time-frequency experimental system, which includes a space cold atomic optical lattice clock. The National Time Service Center of the Chinese Academy of Sciences conducted research on transportable optical clocks and the miniaturization of optical clocks in 2017 according to the optical clock in the laboratory. The volume of the optical clock vacuum system was reduced to 20 cm×42 cm×90 cm and a frequency stability of 3.6×10-15τ-1/2 and frequency uncertainty of 2.3×10-16 were achieved. In the research on optical clocks for space station applications, the volume of the physical system was further reduced to 15 cm×20 cm×60 cm, which is equivalent to one thirtieth of the volume of the physical laboratory optical clock system. After parameter adjustment, the strontium atomic light clock of the China Space Station will achieve a stability of 1.5×10-15τ-1/2 and an uncertainty of 2×10-17 under the joint efforts of the National Time Service Center of the Chinese Academy of Sciences, the Shanghai Institute of Technical Physics of the Chinese Academy of Sciences, the Hangzhou Institute of Advanced Research of the National University of Science and Technology of China, and the National University of Defense Technology. The Mengtian experimental module was successfully launched in October 2022. The high precision time-frequency experimental system utilizes the combination of an optical lattice clock, active hydrogen maser, and cold atom microwave clock, as well as corresponding comparison links, to perform in-orbit tests and fundamental physics research.Conclusions and ProspectsSince the realization of cold atomic clocks in the 1990s and the subsequent technological development, ground-based cold atomic fountain clocks have been used to realize the definition of the international second and assist in timekeeping. Several space cold atomic clock projects have been realized or are under development. In 2016, the space cold atomic clock developed by SIOM was the first globally to operate in orbit in the Tiangong-2 Space Lab, marking an important milestone in the field of space quantum sensors. The successful operation of space cold atomic clocks in orbit has provided a technical foundation for establishing a high-precision space time-frequency standard, which is of great significance for improving the accuracy and stability of the global satellite navigation system. Cold atomic optical clocks, combined with technologies such as optical frequency combs, have also played an important role in experimentally verifying gravitational redshift and the drift of the fine structure constant over time. Several space optical clock projects are underway, and key technologies are continuously advancing. Despite the significant progress in space cold atomic clock-related technologies, several shortcomings remain, such as the relatively large volume, unsatisfactory reliability, and issues with environmental adaptability. There is still significant room for technical improvement in components with large volume and weight, such as physical vacuum and highly reliable optics, which have poor reliability. The development of these technologies, while applied to space cold atomic clocks, will also promote the wider application of ground-based cold atomic clocks.The development of miniaturized space cold atomic microwave clocks will help improve the performance of the global satellite navigation system. With the continuous improvement in the requirements for accuracy and autonomous operation capabilities of satellite navigation systems, higher requirements have been put forward for the frequency stability and accuracy of on-board atomic clocks. Miniaturized space cold atomic microwave clocks can meet the future needs of on-board atomic clocks, and those carried by deep space craft can also be used for deep space autonomous navigation.Compared with ground-based atomic clocks, space atomic clocks have the advantages of large gravitational potential differences, the ability to significantly modulate the gravitational potential at their location, and being unaffected by ground noise, providing more benefits for such research. The verification of general relativity, the detection of gravitational waves, and the exploration of dark matter are currently the most cutting-edge research directions in physics, and breakthroughs in these areas will very likely lead to the discovery of new physical laws.

SignificanceWith the rapid development of space technology in the 21st century, the frequency of deep space scientific exploration and manned space missions continues to increase. There is an urgent need to establish a high-speed integrated information network. Highly reliable connectivity between space-based satellite networks and ground-based networks is urgently needed. A large number of high-value satellite on-orbit experiment data will be returned to ensure the smooth progress of scientific research missions. Satellite laser communication has the advantages of high speed, good safety, small size, light weight and no electromagnetic spectrum constraint. Satellite laser communication has gradually stepped out of the laboratory to complete the technical verification between satellite and Earth and between satellites. It is expected to solve the bottleneck of data transmission in the current space information network and meet the growing business needs of aviation and aerospace. Institutions from Europe, the United States, and Japan carried out satellite laser communication research earlier. At the beginning of this century, the on-orbit technology verification of point-to-point inter-satellite/satellite-based acquisition, tracking and pointing, and long distance relayless communication modulation and demodulation has been completed. Space communication technology has been gradually moving towards the practical stage. China has achieved rapid development in the field of satellite laser communication technology in the past 20 years. The original verification was completed, the key technical problems were solved, and the technological level of related devices was greatly improved. Under the traction of engineering missions such as HY-2, Mozi and BD-3, various key technologies of satellite laser communication have been verified in orbit. At present, it is developing towards high-speed networking applications.ProgressThis paper focuses on the development status of satellite laser communication in Europe, the United States, Japan and China. By combing through the technical expertise and research priorities of the above regions, we can discover their future development directions. Then the current status and challenges of satellite laser communication technology in target acquisition and tracking, modulation and demodulation, atmospheric turbulence suppression and anti-interference link maintenance are analyzed. Those mainly include the robustness of the mechanical structure of the communication terminal, the adaptability of the platform, the anti-interference, the asymmetry of the upstream and downstream of the satellite and the ground communication, and the low availability in the harsh environment. In view of the above problems, we give a possible solution to the future development according to the current research status of the corresponding technology, so as to provide a certain reference for scientific researchers in related fields, and then promote the development of China’s space field. Finally, according to the needs of space engineering and social development, the development trend of satellite laser communication technology in the future is predicted.Conclusions and ProspectsSatellite laser communication technology has been studied since the 1970s. After several rounds of technological innovation, technological breakthrough of the components, and systematic on-orbit verification, it has become one of the important means of high-speed satellite communication. This paper systematically describes the difficulty existing in the application of satellite laser communication. According to the requirements of space exploration and engineering applications, the development trend of satellite laser communication technology is forecasted. With the coming of the era of high-speed information networking, satellite laser communication technology will play an irreplaceable role.

ObjectiveIn microgravity, atoms can be cooled to very low temperatures, manipulated by a trap with a novel topology structure, and observed over long timescales. This phenomenon has garnered considerable attention, leading to exploration of ultracold atomic physics and its applications in microgravity. Over the past two decades, various state-of-the-art ground-based microgravity facilities and highly reliable ultracold atomic physics experimental systems have been developed to explore the lower temperature limit and applications of cold atoms in microgravity. However, space-based platforms, such as sounding rockets and space stations, have evolved into ideal environments because of their long free-fall time and stable microgravity environment. With the development of the Chinese Space Station (CSS), a Cold Atom Physics Rack (CAPR) that uses an all-optical approach has been deployed to investigate low-temperature and novel physical phenomena in microgravity based on the ultracold quantum degenerate gas of 87Rb Bose‒Einstein condensate (BEC). In addition, the CAPR serves as an open experimental platform for studying ultracold atomic physics and performing precision measurements in microgravity, with the major aim of cooling atoms at the pico-Kelvin scale through two-stage crossed beam cooling (TSCBC).MethodsThe CAPR needs to satisfy the restrictions on its size, weight, and power consumption. In addition, it needs to withstand the vibrations and impact during its launch as well as operate well after the launch. A highly reliable and integrated CAPR that integrated all the hardware for preparing, manipulating, and probing the 87Rb BEC was designed. The designed CAPR included a physical system, a cooling laser system, an optical trap and lattice laser system, an electronic control unit, and a rack supporting system with dimensions of 1820 mm×1050 mm×815 mm. The dimensions and mass of the assembled physical system were approximately 590 mm×930 mm×510 mm and 170 kg, respectively. This system could provide a high-vacuum, optical, and magnetic environment for ultracold atoms. The cooling laser system consisted of a repumping laser, cooling laser, and probing laser, which provided three high-power outputs for cyclic cooling of 87Rb atoms to temperatures of tens of microkelvins as well as for detecting the atoms. The optical trap and lattice laser system provided eight high-power outputs for evaporative cooling to attain the BEC, deep cooling via TSCBC, and manipulation of the ultracold atoms in the optical lattice. The electronic control unit controlled the experimental sequences as well as stored the experimental results and engineering parameters. The sizes and weights of the laser cooling system, optical trap and lattice laser system, and electronic control unit were similar (550 mm×470 mm×270 mm and less than 50 kg, respectively). To achieve the mission target, BEC and TSCBC tests were conducted on the ground before the launch. The realization of the 87Rb BEC and the TSCBC were crucial and confirmed that the output of all the subsystems fulfilled the experimental requirements for the preparation, regulation, and detection of ultracold atoms.Results and DiscussionsThe vacuum apparatus is the main part of the physical system and includes a two-dimensional magneto-optical trap (2D-MOT) chamber and science chamber for atomic cooling, manipulation, and probing. In addition, all the magnetic coils and optical modules, which provide the required magnetic and optical fields for the ultracold atoms, are fixed on the vacuum chambers. In the laser cooling system, the powers of the repumping, cooling, and probing lasers are 200, 600, 800 mW, respectively. The repumping laser is locked to the 87Rb D2 |52S1/2, F=1〉→|52P3/2, F’=0,1〉 crossover transition via modulation transfer spectroscopy (MTS), which is 193 MHz red-detuned from the repumping transition. The frequencies of the cooling and probing lasers are red-detuned by a few natural linewidths (Γ=2π×6.065(9) MHz, which is the natural linewidth of the 87Rb D2 line) from the 87Rb D2 |52S1/2, F=2〉→|52P3/2, F’=3〉 transition. The MOT loading process takes 10 s and more than 1.5×109 atoms can be trapped with a temperature below 500 μK. Furthermore, the atoms can be cooled to a temperature below 30 μK using optical molasses, demonstrating the performance of the 780 nm cooling laser system. As to the optical trap and lattice laser system, the capability of the tight-confining laser is confirmed by loading more than 1.2×106 atoms and successfully cooling more than 1×105 atoms via evaporative cooling to the BEC at a temperature below 30 nK. The performance of the loose-confining laser is verified by deeply cooling the ultracold atoms to 2.4 nK via TSCBC. Additionally, the CAPR performs well in space environmental qualification certification tests.ConclusionsThe CAPR flight model (FM) was installed in the Mengtian laboratory module, which was launched into the CSS on October 31, 2022. The CAPR investigates low-temperature and novel physical phenomena in microgravity based on the quantum degenerate gas of 87Rb BEC. Here, we report the design of the integrated CAPR, which includes a physical system, a cooling laser system, an optical trap and lattice laser system, an electronic control unit, and a rack supporting system. Ground based experiments have been conducted to confirm the ability of the CAPR to realize the 87Rb BEC and lower its temperature from 30 nK to 2.4 nK with the TSCBC.

SignificanceInformation is the basic element of human cognition of the world. Notably, 83% of human access to information relies on vision; hence, imaging technology based on image information acquisition is important for national defense and improving people’s lives. Traditional imaging processes entail a geometric-optical transformation relationship from the target scene to the imaging sensor, facilitating accurate recording of object information. As the imaging technology has evolved, diverse imaging equipment has overcome the limitations of human vision, enabling us to perceive finer, more distant, and broader phenomena. For instance, microscopes allow us to observe the mysterious microworld and astronomical telescopes grant us a glimpse into the expansive cosmos spanning billions of light-years. Moreover, infrared thermal imaging cameras overcome the darkness by enhancing our vision in low-light conditions.However, regardless of the advancement of imaging equipment, when a scattering medium exists between the target scene and imaging apparatus, issues arise due to scattering and absorption. On the one hand, normal imaging light gets blocked, leading to signal energy loss. On the other hand, some of the signal light from the target scene gets disturbed, causing deviation in the original geometric imaging model of point-to-point imaging. In addition, scattered ambient light (referred to as airlight) enters the imaging detector. This light is not encoded by the target scene, does not carry any target information, and is additive noise.Research on scattering imaging has been conducted since the 1950s, with China joining the international efforts around 2000. Since then, advancements in hardware and algorithm development have led to the application of numerous new technologies. However, improving detection depth, expanding the field of view, increasing the imaging speed, and improving the recovery quality remain crucial scientific challenges. Scattering imaging is a crucial technique for image acquisition in complex scenes.Over the past decades, many methods have been proposed to address this practical problem. These methods can generally be categorized as active or passive based on whether or not active illumination is required. Among active methods, the most straightforward and useful method is to select the light that has been scattered the least by gating, wavefront compensation, and pointwise scanning and use it in florescence microscopy. These methods have broad applications across various fields. However, the imaging distance/depth achievable by these methods in scattering media is limited due to the attenuation of ballistic light. To further improve the imaging depth, active methods such as optical phase conjugation, wavefront shaping, optical transmission matrix measurement, speckle correlations based on optical memory effects, and deep learning have been proposed to exploit scattered light for image formation.Passive methods do not rely on active illumination. In particular, scattering particles absorb and scatter light from the object of interest and generate a substantial amount of airlight by directly scattering light from the illumination source, such as the sun. The presence of airlight considerably reduces the contrast of captured images, resulting in poor visibility. Conventionally, image dehazing algorithms are applied to enhance contrast. These algorithms can be broadly categorized into two groups. The first group includes image restoration algorithms based on physical models, such as polarization models, image depth priors, and dark channel priors. The second group comprises image enhancement algorithms that do not depend on physical principles; typical examples include Retinex-based algorithms, wavelet transform, and data-driven deep learning.ProgressThe research progress of various scattering imaging techniques is summarized in Table 1. These techniques are classified into two imaging modes, namely active and passive, based on whether active light illumination is required or not. This classification stems from the different noise models that contribute to image degradation. In the active mode, most scattered light carries disturbed object information or scattering channel information, representing invertible multiplicative noise. Consequently, it can be used to extract ballistic light or utilize scattered light for imaging. Conversely, in the passive mode, scattered light primarily originates from airlight randomly scattered by the atmosphere, constituting additive noise without object signals. Therefore, most current research focuses on dehazing algorithms for extracting ballistic light. Based on different modes and utilization of scattered light for imaging, the summary proceeds as follows: first, techniques for extracting ballistic light in the active mode (Fig. 1) are summarized as simple and effective methods with a high degree of required engineering (Figs. 2 and 5). Next, the summary covers imaging techniques using scattered light in the active mode and non-line-of-sight imaging (Fig. 10). Subsequently, the summary addresses imaging techniques used for extracting ballistic light in the passive mode (Fig. 13). Finally, examples of relevant applications of deep learning in both the modes are presented (Figs. 18 and 19).Conclusions and ProspectsMost of the scattered light in the active mode constitutes invertible multiplicative noise, enabling its utilization for extracting ballistic light or achieving imaging using scattered light. Conversely, scattered light in the passive mode primarily consists of additive noise, necessitating reliance on dehazing algorithms for extracting ballistic light. The application of deep learning as a new technology has demonstrated numerous examples of its effectiveness in both the modes. Looking ahead, advancements in optoelectronic devices and algorithmic arithmetic are anticipated to enable improved integration of the front-end and back-end in scattering imaging research. This integration is expected to generate innovative ideas, leading to the further improvement of imaging depth and recovery quality.

SignificanceWith approximately 71% of the earth’s surface covered by seawater, the marine environment is closely linked to global climate change, the evolution of ecosystems, and the global carbon cycle, among other activities. In recent years, in particular, the increasingly close connection between oceans and human activities, along with the significant impact of human activities on the global marine environment, has sparked considerable interest in understanding the oceans. This interest has led to the development of a wide range of methods and equipment for exploring the oceans.Mainstream programs for ocean exploration research primarily include acoustic and optical programs. Acoustic solutions, such as multibeam sounders, single-beam sounders, and side-scan sonars, are commonly used in ocean exploration. However, because of the substantial difference in viscosity coefficients between water and air, the acoustic impedance values near the air-sea interface vary significantly. Moreover, when an acoustic signal propagates between air and water, it undergoes significant reflection, thereby affecting its propagation in both directions. This limits the application of acoustic oceanographic equipment to submerging below the surface of water and using shipboard platforms. Coastal zones and shallow water areas of sea islands and reefs are the areas where the interaction between sea and land is frequent, and also the areas where human beings participate most in marine activities. Because of the shallow water depth and complex underwater topography in these areas, traditional shipborne multibeam and single-beam sonar surveys are characterized by operational hazards, low efficiency, and high collection costs, resulting in numerous gaps in large-scale mapping data in the shallow waters of China’s coastal zones and sea islands and reefs.Compared to passive remote sensing, airborne LiDAR bathymetric technology, as an optical means of active detection, can function continuously and acquire ocean profile information. This technology uses the good seawater penetration characteristics of the blue-green band laser. It transmits the laser from an airborne platform and receives the laser echo from both the sea surface and seabed, thus realizing shallow-water measurement. Simultaneously, it can also measure the elevation of the land surface. After approximately 60 years of development and application, airborne LiDAR bathymetric technology has demonstrated advantages including high efficiency, high accuracy, high mobility, and low cost in near-shore shallow-water areas with suitable water quality conditions. This technology has become the preferred means of integrated surveying and mapping of land and sea in these regions. With the advancements in laser and unmanned aerial vehicle (UAV) technologies, existing research results should be summarized and the future development of airborne LiDAR bathymetric systems should be discussed.ProgressFirst, the principle of airborne LiDAR bathymetric system is introduced, and studies on system composition and data processing methods are discussed. Subsequently, key technological developments such as the blue-green laser source, large dynamic range optical reception, and high-speed data acquisition are analyzed. The parameters of typical airborne laser bathymetric systems, such as HawkEye-5, CZMIL, and Mapper10K, are compared (Tables 1 and 2). Thereafter, new technological developments in airborne laser bathymetric systems, including photon counting laser bathymetric systems and multi-wavelength laser ocean sounding systems, are introduced. The photon-counting laser bathymetric system developed by the Shanghai Institute of Technical Physics (SITP) was tested in Qiandao lake, Zhejiang province, achieving a 3.1 m water depth measurement (approximately two times the depth of the transparency disk) (Fig. 24). The Shanghai Institute of Optics and Fine Mechanics (SIOM) developed an underwater multi-wavelength ocean detection LiDAR and conducted experiments on the spectral detection of different ores underwater. They also investigated classification algorithms in the laboratory (Fig. 31). Typical applications of airborne LiDAR bathymetric systems are then presented, encompassing four aspects: integrated topographic mapping of land and sea, remote sensing observation of ocean waves, three-dimensional remote sensing observation of optical parameters of seawater, and substrate classification.Conclusions and ProspectsAirborne LiDAR bathymetric technology and its applications have matured, and existing systems can satisfy the demand for integrated large-scale surveying and mapping of land and sea. The future development trend will be towards system miniaturization and unmanned platforms. Technologies that can combine photon counting and waveform sampling, high-efficiency broad-spectrum laser technologies in the blue and green bands, the technology that can generate data by combining simulation and actual measurement, and processing methods handling considerable amounts of data should be developed.

ObjectiveOcean carbon sequestration accounts for one-third of total plant carbon sequestration of the world, and particulate organic carbon (POC) is a major form of ocean-based carbon. Accurate detection of POC content can help China achieve the goal of carbon peak and neutrality. Light detection and ranging (LiDAR) active detection is the only known technology that can directly penetrate ocean bodies to realize the detection of marine euphotic zones. When the characteristic parameters of the phase, frequency, amplitude, and polarization of the optical signal are analyzed and the characteristics of the detected target are inverted, the vertical distribution of ocean POC over a long range can be obtained. A 532 nm frequency-doubled blue-green wavelength is typically used in ocean LiDAR systems, as lower attenuation is found at 532 nm in coastal waters. Additional research shows that if the laser wavelength of the LiDAR operates near the Fraunhofer dark lines of the solar spectrum, such as 518.36, 486.13, and 434.05 nm, the interference of solar background noise on the laser ocean detection system can be effectively reduced. It can also improve the signal-to-noise ratio and extend the working period of the LiDAR system. This study proposes a high-repetition-frequency three-wavelength laser system that provides a new technical route for the development of a LiDAR light source for POC hyperspectral detection in oceans.MethodsA multi-wavelength laser system for POC hyperspectral detection LiDAR with a high repetition rate is developed using a frequency-stabilized seed laser (Fig.1) combined with fiber-bulk hybrid amplification and nonlinear frequency conversion technologies (Fig.2). A 1064-nm distributed feedback (DFB) semiconductor laser with a linewidth of ~1 MHz is used as the single-frequency continuous-wave (CW) seeder, and an iodine molecular absorption pool is designed to control the frequency stability. The seeder laser is chopped into nanosecond pulse trains with a repetition rate of 5 kHz using an acousto-optic modulator following continuous fiber amplification; it is then amplified by a double-clad fiber amplifier. The output is collimated and coupled into a solid-state amplification system for further pulse energy scaling. The solid-state amplification system comprises a two-stage Nd∶YVO4 crystal dual-pass preamplifier and two-stage Nd∶YVO4 crystal main amplifier. The crystals at each level are end-pumped using a fiber-coupled laser diode (LD). The amplified laser is incident on the nonlinear frequency-conversion module after turning mirror. Type-I LiB3O5 (LBO) crystals with a size of 4 mm×4 mm×20 mm and phase matching cut angles of θ=90° and φ=11° are used as second-harmonic-generation crystals. Type-II LBO crystals with a size of 3 mm×3 mm×10 mm and phase matching cut angles of θ=42.7° and φ=90° are used as sum-frequency-generation crystals. Finally, two type-I BBO crystals with a size of 5 mm×5 mm×20 mm and phase matching cut angles of θ=29.6°and φ=90° are used as parametric crystals to produce a 486-nm blue laser.Results and DiscussionsWith a laser wavemeter, the center wavelength of the DFB semiconductor laser is measured to be 1064.49061 nm. After being chopped by an acousto-optic modulator (AOM) and amplified by a fiber pulse preamplifier, the single-pulse energy is approximately 1 μJ. The pulse width is approximately 25.4 ns and the repetition frequency is 5 kHz. When the pump energy of the entire system is approximately 29.3 mJ, the single-pulse energy of the amplified output laser is approximately 6.8 mJ, and the total extraction efficiency reaches 23.2%. At the maximum output energy, the measured amplified laser pulse width is approximately 17 ns (Fig.3). The measured diameter of the near-field spot is approximately 1.2 mm and 1.9 mm in the x and y directions, respectively, which correspond to divergence angles of 2.2 mrad and 2.9 mrad, respectively (Fig.4). The beam quality factors are Mx2=1.03 and My2=1.15. Following frequency doubling by the first LBO crystal, a green laser with a single-pulse energy of 3 mJ is generated. The center wavelength of the green laser is measured to be 532.2448 nm with a spectral linewidth of less than 400 fm, which is less than the resolution limit of the wavemeter. The frequency jitter of the green laser is less than 20 MHz within 30 min (Fig.7). With the type-II phase-matching LBO, a 355-nm ultraviolet pulse laser output with a single-pulse energy of 2.6 mJ is obtained. The remaining 532-nm green laser pulse energy is approximately 0.53 mJ. The ultraviolet pulse laser pumped optical parametric oscillator (OPO) crystal finally achieves a blue laser output with a single-pulse energy greater than 0.7 mJ, corresponding to a conversion efficiency of 26.9%. The laser center wavelength is ~486.1 nm at a linewidth of ~0.16 nm (Fig.8). Table 1 presents the parameters of the successfully developed high-repetition-rate three-wavelength laser.ConclusionsA multi-wavelength, high-repetition-rate laser based on a fiber-bulk hybrid cascaded amplifier is experimentally investigated as a laser source for POC detection LiDAR. An iodine molecular absorption pool is used to control the frequency stability of the DFB laser. An AOM is used to chop the CW output of the DFB laser into nanosecond pulse trains with a repetition rate of 5 kHz. Fundamental frequency laser output at 1064 nm with a single-pulse energy of approximately 6.8 mJ and pulse width of approximately 17 ns is obtained through fiber solid-state hybrid amplification. Following third-harmonic generation and OPO using nonlinear crystals, a three-wavelength laser beam output is obtained. The corresponding single-pulse energies are 2.24 mJ@1064 nm, 0.53 mJ@532 nm, and 0.7 mJ@486 nm. The center wavelength of the green laser is measured to be 532.2448 nm with a spectral linewidth of less than 400 fm. The results of this study provide a new technical route for the high-spectral-resolution detection of POC using LiDAR systems in seawater, with the advantages of a high repetition rate and narrow linewidth.

SignificanceThe global navigation satellite system (GNSS) has been widely used in the modern information society. But it still faces some problems like ground dependence and accuracy improvement. The advantages of optical technology such as optical frequency reference and free space laser link would help to solve the problems. The United States, Germany and China have respectively proposed concepts such as space-time reference based on optical link (O-STR), the third generation global navigation satellite system Kepler, and new generation global navigation and positioning timing system based on optical technology O-GNSS. This paper summarizes the main research progress of their architecture, prototype, and key technologies, providing a reference for promoting the development of space-time reference and navigation positioning timing technology.ProgressThe core idea of O-GNSS which was proposed by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (SIOM, CAS) is to make full use of the outstanding advantages of optical frequency reference and optical inter-satellite link to realize space self-maintenance of the space-time reference. It has an important foundation in the overall design, optical frequency reference, optical frequency comb, space laser link, fiber time-frequency synchronization and many other key technologies. The designed prototype system is based on the Beidou constellation, including inclined geosynchronous orbit (IGSO) and geostationary orbit (GEO) satellites, and integrating low earth orbit (LEO) communication constellation. The GEO/IGSO satellite will be equipped with a cavity stabilized frequency reference, iodine/rubidium long-term stabilized light clock, laser communication-range synchronization integrated terminal (LCT), and space reference interferometry terminal. The medium earth orbit (MEO) satellite is equipped with a cavity stabilized laser, optical frequency comb and LCT. Global synchronization with picosecond level accuracy, absolute ranging with sub-millimeter level precision and communication with Gbit/s level speed among satellites will be achieved through laser links. And the optical frequency and time reference will be transferred to the microwave band through optical frequency combs, and broadcast to the Earth. LEO satellite is equipped with signal monitoring terminal to realize continuous monitoring without atmosphere and relay enhancement of MEO navigation signals. The two ground stations act as backup for each other, and they are equipped with high-performance optical clocks, satellite-to-earth LCT, microwave communication measurement and control terminals, to achieve the alignment between self-maintained constellation and earth rotation, as well as system monitoring and ground maintenance. In addition, based on the ground fiber network, the ground fiber time-frequency synchronization network is constructed, which can cooperate with the constellation to provide high-precision continuous time-frequency and phase information to users with high-performance needs.In terms of key technologies of optical frequency reference, the output frequency noise of Michelson interferometer (MI) cavity stabilized laser based on optical fiber delay line is better than 6 Hz/Hz at 10 mHz, the linewidth is 0.32 Hz, and the frequency stability is 3.2×10-15 at 1 s and 1.1×10-14 at 1000 s. The key technology of optical frequency comb is based on a figure-9 cavity to achieve 0.9 GHz high-frequency output, and based on Brillouin amplification scheme to achieve 1 W single comb tooth amplification. In terms of key technologies of space laser link, it simultaneously achieves 1 Gbit/s communication rate and 5 mm ranging accuracy at a working distance of 40000 km. In terms of the key technologies of ground fiber time-frequency synchronization network, the transmission stability of optical frequency and radio frequency can reach 3.5×10-20 at 1000 s and 5×10-19 at 10000 s, respectively. The synchronization jitter peak of 1 pulse per second time signal can reach 3.3 ps at 10000 s.Conclusions and ProspectsThe new generation of GNSS based on optical technology has a good key technical foundation and feasibility, and can give full play to the advantages of optical technology. It is an important option for the further development of GNSS, which would help to overcome the problems of the current system, such as over-reliance on the ground system and others. And it can realize many advanced functions, such as the space-time reference independently constructed and maintained on the constellation, positioning, navigation, and timing (PNT) with higher precision, integrated communication and navigation, and space-earth integration network for backup and performance enhancement.

SignificanceTime measurement plays an irreplaceable role in modern society, not only in frontier basic research and high-tech fields but also in positioning and navigation, electricity, finance, geodesy, and other fields where it is the core of the technical foundation. The basis of time measurement—the time unit second is defined by atomic time and generated by atomic frequency standards. In recent years, atomic frequency standards have undergone rapid development, and their performance indicators have continuously improved, driving great progress in related fields of precision measurement. Typical examples include the global satellite navigation system based on time measurement achieving sub-meter level positioning accuracy, the time unit second becoming the most accurate and fundamental standard unit among the seven basic units of the International System of Units, and playing an important role in the quantum transformation of the International System of Units. These advances have directly affected all aspects of life.Atomic frequency standards have continuously improved the uncertainty by one order of magnitude every 20 years since they were established in the 1950s. The earliest highest standard was the cesium atomic beam, which had a level of 10-14. In the 1980s, laser cooling enabled atomic frequency standards to achieve a technological leap, giving birth to cold atom frequency standards represented by atomic fountains. This technology effectively reduced the atomic loss caused by thermal motion, and parabolic motion significantly increased the coherence time. Combined with high-sensitivity optical detection and other technologies, the uncertainty of fountain frequency standards was improved to the level of 10-16. At the end of the last century, optical frequency standards emerged, and by raising the working spectral lines from the microwave band to the optical band, frequency measurement achieved another technological leap. The uncertainty of frequency standards, which decreased by two orders of magnitude, reached 10-18. However, time-frequency signals are primarily based on microwaves, and microwave frequency standards are more mature. The time measurement in the world is mainly undertaken by microwave frequency standards, among which atomic fountain is the highest standard for reproducing second and it plays an important role in generating time scales of various countries and establishing international atomic time.With the development of science and technology, time-frequency measurements extend from the ground to outer space, and the space microgravity environment is conducive to achieving frequency standards with narrower spectral lines and better performance. Space cold atom frequency standards have become a current research hotspot, which can not only calibrate the time-frequency signals of navigation satellites in all weather but also greatly help to improve the accuracy of the global space-time system and provide a unique platform for basic physical experiments, which can play an important role in the research of relativity verification, basic constant measurement, etc.In the past few years, with the emergence of the space clock-time system, time reference has not been generated only by the ground laboratory, which will build up an accurate and sensitive space-time system, making time and space more closely integrated. Therefore, it is necessary to summarize the existing research to guide the future development of time and frequency fields more reasonably.ProgressA frequency standard is a device that outputs a standard frequency by locking a microwave or optical frequency signal to the resonance frequency of the atomic (molecular, ion) transition energy level and realizes quantum interrogation by Rabi oscillation or Ramsey effect. The interrogation spectral linewidth is the most important indicator of the frequency standard. Cold atom frequency standards can effectively reduce the influence of thermal motion and often yield excellent performance indicators. Fountain frequency standards are typical cold atom frequency standards. They use cold atoms to interact with microwaves twice in the parabolic motion of throwing up and falling down through the microwave cavity and realize Ramsey interference. Fountain frequency standards can achieve a 1 Hz spectral line, currently the best-performing microwave frequency standard. The short-term stability evaluated by Allan deviation is 1×10-13τ-1/2, and the long-term stability is 10-17. The type-B uncertainty of the fountain frequency standards was obtained by evaluating the physical effects that affect the frequency, which was 10-16. Advanced time-frequency laboratories worldwide are developing fountain frequency standards for frequency traceability and time-frequency signal generation. The stability of fountain frequency standards is often limited by the microwave oscillator, which can be overcome by upgrading the oscillator. A photonic microwave generator is a commonly used scheme (Fig. 6) that transfers the excellent performance of an ultra-stable laser to the microwave band through an optical frequency comb and can achieve 10-15 or even better second stability. Using a photonic microwave generator as the local oscillator, the stability of the fountain clock can reach the quantum projection noise limit. To meet the requirements of space-time frequency measurement, cold atom microwave frequency standards extend from the ground to space. In 2016, the Shanghai Institute of Optics and Mechanics (Chinese Academy of Sciences) realized the worlds first cold atom clock in orbit. The improved in-situ measurement cold atom clock (Fig.11) was launched into space and will play an important role in integrated space-ground time-frequency systems in the future.Conclusions and ProspectsIn recent years, cold atom microwave frequency standards have rapidly developed, and these frequency standards have played an important role in the generation and unification of time-frequency signals in various countries worldwide. The Shanghai Institute of Optics and Mechanics (Chinese Academy of Sciences) has conducted systematic research in related fields, such as fountain frequency standards and optically generated microwaves, and realizes the worlds first space cold atom clock in orbit. The related technical indicators have reached or approached the advanced international level. In the future, cold atom microwave frequency standards will play a more important role in many fields related to the national economy, peoples livelihoods, and national defense security, especially in measurement, geodesy, basic research, positioning, navigation timing, and other fields.

SignificanceUltrastable space laser with an ultranarrow linewidth, ultralow noise, and high frequency-stability is widely used in important space programs such as space gravitational wave (GW) detection, space cold-atom physical experiments, and space laser remote sensing. For example, in space GW detection applications such as the Laser Interferometry Space Antenna (LISA), Taiji, Tianqin, and DECi-Hertz Interferometer Gravitational-wave Observatory (DECIGO), to obtain extremely weak GW signals from the several-million, long-distance, free-floating, test-mass-based optical interferometer, extremely stringent requirements must be imposed on the ultrastable laser specifications. The linewidth should be below 1 Hz, with the frequency stability below 10-15 and the frequency noise below 30 Hz/Hz. Additionally, because the abovementioned interferometer is to be used in space, the laser must withstand the harsh space environment such as vacuum, vibrationsrough, temperatureextreme, and radiationshigh-dose. Furthermore, the laser should feature a small volume, low power consumption, and a low weight to reduce the flight budget. Because of these stringent requirements on ultrastable space laser, researchers worldwide are focusing on its technological development.In 2003, the Shanghai Institute of Optics and Fine Mechanics (SIOM) of Chinese Academy of Sciences (CAS) began developing space-grade laser. In 2007, China’s first space-grade laser, i.e., the ChangE-1 laser, which is a pulsed high-energy solid-state laser, was successfully launched. In 2008, SIOM began developing ultrastable, ultralow-noise, and high-stability continuous space wave lasers. In 2016, China’s first space-grade ultrastable long-life laser was successfully launched simultaneously with the FY-4 weather satellite, which has been operated for 7 years and counting. This paper summarizes the technical operating principal, test results, and onboard performance of these ultrastable lasers.The DQ1 satellite is an integrated path-differential absorption (IPDA) LiDAR system used to measure the global CO2 concentration both day and night for a global-warming project. It demands extremely stringent requirements on the frequency stability of the “on” and “off” laser source, i.e., 10-10 with 8 years lifetime. A 1572 nm DFB diode reference laser is frequency locked to the center absorption peak of CO2 at approximately 1572.018 nm via high-frequency modulation. Meanwhile, the “on” and “off” diode lasers are frequency locked to the reference laser using the optical phase-lock loop (OPLL) technique separately at frequency separations of 760 MHz and 8.08 GHz, respectively. Because of the low absorption strength of CO2 at approximately 1572 nm, a multipass cell was successfully constructed, which featured an absorption length of 10 m, a hermicity of 5×10-10 Pa·m3·s-1, and weight of 5.3 kg. The final frequency stability is approximately 1×10-11@10000 s. The DQ1 laser was successfully launched in April 6, 2022 and has been operating favorably hitherto.The Cold-Atom Physics Rack (CAPR) is a science platform that has been successfully launched and deployed on the China’s Space Station on October 31, 2022. Researchers are planning to use rubidium and potassium in the CAPR to achieve the Bose Einstein condensation (BEC) and Fermi degeneration on the space station. The CAPR laser module provides a cooling, repumping, and detection laser with 10-11 frequency stability and agile frequency tenability for experiments. Three 1560 nm DFB diode lasers are amplified separately using a master oscillator power-amplifier (MOPA) and their separated frequencies are doubled using three PPLN crystals, which are used as the cooling, repumping, and detection laser with 900 mW output power. The frequency of the repumping laser is locked to the saturated absorption peaks of rubidium and potassium based on the modulation transfer spectrum (MTS). The cooling and detection lasers are locked to the repumping laser via the OPLL technique. The final frequency stability is approximately 2×10-12@1000 s.The Taiji program is a space GW-detection program proposed by CAS. Three phases has been proposed and included in the roadmap. The first is the Taiji-1 mission, which is a single satellite for testing key techniques for GW detection such as ultrastable lasers and interferometers. The key component of the Taiji-1 laser source is a nonplanar ring oscillator (NPRO) solid laser. The frequency and power noises of the laser source are improved significantly by applying precision-driving current control and temperature control. Frequency noise measuring 0.1 MHz/Hz@0.1 Hz and power noise measuring 0.02%@0.1 Hz are obtained. In order to improve the frequency stability of the laser to meet the requirement for the ongoing Taiji-2 mission, a ultra stable fiber based on frequency stabilization technique is proposed and demonstrated successfully with 6 Hz/Hz@10 mHz frequency noise,3×10-15@1 s,5×10-15@100 s frequency stability which is enough for the Taiji-2 mission.ProgressThe FY-4 satellite is a weather-forecast satellite that uses a Fourier-transform infrared spectrometer (FTIR) to measure the Earth’s spectrum. To calibrate the frequency of the spectrometer, a laser with a stability level of 2×10-6 and 5 years lifetime is required. To fulfill these requirements, two 852 nm DFB diode lasers with cold redundancy are used as the laser source. A caesium cell is used as the 852 nm frequency reference. Low-frequency dithering is applied to the current of the diode laser. Using the 852 nm frequency-absorption peak of caesium, the frequency of the laser is locked to the center of the peak and stabilized. After 7 years of onboard operation, the laser is still operating favorably, with less than 1% power degradation.Conclusions and ProspectsA series of ultranarrow linewidth, low-noise, and high frequency-stability ultrastable space lasers has been successively constructed at SIOM and used in important applications such as space GW detection, the FY weather satellite, and the Chinese Space Station. The onboard-laser frequency stability improved significantly from 10-7 to 10-12, as well as to 10-15 with a linewidth of less than 1 Hz. The frequency stability of ultrastable space lasers shall be improved continuously to satisfy the major requirements of space programs.

SignificanceThe high-power fiber laser technology is currently one of the rapidly advancing laser technologies, primarily due to its outstanding performance in various fields, such as communication, research, industry, and defense.ProgressBased on the development trend of the laser technology, the Shanghai Institute of Optics and Fine Mechanics (SIOM) took the lead in researching the fiber laser technology in China and decided that two key technologies are the high-power narrow line-width fiber laser technology and high-power fiber laser beam combining technology. Based on the optical fiber-based laser properties, the narrow-line-width fiber laser output power must be increased to suppress different nonlinear effects. This increase depends on the single frequency seed spectrum modulation combined with the multistage power amplifier technology, which can maintain the spectrum width and realize beam quality during the single-fiber laser power amplification process. For high-power fiber laser beam combining, SIOM first proposed and studied the all-fiber ring cavity coherent beam combining technology and spectral beam combining technology in China. This paper reviews the progress in recent years. SIOM has overcome a series of technical difficulties and achieved a series of achievements, which has driven the promotion and development of these technologies.Conclusions and ProspectsIn the future, the narrow-line-width fiber laser technology can be used as an effective light source for the development of fiber lasers and beam combining technology. The main problem of nonlinear width fiber laser continues to be nonlinear effects: SBS and TMI. However, these may be improved via three aspects: using the new optical fiber material technology, new spectral modulation technology, and polarized high-power narrow-line-width fiber laser technology. The future demand for higher power and excellent beam quality is expected to be the comprehensive application of different beam combining technologies, complementing each other. Of course, suppressing the nonlinear effect of the laser, developing high-performance devices, and combining cavity structure optimization and beam quality maintenance technology are the basis of the development of the high-energy fiber laser technology. At present, the narrow-line-width fiber laser combined with the laser beam combining technology is the preferred solution of the high-energy light source.

SignificanceLaser technology is an important tool for basic physics research, cutting-edge scientific and technological breakthroughs in multiple fields. The gain media of solid-state lasers include crystals, glass, and ceramics. Activated ions include transition metal ions and rare earth ions, etc. The matrix material must have good optical, mechanical, and thermal properties. The main characteristics of crystals are high thermal conductivity and anisotropy. The advantages of glass include convenient preparation and easy access to large-sized components, which can also be made into fibers. Ceramics have the characteristics of high thermal conductivity and easy realization of large dimensions, but are limited to cubic materials. Laser diode (LD) pumped solid-state lasers based on solid-state gain media have advantages such as compact structure, high efficiency, and long service life.Laser crystals have milestone significance for the development of laser technology. The first laser based on ruby laser crystals was introduced in 1960s. In 1970s, neodymium doped yttrium aluminum garnet (Nd∶YAG) crystals promoted the vigorous development of solid-state lasers. In 1980s, Ti∶sapphire (Ti∶Al2O3) led to the rapid development of femtosecond laser technology. In 1990s, neodymium doped yttrium vanadate (Nd∶YVO4) crystals further advanced the development of solid-state laser technology. In the 21st century, applications such as national defense, cutting-edge science, and optoelectronics have put forward new requirements for solid-state lasers. Nd∶YAG, Nd∶YVO4, and Ti∶sapphire are three main laser crystals. They can meet the needs of most solid-state lasers. However, they still have shortcomings in some special application areas. So it is necessary to explore new laser crystals.ProgressThis paper introduces the main research progress of laser crystals in different wavelength bands at home and abroad, and focuses on the main research achievements of laser crystals in Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences, in recent years. The wavelength bands involve visible, near-infrared, and mid-infrared. The matrix crystals involved include oxides, fluorides, Ⅱ‑Ⅵ compounds, etc. In addition, research progress on the growth of polycrystalline diamonds on laser crystals is also introduced.Visible solid-state lasers based on laser crystals are compact and light. Lasers from deep red to blue have been reported. Especially, there were a lot of reports on Pr3+ doped laser crystals, and continuous wave lasers around 490 nm have been achieved. Ti∶sapphire is the main gain crystal for ultrafast lasers. Superintense ultrafast lasers with peak power ranging from several hundred terawatts to ten petawatts require high-quality and large-sized Ti∶sapphire crystal. The origin of defect related optical absorption in Ti∶sapphire and the growth of large-sized high-quality crystals are two important issues that urgently need to be addressed. In recent years, we analyzed the mechanism of defect related optical absorption in Ti∶sapphire theoretically, and grew large-sized high-quality crystals through heat exchange method. The main activating ions for 1 μm laser crystals are Nd3+ and Yb3+. Nd∶YAG is the most widely used laser crystal. In recent years, we explored several new Nd3+ doped fluoride and oxide laser crystals, and solved the emission cross section problem of Nd∶Lu3Al5O12. SIOM reported a new type of laser crystal Yb∶GdScO3, of which the gain bandwidth is about 85 nm. The commonly used activation ions for 2 μm laser crystals are Tm3+ and Ho3+. Tm3+ can be directly pumped by laser diode. Ho3+ has larger stimulated emission cross section, and its emission wavelength is longer than 2 μm. We studied the growth, spectroscopy, and laser performance of Tm∶LiYF4, Tm∶LiLuF4, Ho∶LiYF4, Tm,Ho∶LiYF4, and Tm,Ho∶LiLuF4 crystals. In addition, new laser crystals such as Tm∶PbF2, Tm∶LaF3, Ho∶PbF2, Ho∶LaF3, Ho∶CeF3, and Tm,Ho∶LaF3 were also explored. Part of the crystals are shown in Fig. 15. We successfully grew Tm∶LiYF4 crystals with diameter of 3 inches using home-made raw material purification and crystal growth equipment (Fig.16). We reported a new type of laser crystal Tm∶GdScO3. Its emission bandwidth of 2 μm band is about 269 nm, which is, to the best of our knowledge, the widest among all Tm3+ doped crystals. The commonly used rare earth ions in 3 μm laser crystals include Er3+, Ho3+ and Dy3+. We reported the deactivation effect of Pr3+ in Ho,Pr∶LiLuF4 crystal. Based on this crystal, 2.95 μm continuous wave laser was achieved. The main crystals for 4 μm lasers are high concentration Ho3+ doped BaY2F8, Dy3+ doped sulfides and chlorides, and Fe∶ZnSe. Currently, joule level pulse laser output has been reported based on Fe∶ZnSe. Thermal effects such as thermal lenses and depolarization limit the development of high-power solid-state lasers. Diamond has extremely high thermal conductivity and is expected to be applied in the thermal management of high-power solid-state lasers. SIOM proposed a scheme for directly growing diamond on the surface of laser crystals, and successfully grew continuous, well attached, and crack-free polycrystalline diamond films on laser crystals (Figs.19 and 20).Conclusions and ProspectsLaser crystals have the characteristics of high thermal conductivity and anisotropy, making them suitable for high peak power, large pulse energy, and high repetition rate solid-state lasers. Although the comprehensive performance of the three basic laser crystals (Nd∶YAG, Nd∶YVO4, and Ti∶sapphire) is excellent, the development of solid-state laser technology requires new laser crystals. SIOM has achieved several breakthroughs in new laser crystals in the visible light, near-infrared, and mid-infrared bands, effectively promoting the development of solid-state laser technology. In 2017, we developed the world’s largest Ti∶sapphire crystal (Φ235 mm), which supported the 10 PW laser output of Shanghai Superintense Ultrafast Laser Facility. The development of Yb and Tm doped GdScO3 laser crystals with extremely wide emission spectra drives the development of laser diode pumped ultrafast solid-state lasers. With the increase in pulse energy, peak power, and repetition rate of solid-state lasers, laser crystals will develop towards larger sizes, higher crystal quality, and controllable key performance.

ObjectiveThe mid-infrared band of 2?5 μm, which contains the atmospheric transmission window and the fingerprint absorption peaks of many molecules, holds vast potential applications in areas such as biomedicine, precision machining, and mid-infrared photoelectrical countermeasures. Although high-power mid-infrared lasers can be generated using methods such as nonlinear frequency conversion, the existing devices are often bulky and structurally complex, making them unsuitable for scenarios with complex optical paths or spatial constraints. Optical fibers, as an excellent and flexible light guide medium, can realize miniaturized and lightweight systems and have been widely studied. The materials mainly used for optical fibers include silicate glass, tellurite glass, fluoride glass, and sulfide glass. Among them, silicate glass fibers have an infrared cut-off edge at approximately 2.5 μm, making them unable to transmit lasers in the mid-infrared range. Tellurite glass fibers exhibit significant intrinsic absorption in the wavelength range greater than 4 μm, leading to rapid increases in loss and an inability to meet low-loss transmission in the 2?5 μm full spectrum. Sulfide fibers have a broader infrared transmission range; nevertheless, the absorption bands caused by S—H and Se—H bonds are challenging to eliminate, resulting in higher average loss in the 2?5 μm wavelength range. Currently, the main choice for low-loss transmission across the entire 2?5 μm range is fluoroindate fiber (InF3-based fiber). This study introduces the design and preparation of InF3-based fibers suitable for high-power mid-infrared laser transmission.MethodsThe early investigation by the research team on the crystallization behavior of fluoroindate glass indicated that in the molten state of multi-component InF3-based glass, high field strength fluoride cations compete for the nearby fluoride ions, leading to phase separation. The formation of phases reduces the activation energy for non-uniform nucleation in the melt, prompting spontaneous crystallization during the glass cooling stage. Based on this, the research team utilized inorganic glass engineering software for simulation analysis. Combining the team own accumulated experience in fluoroindate glass formulations, the composition of InF3-based optical fiber core/cladding glass is designed. Using the designed glass composition, a precursor for fluoroindate glass is prepared through the melt-quenching method. Combined with the team developed physical-chemical dehydroxylation technique, water is efficiently removed from the glass. Finally, high-power transmission InF3-based optical fibers are drawn by employing a “short heating zone” specialized optical fiber drawing process.Results and DiscussionsFigure 1 displays the mid-infrared transmittance spectrum of a 10 mm thick InF3-based glass precursor, which reveals no significant absorption near the 2.8 μm wavelength, indicating effective elimination of hydroxyl groups in the glass. Through differential thermal analysis (DTA), the transition temperature (Tg) and crystallization temperature (Tc) of the glass are investigated (Fig.2). The glass transition temperatures for the core and cladding glasses are 295 ℃ and 299 ℃, respectively. The glass transition temperatures for the core glass and cladding glass are close, which is favorable for the fiber drawing process. The crystallization temperatures for the core and cladding glasses are 384 ℃ and 381 ℃, respectively. Using the formula ?T=Tc-Tg, the thermal stability parameters (?T) for the core and cladding glasses are calculated to be 89 ℃ and 82 ℃, which indicates that both glasses have good thermal stability, making them suitable for subsequent fabrication of fibers. The optical fiber preform rod is heated to the vicinity of the glass transition temperature. Once the preform rod forms a molten tip, it is drawn and elongated into an optical fiber under the influence of gravity. The dimensions of the fiber are illustrated in Fig.3, with a core diameter of 200 μm and a cladding diameter of 260 μm. The average loss of the optical fiber is ≤0.22 dB/m @ 3?5 μm [Fig.3(b)]. In practical applications, optical fibers may encounter sharp and pointed objects, potentially leading to damage during use. Armoring the optical fiber to create a fiber optic cable can effectively address this issue, significantly enhancing the safety of optical fiber use (Fig.4). The transmission results of optical fibers and cables are shown in Fig.5. Pulsed laser with a wavelength of 3.7?4.8 μm is used as the target light source. The transmission of pulsed laser at the 10 W level has been realized in a laboratory environment through spatial coupling.ConclusionsThe Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences has successfully produced high-power transmitting energy InF3-based optical fibers, demonstrating initial capabilities for independent production of mid-infrared fluoroindate optical fibers. The fiber products with independent intellectual property rights are initially realized the indigenization substitution. The manufactured cables have achieved pulsed laser output at the 10 W level in the 3.7?4.8 μm wavelength range, showcasing excellent mid-infrared laser energy transmission performance. With the ongoing optimization and adjustments to fiber composition and manufacturing processes, the optical and mechanical properties of the fibers will further improve, providing robust support for the development of high-end infrared optical systems in our country.

SignificanceWith the development of technologies such as artificial intelligence, the metaverse, the digital economy, and quantum computing, global data production is growing dramatically, and the amount of stored information has rapidly jumped from the PB level to the EB level or even the ZB level in extreme cases. According to statistics and predictions from the Statista, the total amount of data generated globally will grow at an annual rate of 27% from 2020 to 2025, and it is expected to reach 2142 ZB by 2035. But major storage devices, such as solid-state drives, hard drives, and magnetic tapes, generally face issues of high energy consumption and short operating lives. Additionally, costs increase sharply with time migration, and there is a risk of data loss. In comparison, optical storage technology is more durable, reliable, and energy-efficient, and it presents a promising green solution for long-term data storage. However, the density of traditional optical storage technology is limited by diffraction, making it difficult to meet the storage requirements of massive data.In order to improve storage density, methods commonly used are shortening the laser wavelength and increasing the numerical aperture (NA) of the objective lens to reduce the size of the recording spot, as demonstrated by the development from CD to BD. By using a recording wavelength of 405 nm and an objective lens with an NA of 0.85, a single BD can achieve a maximum capacity of 500 GB and a storage density of approximately 49.1 Gbit/inch2. Wavelengths below 400 nm result in high costs, while increasing NA reduces the working distance, which will easily damage the lens and optical disc. Therefore, it is necessary to develop new technologies to achieve ultra-high density optical storage.In recent decades, researchers have continuously explored innovative mechanisms for optical storage. By introducing parameters such as 3D space, polarization, and wavelength for multiple information reuse, storage space can be further utilized. In addition, with the development of nanotechnology, superresolution nanoscale optical storage can be achieved by breaking through the diffraction limit. Although significant progress has been made, there are still a series of challenges in practical application and industrialization. Therefore, it is important and necessary to summarize existing research in order to better prospect future development in the field of big data optical storage.ProgressWe review the research progress of ultra-high density optical storage over the past 20 years. First, the principle and the storage density enhancement ability of multiplexing optical storage in different dimensions are introduced, including 3D space, polarization, wavelength, and orbital angular momentum. Second, we summarize the existing superresolution optical storage technologies. Figure 5 shows that dual-beam superresolution optical storage technology has the potential for ultra-high capacity. Then, we discuss the problems faced by technology and ongoing research trends. In the end, we demonstrate the latest research progress of our research group in this field. Combining the dual-beam superresolution optical storage technology with the AIE effect, the equivalent capacity of a single disc is increased to 1 Pb.Conclusions and ProspectsMultidimensional information multiplexing technology and superresolution optical storage technology have effectively improved storage density to provide green, cost-effective solutions for long-term information storage in the era of big data. With the continuous deepening and improvement of the light‒matter interaction at the sub-wavelength scale, exploring the combination of the above two technologies will be a future direction. Furthermore, the realization of dual-beam superresolution optical storage technology with ultra-fast recording speed and ultra-high recording precision will offer new opportunities for the innovation of ultra-high density data storage systems.

SignificanceUltrathin two-dimensional (2D) nanomaterials are a new type of nanomaterials. They have a sheet-like structure with a lateral dimension that exceeds 100 nm or as high as several microns or even larger, but the thickness is only one or several atoms. Because electrons are confined in a 2D space, the characteristics of 2D materials are unique, as they exhibit unprecedented physical, electronic, and chemical characteristics. Hence, they have attracted significant attention.The study of ultrafast nonlinear optical (NLO) properties and ultrafast carrier dynamics is crucial for the development of photonics and optoelectronic devices. It is necessary to extensively investigate the NLO characteristics of 2D materials. Under the action of intense lasers, different 2D materials show different NLO responses. With the changes in preparation methods, test environment, and material customization, the NLO characteristics of materials are more colorful. 2D materials are widely used in laser protection, Q-switching or mode-locking, optical modulation, and various miniaturized all-optical devices because of their unique optical characteristics, such as ultrafast optical response, significant optical nonlinearity, and strong exciton effect.ProgressDifferent types of 2D materials, such as monoelemental materials and 2D metal sulfides, exhibit distinct NLO properties because of their unique structures. Clarification of the NLO mechanism of 2D materials and reasonable selection will help further expand the application field of 2D materials.First, this study introduces several methods for preparing 2D materials, the basic principle of NLO, and the experimental devices for common NLO and ultrafast carrier dynamics testing. The experimental devices include the Z/I-scan system, second-harmonic test device, and pump-probe detection system. Next, the NLO properties and principles of different types of 2D materials are comprehensively summarized based on previous studies in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. Cheng et al. investigated the influences of different dispersion solvents on graphene dispersion. They observed that the NLO performance of graphene dispersion originated from the solvent and carbon-vapor bubble-induced nonlinear scattering (NLS). They also showed that the NLO performance of graphene could be improved by reducing the air pressure, as shown in Fig.4(b). Many studies have shown that the NLO properties of materials can be significantly improved via complex or covalent modification. For example, Tong et al. reported that graphene/ZnO composites can change the multiphoton absorption (MPA) by controlling the incident laser intensity, and can also transform the saturated absorption (SA) into the reverse saturated absorption (RSA), as shown in Fig.4(d). Huang et al. investigated the NLO characteristics of black phosphorus (BP) and found that BP is more easily saturated under long laser pulses and exhibits a stronger SA response in the visible light band than in the infrared band. The NLO response of BP dispersion under different laser intensities is attributed to the combined effect of SA and NLS. The problem of material oxidation in air can be minimized by embedding the material into the polymer body as an inclusion to form a composite material. For example, Shi et al. synthesized a mixture of BP∶C60 and embedded it in polymethyl methacrylate(PMMA). After annealing, the heat-induced intermolecular charge transfer effect between BP and C60 was strengthened; hence it exhibited improved NLO characteristics, as shown in Fig.5(c). Dong et al. investigated the NLO properties of transition metal dichalcogenides (TMDs) (MoS2, MoSe2, WS2, WSe2) dispersion. They found that TMDs showed optical limiting response under nanosecond pulses with different wavelengths, and selenide exhibited better optical limiting performance than sulfide in the near-infrared region. In addition, TMDs with different thicknesses may have different energy band structures, which improves the NLO characteristics. More new 2D materials exhibit fascinating NLO properties because of their unique structures, and their NLO properties can be regulated by controlling the preparation environment and modifying the materials. Finally, according to previous research, the application of NLO properties of 2D materials is summarized. 2D materials can be used as saturable absorbers for Q-switched or mode-locked lasers because of their excellent SA properties. In addition, the excellent optical limiting performance also makes 2D materials candidates as laser protection materials. Different materials exhibit different NLO characteristics, and they can be used in various applications, such as optical modulators, optical nonlinear activators, and visible light thresholders.Conclusions and Prospects2D materials exhibit fascinating optical properties because of their unique structures. Materials exhibit different NLO properties because of the different preparation methods, test environment, and other factors. Composite materials and covalent modification of materials prevent material oxidation and enhance the charge transfer, thus significantly enriching and improving the NLO properties of the materials. These excellent NLO characteristics of 2D materials can be applied to different fields, such as laser mode-locking or Q-switching, laser protection, and optical modulators. However, 2D materials face several challenges, and the problem of how to obtain large-area and high-quality ultrathin 2D materials must be solved urgently. In the future, more new 2D materials and structures with excellent properties can be developed and designed to satisfy application needs in different scenarios.

SignificanceIn the last half century, electronic devices and integrated circuits have achieved great success in information processing. Moores Law states that the number of transistors in an integrated circuit doubles approximately every 18 months. However, electronic devices and circuits suffer from inherent problems such as resistance-capacitance time delay and thermal effects. According to Moores Law, by 2020, fewer than one electron will be contained/included in a transistor, which severely limits the development of integrated circuits. Integrated photonics is considered one of the most promising technologies to replace integrated circuits in the post-Moore era. Compared with electronics, photons have advantages such as ultra-high transmission speeds, high parallelism, and wide bandwidths. Photons exist in a highly coherent state as bosons, allowing for parallel transmission without the fear of external interference. In addition, photons have relatively high information capacity and can carry signals at varying emission intensities, wavelengths, and polarization. Semiconductor micro- and nano-lasers are essential components in photonic integration systems as high-performance light sources. Renowned physicist Thomas M. Baer published an opinion in Nature stating that, in the future, scientists will achieve micro/nano-laser outputs with spot sizes of approximately 1 nm, which will facilitate ultra-high-resolution imaging and direct sequencing of biomolecules. Therefore, research on semiconductor micro- and nano-lasers is of significance in fields such as integrated displays, integrated photonics, optical information processing, and biological imaging.Semiconductor micro- and nano-lasers utilize wavelength-scale microcavities to achieve laser emission and have advantages such as small sizes, compact structures, and low cost, making them ideal choices for high-performance light sources. Particularly with the development of information technology and integrated optics, the design and fabrication of high-performance micro- and nano-laser sources have become increasingly important. Similar to macroscopic laser systems, the output of micro- and nano-lasers primarily depends on three components: resonant cavity, gain medium, and pump source. To date, advancements in these three aspects have been driving the progress and innovation of semiconductor micro- and nano-lasers. Currently, known resonant cavity structures include edge-emitting, vertical-surface-emitting, distributed Bragg reflector, microdisk, nanowire, microsphere, photonic crystal, and plasmonic cavities. In addition, they can be classified into random-cavity, Fabry-Perot cavity, and ring-cavity lasers as well as photonic crystal microlasers, distributed feedback cavity lasers, and surface plasmon microlasers based on their different resonance mechanisms. The gain media in semiconductor micro- and nano-lasers mainly consist of organic dyes, quantum wells, quantum dots, and two-dimensional transition metal materials.Since the successful demonstration of optical gain in colloidal quantum dots (CQDs) twenty years ago, CQD-based lasers have rapidly developed. However, due to limitations imposed by the quality factors of microcavites, gain material, and spontaneous emission coupling efficiency, the reported outputs of semiconductor micro- and nano-lasers have generally exhibited multimodal structures with poor monochromaticity, low Q-factors, and high thresholds. To achieve high-quality output from low-dimensional semiconductor micro- and nano-lasers, the exploration of new high-gain semiconductor nano-materials, innovative designs, and the fabrication of efficient novel microcavity structures are critical. This article considers the achievements and research progress in the field of low-dimensional semiconductor micro- and nano-lasers and summarizes the research on micro- and nano-lasers based on novel perovskite materials. Finally, the article provides an outlook on the developmental prospects of low-dimensional micro- and nano-lasers.ProgressWhen the sizes of semiconductor crystals reach a few nanometers, a quantum size effect occurs due to strong spatial confinement of charge carriers, which provides new possibilities for constructing novel and powerful optoelectronic devices. Semiconductor quantum dots are recognized as materials with superior optical gain as compared with bulk and quantum well materials. Unlike traditional top-down fabrication processes such as photolithography, the unique bottom-up synthesis approach of low-dimensional semiconductor materials not only simplifies the preparation of micro- and nano-lasers but also provides high-quality self-resonant cavities. Perovskite nanomaterials, as a new type of semiconductor optoelectronic material, possess excellent optical properties and high carrier transport characteristics, making them ideal optical gain media for on-chip integrated micro- and nano-light sources. In the field of micro- and nano-lasers, perovskite nanomaterials are mainly grouped into two categories: organic-inorganic hybrid structures and all-inorganic structures based on the ABX3 structure. Studied nanostructures include nanoplates, nanowires, and quantum dots (Fig. 1). In 2014, Sum et al. published their findings on amplified spontaneous emission and lasing in MAPbX3 thin-film materials, which initiated research on perovskite micro- and nano-lasers. In 2015, Kovalenkos group reported for the first time spontaneous emission amplification phenomenon in all-inorganic CsPbBr3 perovskite quantum dots. Since then, various low-dimensional semiconductor micro- and nano-lasers have been reported based on different morphological structures, including micro-ring cavities, cubic cavities, nanowires, nanoplates, and quantum dots (Fig. 2). In this article, the developmental status of low-dimensional semiconductor micro- and nano-lasers is first introduced. The research progress of micro- and nano-lasers is then presented based on different gain materials and structures, and their applications in fields such as quantum coding, optical anti-counterfeiting, ultrafast optics, and other fields are described. Finally, we summarize the development of low-dimensional micro- and nano-laser and forecast future developmental trends.Conclusions and ProspectsLow-dimensional semiconductor micro- and nano-structures serve as major platforms for studying the interaction between light and matter. Harnessing the mechanisms of micro- and nano-structures, high-performance research on micro- and nano-lasers involves interdisciplinary collaboration across fields such as chemistry, materials science, and physics. This research has significant applications in areas such as micro- and nano-light sources, optical communication sensing, photonic computing, and quantum information processing. To achieve practical applications of low-dimensional semiconductor micro- and nano-lasers through continuous wave pumping and electrical pumping, further exploration and analysis of the physical mechanisms and resonance modes involved in continuous wave pumping laser formation are essential. In addition, a systematic investigation of the requirements for material structures and gain properties under continuous wave pumping is necessary to ensure beam quality and stability of laser output. Finally, achieving electrical injection lasers requires thoughtful and extensively forward-looking research.

ObjectiveThe high-power continuous laser is in urgent demand in military and industrial fields, and it is an important technical support for national defense and economic development. Because it is limited by nonlinear effects, mode instability, and other problems, the output power of single-mode fiber lasers has a specific limitation. To improve the output power of the laser, researchers have proposed a series of laser power enhancement technology routes, such as spectral combination and dichromatic combination. By combining multiple single-path lasers to output laser, higher laser power can be obtained while maintaining good beam quality. Therefore, the combination scheme becomes the key to realize ultra-high laser power output. The dichroic mirror combination technology scheme based on the spectral characteristics of the dichroic film has become one of the main technical routes for high-power continuous laser synthesis output owing to its advantages of stability, high combination efficiency, and easy integration at low cost. Because of the spectral characteristics of the dichromic film, it can simultaneously transmit and reflect two beams with different wavelengths, and by adjusting the incidence angles of the two beams, it can realize the coincidence of the transmitted and reflected beams to achieve the combining effect of two beams. In addition, the higher the transmittance and reflectivity at the transmission and reflection bands of the dichromic film, the smaller is the loss of laser energy in the beam combination process, and the higher is the achieved beam combination efficiency.MethodsThe differences in spectral properties, such as spectral transmittance, reflectance, and steepness, and the electric field distribution of the film layer between the Fabry-Perot cavity superposition and the long-wavelength-pass basic film structures are compared. The long-wavelength-pass film structure is more suitable for the high-power continuous laser low-absorption transmission. Based on the dual ion beam sputtering thin film deposition equipment, the preparation is carried out using the weighted broad spectrum monitoring method, and the prepared samples are tested using a spectrophotometer. The errors are analyzed based on the test results.Results and DiscussionsFigure 7 shows the spectral matching of the transmittance spectra of the dichromic film using the layer-by-layer inversion function of the OptiRE software. Among them, the transmittance spectrum of the first film layer is poorly matched because the spectral transmittance curve contains limited information. Then, because of the self-compensating effect of the broad spectrum monitoring, the target is gradually corrected during the deposition process of the film layer, so that the matching of the measured and theoretical design spectra improves. In addition, as shown in Fig.8, the thickness of the high refractive index material is generally positively shifted, and the thickness of the low refractive index material is are generally negatively shifted. The main reason for the formation of this phenomenon is the self-compensating effect of the broad spectrum monitoring. Aiming at the design transmittance spectrum, the thickness errors of high and low refractive index films are staggered with each other when the film is adjusted repeatedly. At the same time, according to the inversed deposition rate distributions of the high and low refractive index materials, the deposition rates from layer 13 to layer 46 are stable. The rate gradients of the first few film layers are from the state-stabilizing process at the beginning of ion beam output of double ion beam sputtering equipment. In layer 47, the film layer sensitivity is increased. At this time, to achieve the spectral matching, the broad band spectral monitoring is more active, the overall spectral changes are concentrated near the transition band, and the overall rate fluctuation becomes larger.ConclusionsThe optimized design of the coating structure and the strategy of precision preparation are systematically analyzed from the perspectives of design, precision preparation, and test characterization. This allows the achievement of higher steepness, reflectivity, transmittance, absorption, and other properties of the high-steepness dichromatic coating. The long-wavelength-pass film structure is more suitable for the high-power continuous laser low-absorption transmission. The steepness of the designed and prepared high-steepness dichromatic film is 8 nm; the 1060 nm and 1080 nm laser beam combining efficiency reaches more than 99%; and the temperature rise of the dichromatic film is less than 4 ℃ when the combined continuous laser output power reaches 7.16 kW in 180 s. This is of great significance for the design and precision preparation of complex film systems represented by high-steepness dichromatic films.

SignificanceThe field of high-power lasers is at a critical stage of making major breakthroughs and developing applications, especially for high-peak power, ultra-intense, ultrashort lasers and high-energy continuous lasers with high average power. Over the past five years, ultra-intense, ultra-short laser facilities based on chirped pulse amplification and their derivative technologies have broken through 10 petawatts (PW) of peak power, and spectral beams combining continuous-wave laser equipment with hundreds of kilowatts of average power have been delivered. Reflective holographic surface-relief diffraction gratings are the core components of current high-power lasers. The Shanghai Institute of Optics and Fine Mechanics (SIOM) develops the meter-scale (1620 mm×1070 mm) pulse compression grating (PCG) and the advanced spectral beam combining grating (SBCG) worldwide. In the next five years, global development works for the delivery of 100 PW lasers will be online. In addition, the output power of high-energy continuous lasers will also enter a new stage. Accordingly, the high-power laser will necessitate the development of billion dollar industries. Advanced diffraction gratings will play an important supportive role in contributing to the power growth of high-power lasers. Currently, both domestically and abroad, the field of high-power laser diffraction grating exists in both a growth period and a stable period. Therefore, it is necessary to review and look forward to the research trends in the field of high-power laser diffraction gratings from the perspective of those who have experienced the development of the field in China and are oriented to the real needs and real problems.This paper is dedicated to the 60th anniversary of SIOM and takes this opportunity to salute all those who have participated in and contributed to high-power laser grating in China.ProgressIn this study, the domestic and international research progress of PCG and SBCG over the past 40 years has been reviewed based on the demand (Fig. 2) for reflective holographic gratings by the two major technologies of chirped pulse amplification and spectral beam combining, which are represented by ultra-intense, ultrashort laser and high-power fiber laser systems (Fig. 1). Section II of this paper briefly describes the classification and preparation of reflective surface relief gratings (Figs. 3 and 4). Section III highlights the research progress of metal pulse compression gratings in spectral bandwidth expansion, large aperture fabrication (Fig. 7), and laser-induced damage threshold (LIDT) enhancement. Section IV focuses on the line density increase (Fig. 20) and thermal loading performance enhancement (Figs. 21‒23) of multilayer dielectric polarization-independent SBCGs.Conclusions and ProspectsThe development and construction of high-power lasers are in full swing. This paper reviews the progress of domestic and international research on pulsed compressed gold gratings and all-dielectric merging gratings over the past 40 years.Currently, pulse compression gold gratings are mainly developed around the center wavelengths of 800 and 910 nm to achieve a spectral bandwidth of 100 or 200 nm. The concept of fewer cycles or even single-cycle pulse compression has recently been established, and the implementation of the bandwidth demand has risen to 400 nm. Hence, the development of advanced ultra-broadband meter-scale gratings needs to be accelerated to capture the scientific and technological high ground in China. In addition, in the construction goal of 100 kW and even megawatt lasers, the development of advanced SBCGs will continue to be laid out along the two main tasks of high line density and high heat-load performance improvement in the future. Simultaneously, although the problem of laser-induced damage to optical components has coexisted with the development of high-power lasers, LIDT data and studies of gratings are still insufficient, nearly half a century later. The complexity and multidisciplinary nature of grating damage will inject fresh vitality into the field of grating research in the future as the parametric diversity of high-power lasers increases.Although China ventured into the development of high-power gratings later than did foreign countries, it has made huge progress over the past decade and is now among the elite ranks of high-power-grating nations. SIOM, as a representative of China, developed the largest meter-scale (1620 mm×1070 mm) PCG and advanced SBCG worldwide. Over the next few years, China's high-power laser grating development needs to overcome the large aperture, high LIDT, high diffraction efficiency, broadband, and other key challenges, which are strongly supported by scientific research management institutions and the cooperation of all relevant units.

SignificanceDammann gratings (DGs) are a type of diffractive optical elements used in beam splitting and are capable of generating tens to thousands of sub-beams in parallel with only a single integrated element. The efficiencies of DGs are typically in the range of approximately 70%‒80% for binary gratings, and this efficiency can be further improved. Due to these unique properties, DGs are applicable in a variety of fields ranging from optical interconnections to integrated grating magneto-optical traps. Currently, the computing power required to process large amounts of data doubles approximately every 3.5 months, far exceeding the computing power supplied by electronic integrated circuits (EICs) that follow Moore’s law. Compared with traditional electrons, photons are expected to accelerate computing, particularly customized computing, with high computing power, high energy efficiency, and low latency. DGs provide a powerful method for matching the demands of large-scale fan-in and fan-out in optical computing. Thus, this critical element plays a major role in this revived topic, particularly when fused with new technologies such as liquid crystal-based planar optics, metasurfaces, and planar integrated photonic circuits. With the continual development of the theory, design, and manufacturing technologies of DGs, their application scope has widened considerably in modern optics.ProgressThis review analyzes and discusses the principles, developments, and applications of various types of DGs and Dammann encoding gratings in terms of their historical evolution. First, the principles and theories of various DGs, including classical DGs, circular DGs (CDGs), and Dammann encoding gratings, are introduced. For a classical DG, splitting a single incident beam into multiple sub-beams using a single grating is easy, where the efficiency is typically greater than 70% for a binary pure-phase structure. The splitting ratio can be changed by reoptimizing the transitional points for these binary gratings, and the efficiency can be further improved by choosing a multilevel structure or reducing the period required to transform the gratings into the resonant region. In 2003, Zhou et al. proposed the concept of a CDG, and its rigorous theory was completed. This new diffractive optical element has been applied in many fields, including remote sensing, image coding, and circular pumping vortex lasers. Subsequently, the Dammann encoding method was generalized to various encoding gratings for generating numerous complex beam arrays. A major scenario involves the generation of vortex arrays when a spiral phase is encoded.Second, landmark progress and representative achievements are retrieved in historical order during the 50 years of evolution of DGs. In this review, the evolutionary history of DGs is divided into three main stages: start-up (1971‒1995), developing (1996‒2014), and recently advanced (2015‒present). At every stage, the milestones and representative works are summarized in historical order. In addition to the classical Dammann beam splitters, other types of elements, including CDGs, Dammann vortex gratings (DVGs), distorted Dammann gratings, and Dammann zone plates, are introduced as part of the historical evolution of Dammann beam splitters. Some representative applications are also discussed and the challenges are analyzed.We also consider typical applications of DGs and the challenges under each scenario. In the early stages, DGs were used for star coupling into fiber arrays, optical computing and interconnection, and laser coherent beam combination. In addition, DGs have been used for splitting and measuring ultrashort laser pulses, parallel laser processing, direct writing lithography, three-dimensional measurements based on structured light, structured pumping lasers, multi-shearing for imaging and measurement, complex beam array generation, pattern detection, and grating magneto-optical traps. DGs have also been widely used in many fields of modern optics.Finally, we discuss future trends, directions, and challenges in the further development of DGs.Conclusions and ProspectsAs one of the most important fundamental components of modern optics, DGs have developed into a branch of diffraction optics. Over the past 50 years, the theoretical principles, design methods, and manufacturing technologies of DGs have constantly improved, and a variety of new diffractive optical elements based on the Dammann encoding method continue to emerge. In the future, the fusion of DGs with liquid crystal optics, metasurfaces, and planar integrated optics will accelerate, and the functions of various DGs will be continually enriched. DGs have evolved from single-wavelength to broadband operation, passive fixed functions to active adjustable control, simple beam splitting to complex array control, and traditional bulk devices to integrated miniaturization. With the rise of optical computing, DGs will usher in new opportunities and challenges when combined with new technologies such as liquid crystal-based planar optics, metasurfaces, and planar integrated photonic circuits.

ObjectiveWith the continuous development of laser pulse width limits and improvement of peak power, the size of the pulse compression grating (PCG) must be further increased. However, the high-precision manufacturing and testing of large aperture and long focal distance off-axis parabolic (OAP) mirrors required by reflective exposure systems, has presented a difficult challenge that restricts the manufacturing of large aperture gratings. The method based on computer generated holograms (CGH) does not require a complex design and setup, however, it introduces non-rotational symmetry and complex two-dimensional projection distortion. When correcting distortion, traditional marker point s and analytical methods have limited accuracy or complex calculations, which are not conducive to engineering applications. Therefore, this study proposes a distortion correction method based on numerical calculation, with the advantages of simplicity, versatility, and ease of programming. Based on the CGH test optical path, high-precision surface measurement can be achieved using system error calibration and distortion correction methods, laying the foundation for OAP mirror surface accuracy manufacturing and subsequent establishment of large aperture reflective exposure systems.MethodsInitially, a Φ800 mm folding mirror is adopted to effectively shorten the optical path length, enabling development of a CGH measurement optical path on an 18 m vibration isolation air flotation optical table (Fig.2), and design a beam expansion system that matches the measurement aperture of the interferometer and CGH. Subsequently, the measurement errors introduced by the main optical elements in the optical path, excluding the interferometer, including CGH, beam expansion system, and fold mirror, are analyzed in detail, and the errors introduced are calibrated and removed. Thereafter, to ensure that the machining coordinate system matches the testing coordinate system, to achieve the precise positioning and convolution of the removal function and figure error, it is necessary to correct the distorted surface map measurement. Therefore, the mapping relationship between mirror, CGH, and CCD coordinate points is established based on the imaging distortion model (Fig.9). Finally, according to the corrected surface map, the OAP mirror is fabricated using Magnetorheological Finishing (MRF) technology.Results and Discussion. Once the measurement optical path is established, the surface map measured by the interferometer was compared with that measured by the coordinate measurement machine (CMM). Except for the projection distortion in the interferometric measurement results, the distribution of other features is basically consistent (Fig.3). When the CGH calibratable errors, beam expander system, and fold mirror are superimposed, the measurement optical path system error is 11 nm RMS (Fig.8). After error compensation, the measurement optical path error does not exceed RMS 5.4 nm (Table 5). Among them, the fold mirror figure error is the main error source. The coordinate positions and positional errors of the distortion correction points are listed (Table 6). The maximum correction error is 1.96 mm, which is smaller than the testing interferometer measurement resolution. This measurement accuracy meets the of magnetorheological computer numerical control (CNC) machining positioning accuracy requirements. Post processing, the OAP mirror figure error after distortion correction is PVr: 0.130λ, RMS: 0.013λ (Fig.11). By comparing the surface map distributions before and after distortion correction, it can be observed that the corrected surface map distribution and values are fundamentally consistent with those uncorrected s, and with the mirror coordinates. This result provides a good basis for subsequent light field exposure analysis and performance evaluation.ConclusionsFor the measurement of a 1650 mm×1120 mm long focal distance OAP mirror, an Φ800 mm fold mirror is introduced to shorten the CGH compensation measurement optical path. Using error analysis and calculation, the calibratable error in the optical path is projected onto the CCD, and then calibrated and removed, thereby achieving improved measurement accuracy. Considering the projection distortion error caused by simultaneous measurement, a correction method based on ray tracing and imaging distortion model fitting is proposed. Compared with traditional methods, the proposed method is extremely suitable for numerical programming calculations. Once processing verification completes, the figure accuracy can converge to RMS 0.013λ, and meet the requirements of high-precision OAP mirror specifications and manufacturing, laying a foundation for the subsequent establishment of large aperture reflective exposure systems.

ObjectiveLithographic technology is one of the core technologies for large scale integrated circuit manufacturing. The projection lens is the core component of the lithographic system, and its imaging quality determines the lithographic resolution and critical dimension. Wavefront aberration is an important parameter to evaluate the imaging quality of lithographic projection lens. A high-precision wavefront aberration measurement device is necessary for the installation and adjustment of lithographic projection lens. Phase extraction is an important step in wavefront aberration measurement using double-grating Ronchi phase-shift lateral shearing interferometry, which directly affects the final measurement accuracy. There is parasitic interference of multi-level high diffraction orders in the double-grating Ronchi lateral shearing interference field. The traditional phase extraction algorithms cannot eliminate the impact of high diffraction orders, which seriously reduces the accuracy of phase extraction. The high-precision shear phase extraction algorithm can improve the accuracy of wavefront aberration measurement of projection lens. Eliminating the impact of high diffraction orders is very important to improve the measurement accuracy of Ronchi lateral shearing interferometry. In this paper, a high-precision shear phase extraction algorithm is proposed based on the double-grating Ronchi lateral shearing interferometry.MethodsIn this paper, based on the double-grating Ronchi lateral shearing interferometry for the projection lens wavefront aberration measurement technology and system, the shear phase extraction algorithm is studied. The shear phase between +1 and -1 diffraction orders is calculated directly through the double-grating Ronchi shearing interferograms, and the impact of all high diffraction orders in the double-grating Ronchi shearing interference field is eliminated to improve the measurement accuracy of Ronchi shearing interferometry. In this paper, the phase extraction error of the Ronchi lateral shearing interferometry is simulated firstly, and then the sensitivity of the error of the grating period, the accuracy of the positioning stage, and the vibration error are simulated and analyzed. A double-grating Ronchi lateral shearing interferometry system with a shear ratio of 0.058 is used to carry out the verification experiments, and the measurement results are compared with the dual-fiber point diffraction interferometer, which further verifies the effectiveness of the shear phase extraction algorithm.Results and DiscussionsIn order to verify the effectiveness of the proposed high-precision shear phase extraction algorithm, simulation is carried out for the system with a shear ratio of 0.048. In the simulation, the Z4 Zernike polynomial aberration with a coefficient of 0.1 is used as the nominal wavefront to be measured. The 10-step and 13-step phase extraction algorithms proposed by Wu et al. and the proposed high-precision shear phase extraction algorithm are simulated, respectively. It can be seen that there is obvious impact of high diffraction orders in the phase extraction error of the 10-step and 13-step phase extraction algorithms [Fig. 6(b), (d)]. The phase extraction error of the proposed high-precision shear phase extraction algorithm is in the order of 10-16 when 31 steps of phase-shift are used [Fig. 6(f)], and only high-frequency residuals exist in the results, indicating that the proposed high-precision shear phase extraction algorithm can effectively eliminate the impact of high diffraction orders. Simulation results show that with the relative error of the grating period less than 1% (Fig. 7), the introduced RMS shear phase extraction relative error increases as the shear ratio decreases; the accuracy of the positioning stage is better than 0.1% (Fig. 8), and the lower the positioning accuracy of the stage, the greater the relative error of root mean square (RMS) shear phase extraction; the normalized vibration frequency is greater than 5 (Fig. 9), and the larger the ratio of the vibration frequency to the natural frequency of the mechanical structure of the experimental platform, the smaller the relative error of RMS shear phase extraction. The experiment is carried out on double-grating Ronchi shearing interferometry system with a shear ratio of 0.058, and compared with the measurement result of dual-fiber point diffraction interferometer, both of the results have the same wavefront aberration distribution, which further verifies the effectiveness of the shear phase extraction algorithm (Fig. 14, Fig. 16).ConclusionsBased on the high diffraction orders model contained in the overlapping area between ±1st orders of the double-grating Ronchi lateral shearing interferometry, the shear phase extraction algorithm is studied, and a high-precision shear phase extraction algorithm is proposed. This algorithm can eliminate all high diffraction orders in the overlapping area between ±1st orders, thereby improving the accuracy of the shear phase extraction. The proposed algorithm is compared with the current 10-step and 13-step Ronchi shearing interferometry phase extraction algorithms. The simulation results show that the error of the proposed algorithm is in the order of 10-16, and only high-frequency residuals exist in the shear phase extraction errors and are almost negligible. The error simulation and analysis of the proposed shear phase extraction algorithm show that the relative error of the grating period needs to be less than 1%, the positioning accuracy of the stage needs to be better than 0.1%, and the normalized vibration frequency needs to be greater than 5. The proposed algorithm is verified by experiments and compared with the measurement result of dual-fiber point diffraction interferometer. The measurement results of the two methods have the same wavefront aberration distribution, which further verifies the effectiveness of the proposed high-precision shear phase extraction algorithm.

SignificanceThe automobile industry has a large volume, a high correlation, an extensive industrial chain, and a high consumption proportion. It is an important pillar industry that promotes the development of the national economy and is also a landmark industry that reflects the level of advanced manufacturing technology in a country. Owing to the continuous increase in the number and usage of automobiles, the depletion of natural resources, environmental pollution, and global warming have become increasingly prominent. To reduce fossil fuel consumption and greenhouse gas emissions, countries worldwide have established strict standards for vehicle fuel efficiency and carbon dioxide emissions. Vehicle lightweighting is one of the most direct and effective methods of saving energy and reducing emissions. For passenger cars, the body weight accounts for as much as 30%‒40% of the total vehicle weight, and lightweight bodies can significantly reduce vehicle weight. However, to meet the increasingly stringent automobile crash standard, the collision safety performance of vehicles must be considered, and lightweight bodies have become a very important topic in the automotive industry. The strength of traditional high-strength steel parts is limited, and the manufacturing process is complex, making it difficult to meet the goals of lightweight automobiles and improve automobile safety performance. The development of hot-stamping technology has given rise to press-hardened steel, which offers good formability, high strength, and the ability to form complex-shaped parts. The application of press-hardened steel to a car body can significantly improve the safety performance of the vehicle and reduce the thickness of the steel plate, which is of great significance for lightweight vehicles.Laser welding technology has become the mainstream welding method for automobile body welding owing to its small heat input, small deformation, high precision, and high efficiency. Due to the thinness of press-hardened steel, typically between 0.8 and 3 mm, the automotive industry laser welds press-hardened steel of different strengths or thicknesses to produce customized performance automotive parts, also known as Tailor Welded Blanks (TWBs). Currently, there are two main methods for manufacturing hot-stamped parts in the automotive industry. One is direct laser welding of hot-stamped components; however, this method tempers the martensite in the heat-affected zone, resulting in softening of the heat-affected zone. The other is by initially laser welding blanks, followed by hot-stamping forming. This method prevents the softening of the heat-affected zone and has become the mainstream method for welding press-hardened steel. To prevent oxidation and decarburization of the sheet during the hot-stamping process and to improve the surface quality and forming accuracy of the sheet, the surface of the sheet is usually prefabricated. Owing to the presence of the coating, when laser welding is used, the components in the coating melt and diffuse into the weld, significantly deteriorating the mechanical properties of the welded joint.ProgressThe conventional coatings for press-hardened steel mainly include Al-Si, Zn-Fe (GA), Zn (GI), Zn-Ni, and other composite coatings. The advantages and disadvantages of different coatings vary (Table 1). The Al-Si coating is the most widely used coating for press-hardened steel because of its high temperature resistance, oxidation resistance, formability, and lower cost than Zn-Ni and other composite coatings. The compositions of the Al-Si coating and the base material differ significantly and are greatly affected by the laser welding process. However, a common problem persists in different laser welding processes. Al segregation occurs in the weld seam, and Al enrichment leads to ferrite formation. This is the fundamental reason for the significant decline in the mechanical properties of the welded joints, which limits the further application of press-hardened steel. Several studies have been conducted to solve the problem of Al segregation in laser-welded press-hardened steel caused by Al-Si coatings. At present, methods to suppress Al segregation in welds primarily include 1) removing the coating before welding, 2) optimizing the welding process parameters, 3) increasing the fluidity of the molten pool, 4) modifying the Al-Si coating, 5) adding an interlayer and powder, and 6) filling the welding wire. However, these methods have limitations, such as high cost, low efficiency, complex processes, and unsuitability for large-scale production. The “multi-in-one” press-hardened steel laser-filled wire welding technology, which does not require the removal of the Al-Si coating, was developed by the Yang Shanglu team of the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. This technology does not require the coating to be removed before welding and only fills the “multi-in-one” special welding wire during the welding process, which can achieve efficient and high-quality welding of Al-Si-coated press-hardened steel above 1500 MPa grade. This method has wide applicability, low cost, and broad application prospects.ProgressConclusions and Prospects In this study, the characteristics, applications, and laser welding methods of press-hardened steel are introduced in detail. The advantages and disadvantages of different coatings are summarized. The welding problems of press-hardened steel are noted. In addition, the improvement methods for laser welding defects in press-hardened steel are summarized, and the development trends of press-hardened steel and laser welding technology are discussed. Coating technology provides oxidation resistance in press-hardened steels; however, it also causes problems in laser welding. As press-hardened steel increases in strength and decreases in thickness, the chemical composition of the welding wire must be optimized, and in-depth research must be conducted on the microstructure evolution mechanism of the welding wire composition affecting the coating of press-hardened steel and the mechanical property strengthening mechanism of the joint to explore an efficient, intelligent, widely applicable, and robust laser welding method for lightweight automobiles. In the development of laser welding technology for press-hardened steel, it is also necessary to focus on quality inspection of the welding process to achieve adaptive parameter adjustment of the laser welding process and online quality monitoring after welding.

SignificanceDirected self-assembly (DSA) lithography is a prominent candidate for next-generation lithography techniques. This can extend conventional top-down lithography to advanced technology nodes and enhance the pattern quality of top-down lithography. DSA is driven by interactions between two chemically distinct blocks of block copolymers (BCPs) and between the BCP material and guiding template. This unique capability enables the DSA to form nearly all geometric patterns required in semiconductor manufacturing, offering a cost-effective and efficient patterning approach. The mechanism underlying DSA has gained significant attention in recent years, leading to extensive research and development efforts to incorporate them into semiconductor manufacturing processes. Several companies and institutions, including IMEC, IBM, and CEA-Leti, have established pilot lines for DSA, further driving the implementation of this technology in the semiconductor industry.Simulation techniques play a vital role in the research and applications of DSA lithography. The DSA process can be parameterized using a series of parameters, including the chemical properties of the block copolymer material, the geometry and wetting conditions of the guiding templates, and other relevant factors. DSA can generate a wide range of microphase structures by varying these parameters. Simulation techniques provide an efficient and effective method to explore complex high-dimensional parameter spaces by mapping these parameters to the resulting self-assembled structures. Second, it can be used to investigate the mechanisms underlying defect formation. Addressing the issue of defects in DSA has always been a challenge. DSA patterns may deviate from the target structure, resulting in defects affecting pattern quality. Some defects may even be buried beneath the surface, making their experimental characterization difficult. Simulation techniques play a crucial role in reducing DSA defects to acceptable levels. Furthermore, simulation techniques can be employed to address the inverse DSA problem, which involves deducting the parameters or geometry of the guiding pattern from a target structure. Compared to the role of physical models in computational lithography, simulation techniques have become indispensable in the study and application of DSA, providing valuable insights and fulfilling an irreplaceable role.Many simulation approaches have been developed. These approaches include field-theoretic methods, such as self-consistent field theory and complex Langevin simulation; dynamic models, such as coarse-grained molecular dynamics and dissipative dynamics simulation; and probabilistic simulation methods, such as the Monte Carlo approach and simplified models. Among these methods, the self-consistent field theory (SCFT) is one of the most successful models for studying the phase transitions of BCPs. It can accurately predict self-assembled structures, thereby aiding in understanding the DSA mechanism. The SCFT model has been widely applied in theoretical studies of DSA lithography. However, the SCFT has limitations in that it can only obtain equilibrium structures and lacks information on the system fluctuations and time-dependent behavior of BCPs. More detailed methods, such as Monte Carlo simulation, are required in certain specific applications. Both the SCFT and Monte Carlo approaches are based on the system Hamiltonian to obtain stable phase structures with lower energies, but they cannot provide information about the temporal evolution of the system. Conversly, dynamical models provide accurate predictions of equilibrium structures while also capturing the time evolution of the DSA system based on Newton’s laws of motion. Simplified models are used in applications requiring fast computation, such as the inverse DSA problem, full-chip mask synthesis, and verification. These models are constructed using a phenomenological model calibrated using experimental data or by simplifying intricate physical processes with uncomplicated physical models. Consequently, simplified models offer higher computational efficiency at the expense of accuracy and generalizability. It is important to summarize the existing representative simulation techniques to provide a more rational guide for the future development of this field.ProgressDifferent applications require distinct models. Fredrickson’s research group from the University of California, Santa Barbara, in collaboration with the Intel Corporation, has conducted pioneering studies by applying SCFT to investigate the defect formation mechanism in chemoepitaxy DSA and self-assembled cylindrical morphologies in VIA lithography. Coarse-grained Monte Carlo (MC) simulation, initially developed by Detcheverry’s research group at the University of Wisconsin-Madison, is another powerful tool utilized in the study of DSA lithography. Unlike the SCFT, the MC approach is more accurate and can predict thermal fluctuations because it does not invoke a saddle approximation. Research groups from Tokyo Electron, Global Foundries, and other institutions have widely reported the implementation of the MC method. Dynamical models provide novel insights, particularly regarding their ability to simulate the dynamic pathways through which equilibrium structures are formed. Delony’s research group conducted studies on bridge defects and defect modes induced by underlayer errors in chemoepitaxy DSA, providing a time-evolution analysis of these defects. Additionally, dynamical models have been employed to investigate the shrinking and multiplication of contact holes. These studies contributed to a deeper understanding of the behavior and evolution of DSA systems. The aforementioned rigorous models are accurate to a certain extent. However, they are time-consuming and require several hours to produce reliable results. However, this computational demand may not be feasible for time-sensitive tasks. Many research groups, including IBM, IMEC, and Toshiba, have developed simplified models to accelerate computations significantly by several orders of magnitude to address this issue. These models are constructed using several simple equations or data-driven paradigms. Although these simplified models may have limited accuracy and poor generalizability, they are highly effective in computationally challenging situations where speed is essential.Conclusions and ProspectsSimulation techniques are vital tools for developing DSA lithography. Similar to the physical models used in conventional computational lithography, simulation techniques can predict the final structure, interpret the underlying mechanisms, provide a rational basis for the design, and ultimately achieve higher pattern quality. Currently, the simulation of DSA lithography faces several challenges. These challenges include limited scalability for full-chip simulations, loss of accuracy in simplified models, and integration of DSA simulations into traditional electronic design automation (EDA) flows. Furthermore, with the combination of DSA and extreme ultraviolet lithography (EUV), DSA models must simulate the process of utilizing DSA to rectify EUV patterns. With the assistance of simulation techniques, DSA lithography can extend the application of conventional lithography to more advanced nodes.