Light-matter interaction is always one of the themes of science. With the rapid development of ultra-short and ultra-strong laser techniques, nowadays we can research the internal world in a single atom and control the light-electron interactions to explore the ultrafast dynamics of intra-atomic electrons. Laser-induced tunneling ionization of atoms, as the footstone of many strong-field physical phenomena, has important research significance and is also one of the hot frontier topics. In this paper, we review the recent research advances in strong-field atomic tunneling ionization. The coordinate and momentum distributions of electrons after tunneling process are obtained based on the non-adiabatic tunneling ionization in the natural coordinates (i.e., the parabolic coordinates). We introduce the theoretical description and experimental measurement methods for the initial phase (i.e., the sub-barrier phase) of electrons obtained in the tunneling process. Based on the sub-barrier phase, we can reveal the quantum dynamical information of the tunneling process. We introduce the recent advances in photoelectron spin polarization during the strong-field tunneling ionization. On the basis of the orthogonal two-color fields, the degrees of freedom of photoelectrons in the time and space dimensions can be accurately controlled. Finally, we summarize this paper and predict future research advances.

Absolute gravimeter is a precise metrological instrument for absolute gravimetry. Absolute gravimetry refers in particular to the measurement of acceleration of gravity on the earth directly which finds important applications in earth sciences and metrology. The earliest absolute gravimetry was performed in the year of 1590. From 1590 to 1960, pendulum principle was the main method to perform the absolute gravimetry. From 1960, with the invention of the laser technology, people began to use laser absolute gravimeter to perform the absolute gravimetry by measuring the free motion (falling or rising freely) of an object, which is the big progress in the history of precise gravity measurement. In 1991, the group of professor Steven Chu from Standford University used the free motion of laser cooling atoms and atom interferometry technology to perform the absolute gravimetry for the first time, which successfully developed the first atom interferometry absolute gravimeter in the world. National Institute of Metrology (NIM) China is the first organization to research absolute gravimeter in China. Taking the example of development of absolute gravimeter in NIM, we review the technical development of laser absolute gravimeter and atom interferometry absolute gravimeter, especially reveals the revolutionary contribution to the development of absolute gravimetry due to the invention of laser technology.

With the commercialization of the fifth-generation mobile communication technology, the global network traffic is exploding. However, due to the proximity to the Shannon limit, the capacity of the traditional single-core single-mode fiber communication system is unsatisfactory in the future network construction. In order to meet the upcoming challenges, it is urgent for us to upgrade the communication system for capacity expansion. To this end, fiber communication systems using space division multiplexing (SDM), dividing the channels in the spatial dimension, can accelerate the capacity expansion of communication systems. In an SDM system, the fiber amplifier has always been one of the most indispensable devices and has become a research hotspot. In the amplifier, when the power gain is unbalanced among channels, the power difference will accumulate with the transmission distance, leading to an increase in the outage probability and bit error rate at the receiver. Eventually, the transmission performance of the system is directly affected. Thus, achieving the gain equalization among different channels is a key to the SDM fiber amplifiers. In this paper, according to the theoretical and experimental researches, we review the gain equalization technology of SDM fiber amplifiers, including fiber structure design, refractive index doping profile optimization, and pump mode selection. Besides, the working principle, structural characteristics, and deficiencies are analyzed in detail, which provides solutions for the optimal design and performance improvement of SDM fiber communication systems.

At present, the number of illegal intrusion incidents in the field of public security is increasing rapidly, so new sensing technologies are urgently needed to detect illegal intrusion events. Aiming at the bottleneck problems of accurate positioning, accurate identification, and multi parameter fusion, Tianjin University, Tiandy Technologies Co., Ltd., and other research institutions have proposed the key technology for event recognition and perception of photoelectric information. It closely integrates the advantages of video monitoring and distributed optical fiber sensing technology. This technology has realized all-weather and full coverage monitoring and recognition of abnormal events in key areas through the optical time and frequency domain distributed disturbance positioning technology, interference signal pattern recognition technology, superstar optical image recognition technology, video capture analysis and recognition technology, multi parameter sensing perception and fusion technology, active polarization tracking and sensing technology, etc. This paper focuses on the research progress of key technologies of event recognition and perception in photoelectric information events.

The rapid growth of multi-media and data applications has driven the bandwidth demand for long-haul fiber-optic links at unprecedented rates. Against this background, the combination of time division multiplexing, wavelength division multiplexing, and polarization division multiplexing with multilevel modulation makes the transmission capacity of single-mode fibers in the large-capacity transmission system rapidly approach the Shannon limit. In view of this problem, the space division multiplexing (SDM) technology that can break through this limit provides new solutions for the growth of fiber capacity in the future. In this paper, we discuss three kinds of SDM fibers, namely, multi-core fiber, few-mode/multi-mode fiber and few-mode multi-core fiber, and introduce the research progress and design method of the SDM fibers. Furthermore, the comparison among the above SDM fibers is also discussed to reveal their advantages and disadvantages.

Since the phase change of the scattered light is extremely sensitive to external disturbances, the phase sensitive optical time-domain reflectometer (Φ-OTDR) technology has the characteristics of high response speed and ultra-high sensitivity. Compared with other existing distributed optical fiber sensing technologies, Φ-OTDR technology has obvious advantages in environmental adaptability and convenience of optical cable layout. This article introduces the principle, structure, performance, and application of the Φ-OTDR system, and looks forward to its development trend.

Traditional spherical and aspherical surfaces offer limited degrees of freedom for optical system design. Freeform surface breaks the geometric constraints of rotational or translational symmetry. It can correct the aberrations in non-rotationally symmetric systems while decreasing the system size, mass, and number of elements in optical design. The system specifications, configurations, and functions which are difficult to be realized by traditional spherical or aspherical systems can be achieved by using freeform surfaces. The use of freeform surfaces not only offers great potential in the development of the optical design field, but also introduces new difficulties and challenges. This review briefly summarizes the current status of research on the freeform imaging system design. The commonly used types of freeform surface mathematical expression and the aberration theory of freeform imaging systems are demonstrated. The design methods of freeform imaging systems and the applications of freeform surface in various imaging systems are summarized. Finally, the future research directions of the freeform imaging system design are discussed and analyzed.

Optical microscopy is non-invasive, sample-friendly, and fast, rendering itself the major approach for human beings to explore the microcosmic world. However, the diffraction limit has hindered the resolution of all optical imaging systems to approximately half the wavelength of visible light for over a century, until it was fundamentally broken by the development of super-resolution optical fluorescence microscopy. This technology bridges the gap between the electron microscopes (1 nm) and the ordinary optical microscopes (200 nm to 250 nm), but it is essentially useless for most samples, especially those non-fluorescent-labeled. In recent years, inspired by the synthetic aperture imaging, we have developed the optical super-resolution imaging based on frequency shift, providing a new approach to optical super-resolution imaging. As the technique is not limited by the nonlinear effects of fluorescence, it is applicable for both non-fluorescent-labeled and fluorescent-labeled samples. Besides, due to such advantages as fast imaging, high sample universality, and low phototoxicity, the technique shows good prospects in multiple fields, including materials science, biology, and medicine. In this paper, we deliver a detailed overview on both the principles and methods of optical super-resolution imaging based on frequency shift, as well as our prospects to the future development direction of this technique.

An optical freeform surface has been recognized as a revolutionary element in a modern optical system because of its powerful ability of simultaneous aberration correction and structural optimization. However, the complex shape of its surface brings enormous difficulties and challenges to the precise measurement, which limits its manufacture level and has been one of the bottlenecks of its broad applications. Currently, the main measurement methods of an optical freeform surface are mainly developed from the ideas of aspheric measurement technologies. In this paper, we reviewed the development of the optical freeform surface measurement methods, especially focused on several typical measurement methods and their characteristics, and looked forward to the development trend of freeform surface measurement in future.

Based on published literatures about lunar laser ranging with three retro-reflectors landed on the Moon by the astronauts of Apollo 11, 14, and 15 missions in 1969—1972 and the two retro-reflectors landed on the Soviet roving vehicles in 1970—1971, some related physical and technical problems in lunar laser ranging are analyzed and discussed. These problems include: 1) the direction of the retro-reflector array and the accurate determination of its position on the lunar surface; 2) the identification of laser echo signals from the retro-reflector; 3) the intensity of receiving signals and the signal-to-noise ratio; 4) the precision of the lunar laser ranging results; 5) the difficulty of ranging in the full moon period; 6) the effect of the Earth's atmosphere on lunar laser ranging and its precision. Some key technical details in the ranging loop have not been given in the published literature. There is a lack of direct physical evidences for the identification of signals as the laser back from retro-reflectors. The further analysis on the relationship between the lunar ranging precision claimed and the laser characteristics is needed.

Laser is called “the fastest knife”, “the most accurate ruler”, and “the brightest light”. Together with the atomic energy, computer, and semiconductor, they are called the four new inventions in the 20th century. High power semiconductor lasers are widely used in industrial processing, medical cosmetology, optical fiber communication, unmanned driving, intelligent robot, and so on. How to realize high power semiconductor laser source has always been an international research frontier and a hot topic. Thus, the development history of high-power semiconductor lasers is briefly described. The common technologies of high-power semiconductor lasers, including high-power chip technology and high-power beam combining technology, are summarized. Finally, the development direction of high-power semiconductor lasers is prospected.

Single-frequency fiber lasers (SFFLs) have been widely applied in the fields of laser weapons, laser lidar, laser space communication, coherent optical communication, high-precision spectral measurement, and gravitational wave detection. In this paper, the research progress of SFFLs at home and abroad is reviewed on the three typical working wavelength bands of 1.0, 1.5, and 2.0 μm. The content includes the SFFL generation, noise suppression, linewidth compression, continuous and pulsed single-frequency laser amplification, and other technologies. In addition, combining with our research on SFFLs, we focus on the recent progress of SFFLs based on the single oscillator and main oscillation power amplifier, and prospect the future development of SFFLs.

Superhydrophobic surfaces have aroused tremendous attention due to their broad promising applications. Compared with the traditional micro/nanofabrication, femtosecond laser microfabrication has become an effective tool for fabricating surperhydrophobic surfaces owing to the advantages of ablating a wide variety of materials, high processing precision, strong controllability, etc. In this review, we introduced the features of femtosecond laser microfabrication and the theoretical basis of wettability. Then, the research progress of different femtosecond laser-induced superhydrophobic surfaces and the related applications were summarized. Finally, the existing problems and future prospects in this field were discussed.

As an important frontier in the 3--5 μm mid-infrared lasers, semiconductor interband cascade quantum well laser has important scientific significance and application value in many fields, such as semiconductor optoelectronic device technology, gas detection, medical science, and free space optical communications. The emission mechanism of semiconductor interband cascade quantum well is dominated by the interband emission combination of electrons and holes in the Type-Ⅱ quantum wells, and then cascade amplification is formed in the electron injection region and the hole injection region, so as to realize the reuse of electrons and holes in multiple quantum well periods. In this paper, the development history of semiconductor interband cascade lasers, from the proposed band structure, epitaxial materials to device fabrication technology, was reviewed, and the basic concepts and working principles of each functional area in a device structure were analyzed. Furthermore, the milestone breakthroughs in the design of device structures and the technical difficulties of fabrication process were introduced, and the designs such as rebalancing of carriers and separate confinement layer were explained in detail. Finally, the development direction and trend of semiconductor interband cascade lasers were forecast.

In addition to generating stable ultrashort pulses, mode-locked lasers can also generate a series of important non-equilibrium dynamic processes. These fast-changing dynamics processes are helpful to understand the dynamics of ultrafast lasers and related nonlinear systems, and also have important guiding significance for the stability design of ultrafast lasers. With the development of ultrafast detection technology, a series of breakthroughs have been made in the study of ultrafast dynamics of mode-locked lasers. Several typical non-equilibrium dynamics processes of mode-locked lasers are introduced, including mode-locked initiation process, soliton molecular dynamics, breather ultrafast laser, and explosion dynamics of solitons and breathers. These studies not only reveal new physical mechanisms in ultrafast lasers, but will also further promote the development of theories related to ultrafast lasers, solitons, and breathers.

Shanghai soft X-ray free-electron laser (SXFEL) test facility is the first coherent X-ray light source in China, and its output wavelength is shorter than 9 nm. This free-electron laser (FEL) facility is based on a 0.84 GeV linear accelerator, and its main goal is to develop FEL related technologies and test new FEL schemes, especially the cascaded seeding FEL and short wavelength EEHG FEL. This facility already achieved its design goal and passed the national acceptance in November 2020. Status and main achievements of the SXFEL test facility are introduced in this article.

A ceramic laser is one using transparent ceramic materials as its gain media. Compared with single crystals, transparent ceramics have the advantages such as short preparation cycle and low sintering temperature. They possess good optical uniformity under high doping concentration of active ions and are easily developed into various large size composite structures. In recent years, ceramic lasers have made a rapid progress in high power as well as ultra-short and ultra-strong laser operation, and have induced a series of achievements. In this paper, the development progress of ceramic lasers is reviewed and the latest developments of transparent ceramics in high power, ultra-short pulsed laser operation and laser output with special wavelengths are summarized. In addition, the development trend of novel laser materials based on the advantages of ceramic preparation is also prospected.

Semiconductor lighting is a new industry emerging at the beginning of the 21st century, and is also the first breakthrough for the successful industrialization of the third generation of semiconductor materials in China. The technology development is changing day by day, and it is one of the focal points of international high-tech competition. At present, China's semiconductor lighting industry has formed a complete industrial chain. The core technologies of power white LED, silicon LED, full spectrum LED and others are synchronized with the international markets. Innovative applications such as UV LED, visible light communications, agricultural lighting and photo-medical treatment are leading the world. Here, the research progress of semiconductor lighting in China is introduced, the development history of its related industry is reviewed, and its future is prospected.

Since its publication in 1981, Acta Optica Sinica has always carried the responsibility of publishing the best optical scientific research results in China. As the main window for academic exchanges between Chinese optical researchers and their counterparts at home and abroad, Acta Optica Sinica has experienced nearly 40 years of baptism and has also cultivated generations of China’s best optical researchers. This article first reviews the transformation of Acta Optica Sinica from its start publication to enterprise transfer and cluster publishing, to new media and network publishing. Second, the publication data, database inclusion, citation indicators, and honors of Acta Optica Sinica over the years are calculated. Finally, the future strategy and development direction of Acta Optica Sinica are described.

Nonlinear frequency conversion techniques, which are one of the earliest studied nonlinear optical phenomena, have been developing for several decades. The principles and applications of nonlinear frequency conversion have become increasingly mature with time. In this study, new phase-matching principles associated with nonlinear frequency conversion are proposed and demonstrated. In addition, nonlinear frequency conversion has been reinvigorated by the continuous development of integrated photonics, structural photonics, and quantum optics and has played an irreplaceable role in various fields. Further, we focus on nonlinear frequency conversion and introduce its new principle, platform, and application. Based on the research of our group, we introduce the recent progress of nonlinear frequency conversion in related fields. The contents of this study include the new principle of phase matching at the nonlinear interface, nonlinear harmonic manipulation using structured light, integrated nonlinear optical new platform of lithium niobate thin film, and various new applications such as single-photon frequency conversion and optical quantum interface.

Artificial microstructure, due to its ability to trap electromagnetic waves of a specific frequency, which is one of the important platforms to enhance light-matter interactions and manipulate optical fields. Bound states in the continuum (BICs) are located in the continuous radiation region in the spectra, which are an eigenstate completely orthogonal to the radiation continuum in an open wave system. Originating from the destructive interference of waves, BICs can greatly suppress the radiation loss of micro-nano photonic devices, which offer a brand-new solution to optical binding in artificial micro-nanostructures. In this paper, the historical developments of BICs are briefly reviewed. Besides, the advancements and applications of the theoretical models of BICs in different optical artificial micro-nanostructures are emphatically introduced. In conclusion, BICs are expected to boost further progress in optical communications, integrated photonics, and efficient optical field manipulation.

Synthetic dimension emerges as a new frontier of the researches in nanophotonics and topological photonics. It is general believed that the dimension of a physical system cannot be larger than its geometric dimensionality. With the introduction of additional synthetic dimensions, and combined with the intrinsic geometric dimension, one can investigate higher dimensional physics. Meanwhile, the synthetic dimension is highly controllable which contributes to the observation of many high-dimensional novel phenomena. In this review, we introduce the basic concepts of synthetic dimension in photonics, summarize various proposals and set-ups in generating new synthetic dimension, and briefly discuss potential contributions of synthetic dimension in fundamental physics as well as applications.

Freely controlling electromagnetic (EM) waves in desired manners are not only highly important for scientific researches, but also an urgent need in the fields of communications, energy, national defense, etc. To overcome the limitations of natural materials on controlling EM waves, people proposed to construct artificial metamaterials based on subwavelength-sized microstructures with tailored EM properties. Metamaterials have exhibited many fascinating EM effects, such as negative refraction, optical cloaking, and so on. However, after years of development, metamaterials still suffer from many challenges including complexities in structures, high losses, difficulties in optical integrations, etc. Recently, together with other scientists, we proposed the concept of metasurfaces. Utilizing the abrupt phase changes of EM wave scatterings at the meta-atoms and fully exploring the microscopic order of the meta-atoms arrangement, metasurfaces have exhibited much stronger control capabilities on EM wave amplitude, phase, polarization and wavefront distribute, overcoming the bottleneck issues faced by bulk metamaterials. In this article, we will mainly review our key research works on using metasurfaces to control EM waves, including polarization control, wavefront control and dynamic control.

Traditional microwave transmission lines such as micro-strips cannot precisely manipulate electromagnetic modes, and hence traditional electronic information systems suffer from some bottlenecks such as spatial coupling, dynamic response, and performance robustness. To this end, metamaterials of spoof surface plasmon polaritons (SSPPs) provide a strategy to break these bottlenecks and have attracted many research interests in optical and information fields. To be specific, SSPP metamaterials can mimic the behaviors of optical surface plasmon polaritons and manipulate the electromagnetic fields at microwave and terahertz frequencies. Furthermore, with the configurational characteristics similar to those of planar circuits, the SSPP structures can be used to prepare the basic transmission lines of the next generation of integrated circuits. In addition, SSPPs can be divided into the propagation type and the localized type. The propagation type SSPPs, beginning with the three-dimensional structures, have been developed into the ultrathin corrugated metallic strip configurations. Based on the above configuration, scholars have established a new framework for microwave circuits and prepared typical passive and active SSPP devices including filters, antennas, amplifiers and frequency multipliers. Recently, an SSPP wireless communication system has been reported, which can achieve the non-line-of-sight wireless communications of sub-wavelength-spacing multichannel signals. Similarly, the spoof localized surface plasmon (SLSP) metamaterials have also developed from the three-dimensional structures to the ultrathin configurations, and have adopted spiral configuration, chain configuration, high-order mode, and hybridization mode to provide more degrees of freedom for the sub-wavelength scale control of electromagnetic waves. Finally, we systematically discussed the related theories and applications of SSPP metamaterials in the microwave circuits, including the basic concepts and configuration evolution of SSPP metamaterials, the passive and active SSPP devices, and the wireless communication system.

The main goal of strongly coupled cavity quantum electrodynamics (C-QED) is to study physical phenomena occurring during the interaction between light field and matter confined in a finite space. The C-QED system provides an effective tool for the deep understanding of the dynamic behaviors of atoms interacting with photons. As the core of the C-QED system, the high-finesse Fabry-Perot (F-P) optical cavity plays the basic roles in the realization of strong coupling between light and matter, in the exploring of the interaction between light and matter under the extreme conditions, in the precision control of atoms, and in the sensitive detection of the related processes. We briefly introduce the high-finesse F-P cavity and its applications in strongly coupled C-QED system, including its research background, status and development trend. In addition, the future development and potential applications are prospected.

Based on the features of nonionization, noninvasiveness, high penetration, high resolution, and spectral fingerprinting of terahertz (THz) waves, terahertz spectroscopy has great potential in the biomedical field. Based on terahertz spectroscopy, combined with different analysis algorithms, different research groups have achieved qualitative and quantitative identification of mixture samples. However, actual biological mixture samples often comprise different components, including water, which results in poor spectral signal-to-noise ratio and large errors in the final spectral analysis results. For these problems, the use of noise reduction and reconstruction algorithms is effective solutions. These algorithms improve the signal-to-noise ratio of the spectrum by eliminating invalid information in the spectral data or extracting valid information. Finally, these algorithms can be combined with analysis algorithms to provide high-precision qualitative and quantitative identification of biological samples. In this paper, we discuss the main algorithms applied in terahertz spectroscopy over the past five years and summarize their advantages and disadvantages.

Micro/nano-structured optical fiber laser spectroscopy refers to laser spectroscopy with hollow-core or micro/nano-scale solid-core optical fibers as the sample cells. Light-matter interaction takes place inside or in the close vicinity of the core. This paper reviews the basics of light propagation in micro/nano-structured optical fibers, introduces the theory and construction of gas/liquid cells with these fibers, and reports the recent progress of gas/liquid detection with direct absorption, photothermal, photoacoustic, fluorescence and Raman spectroscopy as well as possible further research directions. A micro/nano-structured optical fiber tightly confines a light mode in or near the fiber core and has a large fraction of mode field in air, which enables strong light-matter interaction over a long distance. The use of micro/nano-structured optical fiber as the sample cells would improve the performance of existing spectroscopic techniques as well as create novel spectroscopic methods. The optical fiber sample cells may be flexibly connected to other photonic components, promoting miniaturization and practical applications of spectroscopic sensing and instrumentation.

Cavity ring-down spectroscopy (CRDS) technology has high precision, high sensitivity, and large linear dynamic range, and is widely used in environmental carbon and water cycle monitoring, human expiratory monitoring, and deep sea/ocean dissolved gas monitoring. This review article briefly introduces the basic principle of CRDS and its development history, and summarizes the recent progress in the application of trace gas and isotope detection in domestic and foreign research institutions. The content, the achieved progress, and the existing problems of our research are given in detail in the fields of environmental atmospheric greenhouse gas detection, Qinghai-Tibet Plateau gas profile detection, and deep-sea dissolved gases and their isotopes detection. Application prospect and future development trend of CRDS in trace gas detection are prospected.

Extreme ultraviolet, X-ray and neutron optics are the high-precision observation methods for the development of modern science, which requires the support of different thin film optical components and systems with high quality. Due to the limitation of the short wavelength and optical constants of materials, the structure, optical performance and fabrication techniques of the short wavelength optical components are significantly different with those of the long wavelength optical components. The Institute of Precision Optical Engineering (IPOE) in Tongji University had 20 years research experience in this field. We have built a high-accuracy fabrication and detection platform based on the short wavelength mirrors, developed interface engineering methods for deposition of ultrathin multilayer film, extended the coated technology of large size mirrors, innovated the diffraction theory and fabrication process for high-efficiency/high-resolution multilayer nanostructures, preliminarily studied the basic damage mechanism of the mirrors under short wavelength irradiation, and formed a complete technology chain to develop thin films and crystal based optical systems. These optical components and systems have achieved a series of successful application in the short wavelength photon/neutron science facilities, both in China and in other countries. This paper will briefly introduce the recent research progress of the above mentioned optical components and systems in IPOE.