Semiconductor laser has been half a century since its birth, tremendous progress has been made in theory, practice, and applications, and the market occupies more than half of the entire laser field. It is widely used in communication networks, industrial processing, medical and beauty, laser sensing, aviation and defense, security protection, and even consumer electronics. On the basis of reviewing the development history of early domestic and international semiconductor lasers, this article mainly focuses on GaAs-based 8xx nm and 9xx nm semiconductor lasers in the field of high-power pump sources, 905 nm tunnel junction lasers and 940 nm vertical cavity surface emitting lasers in the field of three-dimensional sensing, and GaSb-based infrared lasers and InP-based quantum cascade lasers in the field of spectral analysis and infrared sensing, for a brief review. The content includes the main application scenarios, the main goals pursued, the latest developments in the past 10 years at home and abroad, and the possible development trends and directions in the future.

Neural networks, as one of the most representative techniques in artificial intelligence, have been in rapid development towards high computational speed and low power cost. Due to intrinsic limitations brought by electronic devices, it can be hard for electronic implemented neural networks to further improve these two performances. Optical neural networks can combine both optoelectronic technique and neural network model to provide ways to break the bottleneck. In order to have a brighter view on the history, frontiers and future of optical neural networks, optical neural networks of feed-forward, recurrent and spiking models are illustrated in this paper. Challenges and future trends of optical neural networks on in situ training, nonlinear computing, expanding scale and applications will thus be revealed.

Air lasing refers to the coherent emission produced with air as the gain medium. Air lasing has numerous advantages such as high directionality, high coherence, high intensity, and free-space propagation. Therefore, air lasing provides a novel pathway for remote sensing. Air lasing, which is generated by the interaction between a strong ultrafast laser and atoms or molecules in air, includes many new strong-field effects. In this paper, we reviewed the major advances in air lasing in recent years. First, generation methods and basic characteristics of three types of air lasing were introduced. Next, new physical effects involved in air lasing were revealed based on two aspects, gain mechanism of molecular nitrogen ion lasing and quantum coherence. Moreover, applications of air lasing in remote sensing were discussed. Conclusively, the research significance of air lasing was summarized, and the opportunities and challenges in this topic were highlighted.

Optical waveguides are fundamental elements in integrated optics. Compact lasers based on the waveguide platforms are miniature light sources that have gained increasing research attention. Waveguide lasers are expected to play important roles in photonics. Laser crystals are major gain media for solid-state lasers. In this study, we review the state-of-the-art advances in solid-state waveguide lasers, including operation of continuous wave and pulse (Q-switched or mode-locked). The lasing wavelength covers a wide spectral range from visible light to the mid-infrared. Furthermore, a brief perspective on future research directions is provided.

In the past decade, mid-infrared ultrafast mode-locked lasers have made significant progress, effectively promoting their applications in many fields, including mid-infrared frequency combs, molecular spectroscopy, material processing, laser surgery, molecular organism, and chemistry. Herein, starting with the progress of mid-infrared ultrafast fiber lasers in recent years, we introduced the development of various optical fiber lasers in this band and discussed femtosecond fiber laser solutions achieving shorter pulse widths and farther wavelengths. Further, we introduced chirped pulse amplification and nonlinear amplification. Thus, mid-infrared ultrafast fiber lasers are in the high-speed development stage and will have abundant applications in the future.

X-rays produced through picosecond petawatt lasers have high flux, short pulse duration, and tiny focal spot. High spatial and temporal resolution X-ray point-projection backlight radiography developed using such high flux X-rays is an important diagnostic technique for measuring the dynamic response of materials under intense laser load, the inertial confinement fusion, and other high energy density physics. Short pulse X-rays are generated through the picosecond petawatt laser beams on TITAN and OMEGA-EP systems, as well as the upgraded SHENGUANG-II facility and other large picosecond watt laser facilities. The spectrum, conversion efficiency, and resolution of the X-ray backlights produced using the interactions between the picosecond petawatt lasers and solid targets are characterized in the experiments conducted herein. Furthermore, the technology of point projection backlighting is developed, and the dynamic demonstration experiment is performed. Images of the compressed fuel of inertial confinement fusion targets and the ejecta of shock-loaded materials are successfully obtained using backlight radiography.

Fiber lasers have the advantages of high photoelectric conversion efficiency, transmitting laser with flexible medium, high power output, high beam quality, high compactedness and reliability. They are especially suitable as heat sources for two types of metal additive manufacturing technologies: powder bed fusion (PBF) and directed energy deposition (DED). This paper reviews the types, output power, and operating modes of fiber laser heat sources in those two additive manufacturing technologies, and the PBF and DED research status of fabricating typical metal materials. The relative density and mechanical properties of specimen manufactured by PBF and DED are higher than that of traditional casting and close to the level of forging. Finally, the research direction of fiber laser additive manufacturing technology is prospected, and the development trend of fiber laser is prospected according to the demand from those two kinds of metal additive manufacturing technology.

Light can carry spin angular momentum and orbital angular momentum. The spin angular momentum is associated with the circular polarization state of the beam, while the orbital angular momentum is related to the spiral phase of the beam. Since Allen et al. theoretically confirmed the physical concept of the orbital angular momentum of photons in 1992, this novel type of light field with a special spiral phase wavefront has attracted many research interests and found many important applications in both classical and quantum optical realms. This study, from both the fundamental and applied physics, reviews the preparation and detection methods of the orbital angular momentum beam, especially the recent progress in a variety of fields with the orbital angular momentum ranging from spiral-phase contrast imaging and remote sensing of the rotational Doppler effect to optical micromanipulation.

The Shanghai Soft X-ray Free-Electron Laser (SXFEL) Facility is the first coherent X-ray light source in China with a wavelength covering the water window. An elliptically polarized undulator (EPU) will be installed to meet the needs of different users, so that the output laser can be switched between linear polarization and circular polarization. This paper presents the polarization control design for the user facility, the evaluation of bunch energy jitter, and analysis of radiation power stability. Moreover, a switching structure of the circular polarization direction is also designed, so the polarization of the output laser can be switched between opposite directions on the base of permanent magnet oscillator at the rate of tens of Hz.

The phase reconstruction technique plays an extremely important role in various fields, such as materials science, biomedicine, and astronomy. In different phase retrieval technologies and applications, different light sources are needed. The wavelength, coherence, and energy of a light source can all affect the final phase reconstruction. In previous studies, in terms of spatial coherence, a light source has often been considered completely coherent light. However, in the actual experiments, X-ray and electron beams both correspond to partially coherent light. Spatial coherence of fully coherent light propagated through a medium will reduce accordingly. Therefore, it is especially important to identify appropriate ways to realize correct reconstruction under partially coherent illumination. In this paper, we provide a review of the research background and progress in developing phase reconstruction methods under partially coherent illumination. Some common methods, such as mode decomposition, transport of intensity equation, self-reference holography, and focus variation, are introduced and their advantages and disadvantages are compared. Moreover, the application of the self-reference holography method to the measurement of the correlation function for a specially correlated partially coherent beam and the determination of the corresponding topological charge of a partially coherent vortex beam is also summarized.

Recently, air lasing has attracted significant attention owing to its promising applications in atmospheric sensing and environmental monitoring. Air lasing usually refers to strong-laser-induced population inversion of atmospheric constituents, resulting in no-cavity light amplification over a remote distance. It has been revealed that with the excitation of intense laser pulses, molecules of the two primary atmospheric constituents, i.e., nitrogen and oxygen, can exhibit lasing behaviors. The gain media can be atomic oxygen and nitrogen, as well as neutral nitrogen molecules and nitrogen molecular ions. Herein, we present the phenomena, underlying mechanisms, and potential applications of air lasing with a focus on the gain media of neutral and single-ionized nitrogen molecules. Moreover, the influence of the laser polarization state on the N2+ lasing is discussed in more detail.