We predicted diode pumped gas lasers will be the future development direction of high-energy lasers (HELs) by re-investigating underlying physical problems of high energy laser. We further discussed the principle and key requirements of diode pumped gas lasers, through detailed case analysis with diode pumped alkali metal vapor lasers, and the prospect of high energy diode pumped gas lasers was given.

Significance Laser has been widely used in the production and living of human. In remote sensing detection field, using laser to detect targets is probably the most precise method for perceiving the appearance of long-range targets at present. This is because laser is characterized by its high brightness, high collimation, and strong coherence. It can actively, real timely, and precisely acquire the three-dimensional (3D) information of the detected targets.

Laser imaging detection is a target detecting method applying laser beams as detection media, which are radiated to illuminate targets in the first place according to certain spatial distribution law. Then, the data including time delay, intensity, waveform, phase, and polarization of laser echoes reflected from the detected targets are collected. After being processed, these data are presented in images. Finally, feature extraction and object inversion, to obtain the information of targets, including distance, position, reflection attribute, structure size, and motion feature, are conducted based on the images. By doing so, targets are found, identified, and confirmed. This technology can provide the information, including the range images, gray images, and feature images of targets with high resolution that cannot be obtained using general imaging methods. In addition, the laser imaging detection, characterized by high-resolution, high-measurement accuracy, anti-interference ability, and strong anti-shadowing ability, is especially suitable for the detailed detection of targets.

Due to its many advantages such as high spatial resolution, strong anti-interference and so on, laser imaging has attracted more and more attentions in the field of space object surveillance and identification. One of the most promising laser imaging methods is the laser reflective tomography imaging, introduced by Parker, Knight, and Matson et al. According to the provided signature, it can be divided into range-resolved, Doppler-resolved and angle-angle-resolved. Range-resolved reflective tomography imaging technique can be used to obtain cross-sectional image of objects that is angularly unresolved while the data are range resolved, so it is especially applicable for an object that is not rotating fast enough. Meanwhile, it can be realized in both coherent and incoherent detection systems, considering that the Doppler-resolved and angle-angle resolved can only be used in coherent detection systems. When it is realized in incoherent detection systems, it has the characteristic that its spatial resolution is not related with the imaging distance but related with laser pulse-width, bandwidth of detectors and noise, and is also insensitive to turbulence.

According to the published documents, various research institutes at home and aboard, have taken studies on the laser reflective tomography imaging, mainly including Massachusetts Institute of technology Lincoln Laboratory, U.S. Air Force Phillips Laboratory, Swedish Defense Research Agency, Shanghai Institute of Optics and Fine Mechanics, National University of Defense Technology and Space Engineering University. Especially, Charles L. Matson from Air Force Phillips Laboratory, has conducted the first satellite feature reconstruction by use of range-resolved reflective tomography techniques from non-imaging laser radar data collected on an orbiting satellite. Many corresponding advances have been achieved. But there are still many problems need to be solved in terms of practicality and the improvement of reconstructed images quality. Hence, it is necessary to summarize the existing researches to guide the future development of this field more rationally.

Progress The progress of methods, and research development in the field of laser reflective tomography imaging techniques are summarized. First, the requirement of projection sampling, the resolution of reconstructed images, signal to noise ratio calculation, the projection type which can be used to reconstruct image, and the application mode of are introduced. Second, the processing methods of projections are summarized according to previously reported studies. Considering the effects on the reconstructed image, the processing of projection can be divided into projection registration and projection de-convolution. Xiaofeng Jin's research group from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has studied on and putted forward feature tracking method for projection registration (Fig.8). Yihua Hu's research group from National University of Defense Technology has engaged in the studies on projection de-convolution (Fig.9). Third, the studies about the algorithms of imaging reconstruction are elaborated. Then, the imaging experimental systems at home and aboard are comprehensively summarized (Table 1), and the experimental studies of laser reflective tomography imaging on range-resolved and Doppler-resolved are also introduced. The studies about Doppler-resolved laser reflective tomography imaging are limited, and needed to be strengthened. In the end, the key problems needed to be solved and the ongoing research trends in this field are discussed, including the fusing with other laser imaging techniques, the practical projection processing methods, new imaging reconstruction algorithms, the design of practical imaging systems, and the key technologies of long-range Doppler-resolved laser reflective tomography imaging.

Conclusion and Prospect Laser imaging is of great importance for long-range space target detection. In order to well applicated in space target detection, the technology of laser reflective tomography imaging still needs in-depth, engineering and practical explorations to promote the development of the long-range laser imaging detection technology in engineering aspects.

Significance Coherent beam combining (CBC) of lasers is an effective technical approach for scaling the power of a laser while maintaining good beam quality. In 2009, Northrop Grumman Aerospace Systems demonstrated the world's first 100 kW solid-state laser system, in which seven 15 kW master oscillator-power amplifier laser chains were coherently combined. It was an important milestone in the history of laser development. In the past 10 years (2011—2020), CBC has developed rapidly, and many representative results have been produced. In this paper, the research progress of coherent laser beam combining in the past decade is reviewed.

Progress First, the output power of fiber, solid-state, and semiconductor lasers has increased significantly, which has provided high-power combinable laser elements for CBC systems. For example, the output power levels of single-frequency and narrow-line-width fiber lasers have reached 500 W and 4 kW, respectively; the average power of femtosecond fiber lasers has exceeded 1 kW; and Nd∶YAG and Yb∶YAG solid-state lasers have both generated over 20 kW in output power.

Second, enabling technologies for coherent laser beam combining have been developed, including phase control, tip-tilt control, polarization control, optical path difference control, high-order aberration control, and aperture?filling. More than 100 fiber lasers have been phase-locked based on active phase control technology. High-power adaptive fiber optic collimators have been designed that can be used for tip-tilt control of fiber lasers with kilowatt-level output power. Active polarization control of a kilowatt-level fiber amplifier has been realized, which has been employed to increase the combination efficiency of CBC systems. High-precision optical path difference real-time control systems have been designed and used for CBC of broad-spectrum and ultrafast lasers. Mode control technologies and adaptive optics methods have been employed for flexible mode manipulation. Aperture-filling technologies, such as microlens arrays, diffractive optical elements, and polarization beam combiners, have also been developed to increase efficiency of combination.

Third, representative results of the coherent combining of various kinds of lasers have been produced. For semiconductor lasers, coherent combining of 218 elements, with total output power of 38.5 W, has been realized. For solid-state lasers, pulse energy of 15.3 J has been obtained by the coherent combing of 6 solid-state lasers, and peak power of 3.7 GW has been obtained by the coherent combining of 2 ultrafast Yb∶YAG lasers. For fiber lasers, the number of laser channels has been increased to 107, and 16-kW output power has been obtained by the coherent combining of 32 fiber lasers. For ultrafast lasers, 61-fs fiber lasers have been coherently combined, and 10.4 kW average power has been generated by the coherent combining of 12 fs fiber lasers. In order to increase pulse energy, divided-pulse amplification and coherent temporal pulse-stacking technologies have been developed. Based on divided-pulse amplification, femtosecond pulses with energy of 23 mJ have been generated; using temporal pulse-stacking technology, 81 pulses were coherently combined to be one pulse with 10 mJ energy.

Fourth, coherent laser beam combining has been employed in versatile applications. For nonlinear frequency conversion applications, high pump brightness is required. A 600 W, 520 nm laser (second harmonic) and a 300 W, 347 nm laser (third harmonic) were obtained based on 1040 nm with kilowatt output power generated through a CBC system. Similarly, by coherently combining narrow-line-width lasers, sufficient output power was obtained for applications such as laser guide star and laser radar. By controlling the optical parameters of a coherent laser array, structural light fields with special spatial distribution can be obtained, such as vortex beams carrying orbital angular momentum. In recent years, research plans for large scientific facilities based on coherent laser beam combining have been proposed. For example, LIGO needs a low-noise single-frequency laser with hundreds of watts of output power; researchers have demonstrated the feasibility of obtaining such a laser source through coherent combining of fiber lasers. The International Coherent Amplification Network project has been proposed to provide a laser source for the next generation of particle accelerators.

Prospects In future work, increasing the number of coherent laser array elements and scaling the power of single-channel lasers will still be the development tendency. Lasers can be extended to almost arbitrary gain medium and wavelength band (visible light, mid-infrared, and even terahertz). Moreover, with the rapid development of computing technology and computational power, artificial intelligence techniques may be used for CBC-enabling technologies such as phase control and tip-tilt control. In the case of multiple-parameter control for massive laser arrays, high-power laser phased arrays will be realized.

Significance In recent years, with the continually improved power and beam quality of the high-power Ytterbium-doped fiber laser oscillator, laser oscillators are being extensively used in industry, scientific research, and other areas. Compared with the master oscillator power amplification (MOPA) fiber laser configuration, fiber laser oscillators hold the advantages of compact volume, easy control logic, low cost, anti-reflection, and high stability. With the development of fiber components and processing technology, the output power and beam quality of fiber laser oscillators will improve, and may be used instead of the MOPA fiber laser in the future.

Progress In scientific research, early in 2012, the Alfalight company reported all fiber laser oscillators with output power of 1 kW. Since then, the output power of the laser oscillator continued to increase every year and increased a lot in the last two years. In 2014, the Coherent company reported single-mode fiber laser with power of 3 kW in spatial structure. Soon, Laserline Gmbh reported 17.5 kW multi-mode fiber laser in spatial structure in 2019 ( Fig. 2). Compared with the oscillators in spatial structure, all fiber laser oscillators got more attention by the researchers. After 2016, many institutions studied single-mode laser oscillators in detail, and the output power increased from 2 kW in 2015 to 8 kW in 2020. In 2020, Fujikura reported the highest single-mode, all-fiber laser oscillator with output power of 8 kW ( Fig. 9). In our group, we are studying laser oscillator from 2010. In 2012—2020, we also demonstrated all-fiber laser oscillators with output power from 1 kW to 6 kW ( Fig. 6).

In 2010, the CoreLase company launched the fiber laser oscillator product with output power of 1 kW. After 5 years, the CoreLase company launched a 2 kW laser oscillator product. In 2015, Maxphotonics company in China also launched a 1.5 kW laser oscillator product in cooperation with our group. Since 2017, a lot of laser companies such as Lumentum, GW laser, Reci laser, Feibo Laser, and DK laser launched fiber laser oscillator products with output power from 2 kW to 4 kW (Table 2).

As we know, IPG photonics demonstrated a 10 kW single mode fiber laser with MOPA configuration. After that time, fiber laser with MOPA configuration was being developed. Many institutions demonstrated output power from 5 kW to 10 kW in recent years (Table 3). In China, some institutions including CEAP, Tsinghua University, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, and our group demonstrated MOPA laser with power more than 10 kW (Table 3). However, in the industry, only after 2019, Raycus laser and Maxphotonics launched MOPA laser products with power 3--6 kW (Table 3), and most of these were not single mode products. On comparing the production of fiber laser in scientific research and in industry, we can see that the time from scientific reported laser to the industry product in fiber laser with MOPA configuration needs almost three years, which is longer than that of the fiber laser oscillator. The possible reason could be that the laser oscillator holds the advantages of anti-reflection and high stability than the fiber laser with MOPA configuration, which gives the fiber laser oscillator a more practical option in industry. So, we can see that laser oscillators are widely used than the MOPA fiber lasers in industry. And also, in some experiment, researchers found that the fiber laser oscillator held a higher mode instability threshold than that in fiber laser with the MOPA configuration (Fig. 11).

For the future, the develop tendency of Ytterbium-doped fiber laser includes scaling both power and efficiency with good beam quality, generating special beam pattern in practice application, and extending the laser wavelength to short and long wavelengths.

To scale the fiber laser performance, close attention needs to be paid to these key technologies: high efficiency and loss pump & signal combiner are the essential preconditions for high power and good beam quality fiber laser; high efficiency and relative low absorption laser diode as the pump source for the gain fiber is the key component for increasing laser power and efficiency; new types of gain fiber such as spindly fiber are an effective method for balancing the nonlinear effect and mode instability; specific fiber grating is an effective way for lasers with controllable beam quality. Considering these technologies, a technical proposal for 10 kW high-power Ytterbium-doped laser oscillator is provided. In this proposal, we have used most of the above-mentioned technologies.

Conclusions and Prospect With further expansion in the fiber laser, the requirements for the power and beam quality of the fiber laser oscillator will increase. If using the conventional technology method for the near single-mode fiber laser oscillator, technical bottleneck will be encountered during the power increasing. Our new technical proposal combined the special high efficiency and loss pump & signal combiner, high efficiency and relative low absorption laser diode, gain fiber with vibrational core diameter and end cap with tapered fiber, which can provide a breakthrough regarding the power limitation of the conventional fiber laser, and help scale the power of single mode fiber laser oscillator to more than 10 kW.

Significance Laser-induced fluorescence (LIF) can be used to perform non-intrusive, in-situ, and temporally and spatially resolved measurements of combustion characteristics with strong species selectivity and good sensitivity. This paper reviews research progress in the development and application of multi-species planar LIF (PLIF), tracer PLIF and PLIF-based velocimetry in combustion diagnostics to measure instantaneous flame structure, fuel concentration, temperature, and velocity. This work also discusses the typical examples, characteristics, and challenges of conducting PLIF in fundamental combustion diagnostics and practical engine measurements. The technological trends about high-repetition PLIF, volumetric LIF (VLIF), and simultaneous multi-parameter measurements are also presented.

Progress The PLIF can visualize the two-dimensional distributions of multi-species generated during the combustion process and show the instantaneous flame structure. The formaldehyde (CH₂O) can be used as an indicator of the flame preheating zone, and OH radicals can be regarded as a flame marker of the product zone. Simultaneous PLIF measurements of the CH₂O and OH can obtain the heat release zone of a premixed turbulent flame shown in Fig.6(a). Fig.6(b) and Fig.6(c) show CH₂O and OH PLIF images that are acquired simultaneously as well as the distributions of the heat release zone that are obtained by the pixel-by-pixel product of OH and CH₂O. CH radicals in flames are usually employed to indicate the flame reaction zone. The CH radicals are difficult to be measured by PLIF due to their relatively low concentration in flames. The signal-to-noise ratio of the CH PLIF can be significantly improved by using a high-energy tunable Alexandrite laser at ~387 nm and the C-X excitation at ~314 nm. The distributed reaction zone can be identified in a high-speed jet flame by broadening CH distributions that can be observed by the CH-PLIF. The HCO and CH₃ radicals that indicate instantaneous flame structure can be measured by single-shot HCO PLIF and photofragmentation LIF, and the two-dimensional distributions of HCO and CH₃are shown in Fig. 6(c) and Fig. 6(d), respectively. The OH, CH, and CH₂O PLIF can be used to obtain instantaneous flame structures in a cavity-based scramjet combustor, which helps to better understand the flameholding modes and mechanism in a supersonic flow. Feature extraction of the turbulent flame front, the flame surface density, the progressive variable, and the ridge can be achieved from the PLIF images, which gives quantitative information of the flame structure and sheds light on interactions between combustion and turbulence.

The tracer PLIF can measure fuel concentration, equivalence ratio, and temperature during the mixing process by adding fluorescent tracers into small-scale burners or practical combustion systems. Characteristics of the frequently used tracer molecules are described, such as acetone, 3-Pentanone, toluene, and NO. Typical applications of the tracer PLIF in showing the fuel distribution, equivalence ratio, and temperature distribution during the mixing process of different engines are introduced. The two-line atomic fluorescence (TLAF) can be used to indicate the two-dimensional distribution of the flame temperature by seeding atomic elements into a flame. Temperature characteristics of typical atomic elements (e.g. gallium, indium, and thallium) are compared, and different seeding methods for the indium atom are introduced. Linear and nonlinear TLAF methods with the indium atom seeding can be used to show two-dimensional distributions of the flame temperature. Nonlinear TLAF is a promising technique for two-dimensional temperature measurements in a harsh environment with an acceptable signal-to-noise ratio. The challenges of conducting the tracer PLIF in quantitative measurements are presented. Accurate calibrations of the fluorescence intensity in different conditions of temperature and pressure play a key role in the quantitative measurements of the tracer PLIF and TLAF techniques.

The PLIF techniques can be used in molecular tagging velocimetry (MTV) to non-intrusively measure the velocity distribution of the flow field. In the MTV technique, a ‘write’ laser pulse is employed to generate flow tracer (e.g. NO, Kr and OH) with a relatively long-lifetime fluorescence through the process of photodissociation, excitation, or photochemical reaction, and then a ‘read’ laser pulse is used to tag the location displacement and the delay time of the tracers. The NO PLIF, Kr PLIF, and OH PLIF are usually adopted during the ‘read’ process of the MTV technique. The air photolysis and recombination tracking (APART)/vibrationally excited NO monitoring (VENOM), krypton tagging velocimetry (KTV), and hydroxyl tagging velocimetry (HTV) have been widely used in measuring the velocity distribution in a cold or reacting flow ranging from low to hypersonic velocity.

Conclusion and Prospect LIF is a non-intrusive and in-situ technique, which can be used to accurately measure instantaneous flame structure, fuel concentration, temperature, and velocity of flames and engine combustion. The repetition rate and measurement dimension of LIF techniques are required to be further improved. With the development of high-energy and high-repetition pulsed lasers, the high-speed PLIF technology (10-1000 kHz) can show the dynamic evolution of instantaneous flame structure during the process of flame ignition, flameout, and combustion oscillation. The VLIF technology can be employed to demonstrate the three-dimensional structure of the flame and realize four-dimensional (three-dimensional+t) measurements in combination with a high-speed laser. The PLIF can be synchronized with PIV, Rayleigh scattering, and other techniques to realize the simultaneous visualization of instantaneous flame structure, flow velocity, and flame temperature, which helps to further reveal the interaction mechanism of combustion and turbulence.

The applications of the PLIF techniques in practical engines need to solve many problems, such as complex optical path adjustment and optical window design, stray light suppression, and signal-to-noise ratio optimization. The combustion information obtained by the PLIF techniques is still limited. The PLIF techniques need to be combined with other measurement methods, theoretical models, and numerical simulations to better understand the characteristics and mechanisms of combustion.

Significance Thanks to the invention of the chirped pulse amplification technique and the Kerr lens mode-locking technique as well as the discovery of the titanium: sapphire laser medium, the current laser systems can provide picosecond or femtosecond laser pulse duration and simultaneously ultrahigh peak power of terawatt to petawatt levels. Recently, the peak power of multipetawatt (as 10 PW) has already been achieved in several laboratories, such as the extreme light infrastructure-nuclear physics and Shanghai superintense ultrafast laser facility. These laboratories have prompted significant progress on laser-plasma interaction. In the past decade, laser-driven ion sources and their applications have been extensively investigated. One of the significant features of laser-driven ion beams compared with conventional ion accelerators is the sufficiently small valid source size (~10 μm) and ultrashort duration (picoseconds) at the source of the ion bunch. This is attributed to the increased acceleration gradients of the order of MeV/μm, compared with MeV/m provided by the conventional ones, e.g., the radio frequency wave-based accelerators.

Progress Since the 1990s, the research on ion acceleration driven by ultraintense laser pulses has attracted significant attention from national and international professionals. Before 2000, the ion energy of several MeV had been achieved in laser-plasma experiments using different targets, such as thick solid foils, gas jets, and submicrometric clusters. However, these ion beams with a wide energy spectrum and large emission angle are unattractive as ion accelerators for many potential applications. In 2000, high-energy proton beams with a peak energy of 58 MeV are obtained in the Lawrence Livermore National Laboratory. The protons are detected at the rear side of the target and emitted as a collimated beam along the target normal direction. Since then, scientists have proposed a new upsurge of high-quality energetic ion beams driven by relativistic laser pulses. Several groups have demonstrated, over a broad range of laser and target parameters, the generation of multi-MeV proton or ion beams with unique properties, such as low transverse emittance, ultrashort duration, high-energy conversion efficiency, and narrow energy spectrum. To improve the beam quality, scientists have proposed various ion acceleration mechanisms, such as target normal sheath acceleration (TNSA), radiation pressure acceleration (RPA), collisionless shockwave acceleration (CSA), breakout afterburner (BOA), Coulomb explosion (CE), and cascaded acceleration (CA). Among them, some have been demonstrated in experiments, while some are being or will be tested in the future petawatt laser facilities.

The laser-accelerated proton and ion beams have a duration of the order of a few picoseconds, ultrahigh cut-off energy, and a narrow emittance, making them suitable for several applications, including several unique applications. In this study, we first review the historical background of laser-driven ion acceleration, such as laser technology development and laser energy absorption mechanisms in plasmas. Second, we introduce several implemented and proposed laser-driven ion acceleration mechanisms, such as TNSA, RPA, CSA, BOA, CE, and CA, based on three aspects: theoretical models, numerical simulations, and experiments. We also compare the experimental results with theoretical predictions and simulations. Furthermore, we discuss possible ways to manipulate the proton/ion beams by tailoring the target profile and changing the laser parameters, such as the laser intensity, laser contrast, focal size and duration, target thickness, and initial density.

The third part of this review involves the potential applications of laser-driven proton or heavy ion sources. Some of them have already been established, while others are yet to be demonstrated. We analyze their promising applications in fast ignition of precompressed inertial confinement fusion fuel by laser-accelerated protons, in medical therapy, and nuclear physics. The laser-driven ion sources complement the conventional accelerators in these applications, indicating remarkable different properties.

Conclusion and Prospect Laser-driven ion acceleration is a rapidly developing research area, attractive from the fundamental science and applications perspectives. Several theoretical results, experiments, and simulations have been obtained. They contribute significantly to understand many phenomena, such as jet emission, shock formation, and flying mirror generation. As the laser ion accelerators progress, we observe that some of their applications have been successfully demonstrated, while some are being tested in experiments. Besides, with the increasing laser intensity (?10²² W/cm²) in the next years, the relativistic nonlinear effects, such as radiation reaction (RR) and pair production (PP) in the ultrarelativistic regime, become crucial. Thus, the roles of RR and PP in laser-driven ion acceleration should be investigated in detail. It will be one of the attractive topics in laser-based ion accelerators. We tend to see the transition from basic research and proof-of-principle experiments to systematic study and optimization of laser-driven ion sources through some advanced acceleration mechanisms and elaborated target designs. We believe the laser-driven ion beams will find many potential applications with high scientific, industrial, and social impacts in the near future.

Significance In high-power infrared laser systems, monocrystalline silicon reflectors are widely used, and the reflectors need to achieve high-precision, high-stability beam transmission under high-power laser irradiation for long durations. The high accuracy of components and laser load capacity are both highly desired. With the continuous development of high-energy laser technology, the performance of existing monocrystalline silicon components has been unable to support the further improvement of system output power and performance, which has become a technical shortcoming. A high-energy laser system requires optimum reflector performance in systems, i.e., precision and laser load capacity. The pursuit of full-frequency spatial error restraint and reduction of laser energy absorption rate depends on fabrication quality. However, at present, fabrication technology inherits from traditional optical processing, and it is difficult to achieve both precision and laser load capability. Accordingly, it is necessary to investigate manufacturing methods and processes and combine innovative manufacturing techniques with application characteristics. By discussing the present situation and key technology of monocrystalline silicon component manufacturing, we hope technical support for realizing nanoprecision shape control manufacturing of monocrystalline silicon can be provided.

Progress This article summarizes the current status of and difficulties in manufacturing high-energy laser aspheric components and reveals the typical processing defect morphology and generation mechanism that reduce the laser load capacity. Based on the realization of high-precision processing of aspherical components, the role of new methods of controllable flexible body processing based on immersion smooth polishing, ion beam sputtering cleaning, and other methods in controlling defects is discussed to realize the formation of high-energy laser aspherical components. The specific research includes the following aspects:

(1) Nanoprecision surface shape control manufacturing technology. With continuous improvement in energy transmission ratio and irradiation distance of laser systems, the quality of optical parts has also increased. Nanoprecision manufacturing requires the full-frequency band error to converge to the nanometer order of magnitude. At this time, the surface shape error and the medium- and high-frequency roughness at the macroscopic scale will be in the same order of magnitude, and the correlation between them will be obviously enhanced. In recent years, our research group has vigorously developed ultraprecision cutting (Fig.1), magnetorheological methods (Fig.2), ion beam techniques, and cylindrical smoothing techniques (Fig.3) to obtain full-band subnanoprecision optical surfaces.

(2) Nanoprecision intrinsic surface controllability generation method. The surface of monocrystalline silicon generally leads to some typical damage precursors such as absorbable impurities, cracks, and scratches during mechanical polishing. The types and densities of these damage precursors seriously restrict the damage resistance of optical components. As a new form of subsurface defect, the body before nanodamage can be divided into mechanical, pollution, and structure types. Our research group has conducted a series of investigations on the monocrystalline silicon intrinsic surface processing method and the craft. It removes the optical component surface defect (Fig.5) and obtains the nanoprecision intrinsic surface through immersion polishing (Fig.4) and ion beam technology (Fig.6 and Fig.7), which enhances the loading ability of monocrystalline silicon.

(3) Nanoprecision shape control combination process. To achieve the goal of nanoprecision surface shape control, our research group has realized a combination process of high precision and low defect (Fig.8), which is different from the simple connection conversion of traditional machining processes. The combined process reasonably distributes all the indexes to the entire process and realizes the high-precision and low-defect control manufacturing of monocrystalline silicon components (Fig.11). Immersion smooth, ion beam modification, and postprocessing are used to optimize the defect restraint strategy and develop monocrystalline silicon substrates. Compared with the traditional process, the advantages of the combined process in improving the accuracy, reducing the absorption, and achieving good adaptability on paraboloid and cylindrical elements are verified. Full-frequency subnanoscale-accuracy manufacturing is realized on small-diameter planar components. Full-band error convergence and absorption precursor restraint are realized on large-aperture planar, parabolic, and cylindrical components and high-precision, low-absorption, and high-power laser monocrystalline silicon is processed component substrate.

Conclusion and Prospect The main achievements of high-precision and low-defect control manufacturing technology in the Precision Engineering Laboratory of National University of Defense Technology in recent years are reviewed. The high-precision and low-defect combination processing technology developed by our research group has been applied to the processing of monocrystalline silicon, which supports the development of high-power laser systems.

Significance Since gas-stimulated Raman scattering (SRS) was first reported in 1963, it has been recognized as an effective method for extending laser wavelength range from the ultraviolet to infrared bands. However, the effective interaction length between laser and gas is limited, so the threshold for SRS in the traditional gas cavity is very high. Although some methods have been developed in which hollow-fiber capillaries and high-finesse cavities are adopted, the effective interaction length remains limited. This situation has not been improved until 1999 by the production of the first hollow-core fiber (HCF), which provides an ideal environment for the interaction of light and gases. The high-intensity laser is confined to the core area and continues to interact with the gas over a very long transmission distance, thus the effective interaction length is greatly increased. Because the transmission band of the HCF can be designed, it is easy to suppress those unwanted Raman lines and increase the power-conversion efficiency of the first-order Stokes wave, thus the production of efficient gas Raman lasers becomes possible. In 2002, Benabid et al. at the University of Bath conducted the first SRS experiment in a hydrogen-filled HCF, and a new era of fiber gas Raman lasers (FGRLs) is launched.

In the main text of this paper, Fig. 2 shows the typical experimental setup of the FGRL, which includes a pump source, a light-coupling system, two gas cells, and an HCF. The pump laser is coupled with the HCF via lenses and mirrors. Generally both ends of the HCF are sealed in the gas cell, through which the gas medium is pumped into the HCF. The input/output glass window mounted in the gas cell enables laser entry/exit from the HCF. SRS of the gas medium is produced by pump laser transmission along the HCF. When the pump power exceeds the given threshold, the Stokes laser power rapidly increases and can be detected at the output end of the HCF.

The HCF is a key component of the FGRLs, providing an ideal environment for gas SRS. The HCF performance parameters have a decisive influence on the laser output characteristics. For example, the transmission band range of the HCF affects the output laser wavelength, the transmission loss affects the Raman conversion efficiency, the core diameter affects the Raman threshold, the damage threshold affects the upper limit of Raman power, and the bending resistance affects the volume of the laser source system. Therefore, the recent rapid development of the HCF has greatly advanced the development of FGRLs. In general, the inside of the HCF contains air, and laser transmission in the HCF no longer meets the principle of total internal reflection. Two main mechanisms are involved: photonic band gap effect and anti-resonance reflection. Thus, HCFs can be roughly divided into two categories: photonic band gap hollow-core fibers (PBG-HCFs) and anti-resonance reflection hollow-core fibers (AR-HCFs). PBG-HCFs feature a relatively small core region and narrow transmission band, suitable for efficient rotational SRS generation. AR-HCFs feature multiple transmission bands, suitable for gas SRS with a large Raman shift and are mainly used in the mid-infrared region. The microstructures of AR-HCFs include the Kagome type, ice-cream type, nodeless revolver type, conjoined-tube type, and nested type.

Progress Since its introduction in 2002, the FGRL has been vigorously developed. Thus far, hydrogen, deuterium, methane and other gases have been used as Raman media to achieve laser output at various bands from ultraviolet to mid-infrared. The pure rotational SRS of hydrogen in HCF with 1.1μm output was reported in 2004. In 2007, researchers reported the development of a continuous-wave-pumped FGRL based on a hydrogen-filled HCF. In 2014, the 1.9μm FGRL based on hydrogen-filled HCFs was reported. In 2016 and 2017, 1.5μm FGRLs based on ethane-filled and methane-filled HCFs were reported, respectively. The 4.4μm FGRL based on hydrogen-filled HCFs was reported in 2017. In 2018, the SRS of deuterium in HCFs was reported. In the same year, the 2.8μm FGRL in a methane-filled HCF was achieved, and the cascaded system was proposed. In 2019, the SRS of carbon dioxide in HCFs was reported, and the SRS of SF₆ and SF₄ in HCFs was reported. In 2020, the 1.7μm FGRL based on a hydrogen-filled HCF was developed. The ultraviolet laser is typically generated by Raman frequency combs. Table 2 in the main text summarizes the development of representative FBGLs worldwide.

Conclusion and Prospects Although FGRLs have rapidly developed in response to the fast development of HCFs in recent years, research remains at the very initial developmental stage. Currently, the main problems are: the limitations imposed by the instability of the spatial structure on the practicality of FGRLs; the technique used to manufacture HCFs, especially AR-HCFs, which has not reached a commercial level; the lack of development of HCF-related devices; and the urgent need to resolve the low-loss coupling of the solid-core fiber and HCF. To resolve these problems, the development of FGRLs has four main future directions: 1) development of an all-fiber system for the practical use of FGRLs; 2) extension of the output laser wavelength to the mid-infrared or even far-infrared band to enable mature FGRLs to fill the current gap of fiber lasers in the mid-infrared band, especially that above 4μm will become an important mid-infrared laser source; 3) achievement of continuous-wave Raman laser emission, especially in the mid-infrared range; and 4) achievement of high-power laser output. With the key technology breakthrough of the all-fiber gas cavity, the realization of high-power all-fiber gas Raman lasers should greatly promote the development of these light sources toward their practical applications.

Significance A fiber gas laser (FGL) based on gas filled hollow-core fibers (HCFs) is a laser source that combines the advantages of traditional gas and fiber lasers. According to the operating mechanism, FGLs can be divided into two categories: one is based on the stimulated Raman scattering (SRS) of gas molecules, and the other is based on population inversion realized by intrinsic absorption of gas molecules between the vibrational-rotational energy levels. The threshold power for population inversion is much lower than that for the SRS effect, making it easier to realize continuous-wave (CW) laser emission. The laser wavelengths corresponding to the vibrational-rotational energy level transition of most gas molecules are in the mid-infrared waveband; thus, the FGLs based on population inversion provide a novel method for the mid-infrared fiber lasers, which have wide applicability in military, biomedicine, and atmospheric communication fields. In HCFs, most of the mode energy is concentrated in the hollow-core. The laser mode edge overlaps with a small amount of glass material in the cladding. As the field intensity in the glass material is at least one order of magnitude smaller than the peak field intensity in the core region, the theoretical damage threshold of HCFs is much higher than that of solid-core fibers, making HCFs ideal to operate at much higher power level. Meanwhile, the hollow-core structure can be filled with various gain gas media to achieve plenty of laser wavelengths, especially beyond 4 μm, which is very difficult for traditional rare-earth-doped fiber lasers. Because of the intrinsic properties of gas molecular energy levels, laser output with narrow linewidth (several-hundred MHz) can also be obtained without additional linewidth control technology for FGLs, which has great advantages in maintaining laser linewidth at high power compared with lasers using solid-core fibers. Hence, FGLs provide a universal solution for the technical bottlenecks encountered by traditional mid-infrared fiber lasers in power enhancement and wavelength expansion.

Progress The advent of HCFs greatly promotes gas laser development, as it provides an ideal interaction environment between light and gas molecules. Since the first FGL based on population inversion was reported in 2011, it has obtained great attention because of its potential advantages in generating effective mid-infrared laser emission. In recent years, with the fast development of anti-resonant HCFs with low transmission loss in the mid-infrared waveband, FGLs operating at the mid-infrared waveband have been intensively studied recently. FGLs based on C₂H₂-, CO-, CO₂-, N₂O-, I₂-, HBr-, and HCN-filled HCFs have been reported, and most laser wavelengths are within the range of 3--5 μm, except for the I₂ laser, which operates at the 1.3-μm band.

In 2017, Xu et al. achieved the highest laser power using C₂H₂-filled anti-resonant HCFs pumped by a narrow linewidth 1.5 μm diode laser amplified by an erbium-doped fiber amplifier (Fig. 7). The highest continuous output power at 3.1 μm is 1.12 W at 0.6 mbar pressure, and the slope efficiency is as high as approximately 33%. The HCF can effectively confine both the gases and the pump light within the core area over a distance much longer than the length of traditional gas cells, greatly reducing the pump threshold and improving the conversion efficiency. In 2017, Dadashzadeh et al. studied the output laser beam quality of FGLs based on C₂H₂-filled Kagome HCFs (Fig. 8). The experimental results show that the mid-infrared FGL has good beam quality as traditional fiber lasers, the best M² factor measured is less than 1.4, and the best value is approximately 1.15, showing the beam quality near the diffraction limit. In 2019, we achieved the 4.3 μm CW FGLs based on CO₂-filled anti-resonant HCFs (Fig. 10), which is also the first CW fiber laser with output wavelength larger than 4 μm. The pump source is a self-developed thulium-doped fiber amplifier seeded by a tunable narrow linewidth 2-μm diode laser. And the pump source is employed to pump a low transmission loss anti-resonant HCF with a length of 5 m, which is filled with low-pressure (several mbar) CO₂. At the optimal pressure of 500 Pa, the laser threshold and the maximum output power are approximately 100 and 80 mW, respectively, with a laser slope efficiency of approximately 9.3%. In 2019, Aghbolagh et al. reported that a 45-cm long Kagome HCF filled with N₂O gas was pumped with a 1.517-μm-band OPO to produce a 4.6-μm band laser with maximum output energy of 75 nJ under a pressure of 80 Torr (1 Torr≈133 Pa). However, the laser slope efficiency is only 3% because of high transmission loss of HCFs.

Conclusions and Prospect While fiber laser and gas laser technologies have reached a high level of maturity because of intense research over the last 50 years, the FGL is just in its infancy, and there are still many basic physical issues and key technologies that need in-depth investigation, such as theoretical FGL models especially at high-power, basic gas parameters' measurement, further reduction of HCFs' transmission loss, and efficient and high-power coupling technology between HCFs and solid-core fibers.

An all-fiber structure is one of the major development directions of FGLs in the future as it is an ideal choice in practical applications. However, presently, the pump light is usually coupled into HCFs by the spatial optical path coupling method. The spatial coupling structure is unstable and easily influenced by the external environment, leading to decreased coupling efficiency. To realize all-fiber FGLs, we need to resolve the following key issues: low-loss coupling between HCFs and solid-core fibers and fabrication of high-stability low-pressure all-fiber gas cells.

Another important direction for the development of mid-infrared FGLs is to achieve high-power output. Compared with the traditional solid-core fiber, most of the mode field energy in HCF is concentrated in the hollow-core region. The overlap area between the glass material and core area is very small. Therefore, theoretically, the damage threshold is much higher than that of the solid-core fiber, which is very potential for its high-power output. However, the highest power reported is only at the watt level. In the future, we will resolve several key issues to achieve higher power output, mainly including a high-power theoretical model, suitable narrow linewidth high-power pump source, and low-loss coupling of the high-power pump laser.

Obtaining more abundant laser wavelengths is also an important direction in the future. Compared with solid-core rare-earth-doped fiber, gas gain media are more convenient to be replaced in GFLs, and there are more choices. Suppose the HCFs' transmission bands are designed properly, with suitable gases and pump sources. In that case, we can obtain lots of laser wavelengths, especially in the mid-infrared band, which are not easy to achieve with traditional fiber lasers. In the future, if we use soft glass to manufacture HCFs, it is expected that far-infrared FGLs can be realized. On the other hand, FGLs also have certain advantages in realizing laser output in the visible and ultraviolet bands. Especially in the ultraviolet band, the photon darkening effect of HCFs is much weaker than that of solid-core fibers. The choice of gain media in the visible and ultraviolet bands is very rich, including common inert gases, various chlorides, and metal vapors. The pumping method can use optical pumping and electric excitation.

Significance High-power fiber lasers offer many advantages: high-power output, excellent beam quality, high optical efficiency, high degree of integration, high reliability, and spatial compactness. Fiber lasers have therefore been widely adopted for both scientific and commercial applications including material processing, free space communication, and military defense. Among the different kinds of fiber laser, the GTWave is especially popular for its significant power scalability and flexible structure design. This paper reviews the results of high-power GTWave fiber lasers across the world; their characteristics and merits are analyzed through comparison with conventional double cladding fiber lasers. Future directions of research on GTWave fiber lasers are additionally discussed.

Progress The construction of GTWave fiber is detailed in Fig.1 of this paper. The results of high-power GTWave fiber lasers all over the world, shown in Table 1, indicate that most of the results are made by four organizations: Southampton Photonics Inc., the IPG Photonics Corporation, China’s National University of Defense Technology, and the China Academy of Engineering Physics.

The concept of using optical fibers to evanescently couple pump energy from laser diodes to a solid-state laser rod was proposed by U.S. Naval Research Laboratory in 1991, but the University of Southampton made the first GTWave fiber, and Southampton Photonics Inc. manufactured many high-power fiber lasers that demonstrated the high injected pump power and high-power scalability of GTWave fiber lasers. Southampton Photonics Inc. manufactured a 2 kW GTWave fiber oscillator in 2016; its stimulated Raman scattering (SRS) level was low, and the transverse mode instability (TMI) had been mitigated. The IPG Photonics Corporation also developed GTWave fiber very early, and manufactured a 2 kW fiber laser in 2006, which showed the flexible structure design of GTWave fiber and displayed massive injected pump power. They created the first 10 kW high-power GTWave fiber laser in 2009, which was much more powerful than other fiber lasers. The Chinese National University of Defense Technology developed a homemade 1 kW GTWave fiber laser in 2014, and obtained a 4 kW GTWave fiber oscillator and main oscillator power amplifier (MOPA) by multi-stage bidirectional pumping in 2018. Although the China Academy of Engineering Physics’ research on GTWave fiber started relatively late, they adopted multi-pump fiber schemes to develop GTWave fiber and developed a (2+1) GTWave fiber 2 kW MOPA laser in 2016. They obtained a 10.45 kW (8+1) GTWave fiber MOPA laser in 2018.

Many other organizations have paid much attention to GTWave fiber lasers; however, the technology requires much investment and technology, and as a result most can do only theoretical research. The Russian Academy of Sciences developed a 100 W GTWave fiber laser in 2005, and made many low-power fiber components in early years, but no results have been reported of late.

Conclusion and Prospect The above research shows that the development of GTWave fiber lasers is very fast, and that their power scalability is so strong that their output power is much higher than that of other fiber lasers. The output power of GTWave fiber has grown to 10 kW within 10 years. We analyze the characteristics and merits of GTWave fiber lasers through comparison with conventional double cladding fiber lasers. Although the conventional double cladding fiber laser has exceeded 5 kW for the reverse combiners commercialized in recent years, GTWave fiber lasers are still more suitable for bidirectional pumping and multi-injected ports by means of multi-stage cascaded amplifiers or multi-pump fiber schemes. As the pump light is coupled gradually to the active GTWave fiber, the heat is well-distributed along the fiber, which is more suitable for a high-power fiber laser.

The development process of GTWave fiber is very complicated, and the price is very high at present. This limits the research of GTWave fiber lasers to only a few organizations. With the development of high-power fiber lasers, people have paid more attention to GTWave fiber both theoretically and experimentally. We believe that the commercialization of GTWave fiber will come in the near future, as more and more people conduct research on GTWave fiber lasers.

Significance Photons can generate radiation pressure on objects due to optical momentum transfer. For light sources existing in nature, the mechanical effects are very weak, and thus difficult to be directly observed and utilized. Until the 1960s, the emergence of laser has provided strong collimating light sources for the study of optical radiation pressure, which finally resulted in the birth and development of optical tweezers technology.

As a pioneer of optical tweezers, Arthur Ashkin from Bell Laboratories successfully captured SiO₂ particles in water with double laser beams in 1970. Later, he successfully captured SiO₂ microspheres in air environments, and oil droplets in the environments of low gas pressures. In 1986, Ashkin used a highly converged single laser beam to trap the particles, which is well known as optical tweezers nowadays. The technology of optical tweezers is subsequently applied in biology, and Ashkin received the Nobel Prize in Physics in 2018.

In the past three decades, optical tweezers have been diversified into many schemes, including holographic optical tweezers, time-modulated optical tweezers, femtosecond optical tweezers, optical tweezers in vacuum, etc. Due to the features of non-contact and low damages, this manipulation technology has reached outstanding achievements in biology, nanotechnology, fundamental physics, quantum science, precision measurement, and so on. Especially in the last decade, the rapid development of optical tweezers in vacuum has attracted many top research teams. In 2010, the instantaneous velocity of Brownian particles have been measured in the experimental system of optical tweezers for the first time since Einstein's conclusions in 1907. Later, particles in micron-sizes are stably trapped in dual-beam optical tweezers in high vacuum, and the equivalent temperatures of the mass center motions can be cooled into several mK by the scheme of optical momentum modulations. At almost the same time, nanospheres are also stably trapped in single-beam optical tweezers in high vacuum, and a quite different scheme known as parametric feedback cooling is proposed and employed to decrease the center-of-mass temperatures into sub-Kelvin. For the particles, both in micron-scales and nano-scales, trapped in vacuum optical tweezers, the absence of collisions from the fluid molecules will provide robust decoupling from the heat bath of the fluid environments and no longer need cryogenic precooling. Thus center-of-mass temperatures close to the quantum ground state can be within reach in relatively miniaturized systems in room temperatures. Meanwhile, the trapped particles can be approximated as ideal harmonic oscillators, which is considered as the “ideal platform” for many physical quantity precise measurements, including weak force detection with resolutions in almost 1 aN/Hz^1/2, acceleration sensing with resolutions of about 100 ng/ Hz^1/2, milli-charge measurements, torque detection with a new sensitivity of 4.2×10^-27Nm/Hz^1/2, measurements of high frequency gravitational waves, and so on.

Progress Here we will introduce the fundamental theories, the main experimental setups, and typical applications of optical tweezers in vacuum. First, the introduction of the theories are described in two parts: the calculation models of optical forces and the principles of thermodynamics in optical tweezers. Later, the main experimental setups are introduced including optimized optical structures, efficient particle loading, precise position detection, reasonable stiffness calibrations and effective cooling schemes for center-of-mass temperatures. At last, recent applications in precision measurement are summarized.

Conclusion and Prospect In the past ten years, the quality factors and the trapping duration of the oscillators in optical tweezers in vacuum increase with the advance of experimental technologies. Great potentials have been shown in the following applications including sensing extremely weak forces and accelerations, rapid spinning control, fractional charge calibration, micro torque detection, and high frequency gravitational waves detection, etc. Thus the harmonic oscillators can be used to test the laws of thermodynamics, or to search the evidence of dark matter, or to explore macroscopic quantum effects. The realization of the macroscopic quantum states can further enhance the sensitivity of the precision measurement. In addition, optical tweezers in vacuum can work at room temperatures, without the need for additional refrigeration equipment. Recently, it is predicted that feedback cooling schemes might be eliminated to achieve stably trapped particles when the noise is low enough in ultra-high vacuum environments. This will make the whole system more concise and efficient to have more broad application prospects.

Currently, there are two main ideas for the future development of optical tweezers in vacuum: one is the system consisting of common optical components (named common system) and the other is the system to be built based on optical fibers and integrated optics (named integrated system). The common system will continue to pursue sensitivity breakthroughs in high precision measurement, and find possible applications in the exploration of multidisciplinary frontiers and cutting-edge technologies. Several issues are still waiting for better solutions, including further cooling of center-of-mass temperatures, more precise position detection of the particles, longer trapping duration, fewer laser trapping powers, and so on. A main focus could be how to realize a possibly simpler and more efficient cooling scheme for almost all particles, and thus reduce the system noises in a near future. At the same time, with the rapid development of fiber communications and the etching technologies in integrated optics, the integrated system can be made towards small volumes, miniaturization and low-power consumption. It is an important technical route for practical applications in the future human life. At present, there are still some unsolved problems, such as repeatable loading of single particles and integratable position detection of particles in chip-scale systems.

Significance The development and application of many optical imaging and measurement systems have been promoted based on the principle of straight-line propagation of light, and the artificial intelligence technologies have also been developed rapidly. However, when the target to be detected is blocked by the scatterers such as clouds, haze, suspended dust, and turbid water with large optical thickness, due to the lack of point-to-point direct mapping between the object domain and the image space pixels, how to obtain the target image effectively becomes a difficult problem to be solved in the field of optical imaging.

The influence of scatterers on optical signal transmission mainly includes absorption and scattering, especially scattering. Scattering is due to a large number of scattering media or scattering particles with different refractive indexes and particle size distributions in the scattering body, which makes the light wave front from the target subject randomly interfere in the process of penetrating scatterers, resulting in the reduction of signal-to-noise ratio of the target and distortion of direct detection images. Optical imaging of penetrating scatterers is suitable for complex and diverse application scenarios, long target distance, and large optical thickness of scatterers. It is of great significance to solve the problem of optical scattering imaging with wide field of view and long distance.

Progress Various imaging methods based on ballistic light (non-scattering light) and non-ballistic light (scattered light) have been developed to solve the problem of optical scattering imaging. Scattering imaging technologies based on ballistic light acquisition, such as range gated imaging, polarization imaging, and adaptive optics imaging, have played important roles in astronomical imaging, transportation, underwater exploration, and biological imaging. For example, in 2017, Li et al. from National University of Defense Technology (NUTD) proposed a method for degraded matrix estimation and target image reconstruction based on laser longitudinal tomography, which effectively solved the multiplicative interference problems such as the non-uniform attenuation of the scattering medium to the target signal in range gated imaging. In 2019, Zhao et al. from Zhejiang University proposed a multi-guide-star conjugate adaptive optics correction method. By using multiple navigation satellites, the correction area of pupil adaptive optics method was increased, the correction efficiency was improved, and the field of view was expanded.

With the rapid development of computational imaging technology, a variety of new scattering imaging technologies have been developed by combining the scattering imaging with the computational imaging. In 2010, Popoff et al. proposed the scattering imaging technology based on optical transmission matrix, and in 2012, Bertolotti et al. developed the scattering imaging technology based on the optical memory effect (OME) and speckle correlation. In 2014, Katz et al. proposed the noninvasive single frame scattering imaging technology to overcome the mechanical instability of angle scanning and acquisition time-consuming defects, which promoted the development of computational scattering imaging technology.

Coherent diffraction imaging (CDI), ptychographic iterative engine (PIE), correlation imaging, non-line-of-sight (NLoS) imaging, and other new computational imaging methods are gradually combined with the requirements of scattering imaging, and have been developed rapidly, gradually solving many problems in the application of scattering imaging. In 2016, Zhou Jianying’s team from Sun Yat-sen University modeled the scattering imaging process as a convolution operation, realizing the field of view beyond the limited scope of OME. In 2018, Sahoo et al. from Nanyang Technological University in Singapore, based on the single frame speckle correlation imaging technology, used the speckle image generated in the area beyond the limited range of OME to realize scattering imaging of hidden objects in wide field of view. In 2019, Shao Xiaopeng’s group from Xidian University realized scattering imaging beyond the limited range of OME through the use of prior information; Dai Qionghai’s research group from Tsinghua University proposed and implemented a wide field of view speckle correlation imaging technology. In 2019, Li Xiujian’s group from NUDT proposed a single frame coherent power spectrum scattering imaging technology and a variable aperture Fourier PIE imaging technology, which effectively improved the imaging efficiency and enlarged the width of field of view; Shanghai Institute of Optics and Fine Mechanics proposed the idea of realizing scattering imaging in different regions to broaden the imaging field of view; Yao Baoli’s research group of Xi'an Institute of Optics and Precision Mechanics proposed a scattering imaging technology based on PIE and shower curtain effect, which broadened the imaging field of view. In 2018, Chen Pingxing’s research group from NUDT realized scattering imaging beyond the limited range of OME based on the optical path of correlation imaging. In 2019, Charles Saunders from Boston University in the United States proposed the computational perimetry using ordinary digital cameras. In 2020, Metzler at al. from Stanford University in the United States completed the NLoS imaging with remarkable effect by using the speckle correlation technology and deep learning method. These are beneficial attempts of computational scattering imaging.

Conclusions and Prospect In recent years, various optical scattering imaging technologies have been developed rapidly, have played important roles in biomedical microscopic imaging, military and civil target detection, aviation and road traffic monitoring, and other fields, and will play a greater role. However, there are still many problems in the application of wide field of view and long-distance scattering imaging. Combined with the use of ballistic and non-ballistic light, the computational scattering imaging technologies based on OME and speckle correlation, combined with CDI, PIE, and other computational imaging technologies, are expected to promote the development and application of wide field of view and long-distance scattering imaging. According to the technical development and application requirements, it is worth developing scattering imaging technologies which have the characters of the combination of active and passive imaging methods, facing non-sparse targets, moving targets, and dynamic scattering media, have three-dimensional imaging ability, or have color and spectral imaging capabilities. And it is necessary to develop the abstract mathematical model which can express the whole optical effect of scattering media.

Significance Yb-doped fiber lasers operating around 980nm have attracted much attention because of their extensive application as high-brightness pumping sources. In addition, Yb-doped fiber lasers operating at this wavelength are potential sources for blue-green light. They can generate blue light around 488nm for underwater communications and sensing by frequency doubling. However, it is challenging to achieve a high brightness with Yb-doped fiber lasers operating near 980nm. The main reasons for this are that Yb-doped fiber lasers when operating near 980nm not only have a high pumping threshold but also have serious gain competition with the amplified spontaneous emission (ASE) at around 1030nm. A number of studies have been conducted on achieving high brightness with these lasers because of their wide application potential. The output power of continuous-wave (CW) Yb-doped fiber lasers operating near 980nm was previously low, which limited their applicability. Researchers have enhanced their peak power by pulsing, realizing blue-green light output by frequency doubling. Although development of pulsed Yb-doped fiber lasers is advancing, people are still aiming to achieve a higher output power with CW fiber lasers. Early research progresses of CW Yb-doped fiber lasers operating near 980nm have been reviewed in Ref.[16] and Ref.[17]. However, these studies only review the research progress before 2011. In recent years, significant progress has been made with CW Yb-doped fiber lasers operating near 980nm. In this study, the development of CW Yb-doped fiber lasers operating near 980nm after 2011 is reviewed. The technical schemes and important experimental results are summarized and discussed. The future development of CW Yb-doped fiber lasers operating near 980nm is also discussed.

Progress In recent years, research outside China on CW Yb-doped fiber oscillators operating near 980nm has involved optimization of the fiber design and significantly progressed fiberization of the fiber laser system. Thus far, an output power of 151W at 978nm with slope efficiency of 63% has been achieved with a CW single-mode all-fiber oscillator operating near 980nm. In this scheme, the high output power was obtained by using a Yb-doped, all-solid, double-cladding photonic bandgap fiber. However, because of the complex structure of the Yb-doped all-solid double-cladding photonic bandgap fiber, optimizing the design of this fiber will also be an important research direction. Most recently, a scheme based on a Yb-doped fiber with core and inner cladding diameters varying along the fiber length in the shape of a saddle has been used for a fiber oscillator. This oscillator achieves a continuous output power of 10.6W at 976nm with laser slope efficiency of 18.4%. Compared with the Yb-doped all-solid double-cladding photonic bandgap fiber, the saddle-shaped double-cladding Yb-doped fiber is easier to fabricate. However, the limited pump power and low efficiency are common problems in this kind of scheme. Improving the output power and slope efficiency by further optimizing the double-cladding Yb-doped fiber should be the next research focus in this direction. In addition to fiber oscillators, fiber amplifiers operating near 980nm are also important. With the main oscillator power amplifier (MOPA) structure, the amplifier can achieve higher output power. Furthermore, the amplifier with the MOPA structure allows for flexibility with the chosen seed laser. At present, double-cladding phosphate fiber is mainly used in CW Yb-doped fiber amplifiers, and the output wavelength is around 976nm. There are two research directions for fiber amplifiers operating near 980nm. First, with regards to single-mode amplifiers, based on the optimization design of Yb-doped fibers, improving the output power and slope efficiency are research focuses. The latest single-mode amplifier reported has achieved an output power of 39W at 976nm with laser slope efficiency of 19%. In this scheme, the slope efficiency of the amplifier is low. Therefore, increasing the slope efficiency will be the emphasis of future research. Second, for multimode fiber amplifiers, the research direction has been to achieve high efficiency output power by increasing the core-cladding diameter ratio of the fiber. More recently, the achieved output power of this kind of fiber amplifier has been over 30W with a slope efficiency of 66%. The emphasis of future research will be to improve the beam quality together with the efficiency.

Nationally,there has been significant research progress on CW Yb-doped fiber lasers in recent years. Most recently, a CW output power of 113W at 976nm with a slope efficiency of 45% has been achieved. It is the first time that a 100-W fiber amplifier operating near 980nm has been achieved, and it provides an important basis for the next step of power upscaling. With regards to fiber oscillators operating near 980nm, progress is relatively poor because of limitations with the fabrication technology of special Yb-doped fibers (e.g., Yb-doped photonic bandgap fibers). In addition, another important research direction is optimization of the design of the double-cladding fiber based on current fiber fabrication technology to improve the output characteristics.

Conclusion and Prospect In summary, the development of CW Yb-doped fiber oscillators and amplifiers operating near 980nm has progressed rapidly in the past 10 years, where those with an output power around 100 W have been demonstrated. However, scaling up of the output power is still a challenge. The design and fabrication of Yb-doped fibers will also play an important role in the future development of fiber lasers operating near 980nm.

Significance Ultra-precision optical processing technology is a necessary means to manufacture high-performance optical components, affecting all aspects of human production and life. With the development of modern science and technology, it has been widely and profoundly applied in various fields, from the glass screen for mobile phones to the ultra-short and ultra-powerful laser experimental devices of PW order, which are supported by ultra-precision optical processing. In the fields of integrated circuit manufacturing, chip manufacturing and other ultra-precision optical processing, ultra-precision optical processing is even the “soul of technology”, its technical level directly determines the processing quality and service life of optical elements, furthermore determines the performance limit of optical system.

As an important research direction of ultra-precision optical manufacturing, ultra-polishing has always got the researchers' attention. Many of the core techniques used in the polishing process have been studied in the past, but the acquisition of ultra-smooth surfaces severely depends on the experience of the processors. Considering the significance of ultra-polishing in exploring technical limits in many fields and the current situation that actual polishing processing relies heavily on operational experience, it is necessary to deeply discuss the physical and chemical mechanisms of material removal in the ultra-polishing process represented by classical polishing and chemical-mechanical polishing.

Progress The material removal mechanism in ultra-polishing can be simply divided into two aspects: physical process removal and chemical process removal. The physical process removal mechanism mainly includes mechanical removal mechanism and fluid mechanics removal mechanism.

In terms of the research on mechanism of mechanical removal, Preston and other researchers initially summarized the “Preston equation” and its modified equation by the relationship between macroscopic physical quantities such as workpiece surface pressure, relative velocity of workpiece and material, and removal rate in glass polishing. Zhao's research group used the Greenwood-Williamson theory to reveal the material removal process under the action of abrasive wear from a microscopic perspective, and predicted the material removal rate by calculating the actual contact area of the workpiece and polishing pad, the number of abrasive particles involved in polishing, and the embedding depth of abrasive particles, respectively. For the research on the mechanism of material removal under the action of fluid, Runnels proposed a tribology-based 3D fluid dynamics model and calculated the expression of normal material removal rate caused by fluid erosion. Sundararajan and Thakurta et al established the fluid velocity field of polishing fluid through fluid lubrication model, and then obtained the average polishing rate through mass transfer model.

The above physical removal models all ignored or simplified the chemical process of the workpiece in the polishing solution environment, and didn't reveal the important process of chemical action in the realization of the material microscopic removal process. This problem was well explained in Cook's paper, where Cook proposed a chemical tooth model for material removal (Fig.9). In order to test and verify the correctness of the model, many researchers have followed in Cook's model, carrying out numerous experiments. Yu used atomic force microscope to verify the promoting effect of aqueous solution environment on material removal (Fig.10). Zhou research group conducted experiments on this process in combination with X-ray photoelectron spectroscopy and atomic force microscope. The experiment results showed that water molecules reacted with sapphire to generate hydroxy-rich AlO(OH) and Al(OH)₃ products, thus achieving hydroxylation of sapphire surface (Fig.12); Katsuki, Shi and other researchers combined atomic force microscope and infrared spectroscopy to carry out experimental research on the important process of chemical bonding, the experimental results show that there were a lot of chemical bonds between the workpiece and the probe surface after polishing (Fig.17).

Conclusion and Prospect Each mechanism of material removal mentioned in this paper has its limitations. In the related theory of physical removal mechanisms,the “Preston” equation of phenomenology only explained the influence of material removal rate with macroscopic physical parameters such as pressure and flow rate from a macroscopic perspective. The abrasion theory based on the Greenwood-Williamson model ignored the powerful chemical action in polishing and failed to explain the mechanism of material removal at the atomic and molecular level in the final polishing. The fluid action of the polishing slurry is more concerned with the contact model between the workpiece and the polishing pad, not with fluid erosion. The chemical removal mechanism makes up for the deficiency of physical removal mechanism, but the relevant model can only give a rough description of the material removal process at the atomic and molecular level in the polishing process, which still needs to be further studied. At the same time, in the description of the mechanism of material removal in the whole process, almost all researchers hoped to use a theoretical model to explain the material removal phenomenon in the whole polishing process. However, it seems that the whole process involves multiple material removal mechanisms at the same time, and different material removal mechanisms play a key role in different processing stages. To sum up, it is still necessary to carry out continuous and in-depth research on the material removal mechanism in super polishing, so as to promote the development of the ultra-precision optical processing technology.

Objective Lasers operated at 3.5 μm stretch the absorption peaks in the C—H bonds and exhibit high transparency in the atmosphere windows, making the lasers that generate beams with frequencies of 3.5 μm useful in applications using spectroscopy, remote sensing, environmental monitoring, and infrared (IR) countermeasures. Optical parametric oscillation (OPO) is an effective method to radiate mid-IR laser beams that can transform near-IR lasers into mid- and far-IR radiation. The KTiOAsO₄ (KTA) crystal is a member of the KTiOPO₄ (KTP) crystal family and an excellent nonlinear material. The spectroscopy transmittance range of KTA is 0.35--5.30 μm. The absorption loss in KTA crystals in the 3--5 μm bandwidth is much lower than that of the KTP crystal. The KTA crystal has a large nonlinear coefficient, a wide-angle and temperature-matching bandwidth, a high damage threshold, and stable physical and chemical properties. Experiments on noncritical phase matching (NCPM) demonstrated that the KTA optical parametric oscillator (OPO) pumped by a Nd∶YAG 1064-nm laser can radiate a 3.5-μm laser beam. Due to the influence of temperature dispersion, the wavelength value of KTA-OPO changes as a function of temperature. The detailed study of KTA-OPO temperature tuning properties presented herein can guide the application of KTA-OPO lasers. To date, there are few theoretical or experimental studies concerning the temperature tuning performance of KTA-OPO.

Methods Temperature tuning properties of KTA-OPO are studied theoretically and experimentally. Based on the normal temperature dispersion equation and the temperature dispersion equation of the KTP crystal, the temperature tuning properties of KTA-OPO at different cutting angles (θ) are calculated. The wavelength of idle light (λ_i) at T=25 ℃ and the temperature tuning slope (Δλ_i/ΔT) of KTA-OPO at different cutting angles are calculated with an accuracy of 1° when the wavelength of the pump laser is set at 1064 nm. An experimental study using a 3.5-μm NCPM KTA-OPO laser pumped by a Nd∶YAG 1064 nm laser is conducted. The pump source is an SL800 series pulsed Nd∶YAG laser with a pulse width of 13 ns, a spot diameter of 8 mm, and a repetition rate of 1 Hz. A small hole is placed behind the laser for dimming, and the spot diameter is compressed to 4 mm using a telescope system to increase the energy density of the pump laser. The temperature of the KTA crystal is controlled in a temperature-controlled furnace with an accuracy of 0.1 ℃. The wavelengths of the KTA-OPO idlers at 30, 80, 130 and 180 ℃ are measured using an Omni-300λ spectrometer (Zolix Instruments Co., Ltd) with an accuracy of 1 nm. At the back end of the grating spectrometer, a DEC-M204-InSb detector and a ZAMP amplifier (Zolix Instruments Co., Ltd) are used to detect and amplified the laser signal at the specified transmission wavelength. A DSOX3054T oscilloscope is used to display and measure the signal amplitude from the ZAMP amplifier. The value of the maximum wavelength of the laser signal is set equal to the peak wavelength of the output idler by the grating spectrometer.

Results and Discussions The temperature tuning properties of KTA at different θ angles and the OPO pumped by a 1064-nm laser are studied (Fig. 1). Comparison of the laser wavelength and Δλ_i/ΔT of the idler at different θ angles revealed that the wavelength of the idler increased monotonically as the value of θ increased, while Δλ_i/ΔT decreased monotonically (Fig. 2). When the temperature is 30, 80, 130, and 180 ℃, the measured values of λ_i are 3463, 3466, 3469, and 3474 nm, respectively, and Δλ_i/ΔT is 0.073 nm/℃. The experimental results show that the wavelength of the output idler of KTA-OPO (θ=90° and ?=0°) is less affected by changes in temperature under conditions of type II phase matching, which confirms the theoretical conclusions (Fig. 4). The results demonstrate that the temperature dispersion equation (Table 1) can be extended from near-IR to mid-IR band spectroscopy.

Conclusions The laser wavelength value of KTA-OPO is relatively stable at different temperatures. In type I and II phase matching, with the increase of θ, the idler wavelength value increased monotonically, but Δλ_i/ΔT decreased monotonically and ranged from to 0.0774 to 1.4968 nm/℃. The Δλ_i/ΔT value of the type I phase matching KTA-OPO is generally larger than the Δλ_i/ΔT value of the type II phase match. Among all phase matching points for angle θ, the temperature tuning range is smallest when θ=90° under type II phase matching, and the theoretical value of Δλ_i/ΔT is 0.0774 nm/℃. The experimental results are consistent with the theoretical calculations. Both theoretical and experimental studies show that the wavelength of the mid-IR laser beam of the NCPM KTA-OPO is less affected by temperature. This results of this study demonstrate the potential application of KTA-OPO pumped by a 1064-nm laser.

Objective Photon ranging exhibits the advantages of high sensitivity and long-distance detection. Compared with laser ranging in the linear mode, the photon detection exhibits the first photon bias effect owing to the dead-time of the single-photon detector, which results in greater distortion of the probability distribution of photon echo. This distortion is closely related to the intensity and distribution of the laser echo. There is a close relationship between the target shape and posture and the probability distribution of photon echo. As a result, the range errors in photon ranging caused by the target shape and posture cannot be ignored. Most researchers have focused on analyzing the modulation effect of target characteristics on the laser pulse echo. However, there is a lack of research on the range errors of extended targets in the photon-detection mode. Therefore, we discuss the relationship between the target shape and inclination and photon ranging for three typical extended targets.

Methods Based on the Poisson probability response model and the traditional laser radar equation, the probability distribution model for the photon detection of an extended target is established herein. Combining this with the coordinate-rotation transformation formula, the general probability distribution equation mixed with spatial and temporal distribution at different inclinations is derived for the three typical extended targets: a plane, a sphere, and an aspheric. Experimental results reveal that the probability distribution of this photon echo is consistent with the numerical results. We then simulate and analyze the differences in the photon echo probability distribution and laser pulse echo characteristics of the three typical extended targets. Finally, the variation between the range errors in photon detection and the types and inclinations of the extended targets is discussed theoretically.

Results and Discussions Compared with the laser pulse echo, the probability distribution of the photon echo moves forward as the inclination increases, and the variance decreases. At the same time, the pulse width of the laser echo modulated by the extended plane is wider than those of the extended spherical and aspherical surfaces. Furthermore, the probability distribution of the photon echo of the extended plane moves forward the most. The photon ranging error of the extended targets exponentially increases with the increase in inclination. The average number of echo photons is 3.9, the laser spot radius of the target is 0.2 m, and when the inclination is less than 20°, the difference in the photon ranging errors between the three extended targets is less than 1.23mm. As a result, the range errors in the photon detection caused by different extended target types could be ignored. In addition, when the inclination is greater than 20°, the photon ranging accuracy of the extended plane is most affected by the inclination, whereas that of the aspheric surface is the least affected. When the target inclination is 70°, the photon ranging errors for the extended plane and spherical and aspheric surfaces are 12.5cm,10.6cm, and 8.9cm, respectively.

Conclusions Based on the center-of-mass detection method, the range errors in the direct detection vary slightly with the inclination of the extended targets, which is negligible compared with those in photon ranging. The range errors in photon detection increase as the inclination of the target increases. The photon-ranging errors for the extension plane are most affected by the inclination. If the inclination was smaller, the photon ranging of the extended target would be almost independent of the shapes of the extended targets. These conclusions provide a theoretical basis for the photon ranging performance and error analysis and provide a reliable information support for range-error correction and performance improvement. Furthermore, the posture information of the extended target can be acquired by combining the equation of photon echo probability distribution with the measurement results of the photon echo.

Objective Metal films have lots of excellent characteristics such as higher mechanical strength and damage threshold, better toughness, and thermal conductivity. They are widely used in modern optical industry. Moreover, copper films are used as infrared stealth material because the emissivity of copper is very low in mid-infrared band. This property can reduce the detection efficiency of passive mid infrared detector. The characteristic has extensive applications in military and many groups have performed research about it. However, they only focused on production of high quality copper film. However, the methods to destroy this film are ignored, which is presented in this paper. Laser-induced periodic surface structure (LIPSS) is special surface grating structures which is induced by polarized pulses that appear on nearly all kinds of solid materials. The period and direction of the grating only depend on the wavelength and polarized direction of laser. The structures can change the surface properties of materials such as super hydrophilic/hydrophobic, suppress the growth of the miscellaneous bacteria, as well as high emissivity. LIPSS has drawn attentions of many researchers. Lots of new materials with special characters have been produced by inducing LIPSS on the surface of the materials. However, the study of changing emission characteristics within mid-infrared band of metal films is lacking. Some researches are performed about the effect of LIPSS on infrared emission characteristics of copper films in this paper.

Methods First, the production of LIPSS on copper films is investigated using the surface interference between plasmons and incident laser model (Sipe model). The Sipe model involves two processing: softening and migration of materials. A two-temperature model is used to illustrate the copper-softening process. The theory of the two-temperature absorption of metals can be applied to all types of incident lasers because the processing is performed by the distribution of a large number of free electrons on the material surface, which is different from other types of materials. Therefore, a linear pulse at a center wavelength of 1064 nm with a pulse duration of 100 fs and energy density of 5 J/m ² is used in the simulation experiment. Then, LIPSS is induced on a copper film that covered a quartz substrate using nanosecond linear pulses at a center wavelength of 1064 nm. Additionally, a simulation model is established according to the surface topography of the sample induced in the experiment. The emissivity is within the 1--5 μm band.

Results and Discussions The results of the two-temperature model experiment show that the temperature of the copper free electrons reaches 7073 K after the pulse introduction is finished, which is very much higher than the melting point of copper (1375.8 K). The high temperature softens the target, which means that materials can be rearranged under a periodic space electromagnetic field according to the Sipe model. Then, the temperature of the electronic system quickly decreases, whereas that of the lattice system gradually increases. At 12 ps, the temperatures of the two systems are balanced at 1017 K, which is less than the melting point (Fig. 2), indicating that classical heat damage does not occur. LIPSS is induced by linearly polarized nanosecond pulses (Fig. 4). The direction of the gratings is perpendicular to the polarization direction of the laser. The results show that lowsurface-frequency LIPSS (LSFL) is induced on the film. The electric field distribution of the reflected and transmitted fields of the simulation model show that the laser is modulated by the gratings (Fig. 6). Therefore, the emissivity of the model can reach 0.365 and 0.119 when the laser wavelength is 3 μm and 5 μm, respectively (Fig. 7), which is very much higher than that of smooth copper (Fig. 1). The results show that LIPSS can improve the emissivity of copper films.

Conclusions This paper has presented three main studies. The first one is a brief explanation of how LIPSS can be induced by pulses lower than the damage threshold of materials. The Sipe model is used to describe the process of inducing LIPSS on metals. The material-softening step due to the pulses, whose influence is lower than the damage threshold, is achieved using a two-temperature simulation model. The second study induces LIPSS on a copper film over a quartz basement. The LIPSS is of LSFL type. The results agree with those in the previous studies. The third one proves that LIPSS can improve the emissivity of copper films through a simulation experiment. The results show that a much higher emissivity of films is achieved with LIPSS because the space electromagnetic field is periodically modulated by the micro gratings. The effect on the improvement is significant. These results show that inducing LIPSS on the surface of copper films is a feasible technique to destroy the stealth characteristics of a material in the mid-infrared band.

Objective Atmospheric turbulence causes a random fluctuation in the refractive index. When a laser propagates in atmospheric turbulence, the light intensity fluctuation phenomenon during beam propagation occurs, seriously influencing laser propagation. Because different atmospheric turbulence intensities have different effects on laser propagation, it is significantly important to estimate the atmospheric turbulence intensity. In general, the refractive index structural constant Cn2 of the atmospheric turbulence is used to measure the turbulence intensity. The value of Cn2 is directly proportional to the impact of turbulence on laser propagation. Traditional estimation methods include instrument measurement and model estimation. The instrument measurement allows building an experimental platform to directly measure Cn2, in contrast, the model estimation allows obtaining Cn2 by measuring other atmospheric parameters and establishing a model. In recent years, deep learning has allowed achieving good results in the field of image processing, which can extract the feature information of an image layer by layer. This study proposes a method to estimate the refractive index structural constant Cn2 of atmospheric turbulence based on deep convolutional neural networks. The neural network model is built to extract the features of the light spot images under the influence of atmospheric turbulence and the turbulence information is obtained to estimate the turbulence intensity.

Methods A spot image under the turbulence influence contains the turbulence information. In deep learning, neural networks can extract the characteristic parameters of an image. Based on the above mentioned information, neural network models are built to estimate the turbulence intensity. According to the phase screen theory, the Gaussian beam spot images under the influences of different turbulences are simulated. The spot images are divided into a dataset and a test set. Three-thousand images are selected as the training set, and a neural network model is used to obtain the estimation models. Three-hundred images are used as a test set to analyze the estimated results. In addition, the influences of different network structures on the estimation results are analyzed, which provides a new way for estimating turbulence intensity.

Results and Discussions In this study, a traditional AlexNet network model and a VGG16 deep convolutional neural network model are established. VGG16 is optimized on the basis of the traditional convolutional neural network, which increases the layer numbers of the network, reduces the size of the convolution kernel, and has more advantages on feature information extraction of images. The light spot images at different moments under the same turbulence intensity are selected as the inputs of the neural network to verify the feasibility of the above mentioned method and obtain the corresponding estimation results. Moreover, the standard deviation is calculated, and the estimation results are analyzed. The results show that the method can well estimate the turbulence intensity, and the standard deviation increases with the turbulence intensity. To better analyze the results of the neural network model and measure the estimation results, four statistics, i.e., mean absolute error (E_MAE), mean relative error (E_MRE), root-mean-square variance (E_RMSE), and correlation coefficient (R_xy), are selected. The spot images under the influences of different turbulence intensities are randomly selected as the inputs of the neural network model to obtain the corresponding output. The estimation results of the two neural network models are shown in Table 5. After 20 iterations, the estimation result of the VGG16 neural network model is relatively ideal, the correlation coefficient reaches 99%, and E_MAE, E_MRE, and E_RMSE are controlled within 5%. After 500 iterations, E_MAE, E_MRE, and E_RMSE are further reduced to 2%. By analyzing Table 5, it can be seen that both models can well estimate the turbulence intensity after 500 iterations, and the estimation effect of VGG16 is better than that of the AlexNet neural network model. When the number of iterations is the same, E_MAE,E_MRE, and E_RMSE estimated by the VGG16 neural network model are less than half of those of the AlexNet neural network model. Compared with the traditional AlexNet neural network model, the VGG16 neural network model optimizes the network structure and improves the estimation effect to a certain extent.

Conclusion In this study, a method based on deep convolutional neural network model is proposed to estimate turbulence intensity. First, the laser spot images under the influence of turbulence can be simulated according to the classical phase screen theory. Then, the laser spot images under the influence of turbulence are taken as the inputs of the deep convolutional neural network model, and the convolutional layer of the deep convolutional neural network model is used to extract feature information of images layer by layer. After the training of a large number of datasets, the network model is obtained, and the turbulence intensity is estimated. Finally, the estimated effect is analyzed. Compared with the traditional AlexNet neural network model, the VGG16 model adopts a small convolution kernel, which can better retain the image properties, and has high advantages on image feature extraction and better estimation effect. Therefore, the neural network model can be further optimized to improve the estimation effect, which provides a new way to estimate turbulence intensity.

Objective Compared to traditional inorganic extinction materials, biomaterials have advantages relative to environmental protection, non-toxicity, low cost, and controllable form. Thus, biomaterials have become a new type of extinction material. Research into the extinction properties of biomaterials has received increasing attention. And research into the extinction properties of biological aggregate particles is key to studying the extinction properties of biomaterials. The current research results primarily focus on monodisperse biological agglomerate particles; however, in the real-world, there are almost no agglomerate particles with the same radius. Therefore, polydisperse biological agglomerate particles are closer to the actual situation and have greater research value. This paper simulates the polydisperse biological aggregation particles model and calculates the influence of the number of original particles, porosity, and particle size distribution on the extinction performance of polydisperse biological aggregate particles. The purpose of the paper is to calculate factors affecting the extinction performance of biological aggregate particles and provide a reference for future in-depth studies of the extinction properties of biomaterials.

Methods The polydisperse biological aggregation particles model is simulated based on the ballistic particle-cluster aggregation model. The influence of the gyration radius on the porosity and equivalent complex refractive index of aggregate particles is studied. Using the discrete dipole approximation method, we analyze the influence of porosity accuracy on extinction coefficient. In addition, we calculate the extinction coefficient of the polydisperse biological aggregation particles model with different porosity, different numbers of original particles, and different particle size distributions, and we analyze the influence of porosity, particle number, and size distribution on the extinction characteristics of aggregate particles.

Results and Discussions The results demonstrate that the porosity of polydisperse biological aggregate particles increases with increasing gyration radius, and the real and imaginary parts of the equivalent refractive index decrease with increasing gyration radius (Table 1). To study the effect of porosity accuracy on the extinction coefficient, we analyze the influence of porosity accuracy of 0.01, 0.001, and 0.0001 on extinction coefficient. The results demonstrate that, when the porosity error range is within 0.001, the influence can be negligible (Fig. 4). For aggregate particles with the same size distribution and the same number of original particles, the extinction coefficient at the wavelength of 10.6 μm decreases with increasing porosity (Fig. 3). For polydisperse biological aggregate particles with the same particle size distribution and porosity error range within 0.001 at a wavelength of 10.6 μm, the extinction coefficient of aggregate particles increases with an increasing number of original particles (Fig. 5). For polydisperse biological aggregate particles with the same number of original particles, the same mean of the particle size distribution, and error range in porosity within 0.001 at a wavelength of 10.6 μm, the variance of the particle size distribution has nearly no effect on the extinction characteristics (Fig. 6). For polydisperse biological aggregate particles with the same number of original particles, the same variance in particle size distribution, and an error range in porosity within 0.001 at a wavelength of 10.6 μm, the extinction coefficient increases with the increasing mean of the particle size distribution (Fig. 7).

Conclusions Based on the ballistic particle-cluster model, which is used to simulate polydisperse aggregate particles, this paper discusses the pore characteristics of the aggregation particles model comprising the same number of original particles and analyzes the influence of the radius of aggregate particles on porosity and equivalent complex refractive index. The discrete dipole approximation method is employed to calculate the extinction coefficient of different aggregate particles, and the influence of porosity, the number of original particles, and the particle size distribution on the extinction performance at the 10.6-μm laser wavelength is analyzed. The study finds that the porosity of polydisperse biological aggregate particles increases with increasing radius of gyration, and the real and imaginary parts of the equivalent refractive index decrease with increasing radius of gyration. The extinction performance of aggregate particles decreases with the increasing porosity and increases with an increasing number of original particles and increasing mean of the particle size distribution. The results provide a reference for comprehensive understanding of the extinction properties of biological aggregate particles and for the preparation of biological extinction materials. The extinction performance of biomaterials can be improved by changing the porosity of biological aggregate particles, the number of original particles, and the distribution of particle size.

Objective Computational ghost imaging (CGI) uses a single-pixel detector to realize imaging. It has received a great deal of attention in recent years because it is a low-cost invisible spectrum imaging technology that can make the light transmit via a scattering medium. Researches on CGI through scattering media are mainly conducted on static scattering media such as water and ground glass. In contrast, few studies have been conducted on some complex scattering media, and the imaging situation for dynamic scattering media is unknown. A smoke screen is a type of scattering medium with diffusion and subsidence movements. At the same time, the heat of smoke changes the refractive index of the light. This is a representative of a complex scattering medium. Further, smoke imaging is of research value in the fields of military and life sciences. Dynamic smoke screens distort light intensity measurements and cause image deterioration, which is the difficulty in CGI. In this paper, we choose the smoke screen as the scattering medium for studying the imaging effects under static and dynamic smoke conditions. Smoke screen alleviates dynamically induced imaging degradation and provides a useful reference for CGI in permeable scattering imaging applications.

Methods A CGI system with a smoke screen in the detection path was built. A smoke chamber was used as the scattering medium, and a 532-nm laser was used as the light source. Scattering media were classified according to the motion state of smoke particles. The error factors of the dynamic smoke screen environment were analyzed by simulation. Under experimental conditions, imaging results were analyzed using various frame-rate projections. A point-by-point compensation (PPC) method was proposed and used to track the attenuation process of the light intensity caused by smoke motion, with the addition of a ″1″ measurement matrix before and after each CGI measurement matrix. The projected light intensity of these measurement matrices showed attenuation changes caused by the smoke screen motion. The distortion of light intensity due to the scattering medium was corrected according to the attenuation curve, and the results were compared. Based on analytical and comparative results, the feasibility of the CGI technique was verified, and the applicable range of PPC was obtained.

Results and Discussions The moving state of the smoke medium was classified into the static state, slow dynamic state, and fast dynamic state. The imaging results under three types of motion conditions were compared, and the following conclusions were obtained.

1) Simulation results show that CGI has strong robustness in a slow dynamic smoke environment, and the imaging results do not change significantly due to the scattering distortion of the light intensity. However, if smoke motion causes significant change in the light intensity before and after measurements, images will deteriorate significantly. At this moment, the images obtained by the PPC method are sufficiently improved, but the proposed PPC method is also affected by noises, such as ambient light intensity and light source power. If the signal-to-noise ratio of the intensity measurement is less than 60, the CGI and PPC will fail. To improve imaging with PPC, these noises need to be strictly controlled.

2) The CGI system achieves relatively clear images in a static smoke environment, but the traditional imaging method fails.

3) In a slow dynamic smoke environment, the CGI imaging results are not significantly different from that in a static environment.

4) In a fast dynamic smoke screen environment, the lower the frame frequency, the weaker the CGI imaging effect. Conversely, suppose the frame frequency is too high. In that case, it will be difficult for the laser to pass through, making it impossible to image in the initial stages of smoke emission and making the function of CGI uncertain.

5) In a dynamic smoke screen environment, the PPC method can significantly improve the imaging quality when the projected frame frequency is low. This comparisonis are more pronounced at 10 and 50 Hz.

Conclusions CGI has obvious anti-scattering imaging capabilities. Imaging results under static and slow dynamic conditions are better, and there is no significant difference. In a fast dynamic smoke environment, CGI cannot work due to the distortion of the measured light intensity, and the lower the frame frequency, the greater the distortion. There is a certain tolerance for the degree of motion of the CGI scattering medium. Suppose the degree of motion is within a tolerance range. In that case, the imaging results show no significant quality reduction or are the same as that in static states, even though the measurements show detectable distortion. The PPC method corrects measurements by tracking the process of light intensity distortion to achieve considerable imaging effects. A powerful and stable laser is a suitable light source for transmitting smoke in a CGI system. In conclusion, CGI has unique advantages in anti-scattering imaging and can be improved by the PPC method using dynamic scattering media.