Liquid-assisted laser machining is a composite manufacturing technology. Depending on its unique characteristics and advantages, it already has been received widely attention and applications in the field of manufacturing. The technology has become a hot spot in the interaction between laser and liquid medium. In this paper, the research status of shock wave and high-speed micro-jet pressure in liquid-assisted laser process is re-viewed. The mechanism, the basic characteristics of pressure and the factors that affect the pressure in the pro-cess of liquid-assisted laser machining are resumed. The research method and the latest progress of the pres-sure phenomenon in the laser wet machining are mainly introduced. The advantages and disadvantages of these pressure detection methods are summarized. Finally, the applications of liquid-assisted laser machining in related fields are introduced and the prospects of this technology are summarized.

Laser engineered net shaping is a promising technique to fabricate high-performance components with complex geometry rapidly. Excellent properties of fabricated specimen and high deposition efficiency are both important for this additive manufacturing method, but few researches have been done on the relationship between them. In this paper, single-bead multilayer structures of 316L stainless steel are fabricated by laser en-gineered net shaping (LENS). Using the same laser power and scanning speed, different deposition efficiencies are achieved by adjusting powder flow rate and layer increment. Microstructures and mechanical properties of the deposited structures under different deposition efficiencies are discussed. The results show that, for certain laser power and scanning speed, the deposition efficiency increases from 12.41 mm3/s to 22.62 mm3/s with the increase of the powder flow rate and layer increment , which increases by 86.3% compared with the initial pro-cess. Laser energy consumed by depositing unit effective volume reduces from initial 98.84 J/mm3 to 53.06 J/mm3 and the energy efficiency increases by 46.32%. Microstructures of the specimen consist of columnar den-drite and the dendrite length increases obviously with the deposition efficiency. Property test shows that proper-ties of the specimen under different deposition efficiencies are consistent and do not decrease with the deposi-tion efficiency. Tensile strength and yield strength are stable in 510 MPa and 290 MPa, respectively. The elonga-tion rate is around 40% while the micro-hardness is about 180 HV, all of which have reached the same level of forging. The results illustrate that the deposition efficiency and mechanical property can be optimized, which achieves fabrication of high performance parts with low energy consumption and high efficiency.

In order to carry out the monitoring experiment of Geo-synchronous orbit (GEO) targets, Shanghai As-tronomical Observatory developed an equipment with 100-square-degree field of view, called “optical prototype of Geo-synchronous orbit dynamic monitoring system”. There are a large number of targets in the field of view, how to recognize the GEO targets effectively from the complex observation images is the focus of this paper. GEO targets appear motionless, at a fixed position in the sky, to ground observers. Due to the earth rotation, stars move at a speed of 15″/s relative to the ground observers. We propose the combination of frame difference method and track correlation method. Frame difference method is used to remove a large number of stars from the images, and track correlation method is used to confirm the GEO targets and connect them from different images. The feasibility and accuracy of the method are verified by the analyses of the observation data. The method can recognize more than 50 GEO targets in the field of view at the same time, and the recognition accu-racy exceeds 95%.

With the initial applications of unmanned aerial vehicle technology in various fields, the infrared (IR) radiation characteristic of the UAV becomes an issue of mutual concern. In this experiment, the radiometric cali-bration of middle wave and long wave infrared detectors has been done firstly. The radiation characteristics of the UAV are measured while flying, and the temperature of UAV target is inverted by using single-band method, dual-band colorimetric method and dual-band method based on black-body calibration. Combined with the real measured data, the above methods are analyzed and compared. The measurement precision is analyzed and error source is given. The results show that radiation intensity of UAV is about 0.04 W/sr in middle IR wave, and 0.5 W/sr in long IR wave. Dual-band method based on black-body calibration can improve the precision of UAV temperature inversion greatly, the absolute error of temperature retrieval reduces to 2 K, and relative error is about 0.5%.

In the study of autofocus technology, we propose an image fusion method based on saliency analysis, which can solve the problem of all in focus. First, the focal area in the source image is positioned by the graph-based visual saliency (GBVS) algorithm, and then the watershed and morphological methods are used to obtain the closed area of the saliency graph and the pseudo-focus region is removed. The defocused region is processed by the Shearlet transform, and the SML operator is used to choose the fusion parts. Finally, the precise focused region and the processed defocused region are fused into an all in focus image. Experiments show that the fused image of our method is clear and rich in detail, which has the best visual effect, and improves more than 5% in definition and fusion compared with traditional methods.

Aiming at the characteristics of long-distance tunnel, long maintenance intervals, limited repairing time and small changes in the deformation data, this paper presents a detection method of tunnel deformation based on active omni-directional vision tunnel section deformation. Firstly, the active panoramic vision sensor (ASODVS) is installed at the tunnel and detecting device scans the cross section of tunnel to acquire tunnel section pano-ramic images. Secondly, the sub-pixel center of the panoramic image is extracted by the improved Gauss curve fitting method, and the smooth processing is performed by using the Bezier curves. Thirdly, the system analyzes the geometry information of the tunnel cross-section of the inner wall through calibration results of omnidirec-tional vision sensor. The tunnel cross section 3D point cloud data are used for 3D reconstruction. Finally, the pre-cision of the tunnel reconstruction model is analyzed. The experimental results show the method has the ad-vantages of high speed acquisition, real-time, comprehensive data and good visualization. It can meet the needs of the rapid qualitative and quantitative analyses.

A novel electromagnetically induced transparency (EIT)-like metamaterial of terahertz domain is pro-posed. The metamaterial is composed of a single metal wire and a couple metal wires above it. Numerical sim-ulations demonstrate that by rotating the single metal wire around its center, EIT phenomenon is created. The amplitude of the transparent peak increases as the rotation angle increases. When the rotation angle is 60°, the amplitude of the transparent peak reaches its maximum. However, as the rotation angle keeps increasing, the amplitude of the transparent peak gets lower and the peak finally vanishes as the rotation angle equals 90°. We also analyze the sensing performance of the metamaterial with the rotation angle of 60°. The proposed structure is simple and adjustable, with high Q-factor value and good sensing performance.

A novel electromagnetically induced transparency (EIT)-like metamaterial in terahertz domain is proposed. The metamaterial is composed by a single metal wire and a couple metal wires above it. A plane wave propagates along the perpendicular direction to the structure with its electronic field direction paralleled to the double metal wires. The structure’s transmission spectra are simulated by CST Microwave Studio. In the simulation, the solver is chosen to be frequency domain solver while the boundary conditions are set as unit cell in x-y plane, and open (add space) in z-direction. The metal strips are set to be copper with its conductivity of 5.88×109 S/m and its thickness Δh=0.3 μm. The substrate material is quartz with its permittivity of 3.75 and thickness h=50 μm.

The tunnel is a civil engineering structure and the product for the human breaking through the boundaries of the nat-ural environment, to improve the utilization of underground space and the traffic conditions. In 1970, the Organisation for Economic Co-operation and Development held a tunnel meeting that, after synthesizing the various factors, de-fined the tunnel as follows: "In some applications, under the ground in any way, shape and the size of cross-sectional area is larger than 2 square metres according to regulations".

In the study of autofocus technology, it is always difficult to obtain the all-in focus images because of optical system’s limited focus depth, but a high definition image is very important to scientific research. In the research of autofocus technology, we know that detection of the focused region is the key issue of the multi-focus image fusion algorithm, the blurred boundary of the focused region increases the more difficulty of identifying focused regions accurately. Accord-ing to these principles, the focused region of the source image should be directly fused as much as possible.

With the initial applications of unmanned aerial vehicle technology in various fields, the infrared (IR) radiation charac-teristics of the UAV become an issue of mutual concern. In this experiment, the radiometric calibration of middle wave and long wave infrared detectors has been done. SR800R 12D~~ET extending blackbody made in Israel is used for the radiometric calibration of middle wave and long wave infrared detectors. The quantitative connection between the optical entrance pupil radiation and the output of detector is established, and the responsivity of the infrared detector is obtained. The radiation characteristics of the UAV is measured while flying, and the temperature of UAV target is inverted by using single-band method, dual-band colorimetric method and dual-band method based on black-body calibration.

Geo-synchronous orbit (GEO) has been highly valued because of the increase of satellite launch activities and the limit of the orbital slots. In order to protect space activities in our exclusive orbit space, it is necessary to carry out the dy-namic monitoring of Geo-synchronous orbit. Shanghai Astronomical Observatory developed an equipment with 100-square-degree field of view, called “optical prototype of Geo-synchronous orbit dynamic monitoring system”. The prototype was set up in Gaomeigu observation site in Lijiang. Observation experiment was carried out from December 2015 to February 2016. There are a large number of targets including GEO targets and stars in the field of view. The movement forms of GEO targets are various, including satellites working on orbit, new satellites entering orbit, orbit maneuver and deorbit. Some GEO targets with an inclination not equal to 0 degree move at a very low speed due to the Earth perturbation.

laser engineered net shaping; 316L stainless steel; deposition efficiency; microstructure; mechanical prop-erty

Solid-state avalanche photodiodes (APDs) based on AlGaN with Al composition exceeding 40% are being heavily stud-ied because they have intrinsic solar-blindness, which could be a viable alternative to Si-based photodiodes or photo-multiplier tube (PMT) used in ultraviolet (UV) military, civilian and scientific areas. However, the development of the solar-blind AlGaN APDs with high gain has been still suffered from some problems, such as low p-type doping effi-ciency and high dislocation densities for high-Al content AlGaN layer. In addition, the breakdown voltages of the con-ventional AlGaN APDs are generally more than 90 V, which results in a large leakage current. Large dark current can increase the device noise, as well as confine the APDS avalanche gain.

International and domestic academics are attracted by biometric features recognition, because the intelligent monitor-ing, which is the basis of social security, is more and more required by people. Compared with other biometric features (face, iris fingerprint and so on), gait feature has its advantages, such as acceptability, noninvasiveness, hard to hide and easy to be collected. Gait recognition, recognizing different persons by the way of his/her walking, which has some ad-vantages such as non-contact, low demand in image quality and difficult disguise, is the most potentially biological recognition technology by the way of one’s walking. Recently, the research of gait recognition is a hot topic in the field of computer vision, entrance guard system and medical diagnose, which has extensive realistic significance and wide ap-plying foreground.

Arbitrary micro-scale three-dimensional (3D) structures fabrication is a dream to achieve many exciting goals that have been persuaded for a long time. Among all these applications, the direct 3D printing to fabricate human organs and integrated photonic circuits are extraordinarily attractive as they can promote the current technology to a new level. Among all the 3D printing methods available, two-photon polymerization (2PP) is very competitive as it is a unique method to achieve sub-micron resolution to make any desired tiny true 3D structures. To achieve it, 2PP relies on the direct laser writing to transfer the pattern into the photoresist material by the two photon absorption to initialize the in-situ polymerization. However, the requirements for the building blocks are different for different purposes. Even though 2PP is of many superior properties, it is often considered as a supporting role to make the initial polymer structural molds or backbones for further processing steps.

Liquid-assisted laser machining (LALM) is a composite manufacturing technology. During liquid-assisted laser ma-chining, the liquid has the functions of cooling and cleaning on the processing area. It makes the heat affected area of the work piece become smaller. The thermal stress is reduced, and the processing incision is smooth and cleaning. So the liquid-assisted laser machining has unique advantages in the processing of heat sensitive materials, high hard-en-brittle materials and materials of high precision requirements. Due to the addition of liquid in the laser processing, the laser interacts with the machined and the added working liquid. Some researchers have found that the pressure in the process of liquid-assisted laser machining is produced through the study of the experiments and simulation. The existence of pressure results in the complexity of the laser processing even the processing mechanism would be changed. Then the process is difficult to be controlled. Therefore the process of liquid-assisted laser machining should be detected. In order to better master and control this technology, many researchers have studied the existence of pressure in the process. And it has become a research hotspot in the field of laser composite processing. At present, most of the numerical simulation and experimental studies on the pulsation, shock wave, micro-jets and acoustic radia-tion of single bubble generated by single pulse laser are carried out. However, multi-bubble is usually produced in the process of liquid-assisted laser machining, which makes the phenomenon and the mechanism of laser processing be-come more complex. These complex phenomenon and mechanisms should be further explored and studied by re-searchers.

Recently, researchers of Nanyang Polytechnic in Singa-pore changed the surface morphology by polystyrene (PS) with femtosecond (fs) laser irradiation to modify the sur-face water contact angle (WCA) from the original 88.2° to 12.7° or 156.2°.

Space-division multiplexing (SDM) technology can effec-tively expand the capacity of optical fiber communication networks, but few-mode fiber (FMF) for SDM must use multiple-input multiple-output (MIMO) signal processing to withstand the induced cross talk, resulting in system complexity and cost rise. Zhou Bingkun academician's research team from Tsinghua University designed a po-larization-maintaining PANDA ring-core fiber with a ring-core structure and two stress-applying rods to solve the problem.

The femtosecond (fs) laser-induced surface morphology change often drastically alters a material’s physical prop-erties. But the existing research has not yet been able to observe a complete evolution of the material during fs laser-material interactions. For this problem, Professor Guo Chunlei from the University of Rochester, USA de-veloped an ultrafast optical imaging technique.