Large Divergence Angle Research Articles

Double-Layer Metasurface Integrated with Micro-LED for Naked-Eye 3D Display.

Naked-eye 3D micro-LED display combines the characteristics of 3D display with the advantages of micro-LED. However, the 3D micro-LED display is still at the conceptual stage, limited by its intrinsic emission properties of large divergence angle and non-coherence, as well as difficulties in achieving large viewing angles with high luminous efficiency. In this work, we propose a double-layer metasurface film integrating functions of collimation with multiple deflections, constituting a micro-LED naked-eye 3D display system. The system is characterized through numerical simulations using the 3D finite-difference time-domain method. The simulation results show that the double-layer metasurface film restricts 90% of the emitted light of the micro-LED to the vicinity of the 0° angle, improving its spatial coherence. Subsequently, a large-angle, low-crosstalk outgoing from -45° to 45° is achieved, while providing a deflection efficiency of over 80% and a pixel density of up to 605. We believe this design provides a feasible approach for realizing naked-eye 3D micro-LED displays with a large field of view, low crosstalk, and high resolution.

Compound parabolic concentrator for LED-based underwater optical communication transmitter

Laser sources have been used at the transmitter unit of underwater wireless optical communication (UWOC) systems since their invention. In recent years, light-emitting diodes (LEDs) have begun to be used as UWOC transmitters due to their low cost, long lifespan, and high energy efficiency. However, data transmission distances in UWOC systems using bare LEDs and LED arrays with collimating lens are limited due to LED’s large divergence angles and insufficient gains provided by the low cost lens designs. In this paper, we propose to use a compound parabolic concentrator (CPC), whose transmission efficiency curve is very close to the ideal concentrator, as a collimator that narrows the divergence angle of LEDs to design low-cost UWOC transmitter with long communication distance. Using the Bouguer–Beer–Lambert channel model, we derived the received optical power expression at the receiver side for the LED-based UWOC system with CPC at the transmitter. Furthermore, unlike existing studies in the literature, the full spectrum of the LED has been taken into account when deriving the power expression. The results show that using the CPC collimator instead of a typical collimation lens in an LED-based UWOC system can narrow the divergence angle by approximately 10 times, resulting in a 70 dB increase in signal-to-noise ratio (SNR) at the receiver and up to a 20 times increase in the communication distance.

High transmission efficiency collection system for laser-accelerated proton beams based on permanent magnet quadrupoles prefocusing

A new high transmission efficiency, easily tunable, and cost-effective beam collection system is proposed for laser-accelerated proton beams with large divergence angles and wide energy spectra. In previous experiments conducted at the compact laser plasma accelerator platform of Peking University, a beamline was initially constructed with a collection system based on an electromagnetic quadrupoles (EMQs) triplet. However, due to the limited acceptance angle, the EMQs exhibited a monoenergetic transmission efficiency of only 10% to 20% for laser accelerated proton beams. Permanent magnet quadrupoles (PMQs), known for their high magnetic field gradients, compact size, and lower expenses, are well suited for integration with other readily adjustable transmission elements like EMQs and solenoids, effectively enhancing the beam collection capabilities of the system. In this paper, we show that by introducing a pair of centimeter-sized PMQs in front of the EMQs for beam prefocusing, the beam’s transverse size is quickly compressed, enabling transmission of highly divergent protons. Experimental results demonstrate that the prefocusing by PMQs increased the system’s transmission efficiency by a factor of 2.44 to 6.01 compared to the original setup, while also enhancing the energy selection based on stronger chromatic effect. This method can be extended to 100 MeV high-energy proton beamlines and is crucial for applications of laser plasma accelerators. Published by the American Physical Society 2024

Research on defect detection of bottle cap interior based on low-angle and large divergence angle vision system.

During the machine vision inspection of the inner section of bottle caps within pharmaceutical packaging, the unique conca bottom and convex side walls often create obstructions to the illumination. Consequently, this results in challenges such as irregular background and diminished feature contrast in the image, ultimately leading to the misidentification of defects. As a solution, a vision system characterized by a Low-Angle and Large Divergence Angle (LALDA) is presented in this paper. Using the large divergence angle of LED, combined with low-angle illumination, a uniform image of the side wall region with bright-field characteristics and a uniform image of inner circle region at the bottom with dark-field characteristics are obtained, thus solving the problems of light being obscured and brightness overexposure of the background. Based on the imaging characteristics of LALDA, a multi-channel segmentation (MCS) algorithm is designed. The HSV color space has been transformed, and the image is automatically segmented into multiple sub-regions by mutual calculation of different channels. Further, image homogenization and enhancement are used to eliminate fluctuations in the background and to enhance the contrast of defects. In addition, a variety of defect extraction methods are designed based on the imaging characteristics of different sub-regions, which can avoid the problem of over-segmentation in detection. In this paper, the LALDA is applied to the defect detection inside the cap of capsule medicine bottle, the detection speed is better than 400 pcs/min and the detection accuracy is better than 95%, which can meet the actual production line capacity and detection requirements.

All-optical photoacoustic detection of SF6 decomposition component SO2 based on fiber-coupled UV-LED

An all-optical photoacoustic sensor for measuring the concentration of SF6 decomposition derivative SO2 was designed, whose detection limit was in the order of ppb-level. An UV-fiber and a single-mode fiber were utilized to transmit excitation light and probe light, respectively. The technical scheme of all-optical gas sensing has realized the competitive advantages of long-distance detection and anti-electromagnetic interference. As an UV-LED was the excitation light, the challenge of designing all-optical SO2 sensor was to effectively couple UV-LED with large divergence angle into an UV- fiber. Theoretical calculation and experiment have proved that the lens assembly as an effective strategy to realize the efficient coupling and beam collimation, which can achieve high excitation power and low solid photoacoustic background interference. The lens assembly with high focusing property was the key to improve the signal-to-noise ratio (SNR) by 5.2 times. To detect photoacoustic pressure with high sensitivity, a fiber optic cantilever acoustic sensor based on F-P interference was designed. Based on the demodulation algorithm of white light interference, dynamic cavity length of F-P was demodulated with high-resolution. Allan-Werle analysis result indicate that the detection limit of SO2 gas was 48 ppb with an averaging time of 100 s

Research on a Blue–Green LED Communication System Based on an Underwater Mobile Robot

Underwater robots have been widely used in ocean exploration, deep-sea observation, seabed operations, marine scientific research, and other fields. Underwater low-latency, efficient, and safe communication modes are key to realizing the application of an underwater robot data transmission system. This paper mainly studies the optical communication between underwater mobile robots, including the large-dispersion-angle light-emitting diode (LED) design, large field of view receiving technology, weak light detector technology, etc. By designing a 120° large divergence angle underwater optical communication system in this study, the receiving field-of-view angle of the receiving end can reach 60°, which is suitable for the optical communication system of an underwater mobile platform. The high-power LED driver circuit is designed to drive the high-power LED and adopt weak light detection technology to ensure its stability and reliability. The experimental results show that, in the case of incomplete alignment between the transmitter and receiver, stable communication of underwater robots in motion is achieved through the design of a large divergence angle and a receiving field-of-view angle and the use of an underwater weak light detection technology. The communication distance is 30 m, and the communication rate remains above 10 Mbps. The information transmission content can include network data transmission, real-time video, high-definition video, high-definition images, and other data types. This equipment provides a solution for cableless data transmission of remotely operated vehicles (ROVs) and substantially enhances the application field of ROVs.

Anterior-posterior characterization of intraglottal flow fields in excised canine larynges

It has been espoused that intraglottal flow separation vortices (FSVs) play a critical role in the rapid closure of the vocal fold vibrations by acting as an additional pulling force that affects the maximum flow declination rate (MFDR). Clinically, MFDR is important because studies showed it highly correlates with loudness, intelligibility, and vocal efficiency. Previous studies that aimed to characterize the influence of FSVs were focused on their role in the mid-coronal plane. In the current study, particle imaging velocimetry was used to measure the intraglottal flow, specifically the FSVs, in the anterior–posterior aspects of the glottis during the closing phase of the vocal fold vibrations. The results show that the FSVs are extending along the length of the vocal fold in the anterior–posterior direction and that there is a direct relation between the size of FSVs and the magnitudes of the glottal divergence angles. The results also show that the centroid of FSVs occurs towards the anterior direction, where larger divergence angles are observed. These findings suggest that the FSVs in the anterior aspect have a more significant effect on the aerodynamic force acting on the glottal wall than the FSVs in the posterior aspect.

Design and simulation of a radius-varying active plasma lens for focusing laser-accelerated protons

With plasma as the medium, laser driven accelerator can generate high energy pulsed particles with initially micron scale source size and broad energy spread. Efficient transport of the beam with above characteristics is a challenging task. Active plasma lenses (APLs) provide strong, tunable, axisymmetric focusing with weak chromatic dependencies, making them highly beneficial for compact laser accelerators. However, due to the large divergence angle of the laser-accelerated proton beams, a majority of the protons cannot be transmitted through the APLs, resulting in losses. In this study, we propose a radius-variable APL (RV-APL) that optimizes the capillary’s geometry based on the beam envelope. The collection angle of RV-APL under the design parameters in the article can be increased by about 80% compared to a cylindrical APL (C-APL) with a constant radius, achieving the transmission capacity of traditional magnets. The magnetic field distribution of the RV-APL was predicted using a two-dimensional axisymmetric plasma discharge model, and the focused beam’s chromaticity and emittance properties were theoretically analyzed through particle tracking simulation.

Magnetic focusing of low-power Hall thruster with a center-mounted hollow cathode

Magnetic focusing with xenon propellant has been verified in orbit from medium-to high-power Hall thrusters but has yet to be achieved in low-power Hall thrusters. A magnetically focused Hall thruster was designed to solve the common issue of large plume divergence angle in low-power Hall thrusters, which realizes the plasma lens magnetic field topography with permanent magnet excitation on the wide channel and adopts the center-mounted cathode layout. Experimental results indicate that the HEP-300CHC thruster exhibits an optimal magnetic focusing mode when operated at 288W with plume divergence half-angle approximately 11.7°. The magnetic focusing range is relatively narrow, concentrated within discharge voltage between 210V and 230V, and anode flowrate between 18sccm and 20sccm.The anode efficiency is higher than 40% at the nominal power, and the maximum anode efficiency reach 52% at the discharge power of 300W. This study provides a reference basis for ion beam control of low-power Hall thrusters.

Mode selection and unidirectional emission in multiple-deformed microcavity through dynamic tunneling

Microcavity holds great promise in demonstrating coherent light sources and photonic circuits due to its superior properties such as high-quality factor, small mode volume, etc. Generally, the rotationally symmetric geometry of microcavity naturally makes its output beams isotropic, which is hard to efficiently collect the emissions. Though several solutions have been proposed to tackle this issue, for instance, boundary deformation, nanoparticle scattering, and mode coupling, at the cost of quality (Q)-factor degradation, large divergence angle of output beams, and complex designs, therefore, which more or less hinders the practical potential of microcavity. Herein, to examine the possibility of simultaneously achieving high-Q and unidirectional emission with narrow angular divergence, we employ a spirally deformed quadrupolar microcavity to manipulate the intracavity physics. Our numerical results show that the chaotic-to-regular tunneling and unstable manifolds synergistically dominate the chirality and the corresponding far-field patterns. The spiral implies sufficient backscattering onto the chaotic modes, which sequently breaks the balance of clockwise and counterclockwise waves and brings such non-equivalence into diamond modes without spoiling their Q-factors. Then strong chirality is formed in the chaotic microcavity that initially reduces the output beams. Meanwhile, the geometry deformation from pristine quadrupolar further alters the unstable manifolds, finally yielding highly unidirectional emission with narrower divergence angle (≤30°). As a result, mode selection and unidirectional emission are constructed simultaneously in the presented microcavity through a simple but robust method. This research is of great significance to the research of microcavity and the practical application of optical sensing and single-photon sources.

Large Eddy Simulation of Cavitation Jets from an Organ-Pipe Nozzle: The Influence of Cavitation on the Vortex Coherent Structure

High-speed water jets are widely used in deep mining and the in-depth study of jet characteristics helps to improve drilling efficiency. Three-dimensional Large Eddy Simulation is used to simulate turbulent flows generated by an organ-pipe nozzle. The simulation is validated with existing experimental data and is focused on the evolution and interaction of cavitation bubbles and vortices. Dynamic mode decomposition is performed to extract structural information about the different motion modes and their stability. Results show that the dominant fluid frequency is positively correlated with inlet pressure while unrelated to the divergence angle. Meanwhile, jets’ oscillation is amplified by a large divergence angle, which facilitates the occurrence of cavitation. Results about the flow field outside of an organ-pipe nozzle advance the understanding of the basic mechanism of cavitation jets.

Measurement method and results of divergence angle of laser-controlled solid propellants used in space propulsion

The plume divergence angle is an important reference index for evaluating the thrust efficiency and propellant utilization of space propulsion systems. However, the characteristics of the dynamic variation of plume divergence angle over time cannot be measured using current methods. This paper utilizes high-speed photography and image processing methods to develop a strategy that can give a quick, non-destructive and real-time detection of the divergence angle. Effectiveness of the strategy is verified, and the characteristics of plume divergence angles of different laser-controlled solid propellants were further analyzed and fitted. The experimental results indicate that graphene could effectively reduce the divergence angle, while oxide-doped samples had larger divergence angles than alloy-doped and carbon-doped samples.

Compact structured light generation based on meta-hologram PCSEL integration

Metasurfaces, a catalog of optical components, offer numerous novel functions on demand. They have been integrated with vertical cavity surface-emitting lasers (VCSELs) in previous studies. However, the performance has been limited by the features of the VCSELs such as low output power and large divergence angle. Although the solution of the module of VCSEL array could solve these issues, the practical application is limited by extra lens and large size. In this study, we experimentally demonstrate reconstruction of a holographic images using a compact integration of a photonic crystal surface-emitting laser and metasurface holograms designed for structured light generation. This research showcases the flexible design capabilities of metasurfaces, high output power (on the order of milliwatts), and the ability to produce well-uniformed images with a wide field of view without the need for a collection lens, making it suitable for 3D imaging and sensing.

Research Progress of Horizontal Cavity Surface-Emitting Laser.

The horizontal cavity surface emitting laser (HCSEL) boasts excellent properties, including high power, high beam quality, and ease of packaging and integration. It fundamentally resolves the problem of the large divergence angle in traditional edge-emitting semiconductor lasers, making it a feasible scheme for realizing high-power, small-divergence-angle, and high-beam-quality semiconductor lasers. Here, we introduce the technical scheme and review the development status of HCSELs. Firstly, we thoroughly analyze the structure, working principles, and performance characteristics of HCSELs according to different structures, such as the structural characteristics and key technologies. Additionally, we describe their optical properties. Finally, we analyze and discuss potential development prospects and challenges for HCSELs.

Chromaticity-Tunable All-Inorganic Color Converters Fabricated by 3D Printing for Modular Plant Growth Lighting Devices

In photopolymerization-induced 3D printing of glass and ceramics, the demand for a slurry that has high photosensitivity, low viscosity, and high solid content leads to a limited selection of suspended particles. To this end, ultraviolet-assisted direct ink writing (UV-DIW) is proposed as a new 3D printing compatible approach. A curable UV ink is synthesized, which overcomes the material limitation. Benefiting from the advantage of the UV-DIW process, CaAlSiN3:Eu2+/BaMgAl10O17:Eu2+ phosphors in glass (CASN/BAM-PiG) as chromaticity-tunable specially shaped all-inorganic color converters are prepared for plant growth lighting using an optimized heat treatment procedure. Size compatible dome-type and flat-type CaAlSiN3:Eu2+ phosphors in glass (CASN-PiG) are constructed in batches. The manufactured dome-type PiG-based light-emitting diodes (LEDs) exhibit better heat dissipation capacity and a larger divergence angle. The advantage of CASN/BAM-PiG in plant growth lighting is confirmed by the high degree of resemblance between the emission spectra of CASN/BAM-PiG and the absorption spectra of carotenoid and chlorophyll. A series of dome-type CASN/BAM-PiG based LEDs with selective region doping are constructed, which can weaken reabsorption effects and scientifically match the requirements of different plants. The excellent color-tunable ability and high degree of spectral resemblance indicate the superiority of the proposed UV-DIW process in all-inorganic CASN/BAM-PiG color converters for intelligent agricultural lighting.

Characteristics of penetration and distribution of a liquid jet in a divergent cavity-based combustor

The atomization process of a liquid jet in a divergent cavity-based combustor was investigated experimentally using high-speed photography and schlieren techniques under a Mach number 2.0 supersonic crossflow. Gas-liquid flow field was studied at different divergent angles and injection schemes. It is found that complex wave structures exist in the divergent cavity-based combustor. The spray field can be divided into three distinct zones: surface wave-dominated breakup zone, rapid atomization zone and cavity mixing zone. A dimensionless spray factor is defined to describe the concentration of spray inside the cavity qualitatively. As a result, it is revealed that for the large divergent angle cavity, the injection scheme near the upstream inlet has a higher penetration depth but a lower spray distribution, where the injection scheme near the cavity has a more spray distribution. For the small divergent angle cavity, the injection scheme near the upstream inlet also has a higher penetration depth and the injection scheme near the start point of the divergent section has a more sufficient spray distribution. The small divergent angle cavity-based combustor with the upstream wall transverse injection is an optimized injection scheme to improve both penetration and spray distribution inside the cavity. Finally, a penetration depth formula is proposed to explain the spray and distribution behaviors in the divergent cavity-based combustor.

Introduction of Research Work on Laser Proton Acceleration and Its Application Carried out on Compact Laser–Plasma Accelerator at Peking University

Laser plasma acceleration has made remarkable progress in the last few decades, but it also faces many challenges. Although the high gradient is a great potential advantage, the beam quality of the laser accelerator has a certain gap, or it is different from that of traditional accelerators. Therefore, it is important to explore and utilize its own features. In this article, some recent research progress on laser proton acceleration and its irradiation application, which was carried out on the compact laser plasma accelerator (CLAPA) platform at Peking University, have been introduced. By combining a TW laser accelerator and a monoenergetic beamline, proton beams with energies of less than 10 MeV, an energy spread of less than 1%, and with several to tens of pC charge, have been stably produced and transported in CLAPA. The beamline is an object–image point analyzing system, which ensures the transmission efficiency and the energy selection accuracy for proton beams with large initial divergence angle and energy spread. A spread-out Bragg peak (SOBP) is produced with high precision beam control, which preliminarily proved the feasibility of the laser accelerator for radiotherapy. Some application experiments based on laser-accelerated proton beams have also been carried out, such as proton radiograph, preparation of graphene on SiC, ultra-high dose FLASH radiation of cancer cells, and ion-beam trace probes for plasma diagnosis. The above applications take advantage of the unique characteristics of laser-driven protons, such as a micron scale point source, an ultra-short pulse duration, a wide energy spectrum, etc. A new laser-driven proton therapy facility (CLAPA II) is being designed and is under construction at Peking University. The 100 MeV proton beams will be produced via laser–plasma interaction by using a 2-PW laser, which may promote the real-world applications of laser accelerators in malignant tumor treatment soon.

Serrated periodic electrode for high energy efficiency and large bandwidth acousto-optic modulators

In an acousto-optic modulator, the electrode shape plays an important role in performance, since it affects the distribution of the acoustic field. The acousto-optic modulator based on the conventional rectangular electrode has the problems of low energy efficiency and small modulation bandwidth due to an imperfect acoustic field. In this paper, a new serrated periodic electrode has been proposed for using acousto-optic modulator transducers. The proposed electrode has the following advantages. By using serrated periodic electrodes to suppress the sidelobes, the collimation of the acoustic field in the direction perpendicular to the light incidence is improved. This makes the acousto-optic modulator have a stable diffraction efficiency fluctuation and high energy efficiency. In addition, the electrode has a large divergence angle in the direction of light incidence, so a large bandwidth can be obtained. The simulations and experiments demonstrate that the serrated periodic electrode has an increased bandwidth and high energy efficiency.

The azimuthal currents in the ion-driven magnetic nozzle

Ion-driven magnetic nozzles (Ti > Te) are designed as intrinsic parts of cutting-edge propulsive technologies such as variable specific impulse magnetoplasma rockets (VASIMRs) and applied-field magnetoplasmadynamic thrusters. Employing a two-dimensional axisymmetric particle-in-cell (PIC) code, in the ion-driven magnetic nozzle, the compositions and distributions of azimuthal currents in different axial regions are investigated under various inlet ion temperatures Ti0 and found to differ dramatically from that in the electron-driven magnetic nozzles. Previously reported to be all paramagnetic and vanishing under a high magnetic field, the azimuthal currents resulting from the E × B drift are shown to turn diamagnetic and sustain a considerable magnitude when Ti0 is considered. The previously reported profile of diamagnetic drift current is altered by the introduction of inlet ion temperature, and the paramagnetic part is significantly suppressed. Moreover, a wide range of paramagnetic currents appear downstream due to the inward detachment of ions, which can also be reduced by increasing inlet ion temperature. Albeit considered in this paper, the azimuthal currents resulting from grad-B and curvature drift are still negligible in all cases of interest. The magnitude of diamagnetic azimuthal currents increases with amplifying Ti0, indicating a clear physical image of energy transformation from ion thermal energy to the directed kinetic energy through electromagnetic processes in the magnetic nozzle. Additionally, the magnetic inductive strength also has noticeable impacts on the azimuthal currents, the current magnitude tends to decrease as the magnetic field increases, and over-increment of it may result in larger divergence angles and lower nozzle efficiency.

Static Beam Steering by Applying Metasurfaces on Photonic-Crystal Surface-Emitting Lasers

Photonic-crystal surface-emitting laser (PCSEL) holds promise for laser beam with high efficacy while retain the small divergence angle, being now all the rage as laser source in modern opto-electronic technologies to replace edge emitting lasers and vertical-cavity surface-emitting lasers (VCSELs). Arbitrary beam shaping and deflection of VCSELs with ultra-thin flat optics, aka metasurfaces, has recently demonstrated, making possibility to realize a compact system in many technologies. However, the large divergence angle and limited output power at single fundamental mode of the VCSEL limits its application. Here, we report the static steering of laser beam with high efficiency and near 1° divergence angle by shining the laser beam of the PCSEL through the metasurface array placed on top. The small divergence angle is contributed from PCSEL. On the other hand, the metasurface array offers the higher efficiency beam deflection, without considering additional collimating or focusing. This provides highly flexible platform and is capable to manipulate the laser emission through the design of the metasurfaces and the PCSELs. Moreover, the beam proprieties can be further modified for specific application through the metasurfaces. The integration of metasurface with PCSEL provides the compact and collimated laser source, promising more accurate generation of versatile laser sources on demand for automotive ultra-compact light detection and ranging (LiDAR), depth sensing, optical communication, etc.