Small Divergence Angle Research Articles

Directed pulsed neutron source generation from inverse kinematic reactions driven by intense lasers

Neutron production driven by intense lasers utilizing inverse kinematic reactions is explored self-consistently by a combination of particle-in-cell simulations for laser-driven ion acceleration and Monte Carlo nuclear reaction simulations for neutron production. It is proposed that laser-driven light-sail acceleration from ultrathin lithium foils can provide an energetic lithium-ion beam as the projectile bombarding a light hydrocarbon target with sufficiently high flux for the inverse p(Li7,n) reaction to be efficiently achieved. Three-dimensional self-consistent simulations show that a forward-directed pulsed neutron source with ultrashort pulse duration 3 ns, small divergence angle 26°, and extremely high peak flux 3 × 1014n/(cm2⋅s) can be produced by petawatt lasers at intensities of 1021 W/cm2. These results indicate that a laser-driven neutron source based on inverse kinematics has promise as a novel compact pulsed neutron generator for practical applications, since the it can operate in a safe and repetitive way with almost no undesirable radiation.

Research on ash accumulation surface measurement methods based on the dot laser principle

Dust collectors are essential environmental protection equipment in thermal power plants. However, in recent years, collapses caused by excessively high material levels in the ash hopper in dust collectors have increased frequently, resulting in serious casualties and significant economic losses. The level gauge of the ash hopper is a crucial device for detecting the material level in the ash hopper and plays a vital role in preventing the ash hopper from collapsing due to excessive material load. However, level gauges of the ash hopper currently in use have various problems. Laser-ranging technology is characterized by concentrated energy, a small divergence angle, and the ability to penetrate dust. We have proposed a new material-level measurement method based on the dot matrix laser ranging principle. This method utilizes dot-matrix laser ranging and visualizes the optimized measurement data to ultimately derive information about the material surface's morphology and level. A physical hopper model is used to conduct verification experiments on various material surface morphologies, material levels, and the material hanging on the hopper wall. The results indicate that this method can accurately identify the material level and the material hanging on the hopper wall, preventing false alarms due to “false material levels.”

Service spacecraft for space debris removal

The problem of space debris removal from the near-Earth space is being studied in almost every country taking part in space exploration. This is due to the global threat of critical increase in the number of space debris objects predicted for the near future, in particular as a result of significant growth in demand for multi-satellite constellations for various purposes. To date, researchers have proposed a large number of different methods for space debris removal. The work presented is the development of one of the promising contactless methods of space debris removal by an ion beam. The results of modeling and optimization of physical processes in the ion source to be mounted on board a service spacecraft that is necessary for implementing the considered method of space debris removal are presented. Besides, the results of thermal modeling and thermal mapping of the ion source during its operation are presented also, and the calculation results of its output parameters are compared to experimental data, which verified the small divergence angles of the generated ion beam. To assess the prospects of using a system for contactless space debris transportation by an ion beam using the obtained data on the operating parameters of the ion source, the trajectory design and mission analysis were carried out, which revealed the feasibility of removing seven space debris objects out of the protected region in geostationary orbit by a single service spacecraft. Besides, the preliminary service spacecraft design is presented.

Nonuniformity of diffractive splitting and receiver-transmitter alignment for large-field-of-view hundred-beam-scale LiDAR

To meet the spatial perception requirements for autonomous ship navigation in common scenarios such as ocean navigation, port entry and exit, and lock passage, commercial vehicle-mounted LiDAR technology falls short of demands in aspects such as sensing distance, field of view, angular resolution, and spatial sampling density. This paper proposes a hundred-beam-scale LiDAR scheme based on large-field-of-view diffractive beam splitting and a fiber array for echo reception and presents an in-depth investigation of the angular nonuniformity of diffractive beam splitting and the microradian-scale alignment for such hundred-beam-scale large-field-of-view LiDAR. This paper considers a combination of split-beam transmission based on a high-order diffractive optical element (DOE) and echo reception based on a high-precision fiber-optic line array. The nonlinear angular characteristics of the DOE are deduced and analyzed for a large field of view. The units of the receiving fiber-optic array are designed to offset the influence of the angular nonlinearity of the DOE, ensuring high-precision receiver–transmitter alignment of the hundred-beam-scale LiDAR for beams at any order of diffraction and helping to reduce system errors. The above-described LiDAR system has undergone laboratory testing and practical engineering verification, and it provides a new optical solution for LiDAR systems at the hundred-beam scale with a large field of view, a small divergence angle, and high sampling density. The presented system achieves a registration accuracy of 66.5 μrad with 128 beams and a 10-degree field of view, greatly improving signal reception efficiency. Such LiDAR systems have a wide range of applications, including space docking, target identification, lunar and planetary exploration, and ground-based vehicle-mounted LiDAR.

Enhanced emitting aperture in optical phased arrays with weak-coupled grating antennas via large critical dimensions

We propose what we believe to be a novel strategy to design optical phased arrays (OPAs) antennas by inserting a narrow waveguide between the bus waveguide and grating teeth. When the minimum critical dimension (CD) is 200 nm and the grating length is 1.5 mm, we achieve a quite small divergence angle of 0.05°, a reduction from 0.11° using the conventional method with the same CD. This design is compatible with 248 nm deep ultraviolet lithography, offering a more accessible fabrication process. By the gap’s increase from 180 nm to 410 nm rather than minimizing grating tooth dimensions, the divergence angles (δθ) have been demonstrated to be reduced from 0.28° to 0.05°. Furthermore, the designed OPA exhibits well divergence angles consistency, with a low standard deviation of 0.35% across wavelengths ranging from 1500 nm to 1560 nm. This innovation simplifies OPA fabrication, paving the way for cost-effective, high-resolution light detection and ranging (LiDAR) systems with extended detection capabilities.

Non-uniform distributed silicon optical phased array for high directionality and a wide steering range.

A non-uniform distributed silicon optical phased array (OPA) is proposed and numerically demonstrated to realize high directionality and a wide range for beam steering. The OPA is composed of grating antennas with dual-layer corrugations along silicon strip waveguides, which can achieve a high directionality of 0.96 and a small divergence angle of 0.084°. To reduce the crosstalk between adjacent antennas and realize a wide steering range, the genetic algorithm is improved and utilized to arrange the locations of grating antennas. As a proof of concept, a 32-channel non-uniform distributed OPA is designed and thoroughly optimized. The simulation results successfully demonstrate a two-dimensional wide steering range of 70∘×18.7∘ with a side-mode suppression ratio (SMSR) over 10dB.

High-Power Terahertz Photonic Crystal Surface-Emitting Laser with High Beam Quality

The photonic crystal surface-emitting laser (PCSEL) has attracted much attention due to the advantages of a small far-field divergence angle and high output power. Here, we report a high-power terahertz (THz) photonic crystal laser with high beam quality through the optimization of the absorption boundary condition and the introduction of the symmetrically distributed electrodes. Single-mode surface emission at 3.4 THz with the maximum peak output power of 50 mW is demonstrated. Meanwhile, a high symmetric far-field pattern with C6 symmetry and a small divergence angle is achieved. In this device, the integration of the stable single-mode operation, high beam quality and high output power is realized, which may have great significance for practical applications.

Metasubstrate-based SH guided wave piezoelectric transducer for unidirectional beam deflection without time delay

Focusing the energy of a guided wave along a desired direction is of great significance in structural health monitoring (SHM), since it can improve the sensitivity to defects and reduce the complexity of signal interpretations. The phased array method is the most common approach for beam deflection, which requires a high uniformity of each element and the support of expensive and complex electronics. Moreover, the mirrored wave beam cannot be avoided for a linear array, resulting in the difficulty of distinguishing the defects at the symmetric positions. In this work, a metasubstrate-based piezoelectric transducer (MSBPT) is developed for unidirectional SH0 wave (the fundamental shear horizontal wave) beam deflection under a single driving source. The proposed MSBPT is constituted by a metasubstrate and two columns of thickness-shear mode PZT wafers. The metasubstrate is designed to provide the required phase gradient for unidirectional beam deflection, so no extra time delay is needed during excitation and reception. An analytical model is proposed to guide the design of the transducer to obtain the desired wave beam. The beam deflection properties of the MSBPT are validated by finite element simulations and experiments. It is observed that the MSBPT can concentrate the energy of the generated SH0 mode only in one desired direction with a small divergence angle. Moreover, the MSBPT can also serve as a sensor that only receives the SH0 mode propagating from the deflection angle and filters out the wave energies from other incident angles. Due to the simple configuration, the proposed MSBPT will be helpful in the controllable generation and reception of SH0 mode in SHM.

Recent Advances in Light Detection and Ranging: Optical Modulation Solutions and Novel Nanotechnologies

AbstractLight detection and ranging (LiDAR) sensor is widely recognized as a critical component for accurate perception. However, there are a host of challenges that impede their performance, including low spatial resolution, high costs, large size, low reliability, and susceptibility to interference. It is challenging to overcome these issues using a single LiDAR module, necessitating the need for a review of current LiDAR technologies. The paper commences by introducing the fundamental principles of various laser rangefinders and discussing the optical modulation technologies used to prevent interference and ghost images. Next, the paper delves into the latest developments in laser technology, with a focus on enhancing the switching rate, compliance with eye safety regulations, miniaturization, and improving stability. One highly promising innovation is the photonic crystal surface emitting laser (PCSEL), a novel light source that boasts high‐speed, small divergence angles, and high‐power output. Finally, the paper discusses the advancements made in non‐solid‐state scanning and solid‐state scanning, such as improving stability, increasing scanning angles, and optimizing the manufacturing of mechanical and micro‐electromechanical systems (MEMS). Additionally, the paper highlights the recent advancements in nanotechnology, specifically metasurface technology, which offers superior capabilities such as beam deflection, enhanced field‐of‐view (FOV), and dynamic modulation.

On-orbit space optical communication demonstration with a 22 s acquisition time.

The inherent small divergence angle of space optical communication poses significant challenges to establishing communication links. Therefore, laser link acquisition becomes a crucial technology in optical communication, with a primary focus on reducing the acquisition time effectively. Conventional optical communication acquisition techniques fail to address the error effects caused by satellite insertion into the Earth orbit, thus unable to meet the rapid establishment demands of laser links in satellite optical communication networks. In this regard, we propose a novel, to the best of our knowledge, on-orbit calibration method for laser communication terminals to our knowledge. Leveraging stellar observations, this method achieves efficient on-orbit calibration with only a single observation. Theoretical analysis and on-orbit experimental results demonstrate that this approach effectively enhances an open-loop pointing accuracy, offering promising prospects for achieving a rapid establishment of optical communication links in ultra-long-distance optical communication missions.

Design of low-threshold photonic-crystal surface-emitting lasers with confined gain regions by using selective area intermixing

Photonic-crystal surface-emitting lasers have many promising properties over traditional semiconductor lasers and are regarded as the next-generation laser sources. However, the minimum achievable lasing threshold of PCSELs is still several times larger than that of VCSELs, and limiting its applications especially if the required power is small. Here, we propose a new design that reduces the gain region in the lateral plane by using selective quantum-well intermixing to reduce the threshold current of PCSELs. By performing theoretical calculations, we confirmed that the threshold current can be lowered by a factor of two to three while keeping the PCSEL’s advantage of small divergence angle.

Coupling Between Lattice and Waveguide Modes in Semiconductor Plasmonic Crystal Lasers

Semiconductor-based plasmonic crystal lasers (PCLs) have narrow linewidth, small divergence angle, excellent thermal stability, and great potential for ultra-high-speed communications. The underlying mechanism and lasing modes for their operation is unfolded in this work. We find that the waveguide mode coupling with the surface plasmonic mode plays an essential role in the semiconductor-based PCLs. The simulation and experimental data reveal that the cooperation of the dielectric layers and the metallic nano-pillar array gives the needed resonant mode of high-quality factor. The mode coupling strength has to be properly designed to make these intriguing devices working.

A new mathematical model of phyllotaxis to solve the genuine puzzle spiromonostichy.

Arrangement of plant leaves around the stem, termed phyllotaxis, exhibits beautiful and mysterious regularities and has been one of the most attractive subjects of biological pattern formation. After the long history of studies on phyllotaxis, it is now widely accepted that the inhibitory effect of existing leaf primordia on new primordium formation determines phyllotactic patterning. However, costoid phyllotaxis unique to Costaceae of Zingiberales, displaying spiromonostichy characterized by a steep spiral with a small divergence angle, seems to disagree with the inhibitory effect-based mechanism and has remained as a "genuine puzzle". We developed a new mathematical model, hypothesizing that each leaf primordium exerts not only the inhibitory effect but also some inductive effect. Computer simulations with the new model successfully generated a spiromonostichous pattern when these two effects met a certain relationship. The obtained spiromonostichy matched the real costoid phyllotaxis observed with Costus megalobractea, particularly for the decrease of the divergence angle associated with the enlargement of the shoot apical meristem. The new model was also shown to be able to produce a one-sided distichous pattern that is seen in phyllotaxis of a few plants of Zingiberales and has never been addressed in the previous model studies. These results implicated inductive as well as inhibitory mechanisms in phyllotactic patterning, at least in Zingiberales.

A comparative analysis of intraglottal geometry and velocity flow fields: Excised human, canine, and synthetic vocal fold models

Synthetic vocal fold models imitate the characteristics of human vocal folds and offer advantages over tissue models, such as accessibility and prolonged lifespan, all while producing comparable vibration frequencies to those in humans. Nonetheless, due to their simplified design, they may not fully capture the intricate behavior observed in tissue models like the canine larynx model. Canine larynges, like human’s, exhibit a vertical mucosal wave, which yields a phase delay between the inferior and superior edges of the vocal folds free margin. Consequently, a divergent glottis forms during closing, producing intraglottal flow separation vortices (FSVs) that increase voice loudness and intelligibility. To bridge the knowledge gap between silicone and tissue vocal fold models, we compared the intraglottal geometry and velocity flow fields during phonation of these three models. At low subglottal pressures, the synthetic model displayed small divergence angles and no flow separation, where both tissues models exhibited FSVs. At higher pressures, FSVs observed in tissue models were correlated with increased glottal flow waveform skewing and higher MFDR values. These findings highlight why the excised canine larynx model should be the preferred choice over synthetic vocal fold models for studying the aerodynamics and fluid-structure interaction during phonation.

Energy-Efficient Unmanned Aerial Vehicle-Aided Visible Light Communication with an Angle Diversity Transmitter for Joint Emergency Illumination and Communication.

Unmanned aerial vehicle-aided visible light communication (UAV-VLC) can be used to realize joint emergency illumination and communication, but the endurance of UAV is a key limiting factor. In order to overcome this limitation, this paper proposes the use of an angle diversity transmitter (ADT) to enhance the energy efficiency of the UAV-VLC system. The ADT is designed with one bottom LED and several evenly distributed inclined side LEDs. By jointly optimizing the inclination angle of the side LEDs in the ADT and the height of the hovering UAV, the study aims to minimize the power consumption of the UAV-VLC system while satisfying the requirements of both illumination and communication. Simulation results show that the energy efficiency of the UAV-VLC system can be greatly enhanced by applying the optimized ADT. Moreover, the energy efficiency enhancement is much more significant when the LEDs in the ADT have a smaller divergence angle, or more side LEDs are configured in the ADT. More specifically, a 50.9% energy efficiency improvement can be achieved by using the optimized ADT in comparison to the conventional non-angle diversity transmitter (NADT).

Hybrid Microcavity Lasers: Principle, Design, and Practical Application

AbstractControl coupling and synchronization of lasers has become increasingly attractive for their applications in spectroscopy, imaging, sensing, secret communication, and optical networks. As complex microcavities emerge, more complicated light–matter interactions, along with the resulting distinct lasing emission behavior, is induced in these hybrid microcavities. This review summarizes the most important advances and current deep insights into the inherent relationship between the hybrid microcavity geometries and lasing behaviors, which are classified as the modulated lasing output freedom, including wavelength, angular momentum, mode, direction, and their multiple combinations. Finally, based on scientific research and practical application, the challenges and prospects of future development of hybrid microcavities are proposed, hoping to provide valuable inspiration for further optimizing hybrid microcavities to achieve low thresholds, small divergence angles, and single‐mode microcavity lasers.

Study of particle acceleration by Laguerre–Gaussian ultra intense laser plasma interactions

In this work, simulations of multi-petawatt lasers in the range of ∼0.2 PW–∼100 PW with varying Laguerre–Gaussian (LG) azimuthal modes as well as linearly polarized (LP) and circularly polarized (CP) laser beams striking near critical density targets were studied by using three-dimensional particle in cell (PIC) codes. Particle acceleration mechanisms have a dependence on laser polarization and it affects the energy gained by the particles. It is known that laser pulses can be polarized helically by applying the LG distribution function to the fundamental Gaussian laser profile. In this study, differently polarized laser beams with varying powers were employed to study laser driven particle acceleration and compares accelerated charged particles’ energy spectra and angular distribution. It is seen that LG laser beams can accelerate higher energetic particles due to higher conversion efficiency compared to LP and CP laser beams. It is also seen that LG laser beams can collimate ions with a narrower spread compared to LP and CP beams. Furthermore, ions can have a smaller divergence angle with increasing azimuthal mode index when the laser is LG polarized. We also studied the energy deposition of these particles in a water cell obtained by the PIC codes for different laser parameters by using Geant4 Monte Carlo simulations which suggests that LG laser beam can be useful for the future hadron therapy applications.

Image transmission performance analysis through free space optical communication link using coherent QPSK modulation under various environmental conditions

AbstractFree space optics (FSO) technology has been used recently in many applications due to its various advantages such as high bandwidth, easy deployment, high security, free installation license and immunity to electromagnetic interference. However, this type of communication is vulnerable to meteorological conditions such as rain, haze and fog which significantly degrade the system's efficiency. To enhance the effectiveness of the FSO systems, Quadrature Phase Shift Keying (QPSK) modulation based on coherent detection constitutes a promising technique for achieving high spectral efficiency and long‐distance FSO links with the characteristics of low‐cost implementation and simple design. This work aims to investigate the performance of FSO communication links on image transmission under the presence of atmospheric attenuations. A comparative study has been made using QPSK modulation scheme and on–off‐keying (OOK) modulation scheme to estimate the maximum range that was used to recover an RGB transmitted image over an FSO link under different meteorological weathers. The obtained results clearly show that the quality of the received image has degraded significantly during transmission through free space yielding to limit the propagation range. Moreover, the proposed FSO communication system performs well when using QPSK modulation rather than OOK modulation in different environmental conditions. Thus, to improve the quality of the received image under adverse fog weather condition, high power and small beam divergence angle have been suggested to increase system's performance. The proposed solution increases the communication range of the FSO link by 230 m and 100 m for moderate and heavy fog, respectively.

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.

Science and technologies in targets for laser accelerators

<p indent="0mm">Laser plasma acceleration (also referred to as laser acceleration) employs ultra-strong electric fields generated when an ultra-intense laser pulse interacts with the plasma to accelerate charged particles. As a promising novel acceleration method, its acceleration gradient can be 10³–10⁶ times that of conventional accelerators. Laser accelerators based on the laser acceleration process can not only generate ultra-short pulse particle/photon beams for fundamental research, but also provide high-energy protons/heavy ions for tumor radiotherapy devices, benefiting people’s lives and health. Under the strong national investment around the world, basic and applied research related to laser accelerators has developed rapidly in recent years. Unlike conventional accelerators, which use macroscopic electromagnetic fields to accelerate particles in a vacuum, the laser acceleration process is realized by shooting targets with laser pulses. The scientific and technical problems related to the “target” are the core problems of the laser accelerator. In order to build an accelerator that can stably output high-energy particle beams, on the one hand, developing new target systems and deeply studying laser acceleration physics is highly necessary. On the other hand, overcoming a large number of key technical issues related to the preparation and characterization of targets and shooting is also very crucial. This paper gives an introduction to the key problems in the science and technologies for laser accelerators and highlights the progress in this field made by our team at the Institute of Heavy Ion Physics of Peking University. By exploiting nanotechnology and nanomaterials, we developed novel targets made of carbon nanotubes, which can serve as long-awaited perfect near-critical-density (NCD) targets. With such targets, we observed the laser focusing and pulse steepening in NCD plasmas for relativistic laser pulses for the first time. We also successfully achieved cascaded ion acceleration scheme by designing and building double-layer CNT targets, which solves the long-standing dilemma between the ionization and long-time acceleration of heavy ions acceleration. By collaborating with the Institute for Basic Sciences (IBS, Korea), we generated record-breaking <sc>580 MeV</sc> carbon ions and <sc>1.2 GeV</sc> Au ions in experiments. Recently, we numerically and experimentally studied how an ultra-intense femtosecond laser pulse interacts with a nanowire array target and eventually creates a high-energy-density plasma. The generation of soft-X-rays/EUV radiations and fusion neutrons from such a plasma were measured and studied in detail. Micro-channel targets are very interesting targets for us as well. We revealed how a laser pulse propagates through a micro-channel, peels off and accelerates electrons to GeV level, and produces corpus gamma rays with very small divergence angles. Experiments collaborating with the China Academy of Engineering Physics confirmed our prediction. Besides the above breakthroughs in acceleration sciences, we solved many key technical problems in laser ion accelerators. For example, we developed some mass-preparation methods for high-quality targets, including nanometer-thin diamond-like-carbon foils and metal foils, measured the target damage thresholds of a majority of existing targets in laser plasma experiments, figured out how to precisely locate transparent targets with high-precision. For non-stop running of the laser ion accelerators, we recently developed liquid sheets, which is highly promising to realize continuous beam delivery for weeks.