Showing papers on "Collimated light published in 2020"

Coherent beam combining of 61 femtosecond fiber amplifiers.

Abstract: We report on the coherent beam combining of 61 femtosecond fiber chirped-pulse amplifiers in a tiled-aperture configuration along with an interferometric phase measurement technique. Relying on coherent beam recombination in the far field, this technique appears suitable for the combination of a large number of fiber amplifiers. The 61 output beams are stacked in a hexagonal arrangement and collimated through a high fill factor hexagonal micro-lens array. The residual phase error between two fibers is as low as λ/90 RMS, while a combining efficiency of ∼50% is achieved.

Wide beam steering by slow-light waveguide gratings and a prism lens

Abstract: A lattice-shifted photonic crystal waveguide (LSPCW) maintains slow light as a guided mode and works as an optical antenna when a kind of double periodicity is introduced. Selecting one LSPCW from its array and converting the fan beam to a spot beam using a collimator lens allows non-mechanical, two-dimensional beam steering. We employed a shallow-etched grating into the LSPCW as the double periodicity to increase the upward emission efficiency and designed a bespoke prism lens to convert the steering angle in a desired direction while maintaining the collimation condition for the steered beam. As a result, a sharp spot beam with an average beam divergence of 0.15° was steered in the range of ${40}^\circ \; \times \;{4.4}^\circ $40∘×4.4∘ without precise adjustment of the lens position. The number of resolution points obtained was 4256. This method did not require complicated and power-consuming optical phase control like that in optical phased arrays, so it is expected to be applied in complete solid-state light detection and ranging.

Metasurface for Structured Light Projection over 120° Field of View.

Abstract: Structured light projection is a widely adopted approach for depth perception in consumer electronics and other machine vision systems. Diffractive optical element (DOE) is a key component for structured light projection that redistributes a collimated laser beam to a spot array with uniform intensity. Conventional DOEs for laser spot projection are binary-phase gratings, suffering from low efficiency and low uniformity when designed for a large field of view (FOV). Here, by combining vectorial electromagnetic simulation and interior-point method for optimization, we experimentally demonstrate polarization-independent silicon-based metasurfaces that can project a collimated laser beam to a spot array in the far-field with an exceedingly large FOV over 120° × 120°. The metasurface DOE with large FOV may benefit a number of depth perception-related applications such as face-unlock and motion sensing.

Extremely brilliant GeV γ-rays from a two-stage laser-plasma accelerator

Abstract: Recent developments in laser-wakefield accelerators have led to compact ultrashort X/γ-ray sources that can deliver peak brilliance comparable with conventional synchrotron sources. Such sources normally have low efficiencies and are limited to 107-8 photons/shot in the keV to MeV range. We present a novel scheme to efficiently produce collimated ultrabright γ-ray beams with photon energies tunable up to GeV by focusing a multi-petawatt laser pulse into a two-stage wakefield accelerator. This high-intensity laser enables efficient generation of a multi-GeV electron beam with a high density and tens-nC charge in the first stage. Subsequently, both the laser and electron beams enter into a higher-density plasma region in the second stage. Numerical simulations demonstrate that more than 1012 γ-ray photons/shot are produced with energy conversion efficiency above 10% for photons above 1 MeV, and the peak brilliance is above 1026 photons s-1 mm-2 mrad-2 per 0.1% bandwidth at 1 MeV. This offers new opportunities for both fundamental and applied research.

Search for light long-lived neutral particles produced in pp collisions at √s=13TeV and decaying into collimated leptons or light hadrons with the ATLAS detector

Abstract: Several models of physics beyond the Standard Model predict the existence of dark photons, light neutral particles decaying into collimated leptons or light hadrons. This paper presents a search for long-lived dark photons produced from the decay of a Higgs boson or a heavy scalar boson and decaying into displaced collimated Standard Model fermions. The search uses data corresponding to an integrated luminosity of 36.1 fb- 1 collected in proton–proton collisions at s=13 Te recorded in 2015–2016 with the ATLAS detector at the Large Hadron Collider. The observed number of events is consistent with the expected background, and limits on the production cross section times branching fraction as a function of the proper decay length of the dark photon are reported. A cross section times branching fraction above 4 pb is excluded for a Higgs boson decaying into two dark photons for dark-photon decay lengths between 1.5 mm and 307 mm.

Design of a six-gas NDIR gas sensor using an integrated optical gas chamber.

Abstract: A non-dispersive infrared (NDIR) gas sensor with an integrated optical gas chamber was designed to accurately detect multiple gas concentrations in a complex polluted environment. This chamber consists of a hexagonal prism shell and a reflection plate. In particular, six dual-channel infrared detectors for sensing different gases and one collimated infrared light source are integrated in the gas chamber. The IR light emitted by the collimated IR light source arrives at these detectors after four reflections. The result of simulation and data collection for optical path length and luminous flux shows that the average optical path length in the chamber is 63 mm, and the effective utilization rate of the luminous flux can reach 78.8%. The highly compact NDIR sensor can detect the concentrations of a mixture of gases (CO, CO2, CH4, H2CO, NH3, and NO with a volume fraction ranging from 0 to 4%).

Modelling Fabry-Pérot etalons illuminated by focussed beams.

Abstract: Fabry-Perot (FP) etalons are used as filters and sensors in a range of optical systems. Often FP etalons are illuminated by collimated laser beams, in which case the transmitted and reflected light fields can be calculated analytically using well established models. However, FP etalons are sometimes illuminated by more complex beams such as focussed Gaussian beams, which may also be aberrated. Modelling the response of FP etalons to these beams requires a more sophisticated model. To address this need, we present a model that can describe the response of an FP etalon that is illuminated by an arbitrary beam. The model uses an electromagnetic wave description of light and can therefore compute the amplitude, phase and polarization of the optical field at any position in the system. It can also account for common light delivery and detection components such as lenses, optical fibres and photo-detectors, allowing practical systems to be simulated. The model was validated against wavelength resolved measurements of transmittance and reflectance obtained using a system consisting of an FP etalon illuminated by a focussed Gaussian beam. Experiments with focal spot sizes ranging from 30 µm to 250 µm and FP etalon mirror reflectivities in the range 97.2 % to 99.2 % yielded excellent visual agreement between simulated and experimental data and an average error below 10% for a range of quantitative comparative metrics. We expect the model to be a useful tool for designing, understanding and optimising systems that use FP etalons.

An ultra-compact particle size analyser using a CMOS image sensor and machine learning.

Abstract: Light scattering is a fundamental property that can be exploited to create essential devices such as particle analysers. The most common particle size analyser relies on measuring the angle-dependent diffracted light from a sample illuminated by a laser beam. Compared to other non-light-based counterparts, such a laser diffraction scheme offers precision, but it does so at the expense of size, complexity and cost. In this paper, we introduce the concept of a new particle size analyser in a collimated beam configuration using a consumer electronic camera and machine learning. The key novelty is a small form factor angular spatial filter that allows for the collection of light scattered by the particles up to predefined discrete angles. The filter is combined with a light-emitting diode and a complementary metal-oxide-semiconductor image sensor array to acquire angularly resolved scattering images. From these images, a machine learning model predicts the volume median diameter of the particles. To validate the proposed device, glass beads with diameters ranging from 13 to 125 µm were measured in suspension at several concentrations. We were able to correct for multiple scattering effects and predict the particle size with mean absolute percentage errors of 5.09% and 2.5% for the cases without and with concentration as an input parameter, respectively. When only spherical particles were analysed, the former error was significantly reduced (0.72%). Given that it is compact (on the order of ten cm) and built with low-cost consumer electronics, the newly designed particle size analyser has significant potential for use outside a standard laboratory, for example, in online and in-line industrial process monitoring.

Chromato-axial memory effect through a forward-scattering slab

Abstract: Light propagation through scattering samples involves long optical path lengths typically resulting in short spectral correlation widths. Here, we demonstrate experimentally and theoretically a chromato-axial memory effect through volumetric forward-scattering slabs of thickness lying in the range between the scattering mean free path and the transport mean free path: a spectral shift of the impinging collimated beam results in an axial homothetic dilation of the speckle pattern over large spectral widths. The center of the homothety is shown to lie at a virtual plane located at $ 1/(3n) $1/(3n) of the slab thickness $ L $L, before the output surface. These results provide new tools to blindly control wavefields through scattering samples.

Sideband-Enhanced Cold Atomic Source for Optical Clocks

Abstract: We demonstrate the enhancement and optimization of a cold strontium atomic beam from a two-dimensional magneto-optical trap (2D MOT) transversely loaded from a collimated atomic beam by adding a sideband frequency to the cooling laser. The parameters of the cooling and sideband beams are scanned to achieve the maximum atomic beam flux and are compared with Monte Carlo simulations. We obtain a 2.3 times larger total atomic flux and a brightness increase of a factor 4 compared with a conventional, single-frequency 2D MOT for a given total power of 200 mW. We show that the sideband-enhanced 2D MOT can reach the loading-rate performances of space-demanding Zeeman-slower-based systems, while it can overcome systematic effects due to thermal-beam collisions and hot-black-body-radiation shift, making it suitable for both transportable and accurate optical lattice clocks. Finally, we numerically study possible extensions of the sideband-enhanced 2D MOT to other alkaline-earth species.

Research on Target Deviation Measurement of Projectile Based on Shadow Imaging Method in Laser Screen Velocity Measuring System.

Abstract: In the laser screen velocity measuring (LSVM) system, there is a deviation in the consistency of the optoelectronic response between the start light screen and the stop light screen. When the projectile passes through the light screen, the projectile’s over-target position, at which the timing pulse of the LSVM system is triggered, deviates from the actual position of the light screen (i.e., the target deviation). Therefore, it brings errors to the measurement of the projectile’s velocity, which has become a bottleneck, affecting the construction of a higher precision optoelectronic velocity measuring system. To solve this problem, this paper proposes a method based on high-speed shadow imaging to measure the projectile’s target deviation, ΔS, when the LSVM system triggers the timing pulse. The infrared pulse laser is collimated by the combination of the aspherical lens to form a parallel laser source that is used as the light source of the system. When the projectile passes through the light screen, the projectile’s over-target signal is processed by the specially designed trigger circuit. It uses the rising and falling edges of this signal to trigger the camera and pulsed laser source, respectively, to ensure that the projectile’s over-target image is adequately exposed. By capturing the images of the light screen of the LSVM system and the over-target projectile separately, this method of image edge detection was used to calculate the target deviation, and this value was used to correct the target distance of the LSVM to improve the accuracy of the measurement of the projectile’s velocity.

Design of a focused collimator for proton therapy spot scanning using Monte Carlo methods

Abstract: Purpose When designing a collimation system for pencil beam spot scanning proton therapy, a decision must be made whether or not to rotate, or focus, the collimator to match beamlet deflection as a function of off-axis distance. If the collimator is not focused, the beamlet shape and fluence will vary as a function of off-axis distance due to partial transmission through the collimator. In this work, we quantify the magnitude of these effects and propose a focused dynamic collimation system (DCS) for use in proton therapy spot scanning. Methods This study was done in silico using a model of the Miami Cancer Institute's (MCI) IBA Proteus Plus system created in Geant4-based TOPAS. The DCS utilizes rectangular nickel trimmers mounted on rotating sliders that move in synchrony with the pencil beam to provide focused collimation at the edge of the target. Using a simplified setup of the DCS, simulations were performed at various off-axis locations corresponding to beam deflection angles ranging from 0° to 2.5°. At each off-axis location, focused (trimmer rotated) and unfocused (trimmer not rotated) simulations were performed. In all simulations, a 4 cm water equivalent thickness range shifter was placed upstream of the collimator, and a voxelized water phantom that scored dose was placed downstream, each with 4 cm airgaps. Results Increasing the beam deflection angle for an unfocused trimmer caused the collimated edge of the beamlet profile to shift 0.08-0.61 mm from the baseline 0° simulation. There was also an increase in low-dose regions on the collimated edge ranging from 14.6% to 192.4%. Lastly, the maximum dose, D max , was 0-5% higher for the unfocused simulations. With a focused trimmer design, the profile shift and dose increases were all eliminated. Conclusions We have shown that focusing a collimator in spot scanning proton therapy reduces dose at the collimated edge compared to conventional, unfocused collimation devices and presented a simple, mechanical design for achieving focusing for a range of source-to-collimator distances.

Multi-channel surface plasmon resonance biosensor using prism-based wavelength interrogation

Abstract: A portable multi-channel surface plasmon resonance (SPR) biosensor device using prism-based wavelength interrogation is presented. LEDs were adopted as a simple and inexpensive light source, providing a stable spectrum bandwidth for the SPR system. The parallel light was obtained by a collimated unit and illuminated on the sensing chip at a specific angle. A simple, compact and cost-effective spectrometer part constituted of a series of lenses and a prism was designed for the collection of reflected light. Using the multi-channel microfluidic chip as the sensing component, spectral images of multiple tests could be acquired simultaneously, improving the signal processing and detection throughput. Different concentrations of sodium chloride aqueous solution were used to calibrate the device. The linear detection range was 4.32 × 10−2 refractive index units (RIU) and the limit of detection was 6.38 × 10−5 RIU. Finally, the performance of the miniaturized SPR system was evaluated by the detection of immunoglobulin G (IgG).

Efficient Frequency Mixing of Guided Surface Waves by Atomically Thin Nonlinear Crystals.

Abstract: Monolayer transition metal dichalcogenides possess considerable second-order nonlinear coefficients but a limited efficiency of frequency conversion due to the short interaction length with light under the typical direct illumination. Here, we demonstrate an efficient frequency mixing of the guided surface waves on a monolayer tungsten disulfide (WS2) by simultaneously lifting the temporal and spatial overlap of the guided wave and the nonlinear crystal. Three orders-of-magnitude enhancement of the conversion efficiency was achieved in the counter-propagating excitation configuration. Also, the frequency-mixing signals are highly collimated, with the emission direction and polarization controlled, respectively, by the pump frequencies and the rotation angle of WS2 relative to the propagation direction of the guided waves. These results indicate that the rules of nonlinear frequency conversion are applicable even when the crystal is scaled down to the ultimate single-layer limit. This study provides a versatile platform to enhance the nonlinear optical response of 2D materials and favor the scalable generation of a coherent light source and entangled photon pairs.

On the use of the supporting quadric method in the problem of designing double freeform surfaces for collimated beam shaping.

Abstract: We propose a version of the supporting quadric method for calculating a refractive optical element with two working surfaces for collimated beam shaping. Using optimal mass transportation theory and generalized Voronoi cells, we show that the proposed method can be regarded as a gradient method of maximizing a concave function, which is a discrete analogue of the Lagrange functional in the corresponding mass transportation problem. It is demonstrated that any maximum of this function provides a solution to the problem of collimated beam shaping. Therefore, the proposed method does not suffer from "trapping" at a local extremum, which is typical for gradient methods. We present design examples of refractive optical elements illustrating high performance of the method.

Self-Measurements of Point-Spread Function for Remote Sensing Optical Imaging Instruments

Abstract: A point-spread function (PSF) is able to describe the spatial characteristics of a remote sensing optical imaging instrument (i.e., camera). Specifically, the PSF of a camera characterizes the image quality degeneration inevitably produced by the camera itself link, involving an optical system, an image sensor, and an electronic system in the imaging process of space optical cameras. In the nonblind reconstruction applications, the PSF is the prerequisite for remote sensing image reconstructions. On-orbit PSF measurement approaches are basically based on ground targets hidden remote sensing images, such as edge-based and point-pulse-based approaches. However, external targets used in these measurement methods are easily accessed because the imaging condition is limited by space and time. Here, we propose a real-time self-measurement method using optical fiber illuminations in conjunction with a Lasso-based reconstruction algorithm to measure the on-orbit PSF in real time. First, we construct an infinite object-point generation module by enclosing a laser, a fiber, and optics into a fiber micropoint-illuminator (mFPI), which can produce the collimated thin beams. The mFPI is composed of the head, fiber, and light source. The light source module is located external to the camera, where the lights from a laser are coupled into the input of an optical fiber by using a lens. The head is composed of the fiber output and lens, which is located on the internal of the camera. The light is guided into the head by the fiber, and the head acts as an infinite point target. Second, we propose a PSF reconstruction algorithm based on an extended Lasso model. Finally, confirmation experiments on a camera with optical fibers are performed. The proposed method provides a valuable self-measurement method for on-orbit cameras.

Silicon photonics co-integrated with silicon nitride for optical phased arrays

Abstract: In the current work we present Si and SiN combined photonic built-up for optical phased arrays (OPAs) and other large area photonic integrated circuits. We report low-loss co-integrated SiN waveguides and nearly lossless vertical transitions between Si and SiN layers, as well as efficient Si thermo-optical phase shifter module. OPA consisting of 64 optical antennas forming highly collimated beam with 0.4° × 0.47° divergence is reported. By changing input wavelength, solid-state beam steering of 0°–10° is achieved.

Surface Plasmon Resonance-Based Sensing Utilizing Spatial Phase Modulation in an Imaging Interferometer.

Abstract: Spatial phase modulation in an imaging interferometer is utilized in surface plasmon resonance (SPR) based sensing of liquid analytes. In the interferometer, a collimated light beam from a laser diode irradiating at 637.1 nm is passing through a polarizer and is reflected from a plasmonic structure of SF10/Cr/Au attached to a prism in the Kretschmann configuration. The beam passes through a combination of a Wollaston prism, a polarizer and a lens, and forms an interference pattern on a CCD sensor of a color camera. Interference patterns obtained for different liquid analytes are acquired and transferred to the computer for data processing. The sensing concept is based on the detection of a refractive index change, which is transformed via the SPR phenomenon into an interference fringe phase shift. By calculating the phase shift for the plasmonic structure of SF10/Cr/Au of known parameters we demonstrate that this technique can detect different weight concentrations of ethanol diluted in water, or equivalently, different changes in the refractive index. The sensitivity to the refractive index and the detection limit obtained are −278 rad/refractive-index-unit (RIU) and 3.6 × 10 − 6 RIU, respectively. The technique is demonstrated in experiments with the same liquid analytes as in the theory. Applying an original approach in retrieving the fringe phase shift, we revealed good agreement between experiment and theory, and the measured sensitivity to the refractive index and the detection limit reached −226 rad/RIU and 4.4 × 10 − 6 RIU, respectively. These results suggest that the SPR interferometer with the detection of a fringe phase shift is particularly useful in applications that require measuring refractive index changes with high sensitivity.

Edge-lit sine-shape wedged light guides: Design, optical simulation, laser-remelting-based precision fabrication, and optical performance evaluation

Abstract: Light guiding is an essential functionality of automotive lighting responsible for styling, safety, and artistic effects. The functional performance of optical lighting and illumination products and components depends on novel optical designs and advanced technologies for cost-effective fabrication of tooling with strict surface quality and form geometry accuracy. The present multi-objective study introduces a new design for an edge-lit wedge planar light guide (LG) with sine-shape riblets as well as an advanced microfabrication technology called surface structuring by laser remelting (SSLRM). In the proposed LG design, sine-shape riblets act as optical structures used to redirect - through total internal reflection - the incoming collimated light onto the illuminated surface. Numerical simulations showed that LGs with different amplitudes of sine-shape riblets (e.g., 25 μm, 40 μm, 55 μm) can reach an illumination efficiency of 97.7%. Furthermore, the applicability of SSLRM process for an efficient fabrication of edge-lit LG tooling inserts was demonstrated and several tooling inserts were fabricated with geometric parameters matching the proposed optical designs. Following this, the geometry and surface quality parameters of the fabricated inserts were presented. In addition, functional plastic prototypes of LGs were produced by means of a hot embossing technology that preserved the original form geometry and surface quality characteristics. Their actual optical performance was comparatively evaluated with respect to a LG with a flat-wedged surface and also to LGs with different amplitudes of sine-shape riblets. The maximum actual optical performance was achieved by LGs with a sine-shape amplitude of 55 μm that exhibited a total luminance of 0.1395 cd while the flat wedged LG without light redirecting structures exhibits 0.0006 cd only. It is also demonstrated that the sine-shape amplitude significantly contributes to the overall LG optical performance gaining 1811% improvement with respect to the LG with 25 μm amplitude delivering 0.0073 cd. The results obtained demonstrate the applicability and the high potential of the proposed edge-lit sine-shape wedged LGs as well as that of the SSLRM process used for their high precision microfabrication.

Pattern randomization: an efficient way to design high-performance metallic meshes with uniform stray light for EMI shielding

Abstract: Here, we proposed an ingenious grid pattern design method called pattern randomization to obtain metallic meshes with uniform stray light. The periodicity of a grid is weakened by the pattern randomization. By comparing the diffraction patterns of one-dimensional periodic grid, one-dimensional aperiodic grid and concentric rings structure, we found that the “radial homogenization” and “angular homogenization” can uniform the high-order diffracted energy. The pattern randomization is proposed to achieve the “radial homogenization” and “angular homogenization” two-dimensional grid while ensuring connectivity. For collimated incident beam, the metal grid with a randomness (90%, 90%) obtained by pattern randomization method generates uniform stray light, while it maintains high visible light transmittance and high electromagnetic shielding efficiency (SE). The simulated results are experimentally verified that the high-order diffraction spots can be effectively suppressed. The coefficient Cv is reduced from 1078.14% to 164.65%. Meanwhile, the randomness of the designed grid structure hardly affects the visible light transmittance and shielding efficiency. The metallic mesh with a shielding efficiency about 17.3 dB in the Ku-band, a relative transmittance higher than 94% in the visible light band and an ultra-uniform diffraction pattern is obtained.

Design of optical meta-structures with applications to beam engineering using deep learning.

Abstract: Nanophotonics is a rapidly emerging field in which complex on-chip components are required to manipulate light waves. The design space of on-chip nanophotonic components, such as an optical meta surface which uses sub-wavelength meta-atoms, is often a high dimensional one. As such conventional optimization methods fail to capture the global optimum within the feasible search space. In this manuscript, we explore a Machine Learning (ML)-based method for the inverse design of the meta-optical structure. We present a data-driven approach for modeling a grating meta-structure which performs photonic beam engineering. On-chip planar photonic waveguide-based beam engineering offers the potential to efficiently manipulate photons to create excitation beams (Gaussian, focused and collimated) for lab-on-chip applications of Infrared, Raman and fluorescence spectroscopic analysis. Inverse modeling predicts meta surface design parameters based on a desired electromagnetic field outcome. Starting with the desired diffraction beam profile, we apply an inverse model to evaluate the optimal design parameters of the meta surface. Parameters such as the repetition period (in 2D axis), height and size of scatterers are calculated using a feedforward deep neural network (DNN) and convolutional neural network (CNN) architecture. A qualitative analysis of the trained neural network, working in tandem with the forward model, predicts the diffraction profile with a correlation coefficient as high as 0.996. The developed model allows us to rapidly estimate the desired design parameters, in contrast to conventional (gradient descent based or genetic optimization) time-intensive optimization approaches.

Design of a high-precision, 0.5 m aperture Cassegrain collimator

Abstract: We present the realization of a high-precision, 0.5 m aperture size Cassegrain collimator system. The optical design, the optomechanical design, the mirror manufacturing, and the telescope alignment with a performance evaluation are extensively discussed. The optical design of the collimator is based on the Cassegrain telescope design with two aspheric mirrors. An athermalized, high stability optomechanical structure is conceived for the collimator to meet stringent performance requirements. The high-quality mirrors are made of low-expansion Zerodur glass-ceramic and the primary mirror is light-weighted to 63% of its initial weight. The design of a dedicated five-axis flexure mechanism driven by nanopositioner stages to compensate the secondary mirror misalignments is given. Primary and secondary mirrors with aspheric surfaces are manufactured, and their forms are measured by computer-generated holograms with a phase-shifting Fizeau interferometer. The alignment strategy is based on minimizing Fringe Zernike coefficients of wavefront decomposition measured by an autocollimation test setup. The alignment sensitivity and corresponding Fringe Zernike coefficient terms are determined by the ray-tracing software that introduces the intentional misalignments of the secondary mirror. The on-axis alignment of the collimator is performed with the guidance of sensitivity analysis results. The final root-mean-square wavefront error for the collimated beam is measured to be 0.021λ.

Uncertainty budget assessment for the calibration of a silicon microdosimeter using the proton edge technique

Abstract: The MicroPlus Bridge V2 silicon microdosimeter was exposed to collimated protons in a clinical radiotherapy beam, a mixed photon–neutron radiation field from a sealed 252Cf source and γ -rays from a 137Cs source in order to investigate the accuracy and the uncertainty budget associated with the calibration of this detector by means of the proton-edge technique. At first, the energy values associated with the proton- and electron-edges were assessed for the detector under study by performing radiation transport simulations using the Monte Carlo code PHITS. After calibrating the detector in pulse amplitude using a pulse generator and in energy imparted using the PHITS-determined proton-edge, the accuracy of the calibration was tested by comparing the position of the electron-edge in the experimental microdosimetric spectra with the theoretical value obtained using PHITS. A study on the determination of which marker point (inflection point, maximum of the second derivative, intercept of the tangent through the inflection point) is the most accurate and least affected by the arbitrary choice of the fitting range is included in the article, proving that the detector can be successfully calibrated using the proton-edge technique with a combined uncertainty of 4%.

Three-degree-of-freedom autocollimator based on a combined target reflector.

Abstract: As an angle measuring instrument, the traditional autocollimator has the ability to measure the two-degree-of-freedom angles, namely, pitch and yaw, but fails to measure the roll angle. In this study, we propose a novel autocollimator that can simultaneously measure the three-degree-of-freedom (3-DOF) angles. As a key component, a combined target reflector (CTR) is meticulously designed to split the collimated laser beam into two beams. The 3-DOF angle measurement is achieved by sensing the displacements of the two beam spots reflected from the CTR. The measurement principle and simulation analysis are presented in detail. Experiments are conducted to assess the performance of the proposed autocollimator, and the results indicate that it has an accuracy of better than 0.74 arcsec over a range of ${ \pm 200}\,\,{\rm arcsec}$±200arcsec, and it can be used for 3-DOF angular motion error measurement of a precision displacement stage.

Practical Calibration Method for Aerial Mapping Camera Based on Multiple Pinhole Collimator

Abstract: Calibration of aerial mapping camera has an important influence on the applications of earth observation. However, the traditional aerial mapping cameras calibrations depend on large-scale calibration target or collimated light, moreover, it is difficult to build the large-scale calibration targets and the collimated light method requires an accurate turntable and a high-precision goniometer, which is a kind of expensive instrument. To solve this problem, this paper proposes a novel high-precision calibration method which is not restricted by the rigorous conditions, such as small aperture, unevenly energy distribution and expensive equipment. Specifically, a collimator and an elaborately designed multiple pinhole mask are firstly used to generate the collimated light of a large aperture with known directions to simulate the calibration targets at infinity. Then, the camera takes pictures for the aperture of the multiple pinhole collimator at multiple angles to ensure that the image points cover the entire detector. Thirdly, the final calibrated results are obtained by solving the data acquired from multiple angles. Finally, the proposed method is verified by Monte-Carlo simulation and real experimental data, whose results indicate that our method can reach the same accuracy performance as the existing methods at lower cost and faster speed, and thus is practical for engineering application.

Design and Evaluation of Uniform LED Illumination Based on Double Linear Fresnel Lenses

Abstract: Redistribution of LED radiation in lighting is necessary in many applications. In this article, we propose a new optical component design for LED lighting to achieve a higher performance. The design consists of a commercial collimator and two linear Fresnel lenses. The LED radiation is collimated by a collimator and redistributed by double linear Fresnel lenses to create a square-shaped, uniform distribution. The linear Fresnel lenses design is based on Snell’s law and the “edge-ray principle”. The optical devices are made from poly methyl methacrylate (PMMA) using a high-speed computer numerical control (CNC) machine. The LED prototypes with complementary optics were measured, and the optical intensity distribution was evaluated. The numerical results showed we obtained a free-form lens that produced an illumination uniformity of 78% with an efficiency of 77%. We used the developed LED light sources for field experiments in agricultural lighting. The figures of these tests showed positive effects with control flowering criteria and advantages of harvested products in comparison with the conventional LED sources. This allows our approach in this paper to be considered as an alternative candidate for highly efficient and energy-saving LED lighting applications.

Dose Analysis of Photobiomodulation Therapy in Stomatology.

Abstract: The penetration depth and the power density of photobiomodulation (PBM) in human tissue under real conditions remain unclear to date. A novel quantitative measurement method was proposed in this study. This study aimed to design a noninvasive measurement system for the quantitative calculation of PBM dose on the attached gingiva. A flexible facial fixture appliance (FFFA) and nine piece detectors were mounted on the retainer to detect the real dose of 660 and 830 nm lasers on the attached gingiva. In addition, the angular distribution of light scattering and the light propagation in the biotissue were obtained. Two cases (a female and a male) are presented in this study. Experimental results demonstrated that the real power density of laser in the target tissue can be measured exactly after the laser light penetrates the orbicularis oris. Simulation results match with real conditions. Conversely, slight differences in power density are observed in the tissue radiated with collimated and uncollimated laser. The proposed method can be used to calculate the real dose in the target tissue for stomatology and deep acupoint stimulation.

Augmented reality light field head-mounted displays

Abstract: A near-eye display system includes a transmissive display panel to display a near-eye light field frame comprising an array of elemental images. The transmissive display panel is configured to transmit light rays of the near-eye light field frame away from the user's eye and towards an array of curved beam splitters. The curved beam splitters collimate the transmitted light rays and reflect the collimated light rays back towards the transmissive display panel for passing to the user's eye.