Beam Expander Research Articles

Amplitude gradient-based metasurfaces for off-chip terahertz wavefront shaping

Metasurfaces provide an effective technology platform for manipulating electromagnetic waves, and the existing design methods all highlight the importance of creating a gradient in the output phase across light scattering units. However, in the emerging research subfield of meta-waveguides where a metasurface is driven by guided modes, this phase gradient-oriented approach can only provide a very limited emission aperture, significantly affecting the application potential of such meta-waveguides. In this work, we propose a new design approach that exploits the difference between meta-atoms in their light scattering amplitude. By balancing this amplitude gradient in the meta-atoms against the intensity decay in the energy-feeding waveguide, a large effective aperture can be obtained. Based on this new design approach, three different wavefront shaping functionalities are numerically demonstrated here on multiple devices in the terahertz regime. They include beam expanders that radiate a plane wave, where the beam width can increase by more than 900 times as compared to the guided wave. They also include a metalens that generates a Bessel-beam focus with a width 0.59 times the wavelength, and vortex beam generators that emit light with a tunable topological charge that can reach −30. This amplitude gradient design approach could benefit a variety of off-chip light shaping applications such as remote sensing and 6G wireless communications.

Compact remote Raman system and its applications under strong sunlight research

Raman signal with a high signal-to-noise ratio (SNR) under a strong sunlight environment of 30000∼60000 Lux is very hard to obtain. A compact remote Raman spectrometer (CRS) has been developed for the purpose of detecting diverse types of lunar and earth minerals. The system comprises a spectrometer that is equipped with an ICCD detector, a 30 mm entry pupil diameter beam expander, and a 532 nm Nd: YAG Q-switched laser serving as the source of Raman scattering. The implementation of synchronous trigger and gating technology effectively overcomes the impact of strong sunlight. We obtained Raman spectra using a shorter integration time than in previous studies. The experimental results demonstrate that the detection of remote Raman spectroscopy under intense sunlight conditions facilitates the identification of silicates that have been discovered on the moon.

Reconfigurable coaxial single-photon LIDAR based on the SPAD array.

The single-photon avalanche diode (SPAD) array with time-to-digital converter (TDC) circuits on each pixel is an excellent candidate detector for imaging LIDAR systems. However, the low fill-factor of the SPAD array does not allow for efficient use of laser energy when directly adopted in a LIDAR system. Here, we design a reconfigurable coaxial single-photon LIDAR based on the SPAD array and diffractive optical elements (DOEs). We use the DOE and beam expander to shape the laser beam into a laser dot matrix. The total divergence angle of the DOE spot beam is strictly matched to the total field of view (FOV) angle of the SPAD array. Meanwhile, each focused beamlet is individually matched to every active area of the SPAD array detector, which increases the use of output energy about 100 times compared to the diffusion illumination system. Besides, the system uses the active area as the minimum pixel and can support sub-pixel scanning, resulting in higher resolution images. Through this coaxial structure, two different telescope systems after transceiver switching can be reconfigured for imaging targets at different distances. Based on our single-photon LIDAR system, we achieved 3D imaging of targets at 100m and 180m using two different telescope configurations.

Analysis and Experiment of Laser Energy Distribution of Laser Wireless Power Transmission Based on a Powersphere Receiver

Laser wireless power transmission (WPT) is one of the most important technologies in the field of long-range power transfer. This technique uses a laser as a transmission medium instead of conventional physical or electrical connections to perform WPT. It has the characteristics of long transmission distance and flexible operation. The existing laser wireless power transmission system uses photovoltaic cells as a receiver, which convert light into electricity. Due to the contradiction between the Gaussian distribution of laser and the uniform illumination requirements of photovoltaic cells, the laser wireless power transmission technology has problems such as low transmission efficiency and small output power. Therefore, understanding the energy distribution changes in the laser during transmission, especially the energy change after the laser is transmitted to each key device, and analyzing the influencing factors of the energy distribution state, are of great significance in improving the transmission efficiency and reducing the energy loss in the system. This article utilizes the optical software Lighttools as a tool to establish a laser wireless power transmission model based on a powersphere. This model is used to study the energy distribution changes in the laser as it passes through various components, and to analyze the corresponding influencing factors. To further validate the simulation results, an experimental platform was constructed using a semiconductor laser, beam expander, Fresnel lens, and powersphere as components. A beam quality analyzer was used to measure and analyze the laser energy distribution of each component except for the powersphere. The output voltage and current values of various regions of the powersphere were measured using a multimeter. The energy distribution of the powersphere was reflected based on the linear relationship between photo-generated current, voltage, and light intensity. The experimental results obtained were in good agreement with the simulation results. Simulations and experiments have shown that using a beam expander can reduce divergence angle and energy loss, while employing large-aperture focusing lens can enhance energy collection and output power, providing a basis for improving the efficiency of laser wireless power transfer.

High-velocity measurement method in dual-frequency laser interference tracker based on beam expander and acousto-optic modulator.

The laser tracker, as a new large-scale measuring instrument of combining conventional measurement technology and modern control technology, has the advantages of intelligence, portability, large measurement space, high measurement accuracy and short detection period. However, the laser tracker has strict requirements on the moving speed of the spherically mounted retroreflector. This deficiency not only limits the application of the measuring instrument in the field of high-velocity measurement, but also greatly reduces the measurement efficiency. In this work, we analyze the factors that affect the tracking velocity of the laser tracker, and propose for the first time to use the beam expander device to improve the transverse tracking measurement velocity of the instrument. The experimental results show that the laser tracker miss distance can reach 2.25 mm. The transverse tracking velocity and acceleration can reach 4.34 m/s and 2.4 g, respectively. Additionally, the acousto-optic modulator is used to increase the frequency difference between the reference beam and the measuring beam, so that the value is greater than 19 MHz. The radial tracking measurement velocity can reach 6.2 m/s. The high-velocity laser interference tracker developed by this new method can be used in the field of large-scale space precision measurement such as nuclear power, medical treatment and rail transit.

Retinal capillary and choriocapillaris assessment using a beam modifier optical coherence tomography angiography module to increase lateral optical resolution.

To assess a new optical coherence tomography angiography (OCTA) technology and its contribution to retinal vascularization and choriocapillaris (CC) exploration. A new module, named "Beam expander" (BE), which increases the lateral resolution of OCTA, was used in combination with a prototype software in the PLEX® Elite 9000 Swept-Source OCT instrument (ZEISS, Dublin, CA). This prospective study involved 22 healthy subjects imaged with and without BE. Qualitative analysis of superficial capillary plexus (SCP), deep capillary complex (DCC) retinal and CC angiograms were performed. Perfusion density (PD), vessel density (VD), and foveal avascular zone (FAZ) measurements were also compared. Qualitative analysis of single SCP and DCC retinal angiograms acquired with BE showed significantly better vessel sharpness (respectively, p = 0.0002, and p<0.0001), and greater peripheral image quality (p = 0.028 and p = 0.007) compared to standard OCTA images. Mean VD of whole retina single scans was significantly higher for BE angiograms compared to classic angiograms (28.16 ±1.29 mm-1 and 23.36 ±0.92 mm-1, respectively, p<0.0001). Repeatability of VD, PD and FAZ raw size were found to be similar between the two methods (intraclass correlation coefficient: 0.671, 0.604 and 0.994 with BE versus 0.764, 0.638 and 0.990 without BE). CC image quality was found to be significantly superior with BE, and flow deficits were more visible in all BE scans compared to standard scans. An increase in lateral resolution of the OCT beam resulted in higher quality of retinal and choriocapillaris OCTA images in healthy subjects. These results provide significant insights into the future OCTA imaging enhancements.

Low-loss tunable beam collimator and expander assembly with no moving parts using anengineered diffuser and varifocal lenses.

In this paper, we present a novel design for a tunable beam collimator. A variable collimator assists in achieving an adaptive size of an output collimated beam. Alternatively, it can also provide an adjustable output beam divergence angle for a noncollimated beam output. Tunable collimators are highly desirable for various applications in testing, engineering, and measurements. Such devices are also useful in providing tunable illumination of samples or targets in microscopes and emulating different target distances for characterizing the performance of camera systems in laboratory settings. The proposed collimator has two distinct advantages: it is light-efficient compared with pinhole-based collimator designs, and it delivers a large range of output beam sizes without involving the mechanical motion of bulk components. These attributes are achieved via the use of an engineered diffuser (in the place of a pinhole) and a pair of large aperture tunable focus lenses, which deliver a tunable magnification to the output collimated beam. In laboratory experiments, we achieve an optical transmission efficiency of 90% for the proposed tunable collimator.

Continuous Vat Photopolymerization for Optical Lens Fabrication

Optical lenses require feature resolution and surface roughness that are beyond most (3D) printing methods. A new continuous projection-based vat photopolymerization process is reported that can directly shape polymer materials into optical lenses with microscale dimensional accuracy (< 14.7µm) and nanoscale surface roughness (< 20nm) without post-processing. The main idea is to utilize frustum layer stacking, instead of the conventional 2.5D layer stacking, to eliminate staircase aliasing. A continuous change of mask images is achieved using a zooming-focused projection system to generate the desired frustum layer stacking with controlled slant angles. The dynamic control of image size, objective and imaging distances, and light intensity involved in the zooming-focused continuous vat photopolymerization are systematically investigated. The experimental results reveal the effectiveness of the proposed process. The 3D-printed optical lenses with various designs, including parabolic lenses, fisheye lenses, and a laser beam expander, are fabricated with a surface roughness of 3.4nm without post-processing. The dimensional accuracy and optical performance of the 3D-printed compound parabolic concentrators and fisheye lenses within a few millimeters are investiagted. These results highlight the rapid and precise nature of this novel manufacturing process, demonstrating a promising avenue for future optical component and device fabrication.

Beam expanders for coherent illumination and speckle reduction

Beam expanders for coherent illumination and speckle reduction

Optical Waveguide Fingerprint Recognition Device with Two‐Stage Dual‐Function Gratings

AbstractIn the under‐screen fingerprint recognition of smart cell phones, the tiny image signals are easily deteriorated by the protective membranes of screens. To solve this issue, an optical waveguide comprising two stage dual‐function gratings is proposed to supply extra lighting. The first stage is formed by two adjacent gratings possessing a cross angle of 120°, which serve as couplers and beam splitters to trap and split the incident light into two paths within the optical waveguide film, respectively. The second stage gratings, possessing a 60° included angle to the first stage, act as beam orientators and expanders by consecutively redirecting a portion of the light beams towards the sensing zone. By covering the waveguide film outside on an organic light emitting diode display screen and setting a camera and a laser under the screen, this slim construction shows high efficiency and large area under‐screen fingerprint recognition with high signal contrast. The design inspires a novel method for efficiently and flexibly controlling photons by arranging a series of gratings with the proper orientations and locations on a chip, which is highly demanded in displays and optical communication.

Research on the Performance of an Active Rotating Tropospheric and Stratospheric Doppler Wind Lidar Transmitter and Receiver

This paper investigates the transmitter and receiver performance of an active rotating tropospheric stratospheric Doppler wind Lidar. A 532 nm laser was determined as the detection wavelength based on transmission and scattering aspects. A ten-fold Galileo beam expander consisting of spherical and aspherical mirrors was designed and produced to compress the outgoing laser’s divergence angle using ZEMAX simulation optimization and optical-mechanical mounting means. The structure and support of the 800 mm Cassegrain telescope was redesigned. Additionally, the structure of the receiver was optimized, and the size was reduced. Meanwhile, the detectors and fiber mountings were changed to improve the stability of the received optical path. A single-channel atmospheric echo signal test was used to select the best-performing photomultiplier tube (PMT). Finally, the atmospheric wind field detection results of the original and upgraded systems were compared. The results show that after optimizing the transmitter and receiver, the detection altitude of the system is increased to about 47 km, and the wind speed and wind direction profiles match better with radiosonde measurements.

Hyperspectral full polarization imaging system based on spatial modulation.

The hyperspectral full polarization imaging system (HFPIS) based on spatial modulation and liquid crystal tunable filter (LCTF) can modulate the polarization information of the target into the interferogram by means of interference imaging. It has the advantage of rapid imaging of the hyperspectral full polarization information from the target, and has good real time imaging and practicality. Through the spectroscopic imaging mode of a Savart prism, the corresponding interference optical path and imaging system are designed, including a beam expander, spatial modulator, LCTF, focusing system, and imaging sensor. This system can extract the different information from the target and demodulate it so as to obtain the hyperspectral polarization image. The experiment shows that the HFPIS can reveal the texture, contour, and other details of the target in the fog, and has obvious advantages over the traditional intensity imaging methods.

Design and realization of a miniaturized high resolution computed tomography imaging spectrometer

The computed tomography imaging spectrometer (CTIS) is a relatively unknown snapshot hyperspectral camera. It utilizes computational imaging approaches to gain the hyperspectral image from a spatio-spectral smeared sensor image. We present a strongly miniaturized system with a dimension of only 36 × 40.5 × 52.8 mm and a diagonal field of view of 29°. We achieve this using a Galilean beam expander and a combination of off-the-shelf lenses, a highly aspherical imaging system from a commercial smartphone, and a 13 MP monochrome smartphone image sensor. The reconstructed hyperspectral image has a spatial resolution of 400 × 300 pixel with 39 spectral channels.

ВИЗУАЛИЗАЦИЯ ГОРЕНИЯ НАНОПОРОШКОВ МЕТАЛЛОВ СПРИМЕНЕНИЕМ ИМПУЛЬСНОГО И НЕПРЕРЫВНОГО ЛАЗЕРОВ ПОДСВЕТКИ

The work is devoted to the development of the method for high-speed imaging of high-temperature combustion processes using laser illumination. The object of visualization was a sample of aluminum nanopowder, which combustion temperature reaches 2500 K and is accompanied by a bright glow that varies in a wide range during combustion. It is demonstrated that laser illumination expands the capabilities of a high-speed imaging system and allows a more detailed study of the surface of samples compared to passive imaging by installing a neutral or band-pass filter in front of the high-speed camera lens. Various options for imaging with the installation of light filters with different transmission are considered. To monitor the surface of a burning material with a high-speed video camera in a wide temperature range of an object, it is proposed to use a low-power illumination from a continuous wave solid-state laser or a pulsed laser, as well as a beam expander for uniform illumination of the surface. The results of imaging using illumination from a solid-state laser with a wavelength of 532 nm and a copper bromide vapor laser with a wavelength of 510.6 nm are compared. It is shown that the output power of a solid-state laser of 200 mW with an illumination area diameter of 20 mm is sufficient to visualize “through the flame” the surface of a burning aluminum nanopowder at a low-temperature stage of combustion up to temperatures of ~1950 K. The uniform illumination of the surface can be ensured both by using a diffuser located directly near the sample, and with the use of a telescopic beam expander. Theoretical estimates and experimental results demonstrate that the illumination average power of 200 mW, regardless of the type of laser, is not enough to visualize the surface of a burning aluminum nanopowder at the high-temperature stage. The possibility of complete suppression of illumination during combustion with a temperature above 2500 K will appear when using an illumination laser with an average power of &gt;3,35 W. The proposed illumination technique based on a solid-state laser has advantages in terms of reliability and simplicity compared to known systems based on copper or copper bromide vapor lasers.

Manipulating the optical beam width in topological pseudospin-dependent waveguides using all-dielectric photonic crystals

We propose a width-tunable topological pseudospin-dependent waveguide (TPDW) which can manipulate the optical beam width using a heterostructure of all-dielectric photonic crystals (PhCs). The heterostructure can be realized by introducing a PhC featuring double Dirac cones into the other two PhCs with different topological indices. The topological pseudospin-dependent waveguide states (TPDWSs) achieved from the TPDW exhibit unidirectional transport and immunity against defects. As a potential application of our work, using these characteristics of TPDWSs, we further design a topological pseudospin-dependent beam expander which can expand a narrow beam into a wider one at the communication wavelength of 1.55 µm and is robust against three kinds of defects. The proposed TPDW with widely adjustable width can better dock with other devices to achieve stable and efficient transmission of light. Meanwhile, all-dielectric PhCs have negligible losses at optical wavelengths, which provides the prospect of broad application in photonic integrated devices.

Analyis of Fibre Laser’s Optical Construction from the End of the Beam Guiding Optical Fibre to the Focal Spot

Abstract In material-processing fiber lasers, the resonator in the closed box produces the laser radiation. Even with the same resonator, the diameter of the laser beam transporting fiber optics and the properties of the optical elements in the laser focusing head decide the cross-section of the focused laser beam used for machining. If we summarize the formulas in different sources in the literature, we can predict the effect of each optical element: what will happen if we choose another focusing lens, put a beam expander in the system, set on the beam expander how many times the laser beam expands. The other important point is that if we want to repeat an experiment or start a production process based on a scientific publication, then in addition to the resonator, it would be good to know the data of the optical elements in the focusing head, which is usually incomplete in the presented articles, but we can determine them approximately using the four formulas listed in the article.

First light of BEaTriX, the new testing facility for the modular X-ray optics of the ATHENA mission

Aims. The Beam Expander Testing X-ray facility (BEaTriX) is a unique X-ray apparatus now operated at the Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Brera (OAB), in Merate, Italy. It has been specifically designed to measure the point spread function (PSF) and the effective area (EA) of the X-ray mirror modules (MMs) of the Advanced Telescope for High-ENergy Astrophysics (ATHENA), based on silicon pore optics (SPO) technology, for verification before integration into the mirror assembly. To this end, BEaTriX generates a broad, uniform, monochromatic, and collimated X-ray beam at 4.51 keV. The beam collimation is better than a few arcseconds, ensuring reliable tests of the ATHENA MMs, in their focus at a 12 m distance. Methods. In BEaTriX, a micro-focus X-ray source with a titanium anode is placed in the focus of a paraboloidal mirror, which generates a parallel beam. A crystal monochromator selects the 4.51 keV line, which is expanded to the final size by a crystal asymmetrically cut with respect to the crystalline planes. An in-house-built Hartmann plate was used to characterize the X-ray beam divergence, observing the deviation of X-ray beams from the nominal positions, on a 12-m-distant CCD camera. After characterization, the BEaTriX beam has the nominal dimensions of 170 mm × 60 mm, with a vertical divergence of 1.65 arcsec and a horizontal divergence varying between 2.7 and 3.45 arcsec, depending on the monochromator setting: either high collimation or high intensity. The flux per area unit varies from 10 to 50 photons s−1 cm−2 from one configuration to the other. Results. The BEaTriX beam performance was tested using an SPO MM, whose entrance pupil was fully illuminated by the expanded beam, and its focus was directly imaged onto the camera. The first light test returned a PSF and an EA in full agreement with expectations. As of today, the 4.51 keV beamline of BEaTriX is operational and can characterize modular X-ray optics, measuring their PSF and EA with a typical exposure of 30 min. Another beamline at 1.49 keV is under development and will be integrated into the current equipment. We expect BEaTriX to be a crucial facility for the functional test of modular X-ray optics, such as the SPO MMs for ATHENA.

On‐Chip Metamaterial Antenna Array with Distributed Bragg Deflector for Generation of Collimated Steerable Beams

AbstractThe generation of collimated steerable beams of light is a fundamental function in optics needed in many applications, including free‐space optical communications, remote sensing, and light detection and ranging. Implementing large‐aperture light emitters directly on a photonic integrated chip without using external optics and expensive alignment systems is an outstanding challenge in integrated photonics. Here, the experimental demonstration of a new integrated antenna array architecture that uses a compact feeding circuit is reported. The design is based on an apodized Bragg deflector working as a low‐loss (&lt;0.3 dB) ultra‐compact beam expander, which generates a 40‐µm‐wide on‐chip Gaussian beam that illuminates a one‐dimensional array of 112 optical antennas with a length of 1.5 mm. Each antenna comprises a metamaterial subwavelength grating waveguide core that is laterally loaded with an array of periodic radiative silicon segments. The device is fabricated on a 220‐nm silicon‐on‐insulator platform using a single etch process with a minimum feature size of 80 nm. An antenna array with a power gain of 50 dB, a radiation efficiency of −3.8 dB, and a far‐field angular divergence of 1.8° × 0.2°, in a wavelength range of 1500–1580 nm is presented.

Assessing the feasibility of a low-throughput gated echelle spectrograph for Laser-induced Breakdown spectroscopy (LIBS)-Raman measurements at standoff distances

We have developed a potentially portable tabletop combined remote LIBS-Raman spectroscopy system using a high-resolution gated echelle spectrograph for measuring both atomic and molecular emission spectra at a standoff distance of 6.5 m. The system consists of a frequency-doubled Nd: YAG laser, a 4X beam expander, a Cassegrain telescope, and a gated echelle spectrograph. The unique advantages of simultaneous broadband coverage (250–850 nm) and high resolution of the echelle system have been employed for measuring the non-identical emission spectrums (LIBS and Raman) at remote distances. The viability and practicability of the low throughput gated echelle spectrograph for detecting the remote Raman signals were evaluated critically by recoding a series of organic chemicals and solid samples. Further, the feasibility of measuring Raman signals of organic chemicals through different colored bottles was also demonstrated. Additionally, the combined LIBS-Raman spectra of several medium Raman cross-section solids samples were recorded from a standoff distance of 6.5 m, and the results are presented here. The benefits of merging the LIBS and Raman spectroscopy system and the bidirectional use of these techniques for remote measurements have also been highlighted.

Design, Fabrication, and Characterization of a Laser-Controlled Explosion-Initiating Device with Integrated Safe-and-Arm, EMP-Resistant, and Fast-Acting Technology Based on Photovoltaic Power Converter.

To augment the intelligence and safety of a rocket or ammunition engine start, an intelligent initiation system needs to be included in the data link. A laser-controlled intelligent initiation system with inherent safety and a laser-controlled explosion-initiating device (LCEID) incorporating electromagnetic pulse (EMP) resistant, safe-and-arms fast-acting modular device based on photovoltaic power converter technology is designed and fabricated in this work. LCEID is an integrated multi-function module consisting of the optical beam expander, GaAs photovoltaic (PV) array, safe-and-arms integrated circuit, and low-energy initiator. These components contribute to EMP resistance, fast-acting, safe-and-arm, and reliable firing, respectively. To achieve intelligent initiation, each LCEID has a unique “identification information” and a “broadcast address” embedded in integrated-circuit read-only memory (ROM), which is controlled by encoded laser addressing. The GaAs PV array was investigated to meet the low-energy initiator firing voltage requirements. Experimental results show that the open-circuit voltage, short-circuit current, and maximum power output of the four-junction GaAs PV array illuminated by a 5.5 W/cm2 laser beam were 220 mA, 21.5 V, and 3.70 W, respectively. When the voltage of the 22 μF energy storage capacitor exceeds 20 V, the laser charging time is found to be shorter than 2.5 s. Other aspects of LCEID, such as laser energy coupling efficiency, the firing process, and the energy-boosting mechanism, were explored. Measurements show that the coupling efficiency of the micro lens with a radius of curvature D = 20 μm and size of r = 50 μm reaches a maximum of 93.5%. Furthermore, for more than 18 V charge voltage, the LCEID is found to perform reliably. The fabricated LCEID demonstrated a high level of integration and intrinsic safety, as well as a finely tailored initiation performance that could be useful in military applications.