Array Of Laser Beams Research Articles

Generating Optical Vortex Array Laser Beams of Superimposing Hermite–Gaussian Beams with a Dual–Phase Modulation Digital Laser System

This study presents an efficient and practical intra-cavity approach for selectively generating vortex array laser beams employing a dual-phase modulation digital laser system, which has not yet been completed in single-phase modulation digital laser. The stable optical vortex array laser beams were formed by superimposing cavity Hermite–Gaussian (HG) eigenmodes. In particular, when the selected cavity HG modes shared the same Gouy phase, the resulting optical vortex beam could preserve its light field pattern, thereby maintaining the optical vortex properties in the near and far fields. Numerical results demonstrated that employing dual-phase modulation could establish optimal boundary conditions for the selection of HG modes within the cavity, successfully generating various vortex array laser beams. The experimental validation of the proposed method confirmed the ability to select optical vortex array lasers solely by controlling the loaded phase of the dual-phase modulation digital laser. These results demonstrate the ability of digital lasers to generate and dynamically control optical vortex array lasers.

Optical Phased Array-Based Laser Beam Array Subdivide Pixel Method for Improving Three-Dimensional Imaging Resolution

The small number of pixels in the current linear mode avalanche photodiode (LM-APD) array limits its three-dimensional (3D) imaging resolution. We use an optical phased array-based beam array subdivided pixel method to improve the 3D imaging resolution, using an optical phased array to generate a beam array with the same number of pixels as the LM-APD array and matching positions and controlling each sub-beam in the beam array to scan in the field of view of the corresponding pixel. The sub-beam divergence angle in the beam array is smaller than the instantaneous field of view angle of a single pixel in the LM-APD array. The sub-beam scanning in a single pixel’s field of view realizes the multiple acquisition of the target 3D information by the LM-APD array, thus improving the resolution of the LM-APD array. The distribution of the beam array in the far field is simulated, and the main performance parameters of 3D imaging are analyzed. Finally, a liquid crystal phase modulator is used as an optical phased array device to conduct experiments on a target 20 m away, and the results prove that our method can improve the resolution from 4 × 4 to 8 × 8, which can be improved at least four times.

Total light bending in non-asymptotically flat black hole spacetimes

The gravitational deflection of light is a critical test of modified theories of gravity. A few years ago, Gibbons and Werner introduced a definition of the deflection angle based on the Gauss–Bonnet theorem. In more recent years, Arakida proposed a related idea for defining the deflection angle in non-asymptotically flat spacetimes. We revisit this idea and use it to compute the angular difference in the Kottler geometry and a non-asymptotically flat solution in Horndeski gravity. Our analytic and numerical calculations show that a triangular array of laser beams can be designed so that the proposed definition of the deflection angle is sensitive to different sources of curvature. Moreover, we find that near the photon sphere, the deflection angle in the Horndeski solution is similar to its Schwarzschild counterpart, and we confirm that the shadows seen by a static observer are identical.

Laser beam array spot steering customized trajectories using the acousto-optic effect.

We demonstrate a proof of principle of a technique for array laser beam steering according to a generalized elliptical path, using the acousto-optic effect. We explore the dynamic trajectories of the diffraction orders of a Gaussian beam, diffracted by a dynamic 2D sinusoidal phase grating. The latter is generated by two crossed acoustic waves propagating in a transparent medium, giving rise to 2D dynamic sinusoidal refractive index variation. We particularly emphasize the case where the two crossed acoustic waves are modulated in frequency and have a phase difference. In such case, the resulting diffraction orders are dynamic and follow some particular trajectory's shapes, from linear and circular to generalized elliptical shapes. Inspired by Poincaré and Bloch spheres used to represent polarization and quantum states, we suggest the sphere volume trajectory shape to represent all trajectories' shapes of all diffraction orders.

Laser Doppler multi-beam differential vibration sensor for real-time continuous visualization of buried objects in multiple frequency bands.

Laser-acoustic detection of buried objects, such as landmines, uses elastic waves in the ground and a laser vibrometer to create a vibration image of the ground surface. A decision on the presence of a buried object is made by analyzing vibration images for multiple vibration frequencies. With traditionally used laser Doppler vibrometers, the vibration imaging data are saved to a computer memory to be analyzed, which increases the detection time. A novel laser multi-beam differential interferometric sensor (LAMBDIS) that provides simultaneous real-time visualization of vibration images for multiple frequencies has been developed. The sensor is capable of displaying vibration images for up to 32 frequency bands simultaneously in real time. The sensor employs a digital line-scan complementary metal-oxide semiconductor camera and field programmable gate arraysbased real-time signal processing and is capable of operating continuously by scanning an object with a linear array of laser beams. Performance of the sensor has been verified experimentally. The ability of the LAMBDIS for real-time continuous vibration imaging of the ground surface in multiple frequency bands for laser-acoustic detection of buried objects has been demonstrated.

Laser multibeam differential interferometric sensor for acoustic detection of buried objects

A laser multibeam differential interferometric sensor (LAMBDIS) was developed that provides measurement of vibration fields of objects with high sensitivity, while having low sensitivity to the whole-body motion of the object, or the sensor itself. The principle of operation of the LAMBDIS is based on the interference of light reflected from different points on the object surface illuminated with a linear array of laser beams. The Doppler shift induced by the sensor motion is approximately the same for all beams and is automatically subtracted from the measurements. The performance of the sensor for laser-acoustic detection of a buried object was experimentally investigated. The ability of LAMBDIS to detect buried objects from a moving vehicle has been demonstrated in field experiments.

Fiber-Coupling Efficiency of Laser Array Beam From Turbulent Atmosphere to Fiber Link

Free-space optical communication (FSOC) systems are nowadays integrated with fiber optical components developed for fiber-optic communications. In such integrated systems, the collected portion of the incident beam on the receiver lens is coupled into a single-mode fiber. The process of coupling, however, is mostly affected by the atmospheric turbulence which distorts the coherency of the propagating beam i.e., it results in speckle over the coupling lens causing a reduction in the coupling efficiency. In this article, we aimed at investigating the fiber coupling efficiency of laser array beams propagating in a turbulent atmosphere. For this purpose, using the Huygens-Fresnel principle, mutual coherence function (MCF) for a laser array beam incidence is formulated. In this way, the average power coupled into the fiber and the average received power on the coupling lens are derived for a laser array beam incidence. It is found that the fiber coupling efficiency clearly increases with the increase in ring radius and the number of Gaussian beams in the array and rapidly decreases with increasing structure constant of atmosphere, link distance, and the number of speckles over the receiver aperture. We also demonstrate the effect of various FSOC system parameters on the coupling efficiency.

Laser Doppler multi-beam differential vibration sensor based on a line-scan CMOS camera for real-time buried objects detection.

Laser Doppler vibrometers (LDVs) traditionally used for ground vibration sensing in laser-acoustic detection of buried objects are limited to operation from a stationary platform due to their sensitivity to the motion of the LDV itself. In order to overcome this limitation a novel Laser Multi-Beam Differential Interferometric Sensor (LAMBDIS), has been developed. The LAMBDIS allows for measurements of vibration fields with interferometric sensitivity, while having low sensitivity to the motion of the sensor itself. The LAMBDIS described in this paper is based on a digital line-scan CMOS camera and FPGA based real-time signal processing. The principle of operation of the LAMBDIS employs the interference of light reflected from different points on the object surface illuminated with an array of laser beams. The Doppler shift induced by the sensor motion is canceled out thereby allowing for measurements from a moving vehicle. The ability of the LAMBDIS to detect buried objects in real time from a moving vehicle has been confirmed in field experiments.

Laser array field correlations in underwater turbulence

In underwater turbulent medium, field correlations are found when the incidence is a laser beam array. Variations of the field correlations against the variations in the ring radius of laser array beam, number of beamlets composing the laser array, source size, underwater turbulence parameters, i.e. the ratio of temperature to salinity contributions to the refractive index spectrum, rate of dissipation of mean-squared temperature and rate of dissipation of kinetic energy per unit mass of fluid, are investigated. Field correlations of laser arrays are found to be larger than the field correlations of the single beams. The effect of underwater turbulence is to reduce the field correlation of laser arrays.

Large-Area Fabrication of LIPSS for Wetting Control Using Multi-Parallel Femtosecond Laser Processing.

In this research, the wetting property control of a stainless-steel surface, structured using parallel processing via an array of 64-femtosecond laser beams, is presented. The scanning of an 8 × 8-beam array over the sample was used to uniformly cover the large areas with LIPSS. The static water contact angle and the LIPSS period dependence on processing parameters were investigated. The wettability control of water droplets on laser-patterned stainless steel, ranging from contact angles of ~63°, similar to those of the plain surface, to the superhydrophobic surface with contact angles > 150°, was achieved. The relationship between the static water contact angle and the LIPSS parameters in the Fourier plane was investigated.

Systematically Investigating the Structural Variety of Crystalline and Kaleidoscopic Vortex Lattices by Using Laser Beam Arrays

We theoretically demonstrate that a family of vortex-lattice structures can be flexibly generated using a multi-beam interference approach. Numerical calculation presents a variety of crystalline and kaleidoscopic patterns. Based on the numerical analysis, we experimentally realized these structure beams by combining an amplitude mask with multiple apertures and a spiral phase plate. The excellent agreement between the experimental and theoretical results not only validates the presented method, but also manifests the structure of vortex lattices.

Programmable electron density patterns induced by the interaction of an array laser and underdense plasma

The spatially modulated electron distribution of plasma is the basis for obtaining programmable electron density patterns. It has an important influence on plasma technology applications. We propose an efficient scheme to realize controllable electron density patterns in underdense plasma based on the array laser–plasma interaction. Theoretical evidence for the realization of programmable electron density patterns and the corresponding electrostatic field is provided analytically, which is confirmed by particle-in-cell simulations. Results show that the spatial distribution of electron density in the propagation and transverse directions of the laser can be highly modulated to obtain rich programmable electron density patterns by adjusting the array pattern code and pulse width of the array laser beam.

Experimental phase control of a 100 laser beam array with quasi-reinforcement learning of a neural network in an error reduction loop.

An innovative scheme is proposed for the dynamic phase control of a laser beam array. It is based on a simple neural network included in a phase correction loop that predicts the complex field array from the intensity of the induced scattered pattern through a phase intensity transformer made of a diffuser. A crucial feature is the use of a kind of reinforcement learning approach for the neural network training which takes account of the iterated corrections. Experiments on a proof-of-concept system demonstrated the high performance and scalability of the scheme with an array of up to 100 laser beams and a phase setting at λ/30.

Generation of a Plasma Waveguide with Slow-Wave StructureSupported by the National Natural Science Foundation of China (Grant Nos. 11865014, 11765017, 11764039, 11475027, 11274255, and 11305132), the Natural Science Foundation of Gansu Province (Grant No. 17JR5RA076), and the Scientific Research Project of Gansu Higher Education (Grant No. 2016A-005).

Using the particle-in-cell simulations, we report an efficient scheme to generate a slow wave structure in the electron density of a plasma waveguide, based on the array laser–plasma interaction. The spatial distribution of the electron density of the plasma waveguide is modulated via effective control of the super-Gaussian index and array pattern code of the lasers. A complete overview of the holding time, and the bearable laser’s intensity of the electron density structure of the plasma waveguide, is obtained. In addition, the holding time of the slow wave structure of the plasma waveguide is also controlled by adjusting the frequency of the array laser beam. Finally, effects due to ion motion are discussed in detail.

Small footprint phase locking system for a large tiled aperture laser array.

A phase sensing system fitted to the control of coherent laser beam array of large cross section is experimented. It is based on the use of a fiber bundle that collects a weak part of the synthetic wavefront, that scales it down (1/40) and that reshapes it in a more compact arrangement (2D to 1D array). Then, the reconfigured beam array can be analyzed by a small footprint system making the large laser beam array easier to phase-lock. The discrete laser array wavefront transmitted by the meter long fiber bundle was stabilized thanks to a multiple arm servo loop. Laser array phase locking was further ensured by random scattering through a diffuser, associated to an alternating projection algorithm. Six fiber laser beams constituting a 110 mm diameter synthetic aperture, were phase-locked with λ/16 accuracy.

Time-reversed water waves generated from an instantaneous time mirror

An instantaneous time mirror (ITM) is an interesting approach to manipulate wave propagation from the time boundaries. In the time domain, the reversed waveform is previously proven to be the temporal derivative of the original pattern. Here, we further investigate into the relationship between the wave patterns in the spatial domain both theoretically and experimentally. The refraction of a square array of laser beams is used to determine the three-dimensional (3D) shape of the water surface. The experimental results verify the theoretical prediction that the reversed pattern is related to the Laplacian of the initial wave field. Based on these findings, the behaviors of the ITM activated in an inhomogeneous medium are discussed, and the phenomenon of total energy change is explained.

Laser Doppler multi-beam differential vibrometry

A Laser Multi-Beam Differential Interferometric Sensor (LAMBDIS) for measuring vibration fields has been developed that alleviates one of the major issues of traditional laser Doppler vibrometers: effect on measurements by motion of the vibrometer itself. The LAMBDIS simultaneously measures Doppler shifts of light reflected from different points on the object surface illuminated with a linear array of laser beams. As a result, the LAMBDIS measures relative velocities between points on an object surface, while the Doppler shift caused by the sensor motion is approximately the same for all beams and is automatically subtracted from the measurements. This allows measurements of vibration fields of objects with high sensitivity from a moving platform. Scanning the linear array of laser beams in the transverse direction provides a two-dimensional vibration image of the surface. Performance of the sensor for ground vibration sensing for acoustic detection of buried objects has been investigated in the laboratory and field experiments. The sensor proved effective at detecting buried objects from a moving vehicle. [Work supported by the Office of Naval Research.]

Laser array beam propagation through liver tissue

Laser array beam propagating through mouse liver tissue is investigated. The turbulence power spectrum of the liver tissue is employed in the extended Huygens–Fresnel method to obtain an optical intensity profile and beam broadening at the observation point in biological liver tissue. Variations of the beam profile and the beam broadening are simulated based on the number of beamlets, source size, wavelength and the ring radius of the array. A biological tissue, illuminated by the laser array beam, exhibits different beam profiles and beam spot radius variations when the number of beamlets, source size, wavelength and the ring radius of the laser array beam are varied. Examining these variations observed in the propagated optical beam and comparing them with the test cases, abnormalities such as cancer and tumor in a biological liver tissue can be diagnosed.

All-optical, self-focused laser beam array for parallel laser surface processing.

A single femtosecond laser beam can be focused to process material surfaces. However, the low processing efficiency by a single laser beam prevents broad applications of femtosecond laser surface processing. Here we have demonstrated an array of high-intensity femtosecond laser beams that self-focus in a piece of glass medium, enabling high-efficiency, millimeter scale parallel laser surface processing. Experimental and simulation results suggest that the laser fluence of each focused laser beamlet in the array is more than 1.2 J/cm2, which is sufficient for micro- or nano-structure laser processing. To prove the concept, nanoscale structures on silicon surface are generated by the self-focused laser beam array, which may improve the efficiency of black silicon fabrication for photovoltaic devices.

Random Scattering and Alternating Projection Optimization for Active Phase Control of a Laser Beam Array

A fraction of a laser beam array, whose unknown phase relationships must be set to prescribed values, is launched into a scattering media with random transmission. The resulting output speckle pattern is sampled by an array of photodiodes measuring the local light intensity. The data feed an innovative optimization process which controls a phase modulator array. Few iterations of the opto-numeric loop lead to efficient and fast phase locking on any desired wavefront.