Flat-top Beam Research Articles

A Novel Strategy Achieving GaN Ultra-Smooth Surface via a Square, Flat Top Beam Shaped Femtosecond Laser Processing Combined with Chemical Mechanical Polishing

We propose a strategy for GaN ultra-smooth manufacture by combing of femtosecond laser processing and chemical mechanical polishing. The optimized flat top beam is used to redistribute the laser fluence from Gaussian distribution to square, flat top profile. Surface with high flatness is achieved after femtosecond laser processing through optimization of the technical parameters. The optimum process to control the damages on surface and subsurface reduces negative influence of femtosecond laser processing on chemical mechanical polishing. Roughness (Ra) of processed surface is decreased to less than 0.1 nm when it is be polished for 6 h. In the background of traditional GaN polishing efficiency facing bottleneck, the characteristic of present study is to use a square, flat top beam shaped femtosecond laser to induce surface of GaN. Structures and composition on induced surface are simultaneous removed in chemical mechanical polishing process. It reduces the burden of GaN chemical mechanical polishing. The present method has advantage of high efficiency and easy realization, which has a wide application in GaN ultra-smooth processing.

Pulse Broadening Effects on Ranging Performance of a Laser Altimeter with Return-to-Zero Pseudorandom Noise Code Modulation.

A laser altimeter using code modulation techniques receives a backscattered pulse wider than the transmitted rectangular pulse when scanning a rough or sloped target surface. This leads to degrading the ranging performance in terms of signal-to-noise ratio (SNR) and detection probability. Unlike the pulsed techniques, little work has focused on the pulse broadening effect of the code modulation techniques. In this study, mathematical models were derived to investigate the pulse broadening effect on the ranging performance of a return-to-zero pseudorandom noise (RZPN) laser altimeter. Considering that the impulse response can be approximated by a Gaussian function, the analytical waveform was derived using a new flat-topped multi-Gaussian beam (FMGB) model. The closed-form expressions were also analytically derived for a peak cross-correlation, SNR, and detection probability in terms of the pulse broadening effect. With the use of a three-dimensional model of asteroid Itokawa for practical surface profiles, the analytical expressions were validated by comparing to the results obtained from numerical simulations. It was also demonstrated that the pulse broadening effect dropped down the peak cross-correlation and then deteriorated the ranging performance. These analytical expressions will play an important role in not only designing a laser altimeter using the RZPN code modulation technique but also analyzing its ranging performance.

Large-Scale Sequentially-Fed Array Antenna Radiating Flat-Top Beam for Microwave Power Transmission to Drones

Power beaming is one of the core technologies for microwave power transmission (MPT) systems. Effective plane-to-plane power beaming requires not only high point-to-point efficiency but also appropriately-shaped beam to extract the best performance of receivers (rectennas). Flat-top beam plays an important role in plane-to-plane power beaming for drones where the output dc power from rectennas should be maximized to drive motors. It is challenging to develop a large-scale flat-top-beam array with appropriate distribution circuits. Sequential array is also required to suppress axial ratio on the receiving plane. In this paper, we proposed a simplified but effective way to create a large-scale sequential array for flat-top beam at C-band. The series feed and block-oriented sequential array were adopted to keep the circuit design and fabrication simple while obtaining a sufficient point-to-point efficiency and good axial ratio on the receiving plane. A 196-element phased array with microstrip antennas was developed by subdividing the whole array into four 49-element blocks for simplicity of the circuit design and implementation of block-oriented sequential array. The efficiency between the transmitting ports and the receiving ports was 50.6 % in simulations and 32.0 % in measurements. In the measured flat-top beam, the transmitted power was uniformly concentrated within the receiving plane and the axial ratio of the beam was successfully suppressed to less than 3 dB on most of the receiving area. A flight test of a microwave-powered drone was conducted where a micro-drone successfully flew for seven minutes only with wireless power.

The Error Analysis of the Complex Modulation for Flattop Focusing Beam Shaping

In order to obtain a focusing flattop beam with high uniformity, complex modulation is used to modulate the optical pupil function, and the beam shaping algorithm is designed with a single phase-only spatial light modulator (SLM). Actually, the wavefront aberrations introduced by each element reduce the uniformity of the shaped beam. In particular, the wavefront aberrations in different positions have different effects on this complex modulation algorithm. However, there is a lack of the corresponding error data and robustness analysis. Here, the error and robustness of the complex modulation algorithm are analyzed when different types of aberrations (defocus, astigmatism, and coma) exist in different positions of the shaping optical system, and the mixed area magnification-free (MRAF) algorithm is used as a reference for comparison. The results show that coma has the greatest effect on the beam shaping quality. It is also proved that the impact on the beam quality when there are aberrations in the laser and beam expansion system is greater than those in the SLM and the focusing lens for the complex modulation algorithm, which is different from the MRAF case.

High-Efficiency Grating Couplers for Pixel-Level Flat-Top Beam Generation

We demonstrate a kind of grating coupler that generates a high quality flat-top beam with a small beamwidth from photonic integrated circuits into free-space. The grating coupler is designed on a silicon-on-insulator wafer with a 220-nm-thick silicon layer and consists of a dual-etch grating (DG) and a distributed Bragg reflector (DBR). By adjusting the structural parameters of DG and DBR, a pixel-level (6.6 µm) flat-top beam with a vertical radiation of −0.5 dB and a mode match of 97% at 1550 nm is realized. Furthermore, a series of high-efficiency grating couplers are designed to create a flat-top beam with different scales.

Surface quality of laser paint removal of marine steel: a comparative study using a Gaussian beam and a flat-top beam.

The spot mode has a great influence on the effect of laser paint removal, but it has not been clarified. This paper studies the influence of a Gaussian beam and a flat-top beam on surface quality. The results show that for the laser paint removal with the Gaussian beam, the surface hardness, corrosion resistance, and adhesion of marine steel improved significantly. The surface hardness increased by a factor of 20.8% compared to the original substrate, and the paint adhesion increased by 32.7%. For the flat-top beam, the surface quality of the substrate is unchanged, but the cleaning efficiency was observed to be 20% higher than that of the Gaussian beam. This paper can provide technical support for the control of surface cleaning quality in the laser paint removal process.

Enhanced fiber-coupling efficiency via high-order partially coherent flat-topped beams for free-space optical communications.

The fiber-coupling efficiency of signal beams is crucial in free space optical (FSO) communications. Herein, we derived an analytical expression for the fiber-coupling efficiency of partially coherent flat-topped beams propagating through atmospheric turbulence based on the cross-spectral density function. Our numerical calculation results showed that the fiber-coupling efficiency of partially coherent flat-topped beams in a turbulent atmosphere could be enhanced by increasing the beam order. Under the same conditions, the fiber-coupling efficiency of the high-order partially coherent flat-topped beams was larger than those connected to the Gaussian and Gaussian Schell-model (GSM) beams. Our results will improve the quality of partially coherent beams used in FSO communications.

Coaxial multi-ring optical vortex generation based on compound spiral phase plates

We propose a new kind of compound optical vortex (COV) generator in this paper. The device consists of an inner spiral phase plate (SPP) and an outer annular spiral phase plate. There is an opaque band between two SPPs. Under the flat-top beam irradiation, concentric multi-ring COV rings with different topological charges in different radial radii can be generated. The theoretical analysis lays a theoretical foundation for the design of the COV generator, and the simulation results prove the effectiveness of the design. The unique characteristics of COV are discussed and some of its potential application scenarios are presented. This work provides a design method for generating COV using compound SPPs, and the advanced COV structure can help to expand the scope of utilization of vortex beam in optical tweezers, optical communication and other fields.

All-fiber passively Q-switched laser with flat-top beam emissions.

Flat-top beams have plenty of applications in theoretical and applied research, but they are not eigenmodes of the wave equation. Here, we propose an effective strategy for generating flat-top beams in fibers, which originates from the incoherent superposition of orbital angular momentum (OAM) and fundamental (LP01) modes. The designed all-fiber passively Q-switched laser can realize high-quality pulsed flat-top and OAM beam emissions. The normalized root mean square of the flat-top beam is around 4.7%, and the purity of the OAM mode is calculated to be higher than 98%. For the first time to the best of our knowledge, pulsed emission of a flat-top beam is achieved using an all-fiber laser. Moreover, its repetition rate and duration can be controlled by adjusting the pump.

Dual-Ridge Gap Waveguide-Based Antenna With Diverse Beam Capabilities

In this paper, a dual-ridge gap waveguide (DRGW) based antenna with beam scanning, sum-difference beam, and flat top beam capabilities has been proposed. The proposed antenna is composed of a gap waveguide (GW) with two flared horn-shaped ridges. A metallic plane with the same horn shape as the ridge is inserted between two ridges and extended beyond the waveguide. Two coaxial probes are located on the top and bottom plates of the GW, respectively, to excite the antenna. By independently adjusting the magnitudes and phases of two signals fed into the antenna, the patterns including the beam scanning from θ=60∘ to θ=120∘ in E plane, the sum and difference beams at the endfire direction, and the flat top beam can be flexibly achieved. Measured results are given to demonstrate that the proposed antenna operates in a band of 16.3 18.4 GHz with the average gain about 11 dBi and the front-to-back ratio (FTBR) better than 20 dB for the beam scanning, the sum beam and the flat top beam, and the null depth better than 25 dB for the difference beam.

A Generalized Flat-Topped Beam Synthesis Approach for Uniform Linear Array With Arbitrary Beam Directions

A systematic approach to synthesize the flat-topped beam (FTB) with arbitrary beam directions for uniform linear arrays (ULAs) is proposed, illustrated, and demonstrated. Based on the correspondence between the far-field pattern of ULAs and the response function of finite impulse response (FIR) filters, both the broadside and end-fire FTBs can be synthesized using window method, whereas the oblique FTB synthesis is failed due to the appearance of grating lobes. A mathematical analyzation indicates that two grating lobes are conjugate to each other. Thus, a linear phase method (LPM) is proposed to apply progressive excitation phases to each individual array element, which can eliminate the grating lobes by manipulating the two conjugate beams to overlap with each other. The proposed LPM is generalized to the synthesis of broadside, oblique and unidirectional end-fire FTBs, and the corresponding inter-element spacing constraints are addressed. The proposed approach is experimentally verified using a 16-element substrate-integrated waveguide slot antenna array. Comparing with the conventional Fourier series method and Woodward-Lawson method, the proposed approach has advantages of both smaller ripples in the main beam and controllable side-lobe levels. The proposed approach is promising for offering efficient beamforming functionalities for wireless communications, imaging, and radar applications.

Machine Learning Aided Inverse Design for Flattop Beam Fiber

Machine Learning Aided Inverse Design for Flattop Beam Fiber

High-fidelity parametric amplification of Ince–Gaussian beams

Ince–Gaussian (IG) beams, as eigenfunctions of the paraxial wave equation in elliptical coordinates, are attracting increasing interest owing to their propagation-invariant and full-field properties. Optical amplification via parametric interactions can further expand their application areas, yet it is rarely studied. In this work, we report on a high-fidelity parametric amplifier for IG beams. The nonlinear transformation of the spatial spectra of the signal and associated influences on the beam profiles of the amplified signal, under different pump structures, were theoretically and experimentally investigated. By using a perfect flattop beam as the pump, we show that the transverse structure of IG signals is well maintained, and the distortion induced by radial-mode degeneration is overcome during amplification. This proof-of-principle demonstration paves the way for a mode-independent and distortion-free amplifier of arbitrary structured light and has great significance in relevant areas, such as quantum optics, tunable infrared-laser generation, and image amplification.

Intensity Distribution Characteristics of Flat-Top Beam After Amplitude Modulation in Target Cavity

Intensity Distribution Characteristics of Flat-Top Beam After Amplitude Modulation in Target Cavity

Enhancement periodic regularity of surface nano ripple structures on Si wafer using a square shaped flat-top beam femtosecond NIR laser

Enhancement periodic regularity of surface nano ripple structures on Si wafer using a square shaped flat-top beam femtosecond NIR laser

Optimization of beam shaping for ultrasensitive inertial measurement using a phase-only spatial light modulator.

This study proposes an approach to generate a uniform flat-top beam with a liquid crystal spatial light modulator (LC-SLM) to optimize ultrasensitive inertial measurement. The random incomplete Gaussian beam is modulated into a flat-top beam by uploading a beam shaping optimization algorithm on an LC-SLM. Simulation results verify the effectiveness of the proposed method. The beam obtained from the experimental results with the 4f filter system optimization also conforms to the properties of the generated flat-top beam. Compared to existing beam shaping algorithms for simulation and experimental analysis, the beam shaping design based on the LC-SLM to optimize the ultrasensitive inertial measurement is realized. This method has also been verified to be effective in beam shaping in various beam situations. The application of this method in ultrahigh-sensitivity inertial measurement should prove significant.

Propagation of high-power optical flat-topped beams in strongly nonlinear media

We performed numerical experiments to investigate the propagation of high-power optical flat-topped beams through a strongly nonlinear medium, accounting for the diffraction, nonlinearity, and multiphoton absorption capabilities of the beams. We found that the uniform irradiance distribution of flat-topped beams can remain constant near the centre axis for a given propagation distance. A series of comparisons were conducted for flat-topped beams with different orders, which demonstrates that increasing the order reduces change of beam characteristic. Furthermore, we investigated the influence of input power on the propagation of flat-topped beams. Moreover, we numerically analysed the dynamics of the inner energy transfer and uniform irradiance decay of the flat-topped beam.

Thermoelastic modeling of laser-induced generation of strong surface acoustic waves

Short pulse laser irradiation of a substrate can generate pulses of surface acoustic waves (SAWs) capable of propagating long distances along the surface of the irradiated substrate. In this work, we use thermoelastic modeling of the generation of SAWs on a Si substrate to explore the effect of irradiation parameters, i.e., pulse duration, laser spot size, absorption depth, and spatial profile of the laser energy deposition, on the strength of the SAWs. A particular goal of this study is to establish the optimum conditions for maximizing the strength of the surface waves generated in the nonablative, thermoelastic irradiation regime. The simulations demonstrate that the highest strain amplitude of the laser-generated SAWs can be achieved for a laser spot size comparable to the characteristic length of the SAW propagation during the laser pulse. The amplitude of SAWs increases with the increase in the characteristic laser energy deposition depth, and laser pulses with sharper spatial energy deposition profiles (flat-top laser beams) produce stronger SAWs. For the optimal set of irradiation parameters, the strain amplitude of a SAW generated in Si in the thermoelastic regime can reach the levels of 10−4–10−3, which are sufficiently high for causing nonlinear sharpening of the wave profile and the formation of a shock front during the wave propagation from the laser spot. The computational predictions suggest the feasibility of a continuous generation of strong nonlinear pulses of SAWs, which may be utilized for driving the surface processes in thin film deposition, growth of two-dimensional materials, heterogeneous catalysis, and other applications.

A numerical study of the partially coherent flat-topped vortex hollow beam and the GSM beam propagation under atmospheric turbulence

This paper uses a numerical simulation method to evaluate the propagation performance of the partially coherent flat-topped vortex hollow (FTVH) beam and the Gaussian Schell-model (GSM) beam through atmospheric turbulence along slant links. The atmospheric turbulence is simulated by random phase screens, and the performance of the beam models is evaluated by using scintillation index and signal-to-noise (SNR) on the receiver plane with various simulation conditions and parameters. This paper has shown that there is an optimal coherence length for both the partially coherent FTVH beam and the GSM beam. In different parameter settings, the partially coherent FTVH beam has the best propagation performance at N = 1 M = 1. Compared to the GSM beam, the partially coherent FTVH beam obtains an obvious improvement in SNR. The results show that the partially coherent FTVH beam is able to improve the communication efficiency of an FSOC system.

Parametric Analysis of Linear Periodic Arrays Generating Flat-Top Beams

Several synthesis techniques are available to optimize amplitude and phase excitations of periodic linear arrays to generate flat-top beams. Clearly, the optimal tapering depends on design parameters such as the array length, the number of array elements, the beam flatness, the beam width, the side lobe levels, and others. In this paper, in order to derive useful guidelines and rule of thumb for the synthesis of periodic array antennas, relations between these parameters are derived employing linear programming techniques, which guarantee optimality of the solutions. Such relations are then plotted and used in some design examples.