Laser Beam Homogenization Research Articles

Laser beam homogenization based on a multifocal liquid crystal microlens array.

A novel, to the best of our knowledge, tunable multifocal liquid crystal microlens array (TMLCMA) was fabricated with a triple-electrode structure consisting of a large-hole, a small-hole array, and planar electrodes. The electro-optical performances of the TMLCMA are characterized, demonstrating the monofocal convex, multifocal convex, and multifocal concave functions when the TMLCMA is manipulated with various driving schemes. Furthermore, the homogenization of a laser beam is realized using the fabricated TMLCMA. The multifocal convex and multifocal concave functions of the TMLCMA successfully suppress the lattice phenomenon caused by the monofocal microlens array, homogenize the Gaussian beam to a flattop intensity distribution, and broaden the beam size.

Characterization of semi-infinite delaminations using lock-in thermography: Experimental results

Delaminations are flat subsurface flaws parallel to the sample surface. Lock-in infrared thermography with optical stimulation is proposed to size the geometrical parameters of a delamination. Here we deal with semi-infinite delaminations, which are characterized by three geometrical parameters: length, depth and thickness. In a recent paper we calculated analytically and numerically the temperature oscillation of a sample containing a semi-infinite delamination when illuminated by a homogeneous and modulated laser beam. In this work, we present lock-in infrared thermography experiments performed on calibrated delaminations manufactured on AISI-304 stainless steel. We fit the numerical model to the experiments to retrieve the delamination length, depth and thickness. The agreement between nominal and retrieved values confirms the validity of the method and paves the way for characterization of finite delaminations.

Study of the Change in Surface Wettability for Zircaloy-4 Utilizing Spatially Homogenized Nanosecond Laser Beam

This study introduces laser surface modification technology using a 532 nm Nd:YAG nanosecond laser. A laser beam homogenizer was used to spatially homogenize the laser beam into a square shape with an interference pattern of 30 μm. For laser surface modification processing, primary processing under 110 conditions was first performed, and then, 15 types of main processing conditions were determined based on the three-dimensional surface structure of laser surface modified zircaloy-4. Subsequently, the specimen was post-processed using three methods: left in the air (25 ℃), heated in the oven (200 ℃ for 2 h), and boiled in water (100 ℃ for 2 h). In the absence of post-processing or with heat post-processing, the laser surface modified zircaloy-4 showed super hydrophobicity with a contact angle of 150° or higher for specific laser surface processing parameters. Under specific laser surface processing conditions, the laser surface modified zircaloy-4 with boiling water post-processing exhibited properties close to super hydrophilic with a contact angle of less than 5°. It was confirmed that the surface wettability of zircaloy-4 can be controlled using a spatially shaped nanosecond laser beam.

Integrated Double-Sided Random Microlens Array Used for Laser Beam Homogenization.

Double microlens arrays (MLAs) in series can be used to divide and superpose laser beam so as to achieve a homogenized spot. However, for laser beam homogenization with high coherence, the periodic lattice distribution in the homogenized spot will be generated due to the periodicity of the traditional MLA, which greatly reduces the uniformity of the homogenized spot. To solve this problem, a monolithic and highly integrated double-sided random microlens array (D-rMLA) is proposed for the purpose of achieving laser beam homogenization. The periodicity of the MLA is disturbed by the closely arranged microlens structures with random apertures. And the random speckle field is achieved to improve the uniformity of the homogenized spot by the superposition of the divided sub-beams. In addition, the double-sided exposure technique is proposed to prepare the rMLA on both sides of the same substrate with high precision alignment to form an integrated D-rMLA structure, which avoids the strict alignment problem in the installation process of traditional discrete MLAs. Then the laser beam homogenization experiments have been carried out by using the prepared D-rMLA structure. The laser beam homogenized spots of different wavelengths have been tested, including the wavelengths of 650 nm (R), 532 nm (G), and 405 nm (B). The experimental results show that the uniformity of the RGB homogenized spots is about 91%, 89%, and 90%. And the energy utilization rate is about 89%, 87%, 86%, respectively. Hence, the prepared structure has high laser beam homogenization ability and energy utilization rate, which is suitable for wide wavelength regime.

Fabrication of a diamond concave microlens array for laser beam homogenization

An ultra-smooth diamond concave microlens array was realized using a chemical reflow process assisted by dry etching technique, which has a great performance for homogenization of light beam. Firstly, photolithography was used to produce a regular hexagonal hole array in the photoresist layer on a diamond substrate. Then, the hole array was reflowed in ethanol solvent vapor atmosphere at 20 ℃ to form the concave spherical structure array. Finally, diamond concave microlens array was formed by transferring photoresist concave structures onto diamond substrate through dry etching technique. The fabricated diamond concave microlens array was characterized, demonstrating a well-uniformity of the surface morphology, high quality imaging and focusing performances. Furthermore, laser beam homogenization was realized by the diamond concave microlens array.

Parallel dark-soliton pair in a bistable two-dimensional exciton-polariton superfluid

Collective excitations, such as vortex-antivortex and dark solitons, are among the most fascinating effects of macroscopic quantum states. However, 2D dark solitons are unstable and collapse into vortices due to snake instabilities. Making use of the optical bistability in exciton-polariton microcavities, we demonstrate that a pair of dark solitons can be formed in the wake of an obstacle in a polariton flow resonantly supported by a homogeneous laser beam. Unlike the purely dissipative case where the solitons are grey and spatially separate, here the two solitons are fully dark, rapidly align at a specific separation distance and propagate parallel as long as the flow is in the bistable regime. Remarkably, the use of this regime allows to avoid the phase fixing arising in resonant pumping regime and to circumvent the polariton decay. Our work opens very wide perspectives of studying new classes of phase-density defects which can form in driven-dissipative quantum fluids of light.

Laser beam homogenization with randomly distributed freeform cylindrical microlens

A randomly distributed freeform cylindrical microlens (RFCML) is proposed for laser beam reshaping and homogenization. By introducing a freeform surface in the microlens, the optical field can be controlled with the expected distribution with a single interface. With the help of randomly distributed subapertures of the microlens, the diffraction orders of the microstructure are smoothed and the optical field is homogenized in the far field. The theoretical design of the RFCML is carried out, and the influence of the field smoothing with the random coefficient is analyzed. With the optimized results, the device is fabricated with the laser beam direct writing method and demonstrates the excellent property of laser beam homogenization in the device. The experimentally reshaped homogenized line with a full angle of 40 deg is in good accordance with the theoretical results.

Laser based method for dynamic calibration of thermocouples

In dynamic temperature measurement, thermocouple sensors are widely used in harsh environments, and their dynamic characteristics directly affect the accuracy of dynamic temperature measurement. When the dynamic characteristics do not meet the requirements of dynamic test, dynamic errors will occur. In order to obtain the dynamic characteristics, it is necessary to calibrate the thermocouple dynamically. In this paper, a dynamic calibration system for the thermocouple was built. In the system, a high power laser was used to generate pulse temperature excitation. In order to realize the uniform heating, the laser beam homogenization system based on microlens array was studied and added to the dynamic calibration system. The homogenization system was designed and simulated, and the uniformity of the laser spot was measured after homogenization. A K-type butt-welded thermocouple was dynamically calibrated by using the dynamic calibration system. The dynamic mathematical model of the thermocouple was obtained by the system identification method, and the dynamic characteristics of the thermocouple were analyzed.

Fabrication of Random Microlens Array for Laser Beam Homogenization with High Efficiency.

The miniaturized and integrated microlens array (MLA) can effectively achieve the beam homogenization, compactness and miniaturization of laser systems. When the high-coherence laser beam is homogenized by means of using the MLA, interference fringes will occur in the homogenized light spot due to the periodicity of the MLA, which seriously affects the uniformity of the homogenized light spot. To solve this problem, a novel random microlens array (rMLA) structure was proposed for the purpose of achieving beam homogenization. The coherence in the homogenization process is suppressed by means of breaking the periodicity of the MLA. The homogenized light spot with a high energy utilization is then obtained accordingly. In the fabrication process, a clever method of combining chemical etching with lithography technology is performed to fabricate a honeycomb rMLA and a rectangular rMLA. The experimental results show that the energy utilization rate of the two types of the rMLAs is about 90%, and the uniformity of the homogenized light spots generated by the honeycomb rMLA and the rectangular rMLA are more than 80% and 85%, respectively. Meanwhile, fully cost-effective fabrication is possible to be realized.

Sapphire Concave Microlens Arrays for High-Fluence Pulsed Laser Homogenization

Microlens array has been demonstrated to be an effective method for laser beam homogenization. However, the homogenization of high-fluence laser beam is still a great challenge due to the serious damage to the microlens-array-based homogenizer. Here, an ultra-smooth sapphire concave microlens-array-based homogenizer, which is fabricated by dry-etching-assisted femtosecond laser machining, is proposed for shaping high-fluence pulsed laser beam to a flat-top profile intensity distribution. By taking advantages of high transmittance (>70%) and low surface roughness (1.1 nm), high-fluence (5 J/cm2) and wide-band (266 nm, 532 nm and 808 nm) laser beams homogenization is successfully realized by the sapphire concave microlens arrays.

Analysis of element content in cement by Gaussian and flattop laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is an elemental qualitative and quantitative analytical technique. To improve the stability and accuracy of LIBS in the determination of elemental content in cement, the Gaussian laser beam was transformed into a homogeneous flattop laser beam by a diffractive optical element. Analytical performance was evaluated by comparing their ablation crater morphology and three parameters (R2, SD, and LOD). The flattop LIBS obtained more constant ablation, more stable spectral intensity, better correlation calibration curve and lower LOD. The results demonstrated that the flattop LIBS has great potential for industrial applications.

Accelerated photodegradation of poly(lactic acid) with weathering test chamber and laser exposure – A comparative study

This study presents the comparative study of two different accelerated aging methods of poly(d-,l-lactide acid) (PLA). One is the well-known and widely used accelerated weathering chamber. The other procedure is photodegradation with a homogenized laser beam, which has not been used for the artificial aging of biopolymers so far. The film sample was artificially aged with a xenon test chamber for 200 h, with an acceleration factor of approximately 7:1. We also performed a comparative artificial aging test using a krypton-fluoride (KrF) excimer laser in a way that the amount of energy transferred by the laser is the same as the energy transferred by the xenon lamp; the acceleration factor of the laser is higher than 42000:1. The resulting two types of photodegraded film products were compared and broadly characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), gel permeation chromatography (GPC) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR); the goal was to create comparison and possible conformity between the two methods to achieve higher acceleration factor for accelerated aging.

Fabrication of diamond microlens arrays for monolithic imaging homogenizer

Compact microlens arrays (MLAs) have been fabricated on diamond substrates using thermal reflow and dry etching techniques for laser beam homogenization. Firstly, close-packed hexagonal photoresist pillars developed by photolithography were reflowed upon heat plate to form MLAs mask on one side of diamond substrate. Secondly, the mask pattern was transferred into substrate with inductively coupled plasma etching to form closely arranged diamond MLAs. Then, the same processes were utilized to fabricate MLAs on the other side of diamond substrate. The MLAs on both sides of substrate are aligned. Thirdly, the obtained diamond MLAs demonstrate compact arrangement, well-uniformity, and good imaging performance with projection experiment. Eventually, the double-sides MLAs-patterned diamond substrate was utilized as monolithic imaging homogenizer, exhibiting a good homogenizing performance.

The Effects of Profile Errors of Microlens Surfaces on Laser Beam Homogenization

Microlens arrays (MLAs) are key optical components in laser beam homogenization. However, due to imperfect surface profiles resulting from microfabrication, the functionalities of MLAs in beam modulation could be compromised to some extent. In order to address this issue, the effects of surface profile mismatches between ideal and fabricated MLAs on beam homogenization were analyzed. Four types of surface profile errors of MLAs were modeled theoretically and numerical simulations were conducted to quantitatively estimate the effects of these profile errors on beam homogenization. In addition, experiments were conducted to validate the simulation results, revealing that profile errors leading to optical deviations located on the apex of microlenses affected beam homogenization less than deviations located further away from it. This study can provide references for the further applications of MLAs in beam homogenization.

Dynamics of laser induced metal nanoparticle and pattern formation

Discontinuous metal films are converted into either almost round, isolated, and randomly distributed nanoparticles (NPs) or fringed patterns of alternate non transformed film and NPs by exposure to single pulses (20 ns pulse duration and 193 nm wavelength) of homogeneous or modulated laser beam intensity. The dynamics of NPs and pattern formation is studied by measuring in real time the transmission and reflectivity of the sample upon homogeneous beam exposure and the intensity of the diffraction orders 0 and 1 in transmission configuration upon modulated beam exposure. The results show that laser irradiation induces melting of the metal either completely or at regions around intensity maxima sites for homogeneous and modulated beam exposure, respectively, within ≤10 ns. The aggregation and/or coalescence of the initially irregular metal nanostructures is triggered upon melting and continues after solidification (estimated to occur at ≤80 ns) for more than 1 μs. The present results demonstrate that real time transmission rather than reflectivity measurements is a valuable and easy-to-use tool for following the dynamics of NPs and pattern formation. They provide insights on the heat-driven processes occurring both in liquid and solid phases and allow controlling in-situ the process through the fluence. They also evidence that there is negligible lateral heat release in discontinuous films upon laser irradiation.

CuInSe2 semiconductor formation by laser annealing

One industrially relevant fabrication method for CuInSe2 absorber layers begins with electrodeposition, which is highly resource efficient and easily up-scalable, followed by a high temperature annealing step. Commonly a furnace is used for annealing, although it is possible to reduce the duration of this step by 2–3 orders of magnitude using laser heating. Past work demonstrated that rastering a 1064nm, continuous wave, neodymium-doped yttrium aluminium garnet laser beam over an electrodeposited precursor film with 1s dwell time, promoted CuInSe2 formation, grain growth, and gave a photovoltaic device with 1.6% power conversion efficiency. However, this device showed inhomogeneous current collection correlating approximately to the Gaussian flux profile of the laser beam. This work demonstrates how deviations in the incident laser flux on the precursor lead to local temperature variation in the film. The temperature affects the rates of atomic diffusion, grain growth and chemical reactions, with the hottest region having the largest crystallites and a stoichiometric and constant composition through the film depth. Spatially resolved photoluminescence yield shows a positive correlation to the temperature profile caused by the Gaussian flux. It is expected that with a homogeneous laser beam flux, spatial variations in absorber layer properties would be eliminated, leading to uniform device properties.

High-Performance Laser Beam Homogenizer Based on Double-Sided Concave Microlens

Microlens array (MLA) has attracted increasing interest as its application in micro-optical device and system. In this letter, we proposed a new approach that applies rotational displacement MLA to the laser homogenization system. These are nonregular arrays consisting of close-packed concave MLAs on two sides with specific rotation angle. And, the double-sided MLAs diffusers with new design parameter were successfully fabricated using single pulse femtosecond laser assisted chemical wet etching. Simulation and experimental results reveal the good homogenization performance of the double-sided MLAs. In addition, we fabricated diffusers with different rotation angles (θ), and when θ = 60°, the diffuser obtains the best optical performance.

Fiber-coupled laser-induced breakdown and Raman spectroscopy for flexible sample characterization with depth profiling capabilities

A combined laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy system for depth profile analyses is presented. The system incorporates a single 532nm laser source, which is delivered through an optical fiber to the sample site. The homogenized laser beam results in well-defined cylindrical craters with diameters of 100μm. LIBS depth profiling analyses of metals was performed applying pulse energies of about 1mJ. The application of up to 500 pulses allowed to drill through layers of several tens of microns, while observing sharp transitions at the layer interfaces. The capability of the system for Raman spectroscopy was investigated for various polymer samples by reducing the pulse energies below the respective ablation threshold. A combined Raman/LIBS depth profiling was applied to a polymer-coated metal. Additionally, the capability of the system for calibration-free LIBS quantification (CF-LIBS) was evaluated. Quantification of major elements in metallic reference materials showed good agreement with the certified values with relative deviations of less than 30%. Finally, the optimized system was applied for depth profiling and elemental composition analysis of ancient Roman bronze rings. Overall, the presented setup combines the high flexibility of a fiber-coupled system with Raman and micro-LIBS, making the system interesting for depth profiling and elemental quantification in archaeometric as well as industrial applications.

High power diode laser remelting of metals

This article is focused on the laser surface remelting of the steel samples with predefined overlapping of the laser spots. The goal of our experimental work was to evaluate microstructure and hardness both in overlapped zone and single pass ones for three kinds of ferrous metals with different content of carbon, cast iron, non-alloy structural steel and tool steel. High power fibre coupled diode laser Laserline LDF 3600-100 was used with robotic guided processing head equipped by the laser beam homogenizer that creates rectangular beam shape with uniform intensity distribution. Each sample was treated with identical process parameters - laser power, beam diameter, focus position, speed of motion and 40% spot overlap. Dimensions and structures of the remelted zone, zone of the partial melting, heat affected zone and base material were detected and measured by means of laser scanning and optical microscopes. Hardness progress in the vertical axis of the overlapped zone from remelted surface layer to base material was measured and compared with the hardness of the single spots. The most hardness growth was found for cast iron, the least for structural steel. Experiment results will be used to processing parameters optimization for each tested material separately.

스트레쳐블 폴리머 디퓨저를 이용한 균질 레이저 디스플레이 해석

본 논문은 스트레쳐블 폴리머 디퓨저를 이용하여서 간섭성이 있는 레이저의 단점인 스페클 효과를 감소하는 결과를 비교 분석하였다. 자체 제작한 RGB 모듈을 기반으로 하여 빔 컴바이너를 거쳐서 레이저빔을 집속한 후 고전압 저주파수를 통해서 레이저빔을 균질화 하였다. 평균 180~300Hz 정도의 주파수를 가한 결과 레이저빔의 균질효과가 나타났다. 컴퓨터 시뮬레이션을 통한 디퓨저와 DMD 반사경을 구현 하였으며, 808nm 레이저빔을 가상으로 조사하여서 디퓨저 유무에 따른 빔의 산란도 및 균질도를 검증 하였다. 결과적으로 디퓨저를 통한 빔의 균질도가 산란효과를 감소시켰고, 이를 통한 디스플레이 선명도를 개선할 수 있을 것으로 기대된다. 청색레이저의 투과도가 83% 정도로 적색이나 녹색 레이저에 비해서 상대적으로 취약하였는데, 이는 디퓨저 내부를 통과시 레이저 빔의 굴절의 양이 많아져서 확산 각이 증가하였기 때문으로 분석되었다. 레이저와 광학 장치 사이에 디퓨저를 사용하여 이미지 구현했을 때 스페클 패턴은 제거되었으며, 레드(650nm), 그린(532nm), 블루(405nm) 파장에서 10-20%정도의 밝기 감소가 나타났으며, 전체적인 밝기 감소는 13%정도 이루어졌다. 또한, 스트레쳐블 폴리머를 이용한 경우 시간축 균질도도 30fs에서 110fs로 개선 되었다. 【A stretchable polymeric membrane shaped diffuser was used to homogenize the coherent laser beam. The developed RGB module was used to focus and defocus test by beam combiners. Average oscillation frequency was set at 180Hz~300Hz and it resulted in the best homogenization effect in visual range. The blue laser module turned out to be the worst case for the transmission and primary reason is believed to be the short length of irradiated laser beam. The developed system removed the speckling and the brightness was decreased by 10 ~ 20% for the RGB laser. Overall the brightness was decreased by 13% and homogenity with respect to the temporal axis was improved from 30fs to 110fs.】