Power Laser Sources Research Articles

Laser-induced reconfigurable wavefront control with a structured Ge2Sb2Te5-based metasurface

Phase change materials have been widely exploited in active metasurfaces due to their large index contrast. Despite recent advances in phase-change metasurfaces, it remains a challenge to integrate diverse reconfigurable optical functionalities into a single metasurface. Here, we demonstrate an effective strategy to realize reconfigurable wavefront control by combining a Ge2Sb2Te5-rod array with laser writing technology. Through arbitrarily modifying the position and power of laser source, the laser writing process helps to realize site-selective and multi-level phase change of Ge2Sb2Te5 rods. Due to multi-level switching for optical properties of Ge2Sb2Te5 material, the Ge2Sb2Te5-rod array offers complete phase control and high amplitude modulation. Subsequently, various optical devices are designed in numerical simulation, including a phase-only hologram, dynamic meta-deflectors, a grayscale image and a perfect absorber. The structured Ge2Sb2Te5-based metasurface with the combination of laser writing technology offers an effective way to explore various types of optical functionalities in the same device.

Multiphoton-initiated laser writing of semiconductors using nanosecond mid-infrared pulses

The advent of more powerful mid-infrared nanosecond laser sources allows using them as attractive tools in material laser processing. For semiconductor bulk processing, laser intensities must remain modest to avoid detrimental nonlinear propagation and pre-focal plasma screening effects. Accordingly, nanosecond pulses are usually appropriate to locally initiate energy deposition by multiphoton absorption and subsequent modification, hardly achievable with ultrashort pulses. Employing a 2.8-μm 2.5-ns laser source, we explore the possibility to modify silicon and other semiconductors. With interactions initiated by three-photon absorption, we modify and determine the surface and bulk modification thresholds of Si, GaAs, and InP. For Si, compared to previous studies at telecom wavelengths, we report on a reduction of the energy threshold for volume modification with processing depth, and repeatable rear-surface modifications. Compared to Si, we find for GaAs and InP important fluctuations in the modification responses, which we attribute to an absorption mechanism initially triggered by a greater presence of defects. We also study germanium, as this narrower bandgap semiconductor very interesting for mid-infrared electro-optical applications is transparent at the newly introduced wavelength. Despite the ability to modify the surface, the bulk of Ge remains inaccessible in this two-photon regime, mainly attributed to beam aberrations and nonlinearities. Nonetheless, we demonstrate the ability to process Si chips integrating Ge layers. All these results showcase the potential of mid-infrared wavelengths to offer new degrees of flexibility for 3D laser processing technologies turned to silicon photonics and microelectronics packaging.

Parallel Diffractive Multi‐Beam Pulsed‐Laser Ablation in Liquids Toward Cost‐Effective Gram Per Hour Nanoparticle Productivity

Nanoparticles (NPs) generated by pulsed‐laser ablation in liquids (PLAL) have benefited many key applications due to their versatility, enlarged surface area, and high purity. However, scaling up NPs production represents one of the main requisites to commercialize this technology. The established upscaling strategy demands high power and repetition rate laser source with fast scanning systems, which are not widely available and costly. Herein, a cost‐effective alternative is proposed, the addition of static diffractive optical elements to achieve parallel processing through the multi‐beam PLAL (MB‐PLAL). In MB‐PLAL, the optimum repetition rate is reduced to compensate laser energy splitting, hence achieving a higher interpulse distance, reducing pulse shielding, and increasing NPs productivity. MB‐PLAL with 11 beams reached a factor 4 productivity increase for iron–nickel alloy (Fe50Ni50) NPs compared to the single‐beam setup (0.4–1.6 g h−1), and a factor 3 increase for gold (Au) NPs (0.32–0.94 g h−1). The scalability of the proposed MB‐PLAL technique setup is confirmed by Au and Fe50Ni50 NPs productivity experiments using 1, 6, and 11 beams, showing a linear increase in productivity.

High power tunable Raman fiber laser at 1.2 μm waveband

Development of a high power fiber laser at special waveband, which is difficult to achieve by conventional rare-earth-doped fibers, is a significant challenge. One of the most common methods for achieving lasing at special wavelength is Raman conversion. Phosphorus-doped fiber (PDF), due to the phosphorus-related large frequency shift Raman peak at 40 THz, is a great choice for large frequency shift Raman conversion. Here, by adopting 150 m large mode area triple-clad PDF as Raman gain medium, and a novel wavelength-selective feedback mechanism to suppress the silica-related Raman emission, we build a high power cladding-pumped Raman fiber laser at 1.2 μm waveband. A Raman signal with power up to 735.8 W at 1252.7 nm is obtained. To the best of our knowledge, this is the highest output power ever reported for fiber lasers at 1.2 μm waveband. Moreover, by tuning the wavelength of the pump source, a tunable Raman output of more than 450 W over a wavelength range of 1240.6–1252.7 nm is demonstrated. This work proves PDF’s advantage in high power large frequency shift Raman conversion with a cladding pump scheme, thus providing a good solution for a high power laser source at special waveband.Graphical

Refining the Performance of mid-IR CPA Laser Systems Based on Fe-Doped Chalcogenides for Nonlinear Photonics

The chirped pulse amplification (CPA) systems based on transition-metal-ion-doped chalcogenide crystals are promising powerful ultrafast laser sources providing access to sub-TW laser pulses in the mid-IR region, which are highly relevant for essential scientific and technological tasks, including high-field physics and attosecond science. The only way to obtain high-peak power few-cycle pulses is through efficient laser amplification, maintaining the gain bandwidth ultrabroad. In this paper, we report on the approaches for mid-IR broadband laser pulse energy scaling and the broadening of the gain bandwidth of iron-doped chalcogenide crystals. The multi-pass chirped pulse amplification in the Fe:ZnSe crystal with 100 mJ level nanosecond optical pumping provided more than 10 mJ of output energy at 4.6 μm. The broadband amplification in the Fe:ZnS crystal in the vicinity of 3.7 μm supports a gain band of more than 300 nm (FWHM). Spectral synthesis combining Fe:ZnSe and Fe:CdSe gain media allows the increase in the gain band (~500 nm (FWHM)) compared to using a single active element, thus opening the route to direct few-cycle laser pulse generation in the prospective mid-IR spectral range. The features of the nonlinear response of carbon nanotubes in the mid-IR range are investigated, including photoinduced absorption under 4.6 μm excitation. The study intends to expand the capabilities and improve the output characteristics of high-power mid-IR laser systems.

Investigating the effects of the laser quenching technology on the hardness and microstructure of P18 steel

The article introduces laser quenching technology and presents the results of surveying the hardness and microstructure of P18 steel sample after laser quenching. Laser quenching is a new candidate technique for having precise heat treatment, limiting warping, saving energy, and being environmentally friendly compared to traditional quenching. Before quenching samples (laser quenching or normal quenching), the φ22 mm diameter x 20 mm thickness samples were sequentially ground by abrasive papers with a grit size of #240, 400, 600, 1000, and 1200. This study aimed to investigate the influence of laser quenching power and traditional quenching on the hardness and microstructure of P18 steel... The microstructure and microhardness were observed by the Axiovert 40 MAT optical microscope and the Future Tech Corp FM-700e hardness tester, respectively. The results showed that the quenched sample hardness with a laser source power of 300, 500, 700, and 900 W are 280, 450, 940, and 1050 HV0.1, respectively, and compared to the traditional harden, it only reaches 650 HV0.1. Observation of the microstructure of the laser-hardened sample in cross-section, including a bright white high-hardness region and a dark transition region.

Optimization of Sandblasting to Improve the Surface Finish of 17-4PH Parts Manufactured by SLM Using Different Laser Scanning Strategies

Great advances have emerged in recent years around additive manufacturing techniques, with an increasing number of different materials (polymers, ceramics, metals). However, metal part manufacturing has always been one of the most demanded in engineering. That is due to its ability to create final functional parts with good mechanical properties. One of the most widely used technique is Selective Laser Melting (SLM). The SLM process uses a laser power source to selectively melt metal powder layer by layer. Typically, this manufacturing technique requires mechanical post-processing operations, not only to split the parts from the build-plate, but also to improve the mechanical properties and surface finish of parts or the dimensional accuracy of specific regions to ensure assembly and interchangeability. In particular, sandblasting is a method of mechanical abrasion cleaning commonly used and very useful for improving the surface topology of SLM printed parts. Besides, the laser scanning strategy used in this additive manufacturing process influences the surface quality of parts. Therefore, in this work, the sandblasting post-process has been optimized for surface roughness improving in parts printed using the most common laser scanning strategies (normal, hexagonal, concentric). The role that sandblasting pressure and time plays in the surface quality of parts, indispensable to optimize this SLM post-process, has been evaluated. Thus, surface roughness of different specimens subjected to different sandblasting parameters has been measured to optimize both values related to the laser scanning strategy used in SLM manufacturing. The material used is 17-4PH stainless steel, an alloy that presents an excellent combination of high strength and good corrosion resistance, high hardness, good thermal properties, as well as excellent mechanical properties at high temperatures. This precipitation-hardened steel has important applications in the aerospace sector, chemical and petrochemical industry, energy sector, surgical instruments, high wear components, and general metallurgy, among others.

Enhancing productivity and efficiency in conventional laser metal deposition process for Inconel 718 - part I: the effects of the process parameters

The sustainable energy transition has spurred the development of technologies that minimize material and energy waste, such as additive manufacturing (AM). Laser metal deposition (LMD) is a promising AM technique, but its complexity and limited automation hinder its implementation in production chains. To enhance productivity, the high deposition rate LMD (HDR-LMD) technology has been developed, requiring advanced equipment and powerful laser sources. In contrast, the conventional LMD (C-LMD) process is simpler and less expensive to implement. This study aims to optimize the productivity and efficiency of C-LMD by adjusting laser power, scan speed, powder feed rate, and standoff distance on Inconel 718 single tracks. An innovative approach eliminates the need for cutting specimens to evaluate single tracks, allowing comprehensive geometric and performance characterization with limited operator involvement, making the analysis quicker and more robust. An extensive experimental campaign was conducted to examine the influence of process parameters on track geometry, productivity, and efficiency. A multi-objective optimization procedure identified parameter combinations maximizing productivity while maintaining high efficiency and desirable clad shape. The study attained deposition rates ranging from 700 to 800 g/h, with powder catchment efficiency ranging between 75 and 90%. These results were achieved using parameters including 1775 W of laser power, scan speeds ranging from 960 to 1140 mm/min, powder feed rates between 810 and 1080 g/h, and standoff distance of 9 mm. The study also clearly indicated that further potential for improving C-LMD process performance may be possible. The findings gathered in this paper are the base for the further optimization presented in the second part of the work, which is focused on multi-pass multi-layer and reaches deposition rates of 1500 g/h, promoting the implementation of C-LMD process at industrial level.

Fiber Coupled High Power Nd:YAG Laser for Nondestructive Laser Cleaning

In this study, a fiber coupled high power side-pumped Nd:YAG laser system for laser cleaning is presented. Based on the two-rod structure and two stages amplifiers, the maximum average output power of 783 W with corresponding pulse energy of 52 mJ at 15 kHz has been achieved. The fiber coupling efficiencies after the master oscillator, one stage amplifier and two stages amplifiers reach to 99%, 98.3% and 94%, respectively. A laser cleaning machine prototype composed of the master oscillator and one stage amplifier with an average output power of greater than 500 W has been developed and achieved better nondestructive cleaning effect for thermal control coating removal compared with commercial fiber laser cleaning machines. This study provides a new method for developing high power laser sources for nondestructive laser cleaning equipment.

COMPARATIVE ANALYSIS OF METHODS FOR MEASURING LASER POWER

The application of lasers in many fields of technology and medicine is constantly increasing. This causes the development of new physical methods and principles for correct measurement of power, energy, and other parameters of laser sources. In most cases, the correct measurement of the laser power is important because the quality of the processes in which laser sources are used depends on it. In the paper, a comparative analysis of the existing methods for measuring the power and energy of laser radiation of diverse types of laser sources is made. This research aims to help users of laser equipment to choose the right measurement method depending on the laser source.

Target Aiming Point Focusing Strategy for Destroying a Short-Range Target Using Distributed Laser Systems

Consider a scenario where multiple high power laser weapon systems are positioned closed to a base, in order to protect the base from an aerial target, such as a missile. We consider destroying a short-range flying target which is sufficiently close to the base. Using multiple lasers can reduce thermal blooming, which decreases the effective power that can be delivered to a target site in the atmosphere. Thus, this paper proposes utilizing multiple high power laser sources to aim on a single point of the target to maximize the attack efficiency and avoid directional disturbances. Once a target is detected by a search radar system, then the position of the target can be calculated in global coordinate systems and is shared by all laser systems through fiber-based wired links. Then, each laser aims at the target followed by firing at the target. This paper presents how to make multiple high power lasers aim at a single target point for maximizing the attack efficiency. Considering the case where there are more than one high power laser sources, this paper is novel in addressing the control and synchronization of every laser system. MATLAB simulations are used to verify the effectiveness of the proposed multi-laser systems.

Improvement in measuring losses by interferometric technique for glass waveguides produced by femtosecond laser writing

This study describes a straightforward and non-destructive interferometric technique for measuring the propagation loss in single-mode optical glass waveguides by exploiting a Fabry–Perot cavity effect. The experiment involves coupling a powerful tunable laser source to a single-mode waveguide that is being tested, then using a detector to analyze the transmission fringes of that source after coating the glass end facets with highly reflective coating. Our first experiment, which used a 22.4-mm-long fabricated waveguide on Non-Iox Gorilla glass to demonstrate the technique, resulted in a propagation loss of 0.49 dB/cm at a transmitting wavelength of about 1575 nm, which is completely consistent with the value obtained by a conventional characterization method. The measurement procedure is also independent from the coupling losses, making the characterization easier and reliable after the manufacturing just with a reversible deposition process on the sample under testing.

Safety Analysis for Laser-Based Optical Wireless Communications: A Tutorial

Light amplification by stimulated emission of radiation (laser) sources has many advantages for use in high-data-rate optical wireless communications (OWCs). In particular, the low-cost and high-bandwidth properties of laser sources, such as vertical-cavity surface-emitting lasers (VCSELs), make them attractive for future indoor OWCs. In order to be integrated into future indoor networks, such lasers should conform to eye safety regulations determined by the International Electrotechnical Commission (IEC) standards for laser safety. In this article, we provide a detailed study of beam propagation to evaluate the received power of various laser sources, based on which and the maximum permissible exposure (MPE) defined by the IEC 60825-1:2014 Standard, we establish a comprehensive framework for eye safety analyses. This framework allows us to calculate the maximum allowable transmit power, which is crucial in the design of a reliable and safe laser-based wireless communication system. Initially, we consider a single-mode Gaussian beam and calculate the maximum permissible transmit power. Subsequently, we generalize this approach for higher mode beams. It is shown that the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula> -squared-based approach for analysis of multimode lasers ensures the IEC eye safety limits; however, in some scenarios, it can be too conservative compared to the precise beam decomposition method. Laser safety analyses with consideration of optical elements, such as lens and diffuser, as well as for the VCSEL array, have been also presented. Skin safety, as another significant factor of laser safety, has also been investigated in this article. We have studied the impacts of various parameters, such as wavelength, exposure duration, and the divergence angle of laser sources on the safety analysis by presenting insightful results.

MHz burst mode processing as a tool for achieving removal rates scalability in ultrashort laser micro-machining

The average power of ultrashort laser source has been increasing continuously and, therefore, solutions are required to employ fully these technology advances for improving the ablation efficiency in laser micro-processing. The use of burst mode processing is one of the solutions that has attracted a significant research and industrial interest in the past decade. A novel empirical methodology is proposed and implemented in this research to assess the MHz burst mode impact on the specific removal rate (SRR) and processing efficiency in ultrashort laser micro-machining. Especially, the capability of the MHz burst mode processing is investigated to scale up SRRs achievable on copper and stainless steel while utilising fully the available maximum pulse energy and average laser power. The results showed that the MHz burst mode offer a significant SRR scalability potential that can be attributed to beneficial near optimum fluence level and other side effects such as heat accumulation. Also, it is evidenced from the obtained results that the surface quality attained with the burst mode processing was comparable to that achieved with the single-pulse processing and even better at some specific process settings. Thus, the obtained SRR improvements were not in expense of the surface quality and the MHz bust mode processing represents a promising solution to employ fully the constantly increasing average power in ultrashort laser processing operations.

Optical and Electrical Considerations for Developing Pulsed High-Power LED for Volumetric Particle Tracking Velocimetry

Volumetric Particle Tracking Velocimetry (PTV) in volumes larger than 103 cm3 with 15 μm Air Filled Soap Bubbles (AFSB) requires a powerful pulsed laser source of few tenth of mJ pulse energy, depending on the optical arrangement of the experiment. Initial experiments in our lab with 15 μm AFSB and 1 mJ pulse energy from a 532 nm laser source showed that in order to image turbulence flow phenomena in volumes about 4x10^3 cm^3 at 1 m working distance and an f-number of 8, one would need a laser source with about 315 mJ pulse energy and 1260 W average power at 4 kHz repetition rate. Such high-power 532 nm lasers are not simple to build and are expensive and complicated to develop. By reducing the working distance and increasing the diameter of the camera aperture, the required laser power was reduced by a factor 18 to 17.5 mJ pulse energy and 70 W average power, which is still a very high power laser. As a possible alternative, we report here on the development of a collimated beam, high-power, pulsed 460 nm LED for volumetric PTV measurements of turbulence flow in a large volume seeded with AFSB p articles. We report of the optical and electrical consideration that are required in development of such an LED source. First, we combined a practical model for the output power of an LED with a model that estimates the signal level on a CMOS detector in a volumetric PTV experiment with AFSB particles. It predicted that our LED should be driven by 277 A in order to generate 1 mJ of optical pulse energy with 20 μs pulse duration. In order to avoid power drooping at high currents density in the LED, we chose an architecture of LEDs array where each LED unit of the array operates at 20 μs pulse duration, 4 kHz repetition rate and with peak current of 200 A. Each such LED will deliver 0.72 mJ pulse energy and 24 such LED units will supply the required optical density. Second, we report of the optical and electrical features of the collimated , high-power, pulsed 460 nm LED unit we built. We tested the LED in a PTV experiment where the LED operated with 0.72 mJ pulse energy, 1 kHz repetition rate and 20 A peak current. The measured signal current from a CMOS-pixel due to Mie scattering AFSB particles was in good agreement with the predicated signal level.

Segmentation of laser induced retinal lesions using deep learning (December 2021).

Detection of retinal laser lesions is necessary in both the evaluation of the extent of damage from high power laser sources, and in validating treatments involving the placement of laser lesions. However, such lesions are difficult to detect using Color Fundus cameras alone. Deep learning-based segmentation can remedy this, by highlighting potential lesions in the image. A unique database of images collected at the Air Force Research Laboratory over the past 30 years was used to train deep learning models for classifying images with lesions and for subsequent segmentation. We investigate whether transferring weights from models that learned classification would improve performance of the segmentation models. We use Pearson's correlation coefficient between the initial and final training phases to reveal how the networks are transferring features. The segmentation models are able to effectively segment a broad range of lesions and imaging conditions. Deep learning-based segmentation of lesions can effectively highlight laser lesions, making this a useful tool for aiding clinicians.

Numerical investigation of the nonlinear spectral broadening aiming at a few-cycle regime for 10 ps level Nd-doped lasers

We present a cascaded nonlinear spectral broadening scheme for Nd-doped lasers, featuring with long pulse duration and high average power. This scheme is based on two multi-pass cells (MPCs) and one multiple-plate supercontinuum generation (MPSG), and the numerical investigation is driven by a home-made Nd-doped fiber laser with 12 ps pulse duration, 50 kHz repetition rate and 100 W average power. The MPC-based first two stages allow us to broaden the pulse spectrum to 4 nm and 43 nm respectively, and subsequently, the MPSG-based third stage allows us to reach 235 nm spectral bandwidth. This broadened spectrum can support a Fourier-transfer-limited pulse duration of 9.8 fs, which is shorter than three optical cycles. To the best of our knowledge, it is the first time to demonstrate the possibility of few-cycle pulses generation based on the 10 ps level Nd-doped lasers. Such few-cycle and high average power laser sources should be attractive and prospective, benefiting from the characteristics of structure compact, low-cost and flexibility.

Cascaded Random Raman Fiber Laser With Low RIN and Wide Wavelength Tunability

Cascaded random Raman fiber lasers (CRRFLs) have been used as a new platform for designing high power and wavelength-agile laser sources. Recently, CRRFL pumped by ytterbium-doped random fiber laser (YRFL) has shown both high power output and low relative intensity noise (RIN). Here, by using a wavelength- and bandwidth-tunable point reflector in YRFL, we experimentally investigate the impacts of YRFL on the spectral and RIN properties of the CRRFL. We verify that the bandwidth of the point reflector in YRFL determines the bandwidth and temporal stability of YRFL. It is found that with an increase in the bandwidth of the point reflector in YRFL from 0.2nm to 1.4nm, CRRFL with higher spectral purity and lower RIN can be achieved due to better temporal stability of YRFL pump. By broadening the point reflector’s bandwidth to 1.4nm, the lasing power, spectral purity, and RIN of the 4th-order random lasing at 1349nm can reach 3.03W, 96.34%, and −115.19 dB/Hz, respectively. For comparison, the spectral purity and RIN of the 4th-order random lasing with the point reflector’s bandwidth of 0.2 nm are only 91.20% and −107.99dB/Hz, respectively. Also, we realize a wavelength widely tunable CRRFL pumped by a wavelength-tunable YRFL. This work provides a new platform for the development of ideal distributed Raman amplification pump sources based on CRRFLs with both good temporal stability and wide wavelength tunability, which is of great importance in applications of optical fiber communication and distributed sensing.

Advanced remote laser cutting of battery foils using an interference approach

This work demonstrates how an interference pattern can improve the performance of remote laser cutting of pure copper foils, making the cutting process effective even for a low power laser source. The proof of concept is carried out by using a nanosecond laser source with a pulse duration of 5 ns, coupled with a two-beam scanning interference setup, producing a spatial period of 12.5 µm. In the experiments, processing parameters as pulse-to-pulse distance, laser power and scanning speed are varied, to optimize the foil breakthrough and their effect on the generated material modifications are investigated. A comparison between the processing results employing the interference pattern and single beam with a Gaussian energy distribution is carried out. While the single beam process is not sufficient for cutting a 10 µm thin metallic foil, the interference treatment shows an improvement over 100%. In addition, only small spatter formations are detected, with average particle sizes of 1.75 ± 0.82 µm on the top side of the foil. The bottom side of a fully separated copper foil only depicts small spatter formations of less than 1 µm.

Temperature measurements of long sparks in air using time-resolved moiré deflectometry

This paper presents an original investigation into time-resolved moiré deflectometry (MD) for gas temperature measurements of long sparks in air. In order to perform a rigorous comparative investigation with the widely used quantitative schlieren (QS) method, we set up an orthogonal optical measurement system. The impacts of spatial resolution and exposure time on the measurement accuracy are investigated by comparing the measured radial distribution of the gas temperature and its time evolution during the same spark discharge event. It is found that the time-resolved MD with an exposure time of 1.0 μs can accurately measure the gas temperature evolution during the isobaric heating and relaxation process of long sparks. The measured results of the expansion rate of cross-section and average temperature T g confirms that the energy loss is dominated by thermal conduction during the isobaric heating and relaxation process after the extinction of discharge current. The MD can achieve a finer spatial resolution than the QS method, which is beneficial to the reconstruction of steep radial temperature distribution and the suppression of measured temperature fluctuation. Moreover, the MD can obtain a sub-microsecond exposure time by increasing the power of continuous-wave laser source, which shows its potential to capture the rapid changes in gas temperature during the fast-heating process.