Focused Beam Diameter Research Articles

Laser-machined two-stage nozzle optimised for laser wakefield acceleration

In this paper, the modelling and manufacturing of a two-stage supersonic gas jet nozzle enabling the formation of adaptive plasma concentration profiles for injection and acceleration of electrons using few-cycle laser beams are presented. The stages are modelled using the rhoSimpleFoam algorithm of the OpenFOAM computational fluid dynamics software. The first 200–300 ${\rm \mu}$ m diameter nozzle stage is dedicated to 1 % N2 + He gas jet formation and electron injection. By varying the pressure between the first and second stages of the injectors, the electron injection location could be adjusted, and the maximum acceleration distance could be ensured. By changing the concentration of the nitrogen in the gas mixture, the charge of the accelerated electrons could be controlled. The second nozzle stage is designed for acceleration in fully ionised He or hydrogen gas and forms the optimal plasma concentration for bubble formation depending on the laser pulse energy, duration and focused beam diameter. In order to reduce the diameter of the plasma profile formed by the first nozzle and the concentration drop gap between the two nozzles, a one-side straight section was introduced in the first nozzle. The shock wave reflected from the straight section of the wall propagates parallel to the shock wave of the intersecting supersonic jets and ensures a minimal gap between the jets. The second-stage longitudinal plasma concentration profile could have an increasing gas density gradient to compensate for dephasing between the electron bunch and the plasma wave due to wave shortening with increasing plasma concentration.

All-fiber probe for optical coherence tomography with an extended depth of focus by a high-efficient fiber-based filter

Although methods have been proposed to maintain high transverse resolution over an increased depth range, it is not straightforward to scale down the bulk-optic solutions to minimized probes of optical coherence tomography (OCT). In this paper, we propose a high-efficient fiber-based filter in an all-fiber OCT probe to realize an extended depth of focus (DOF) while maintaining a high transverse resolution. Mode interference in the probe is exploited to modulate the complex field with controllable radial distribution. The principle of DOF extension by the fiber-based filter is theoretically analyzed. Numerical simulations are conducted to evaluate the performances of the designed probes. A DOF extension ratio of 2.6 over conventional Gaussian beam is obtainable in one proposed probe under a focused beam diameter of 4.6μm. Coupling efficiencies of internal interfaces of the proposed probe are below −40 dB except the last probe–air interface, which can also be depressed to be −44 dB after minor modification in lengths for the filter. Length tolerance of the proposed probe is determined to be −28∕+20μm, which is readily satisfied in fabrication. With the merits of extended-DOF, high-resolution, high-efficiency and easy-fabrication, the proposed probe is promising in endoscopic applications.

Optical properties of metamorphic hybrid heterostuctures for vertical-cavity surface-emitting lasers operating in the 1300-nm spectral range

The possibility of fabricating hybrid metamorphic heterostructures for vertical-cavity surfaceemitting lasers working in the 1300-nm spectral range is demonstrated. The metamorphic semiconductor part of the heterostructure with a GaAs/AlGaAs distributed Bragg reflector and an active region based on InAlGaAs/InGaAs quantum wells is grown by molecular-beam epitaxy on a GaAs (100) substrate. The top dielectric mirror with a SiO2/Ta2O5 distributed Bragg reflector is formed by magnetron sputtering. The spectra of the room-temperature microphotoluminescence of these vertical-cavity surface-emitting laser heterostructures are studied under 532-nm excitation in the power range of 0–70 mW (with a focused-beam diameter of ~1 μm). The superlinear dependence of the photoluminescence intensity on the excitation power, narrowing of the photoluminescence peaks, and a change in the modal composition may be indications of lasing. The results obtained give evidence that the technology of the metamorphic growth of heterostructures on GaAs substrates can be used for the fabrication of vertical-cavity surface-emitting lasers working in the 1300- nm spectral range.

Ultrafast transmission electron microscopy using a laser-driven field emitter: Femtosecond resolution with a high coherence electron beam

We present the development of the first ultrafast transmission electron microscope (UTEM) driven by localized photoemission from a field emitter cathode. We describe the implementation of the instrument, the photoemitter concept and the quantitative electron beam parameters achieved. Establishing a new source for ultrafast TEM, the Göttingen UTEM employs nano-localized linear photoemission from a Schottky emitter, which enables operation with freely tunable temporal structure, from continuous wave to femtosecond pulsed mode. Using this emission mechanism, we achieve record pulse properties in ultrafast electron microscopy of 9Å focused beam diameter, 200fs pulse duration and 0.6eV energy width. We illustrate the possibility to conduct ultrafast imaging, diffraction, holography and spectroscopy with this instrument and also discuss opportunities to harness quantum coherent interactions between intense laser fields and free-electron beams.

Investigation on pulsed Nd:YAG laser welding of 49Ni–Fe soft magnetic alloy

Laser welding is one of the most popular methods for joining hot crack sensitive alloys and thin materials because of its low heat input and high heat intensity. In this study, a Nd:YAG pulsed micro laser machine with maximum 6J pulse energy was used for joining 49Ni–Fe soft magnetic alloy. Welding of samples was carried out autogenously by changing the pulse duration, focused beam diameter, voltage, frequency and welding speed. All samples were bead on plate welded. Scanning electron microscopy (SEM), optical microscopy and microhardness measurements were used for survey of results. Results show that weld bead size and grain size increase with voltage and heat input increase with pulse frequency. However, weld penetration depth and weld craters reduce with pulse frequency. The study shows that the average peak power density and pulse frequency are two important parameters that results in the formation of weld crater and porosity in the weld zone. Formation of hot cracks in the weld metal depends on the weld pool shape. Cylindrical weld pools are sensitive to hot cracks. Therefore, in order to reduce hot cracks in the weld bead, it is necessary to decrease voltage and increase pulse duration time, focused beam diameter and pulse frequency.

Two-dimensional Defect Map for the Deformed Pure Iron Samples by Positron Probe Microanalyzer

We have developed positron probe microanalyzer (PPMA) for obtaining two-dimensional map of open-volume type defects. The primary positron beam with 4 mm diameter was guided in a static magnetic field, followed by focusing to the Ni(100) foil with 150 nm thick for the brightness enhancement. The remoderated positron beam re-emitted from the Ni foil was accelerated and focused to the specimen by the magnetic objective lens. The focused beam diameter was estimated to be 14.5 μm. PPMA has been applied to investigate the defect behavior in a plastic deformation for polycrystalline high-purity Fe and Fe–0.5mass%Cu alloy samples, and Doppler broadening of the positron annihilation line has been used to map the defect distribution. The results showed that the defects were heterogeneously introduced by deformation and the concentration of the defects was high in a necking region. This can be attributed to the inhomogeneous deformation in the polycrystalline sample. A formation of vacancy clusters in the vicinity of the fractured point is suggested by a large change in Doppler broadening of the positron annihilation line. It has been, further, proven that Cu precipitation in Fe–0.5Cu alloys by annealing is enhanced by the defects induced by a plastic deformation.

Study on the microstructure and mechanical properties of Nd:YAG pulsed laser beam weld of UNS-C17200 copper beryllium alloy

In this study, the microstructure of Nd:YAG pulsed laser welding of copper beryllium sheet has been investigated by tensile and hardness tests, optical and scanning electron microscopy (SEM) and X-ray diffraction (XRD). The SEM images reveals three distinct solidified structures due to various thermal gradients occurred in the fusion zone. The XRD patterns show that the preferred solidification directions are like other FCC materials. Tensile strength of the weld metal is lower than that of the base metal since the CuBe phase precipitates during solidification. The effects of pulsed laser parameters on the weld pool dimensions were also studied. The results show that by applying constant pulse energy, focused beam diameter should be kept as low as possible to obtain the weld pool with the highest penetration and the least width. Moreover, the effect of passive layer on the reflectivity of surface by incident beam was investigated. It was found that weld pool increases when the laser energy is high due to multiple internal reflections.

Compact all-fiber Bessel beam generator based on hollow optical fiber combined with a hybrid polymer fiber lens

We report a compact all-fiber Bessel beam generator using hollow optical fiber (HOF) and coreless silica fiber based on a self-assembled polymer lens. A nearly diffraction-free Bessel beam pattern was observed with its focused beam diameter of 20 microm maintained over a propagation distance of 550 microm. The generated Bessel beams were experimentally tested under various structural parameters such as the diameters of the HOF and operating wavelengths. A beam propagation method was applied to simulate the proposed device, which shows good agreement with the experimental observations.

Investigation of optimum condition in oxygen gas-assisted laser cutting

Laser cutting characteristics including power level and cutting gas pressure are investigated in order to obtain an optimum kerf width. The kerf width is investigated for a laser power range of 50–170 W and a gas pressure of 1–6 bar for steel and mild steel materials. Variation of sample thickness, material type, gas pressure and laser power on the average cut width and slot quality are investigated. Optimum conditions for the steel and mild steel materials with a thickness range of 1–2 mm are obtained. The optimum condition for the steel cutting results in a minimum average kerf width of 0.2 mm at a laser power of 67 W, cutting rate of 7.1 mm/s and an oxygen pressure of 4 bar. A similar investigation for the mild steel cutting results in a minimum average kerf width of 0.3 mm at the same laser power of 67 W, cutting rate of 9.5 mm/s, and an oxygen pressure of 1 bar. The experimental average kerf is about 0.3 mm, which is approximately equal to the estimated focused beam diameter of 0.27 mm for our focusing lens ( f=4 cm and 100 W power). This beam size leads to a laser intensity of about 1.74×10 9 W/m 2 at the workpiece surface. The estimated cutting rate from theoretical calculation is about 8.07 mm/s (1.0 mm thickness and 100 W power), which agrees with the experimental results that is 7.1 mm/s for 1.0 mm thickness of mild steel at the laser power of 88 W.

IR Lasers and Application Systems for Myringotomy

. The study examines an Er:YAG laser (2940 nm) and different application systems of the CO2 laser (10 600 nm) with regard to their suitability for a one-shot laser myringotomy of an adequate perforation size (∼2 mm). The laser–tissue interaction of the Er:YAG laser and the CO2 laser in fresh tympanic membranes of horses (thickness: 80–100 µm) as well as in formalin-fixed human tympanic membranes (thickness: 100 µm) is studied correlating perforation diameters to the applied power/energy density and the effects demonstrated by light and scanning electron microscopy are analysed. Using the Er:YAG laser with a focused laser beam (spot diameter: 400 µm) or with a maximally defocused laser beam (spot diameter: 1600 µm) perforations of an adequte size (2 mm) can only be achieved with multiple laser pulses. Histological studies disclose only minimal thermic side effects in the adjacent tissue in both specimens. If the CO2 laser radiation is transmitted via a silver halide polycrystalline fibre (diameter: 900 µm) a maximal perforation diameter of 1300 µm is achieved with significant thermic side effects such as coagulation. Using an Acuspot™ 710 micromanipulator (focused beam diameter: 180 µm) combined with a SilkTouch™ scanner a maximal perforation diameters of 1700 µm can be achieved in horse tympanic membrane with one laser pulse. A prototype of a hand-held CO2 laser otoscope in combination with the SilkTouch™ scanner is suitable for performing laser myringotomies with a diameter of 2 mm with a single laser pulse in fresh horse tympanic membrane.

A Study on the Vision Sensor Using Scanning Beam for Welding Process Automation

The vision sensor which is based on the optical triangulation theory with the laser as an auxiliary light source can detect not only the seam position but the shape of seam. In this study, a vision sensor using the scanning laser beam was investigated. To design the vision sensor which considers the reflectivity of the sensing object and satisfies the desired resolution and measuring range, the equation of the focused laser beam which has a Gaussian irradiance profile was firstly formulated, Secondly, the image formaing sequence, and thirdly the relation between the displacement in the measuring surface and the displacement in the camera plane was formulated. Therefore, the focused beam diameter in the measuring range could be determined and the influence of the relative location between the laser and camera plane could be estimated. The measuring range and the resolution of the vision sensor which was based on the Scheimpflug's condition could also be calculated. From the results mentioned above a vision sensor was developed, and an adequate calibration technique was proposed. The image processing algorithm which and recognize the center of joint and its shape informaitons was investigated. Using the developed vision sensor and image processing algorithm, the shape informations was investigated. Using the developed vision sensor and image processing algorithm, the shape informations of the vee-, butt- and lap joint were extracted.

Micro-coating by a laser powder feed process

The aim of this work was to produce NiCrBSi coatings of small dimensions by a modified laser beam powder feed coating technique. Experiments had been carried out in order to examine the influence of some parameters of this process. Power flux density and travelling speed influence heating and cooling of the coating. Furthermore, there is a geometric relationship between focusing conditions (beam diameter) and the width of the coating trace—they should be of the same order. Powder feed rate determines the thickness of the coating, the focused beam diameter determines the width. Undesired dilution of base material could be prevented by applying a multi-pass coating technique.

Laser-induced damage in ZnSe

Pulsed laser damage in CVD-grown ZnSe has been studied at 1.06 and 10.6 μ. At 1.06 μ the intrinsic damage intensity was measured to be 1.2 GW/cm2. Damage at 10.6 μ was observed to be the result of inclusion absorption with the damage intensity dependent on focused beam diameter. A statistical model of inclusion damage is developed to account for this dependence. By comparing the predictions of the model to damage intensities measured under two conditions of external focusing, the average density of potentially damaging inclusions is calculated to be about 107 cm−3.