Focal Spot Diameter Research Articles

Modulated fiber laser welding of high reflective AZ31

Power sine modulation fiber laser welding (FLW) of AZ31 magnesium alloy was conducted. When the diameter of transmission fiber and focal spot was 400 μm and 0.4 mm, respectively, the average laser powers for penetrating a 2.7-mm-thick AZ31 plate at a welding speed of 5 m/min were reduced by about 33 % by using modulated FLW. The relationship between modulation parameters (i.e., amplitude, frequency, and average laser power) and weld depth which was closely related to the transfer efficiency of laser energy was studied through partial penetration laser welding test on 8-mm-thick AZ31 plate based on quadratic regression orthogonal design. It was found that influence of power modulation on laser welding of AZ31 alloy was highly dependent on laser power density. In the low power density range, laser energy coupling efficiency could be significantly improved by combining low amplitude with high frequency or high amplitude with low frequency. With the increasing of laser power density, the optimum frequency corresponding to maximum weld depth decreased, and the positive effect of the favorable combination of high amplitude and low frequency on laser energy coupling continuously weakened. When laser power density was high enough, power modulation had hardly positive effect on weld depth and energy coupling efficiency. It was argued that improvement of energy coupling efficiency in laser welding of AZ31 by using power modulation was due to the reduction in the portion of energy lost into surroundings. Finally, laser butt welding was conducted on 2.7-mm-thick AZ31 under the condition of high beam quality and the tensile strength of both butt-welded joint and base metal that was tested.

High-pressure X-ray science on the ultimate storage ring.

The advent of the ESRF, APS and SPring-8 third-generation synchrotron sources in the mid-1990s heralded a golden age of high-pressure X-ray science. The high-energy monochromatic micro-focused X-ray beams from these storage rings, combined with the new high-pressure diffraction and spectroscopy techniques developed in the late 1980s, meant that researchers were immediately able to make detailed structural studies at pressures comparable with those at the centre of the Earth, studies that were simply not possible only five years previously. And new techniques, such as X-ray inelastic scattering and X-ray nuclear scattering, became possible at high pressure for the first time, providing wholly-new insight into the behaviour of materials at high densities. The arrival of new diffraction-limited storage rings, with their much greater brightness, and ability to achieve focal-spot diameters for high-energy X-ray beams of below 1 µm, offers the possibility of a new generation of high-pressure science, both extending the scope of what is already possible, and also opening ways to wholly-new areas of investigation.

A semiconductor metasurface with multiple functionalities: A polarizing beam splitter with simultaneous focusing ability

We propose a semiconductor metasurface that simultaneously performs two independent functions: focusing and polarization filtering. The wavefronts of the reflected and transmitted light distributions are precisely manipulated by spatial parametric variation of a subwavelength thin-film Si grating, which inherently possesses polarization filtering properties. We design a 12-μm-wide metasurface containing only nineteen Si grating ridges. Under a 10-μm-wide unpolarized Gaussian beam incidence at wavelength of 1.55 μm, the resulting device shows promising theoretical performance with high power efficiency exceeding 80% and polarization extinction ratio of ∼10 dB with focal spot diameters near 1–2 μm.

热丝填充激光焊接光斑直径与对接间隙裕度研究

热丝填充激光焊接光斑直径与对接间隙裕度研究

Discrimination of impurity elements of solids at laser mass-spectrometric analysis

The ion composition of a laser plasma bunch at laser mass spectrometry has been studied depending on the parameters of the laser emission. It has been demonstrated that the diameter of the focal spot plays a significant role in the formation of the ion composition of the laser plasma. In particular, at low focal spot diameters d, singly and doubly charged ions prevail, while, at d ≈ 0.4–0.6 mm, ions with a charge of 3, 4, and even 5 are registered. The correlation between the emission parameters and the ion composition of the plasma makes it possible to promptly predict the conditions of the exposure to the laser emissions under which ions of one or another charge are registered. It has been demonstrated that the value of the coefficient of the relative sensitivity for the impurity elements should be calculated for low d, at which the relative yield of singly and doubly charged ions is 90%.

Spatial mode cleaning in radically asymmetric strongly focused laser beams

We demonstrate that a femtosecond laser pulse strongly focused in air can produce a highly symmetric damage pattern on glass. This damage pattern contains a series of near-perfect radial rings, with diameters much larger than the predicted focal spot diameter. These rings disappear when the experiment is conducted in vacuum, indicating atmospheric involvement. Surprisingly, the shape and size of the rings seem to be nearly independent of the shape of the generating laser beam, showing dramatic spatial mode cleaning. A “half moon” initial laser mode created by obscuring one side of the round beam produces rings of similar quality to those obtained with the unclipped beam. While spatial mode cleaning has previously been reported in filaments, this is the most dramatic demonstration of the effect that we are aware of. We argue that the effect is due primarily to ionization, in contrast to studies in longer filaments that attribute it to self-focusing.

Precise Neural Stimulation in the Retina Using Focused Ultrasound

Focused ultrasound is a promising noninvasive technology for neural stimulation. Here we use the isolated salamander retina to characterize the effect of ultrasound on an intact neural circuit and compared these effects with those of visual stimulation of the same retinal ganglion cells. Ultrasound stimuli at an acoustic frequency of 43 MHz and a focal spot diameter of 90 μm delivered from a piezoelectric transducer evoked stable responses with a temporal precision equal to strong visual responses but with shorter latency. By presenting ultrasound and visual stimulation together, we found that ultrasonic stimulation rapidly modulated visual sensitivity but did not change visual temporal filtering. By combining pharmacology with ultrasound stimulation, we found that ultrasound did not directly activate retinal ganglion cells but did in part activate interneurons beyond photoreceptors. These results suggest that, under conditions of strong localized stimulation, timing variability is largely influenced by cells beyond photoreceptors. We conclude that ultrasonic stimulation is an effective and spatiotemporally precise method to activate the retina. Because the retina is the most accessible part of the CNS in vivo, ultrasonic stimulation may have diagnostic potential to probe remaining retinal function in cases of photoreceptor degeneration, and therapeutic potential for use in a retinal prosthesis. In addition, because of its noninvasive properties and spatiotemporal resolution, ultrasound neurostimulation promises to be a useful tool to understand dynamic activity in pharmacologically defined neural pathways in the retina.

High-precision small-scale laser focal spot measurements

AbstractIn this paper, a method of two-dimensional fine-scanning with charge coupled device has been conducted to precisely measure spatial position and intensity distribution of small-scale focal spot (diameter in microns). The measurement accuracy of the small-scale focal spot position is better than 1 µm when the fluctuations of the light energy and background noise are relatively small. The theoretical analysis is consistent with the experimental results.

Polarization and Droplet Size Effects in the Laser-Trapping-Induced Reconfiguration in Individual Nematic Liquid Crystal Microdroplets

We experimentally demonstrate reordering throughout the inside of an individual bipolar nematic liquid-crystalline microdroplet optically trapped by a highly focused laser beam, when the laser powers are above a definite threshold. The threshold depends on the droplet size and laser polarization. A physical interpretation of the results is presented by considering the nonlocal orientations of the nematic liquid-crystal molecules in the droplets with the dimensions on the order of the focal spot diameter or larger. On the basis of the finite size approximation, we show that the dependence of threshold power on the droplet size is calculated to be in qualitative agreement with the experimental data.

Al plasma jet formation via ion stream compression by surrounding low-Z plasma envelope

In our earlier papers it was demonstrated that the plasma pressure decreases with the growing atomic number of the target material. In this context a question arose about the possibility to collimate the Al plasma outflow by using the plastic plasma as a compressor. For that purpose a plastic target with an Al cylindrical insert of 400 μm in diameter was used. The experiment was carried out at the PALS laser facility. The laser provided a 250 ps (FWHM) pulse with the energy of 130 J at the third harmonic frequency (λ3 = 0.438 μm). The focal spot diameters (ΦL) 800, 1000, and 1200 μm ensured predominance of the plastic plasma, its transversal extension being large enough for the effective Al plasma compression. To study the Al plasma stream propagation and its interaction with the plastic plasma, a 3-frame interferometric system and 4-frame x-ray camera were used. The information on distribution of electron temperature in the outflowing Al plasma was provided by x-ray spectroscopy. The experimental results reported in the paper are discussed by virtue of a simple theoretical analysis.

UV-laser Ablation of Fibre Reinforced Composites with Ns-Pulses

Within this work the ablation behaviour of both carbon and glass fibre reinforced epoxy resin was assessed when ablated by a nanosecond-pulsed laser source emitting radiation in the ultra-violet spectrum. The investigation focussed on the influences of pulse overlap, focus spot diameter and resulting fluence on process quality and machining time.Results showed that ns-pulsed UV-lasers are capable of machining both types of fibre reinforced composites, while achieving good quality surfaces without burn marks or otherwise heat-damaged areas.

Implementation of focal zooming on the Nike KrF laser

In direct drive inertial confinement laser fusion, a pellet containing D-T fuel is imploded by ablation arising from absorption of laser energy at its outer surface. For optimal coupling, the focal spot of the laser would continuously decrease to match the reduction in the pellet's diameter, thereby minimizing wasted energy. A krypton-fluoride laser (λ = 248 nm) that incorporates beam smoothing by induced spatial incoherence has the ability to produce a high quality focal profile whose diameter varies with time, a property known as focal zooming. A two-stage focal zoom has been demonstrated on the Nike laser at the Naval Research Laboratory. In the experiment, a 4.4 ns laser pulse was created in which the on-target focal spot diameter was 1.3 mm (full width at half maximum) for the first 2.4 ns and 0.28 mm for the final 2 ns. These two diameters appear in time-integrated focal plane equivalent images taken at several locations in the amplification chain. Eight of the zoomed output beams were overlapped on a 60 μm thick planar polystyrene target. Time resolved images of self-emission from the rear of the target show the separate shocks launched by the two corresponding laser focal diameters.

Special aspects of subwavelength focal spot measurement using near-field optical microscope

In this paper we numerically and experimentally investigated the influence of the hollow pyramidal shape NSOM cantilever to the measured characteristics of subwavelength focal spot. Using linearly polarized Gaussian beam with wavelength 633 nm and Fresnel zone plate with focal length 532 nm it was shown that hollow cantilever with angle 70° and nanoaperture 100 nm detects preferably the transverse component of the electric field. The focal length was equal to 0,36λ, the smallest focal spot diameter was (0,40±0,02)λ, the depth of focus was 0,59λ and the diffractive efficiency was 12%.

Elongated Photonic Nanojet from Truncated Cylindrical Zone Plate

Previously (Chen et al., 2004), it was shown that dielectric cylinder can form focal spots with small diameters and long depth. This type of focal spot was called photonic nanojet. In this paper, it was shown that dielectric cylinder of radius 595 nm (1.12 of wavelength) forms near the surface a photonic nanojet with diameter equal to 0.31 of wavelength and depth of focus equal to 0.57 of wavelength. Adding truncated concentric rings with radiuses equal to radiuses of zone plate to the cylinder increases the depth of focus to 1.18 of the wavelength. The diameter and intensity of focal spot near the cylinder surface remain unchanged.

Laser-produced aluminum plasma expansion inside a plastic plasma envelope

Previous experimental results demonstrated that the plasma pressure decreases with the growing atomic number of the target material. In this context, a question arose if the Al plasma outflow could be collimated using the plastic plasma as a compressor. To solve this problem, an experiment using a plastic target with an Al cylindrical insert was performed. The focal spot diameter substantially larger than that of the insert ensured simultaneous heating both target materials. This experiment proved that a production of Al plasma jets collimated by an action of outer plastic plasma is feasible [Kasperczuk et al., Laser Part. Beams 30, 1 (2012)]. The results of investigations presented here provide additional information on distributions of electron temperature in the outflowing plasma and time and space characteristics of ion emission, both registered at bare and constrained-flow Al targets. The experiment was carried out at the Prague asterix laser system iodine laser facility. The laser provided a 250 ps (full width at half maximum) pulse with the energy of 130 J at the third harmonic frequency (λ3 = 0.438 μm). A plastic target with an Al cylindrical insert of 400 μm in diameter as well as a bare Al target (for comparison) was used. The focal spot diameter (ΦL) 1200 μm ensured the lateral pressure effect of the plastic plasma strong enough to guarantee the effective Al plasma compression. The electron temperature measurements have shown that such Al plasma compression is accompanied by the increase of its temperature, dominance of which starts at distance of 0.5 mm from the target surface. Measurements of ion emission characteristics confirm the earlier numerical simulation prediction that in these conditions the plasma expansion geometry is closer to planar. The constrained Al plasma jet is very narrow and its axial velocity is considerably larger than the velocity of freely expanding Al plasma stream. It means that the plastic plasma envelope, besides the Al plasma compression, also strongly accelerates the Al plasma in its axial motion.

Development of microscopic magnetometer with reflective objective using magneto-optical Kerr effect

A microscopic magnetometer using magneto-optical Kerr effect (MOKE) was developed to characterize the magnetic properties of hard magnetic nano-structures such as a bit-patterned medium and a magnetoresistive random access memory. Our new type magnetometer has a unique feature that adopts the reflective objective instead of the generally used refractive lens to reduce the unnecessary rotation of polarization axis of the light by the lens with Verdet constant in a magnetic field. A Schwarzschild-type objective consisting of two spherical mirrors was applied as the reflective objective in our magnetometer. The objective was designed specifically for our magnetometer. An actual focusing spot diameter at the sample surface was estimated to be 4.7 μm by the knife-edge measurement. Furthermore, a magnetization curve was measured by MOKE for the Co80Pt20 thin-film line with a width of 75 μm, and demonstrated that our magnetometer can reduce the unnecessary rotation of polarization axis compared with the one measured by the magnetometer with a refractive lens.

Portable alignment device for an off-axis parabolic mirror optical axis adjustment

In this paper we demonstrate a new technique in which we use the portable alignment device of an off-axis parabolic mirror optical axis adjustment and the CCD camera to rapidly and accurately align the optical axis of an off-axis parabolic (OAP) mirror. In this method the optical axis of an OAP mirror is made parallel to the “five incident parallel laser beams” in the plane of incidence, by checking direction of these five reflected laser beams and changing the height and orientation of the OAP mirror. The CCD camera uses to measure the best focus and minimum spot diameter of the OAP mirror. This fast aligning method for finding the optical axis of an OAP mirror can measure the deviation of Slant Focal Length (SLF) to an accuracy of 0.3 %.

Plastic plasma as a compressor of aluminum plasma at the PALS experiment

AbstractIn our earlier papers, we demonstrated that plasma pressure decreases with the growing atomic number of the target material. That experimentally confirmed fact brought about a question whether it would be possible to collimate the Al plasma outflow by using plastic plasma as a compressor. To prove that idea we used in our next experiments a plastic target with an Al cylindrical insert of 400 µm in diameter. The measurements were carried out at the Prague Asterix Laser System iodine laser facility. The laser provided a 250 ps (full width at half maximum (FWHM)) pulse with energy of 130 J at the third harmonic frequency (λ3 = 0.438 µm). The focal spot diameters (ΦL) 800, 1000, and 1200 µm ensured predominance of the plastic plasma amount high enough for the effective Al plasma compression. To study the Al plasma stream propagation and its interaction with plastic plasma a three-frame interferometric system and an X-ray camera were used. The experiment provided a proof that creation of the collimated Al plasma jet by action of outer plastic plasma is feasible. In order to discuss of the experimental results a thorough theoretical analysis was carried out.

Focusing of longitudinal ultrasonic waves in air with an aperiodic flat lens

Modeling and experimental results of an ultrasonic aperiodic flat lens for use in air are presented. Predictive modeling of the lens is performed using a hybrid genetic-greedy algorithm constrained to a linear structure. The optimized design parameters are used to fabricate a lens. A method combining a fiber-disk arrangement and scanning laser vibrometer measurements is developed to characterize the acoustic field distribution generated by the lens. The focal spot size is determined to be 0.88 of the incident wavelength of 80-90 kHz at a distance of 2.5 mm from the lens. Theoretically computed field distributions, optimized frequency of operation, and spatial resolution focal length are compared with experimental measurements. The differences between experimental measurements and the theoretical computations are analyzed. The theoretical calculation of the focal spot diameter is 1.7 mm which is 48% of the experimental measurement at a frequency of 80-90 kHz. This work illustrates the capabilities of a hybrid algorithm approach to design of flat acoustic lenses to operate in air with a resolution of greater than the incident wavelength and the challenges of characterizing acoustic field distribution in air.

The Talbot effect of plasmonic nanolenses

The Talbot effect of an Ag nanolens with five periodic concentric rings that are illuminated by the radially polarized light was numerically studied by means of rigorous finite-difference and time-domain (FDTD) algorithm. It was found that the Talbot effect occurs only when the incident wavelength is at the scale of less than half of period of the grating structures of the nanolenses. Specifically, in this work, the nanolenses with a 500 nm period grating structures has five focal points due to Talbot effect for the incident wavelength of λ = 248 nm. The diameter of the first focal spot after the exit plane in free space is 100 nm. In contrast, we analyzed the corresponding focal points on the basis of Talbot self-imaging by scalar diffraction theory. It was found that the scalar Talbot effect cannot interpret the Talbot effect phenomenon for the metallic nanolenses. It may attribute to the paraxial approximation applied in the Talbot effect theory in far-field region. However, the approximation does not hold in our nanolenses structures during the light propagation. In addition, the Talbot effect appears at the short-wavelength regime only, especially in the ultraviolet wavelength region.