Ｎｄ：ＹＡＧレーザ２ビーム合成による溶込み深さの増大

Two Nd:YAG laser beams were condensed separately and combined at surface of workpiece in order to increase weld penetration depth. Welding characteristics were evaluated by penetration depth and penetration shape in bead-on-plate tests. Transition from penetration with one bottom to penetration with two bottoms occurs as function of welding condition. Penetration with two bottoms might be a sufficient evidence in favor of keyhole bottom separation. A close study of keyhole bottom separation is necessary to understand the increase of weld penetration by multiple laser beams combined. This paper is intended as an investigation on factors of keyhole bottom separation. The authors found that factors of keyhole bottom separation are angle of incidence of laser beams, laser power density, heat input, distance between two laser beams on a surface of workpiece, and welding position. Keyhole bottom separation occurs under conditions determined by high power density of one laser beam and high total heat input, such as 1500 W/mm2 and 104J/cm, respectively, at angle of incidence of 30° in welding using two CW laser beams in combination.

Aluminum alloy is considered to be one of the ideal materials for light-weighting on aerospace and new energy automotive. Laser welding without a filler wire is an efficient way for weight reduction. The common defects in laser welding of aluminum alloys are porosities and cracks, resulting in poor mechanical performances and short service life of joints. In order to investigate the methods to suppress defects, 1 mm thick automotive aluminum alloy AA6014 was performed for single and dual-beam laser lap welding in this study. The effects of two welding processes on microstructure, cracks, and mechanical properties were investigated. The results showed that the welding adaptability of dual laser beam lap welding was much better than single laser beam lap welding. Well-formed weld could be obtained in a suitable line energy range by both welding processes. The dual beam with an energy ratio of 1:1 was at an angle along the welding direction, which could increase the molten pool area and stir the weld pool more sufficiently. Thus, dual laser beam lap welding could significantly increase the welding speed and alleviate collapse caused by single laser beam welding. The gaps of lap welding tended to be crack sources and generate cracks, while dual-beam laser welding was advantageous for reducing such cracks. This research is of great significance to improve the laser welding quality of all-aluminum automotive bodies, increase production efficiency, and further reduce the weight of the automotive.

Noise-free phase conjugation of high power single mode laser beams by stimulated Brillouin scattering

This paper describes the development work on single and multi beam laser grooving technology for 40nm node low-k/ULK semiconductor device. A Nd:YAG ultraviolet (UV) laser diode operating at a wavelength of 355 nm was used in this study. The effects of single and multi beam laser micromachining parameters, i.e. laser power, laser frequency, feed speed, and defocus amount were investigated. The laser processed die samples were thoroughly inspected and characterized. This includes the die edge and die sidewall grooving quality, the grooving shape/profile and the laser grooving depth analysis. Die strength is important and critical. Die damage from thermal and ablation caused by the laser around the die peripheral weakens the mechanical strength within the die, causing a reduction in die strength. The strength of a laser grooved die was improved by optimizing the laser process parameter. High power optical microscopy, Scanning Electron Microscopy (SEM), and focused ion beam (FIB) were the inspection tools/methods used in this study. Package reliability and stressing were carried out to confirm the robustness of the multi beam laser grooving process parameter and condition in a mass production environment. The dicing defects caused by the laser were validated by failure analysis. The advantages and limitations of conventional single beam compared to multi beam laser grooving process were also discussed. It was concluded that, multi beam laser grooving is possibly one of the best solutions to consider for dicing quality and throughput improvements for low-k/ULK wafer dicing. The multi beam laser process is a feasible, efficient, and cost effective process compared to the conventional single beam laser ablation process.

Nano-enhanced optical field of gold nanoparticles allowed the use of a continuous wave (cw) laser beam for efficient delivery of exogenous impermeable materials into targeted cells. Using this Nano-enhanced Optical Delivery (NOD) method, we show that large molecules could be delivered with low power cw laser with exposure time ~ 1sec. At such low power (and exposure), the non-targeted cells (not bound to gold nanoparticles) were not adversely affected by the laser beam. Further, by varying the size of the gold nanoparticles, cells could be exclusively sensitized to selective wavelengths of laser beam. In contrast other nanoparticles, gold nanoparticles were found to have lower cytotoxicity, making it better suited for clinical NOD. Further, as compared with pulsed lasers, cw (diode) lasers are compact, easy-to-use and therefore, NOD using cw laser beam has significant translational potential for delivery of impermeable bio-molecules to tissues in different organs. We will present optimization of NOD parameters for delivering different molecules to different cells. Success of this NOD method may lead to a new clinical approach for treating AMD and RP patients with geographic atrophy in retina.

A single femtosecond laser beam can be focused to process material surfaces. However, the low processing efficiency by a single laser beam prevents broad applications of femtosecond laser surface processing. Here we have demonstrated an array of high-intensity femtosecond laser beams that self-focus in a piece of glass medium, enabling high-efficiency, millimeter scale parallel laser surface processing. Experimental and simulation results suggest that the laser fluence of each focused laser beamlet in the array is more than 1.2 J/cm2, which is sufficient for micro- or nano-structure laser processing. To prove the concept, nanoscale structures on silicon surface are generated by the self-focused laser beam array, which may improve the efficiency of black silicon fabrication for photovoltaic devices.

As usual cylindrical hohlraum with double laser ring cones may lead to serious laser-plasma interaction, such as the simulated Raman scatter and cross-beam energy transfer effect, spherical hohlraum with octahedral 6 Laser Entrance Holes (LEHs) and single cone laser beams, was investigated and reported to have a consistent high radiation symmetry during the whole implosion process. However, it has several potential challenges such as the smaller space left for diagnosis and the assembly of centrally located capsule. In this paper, based on the view-factor model, we investigate the radiation symmetry and the drive temperature on the capsule located in the spherical hohlraum with tetrahedral 4 LEHs and single cone laser beams, since there is more available space for laser disposition and diagnosis. Then, such target is optimized on the laser beam pointing direction to achieve a high radiation performance, i.e., the radiation symmetry and drive temperature on the capsule. Finally, an optimal spherical hohlraum with optimal laser beam pointing has been demonstrated and compared with the spherical hohlraum with octahedral 6 LEHs. The resulting radiation symmetry and the drive temperature shows that it has almost a similar radiation symmetry (the radiation asymmetry variation is no more than 0.2%), and higher drive temperature (the temperature has been increased by 1.73%, and an additional 133 kJ energy of 2 MJ energy for fusion can be saved).

Using a pulsed YAG Laser, meltability of Al-Mg and Al-Mg-Si alloys were investigated by a bead-on-plate method. In order to improve the quality in laser beam welding, the effectiveness of Ultrasonic Vibration Laser Beam Welding (ULBW) method proposed in this paper was investigated experimentally. And, this proposed method was compared with traditional welding method of Normal Laser Beam Welding (NLBW) and Preheating Laser Beam Welding (HLBW). These welding methods were evaluated from the geometry in fusion zone generated by a single laser beam. It was suggested as a fact that ultrasonic vibration was useful for welding defects and fusion characterstics with cavitation effect and dispersion action of particles in the molten pool during laser beam welding. Moreover, the effects of active stirring in the molten pool by preheating were investigated also.In these experiments, ULBW was the most useful laser welding method from a view-point to improve the laser welding-quality in Al alloys.Using a pulsed YAG Laser, meltability of Al-Mg and Al-Mg-Si alloys were investigated by a bead-on-plate method. In order to improve the quality in laser beam welding, the effectiveness of Ultrasonic Vibration Laser Beam Welding (ULBW) method proposed in this paper was investigated experimentally. And, this proposed method was compared with traditional welding method of Normal Laser Beam Welding (NLBW) and Preheating Laser Beam Welding (HLBW). These welding methods were evaluated from the geometry in fusion zone generated by a single laser beam. It was suggested as a fact that ultrasonic vibration was useful for welding defects and fusion characterstics with cavitation effect and dispersion action of particles in the molten pool during laser beam welding. Moreover, the effects of active stirring in the molten pool by preheating were investigated also.In these experiments, ULBW was the most useful laser welding method from a view-point to improve the laser welding-quality in Al alloys.

This paper focuses on the design and development of the laser injection system for the ITER Edge Thomson Scattering system (ETS). The ITER ETS achieves a temporal resolution of 100 Hz by firing two 50 Hz laser beams alternatively. The use of dual lasers enables us to perform the Thomson scattering measurements at a temporal resolution of 50 Hz in case that one of the laser systems stops functioning. A new type of beam combiner was developed to obtain a single beam that is collinear and fixed linearly polarized from two laser beams using a motor-driven rotating half-wave plate. The rotating half-wave plate method does not induce misalignment even if the rotating mechanism malfunctions. The combined beam is relayed from the diagnostic hall to the plasma using mirror optics and is absorbed at the beam dump integrated on the inner blanket. The beam alignment system was designed to direct the laser beam onto the center of the beam dump head. The beam position at the beam dump is monitored by four alignment laser beams which propagate parallel to the diagnostic Nd:YAG laser beam and imaging systems installed outside the diagnostic port.

In the field of heavy industries, many products are made of thick metal parts. Nd:YAG laser has been recently developed up to 10 kW. Nd:YAG laser has the characteristics of the optical fiber transmittance and the good absorption by the metal surface, so that it is expected to apply Nd:YAG laser to thick plate welding. This study presents the thick plate welding with Nd:YAG laser and COIL (Chemical Oxygen-Iodine Laser). We have developed a coaxial beam combining system with beams of Nd:YAG laser and COIL. The maximum average power of the combined beam was 19 kW. Welding tests of 304 stainless steel plates were carried out. The effects of various welding parameters were investigated, such as the laser power and pulse modulation. As a result, it was clarified that the pulse wave has good efficiency of deeper penetration as compared to continuous wave at low welding speed. When the combined beam was used, 20 mm penetration depth on the stainless steel could be obtained in high aspect ratio at welding speed of 1m/min. When the combined beams and another Nd:YAG laser beam whose power was 4 kW were used, both side welding on 30 mm thickness plate could be achieved.

In present paper we investigate the transverse dynamics of frequency-modulated cw laser beam propagated in resonance conditions. At modulation periods comparable with the atomic relaxation times the time dependence of the output intensity exhibits the manifestations of resonance sel f-action effects. The influence of resonance self-induced lens and aperture on cw laser beam transverse profile and the boundary conditions of self-action are analyzed by numerical simulation. Keywords: resonance self-action, cw laser beam propagation, numerical simulation, frequency modulation, propagation length, transverse profile of laser beam, Gauss-Laguerre modes. 1. INTRODUCTION Self-action of cw laser beams due to saturated refraction in resonance media has been studied for a long time (see, e.g. [1-4]), most of investigations was related to observation of lensing properties of the media. Under extremely high absorption and saturation conditions the manifestations of self-induced diffraction have been found to mimic self-focusing and self-channelization at zero frequency detuning when the optical strength of the saturation-induced lens is zero [2-4]. From the theoretical point of view it means that the medium response can be described by means of the nonlinear susceptibility. Obviously, the same approach is valid for time-dependent field whose variation is slow compared with atomic relaxation. Faster field changes are expected to cause transient response of the medium and, therefore, transient self-focusing and self-aperturing effects. Experimental manifestations of non-stationary near-resonant self-focusing of frequency-modulated beams have been reported in saturation spectroscopy [5]. The present paper is a continuation of our previous work [6], where we inve stigate the beams initially modulated in frequency. Passing through resonant absorbing medium, the beam gradually acquires intensity modulation, which is simply caused by different absorption at different frequencies. This effect is neither nonlinear, nor transient, it takes place even at low intensity and low frequency of modulation. Note, that in this case it is not affected by the lensing properties of the medium, since they are due to saturation and not to frequency-dependent refraction as such. First, we study how the saturation of absorption and refraction affects the transmission of modulation from phase at the input to the intensity at the output. We take the frequency of modulation low enough to consider the saturation as adiabatically following the field variation in time. The most prominent manifestations of the induced lens can be seen in moderate saturation regime. In this regime we focus our attention on the resonant self-action dynamics and boundary conditions and on the self-action influence on laser beam transverse profile.

It is well known that arc welding of thin metallic sheets in difficult due to their easier distortion and deformation. On the other hand, in laser welding, such distortion and deformation is greatly reduced due to a smaller spot size and smaller heat input than in arc welding. Even in laser welding, thinner metallic sheets become more difficult to weld due to the loss of materials and the difficulty in supporting a shallow molten pool against evaporation and recoil pressure, leading to the formation of cut or drilled welds. In this study, therefore, lap welding characteristics of thin metallic sheets were investigated by utilizing combined beams or respective single beam of normal pulsed YAG laser and Q-switched SHG YAG laser. It was consequently found that the formation conditions for good lap welds in thin stainless steel sheets were broadened by using combined laser beams. Especially, lap welding of pure copper sheets was very difficult with single pulsed YAG laser; however, it was confirmed to be possible to weld them with combined laser beams of two different wavelengths in the air environment.

We present a compact optoacoustic laser Doppler velocimetry method that utilizes the self-mixing effect in a RF-excited CO(2) laser. A portion of a Doppler-shifted laser beam, produced by irradiating a single wavelength laser beam on a moving object, is mixed with an originally existing laser beam inside a laser cavity. The fine change of pressure in the laser cavity modulated by the Doppler-shifted frequency is detected by a condenser microphone in the laser tube. In our studies, the frequency of the Doppler signal due to the optoacoustic effect was detected as high as 50 kHz. Our measurements also confirmed that the signal varied linearly with the velocity of the external scatterer (the moving object) and the cosine of the angle between the laser beam and the velocity vector of the object.

High precision patterning of ITO using femtosecond laser annealing process

In this study, fast welding of an aluminum alloy AlSi12 and a structural steel S355 was investigated using a single mode fiber laser system delivering a maximum laser power of 10 kW with a laser beam quality of M2 = 2. The laser beam was focused with a welding head OL YW50 F500 HQ from Precitec GmbH & Co. KG resulting in a spot size of 70 μm, thus delivering a maximum intensity up to several 108 W/cm2 to the workpiece surface. Therefore, narrow weld seams and large penetration depths could be achieved. However, detrimental effects like humping, spiking, spatter tendency, pore formation, and hot cracking, reported by Li et al. [J. Mater. Process. Technol. 214, 565–570 (2014)] and Zhang et al. [Optic Laser Technol. 98, 97–105 (2018)], have been observed at some parameter settings. The welding experiments performed on AlSi12 with a laser beam power of 10 kW, focus position on the material surface, and welding speeds from 2 to 10 m/min reveal penetration depths of about 12 mm independently from the distinct welding speed. Therefore, the weld bead width was 5.5 mm for 2 m/min, thus decreasing to 1.1 mm at the highest welding speed of 10 m/min. By shifting the focus position inside the material, the penetration depth rises significantly and reached a maximum of 17.5 mm for a welding speed of 2.5 m/min and a focus position of 6 mm underneath the sample surface. For this, the weld cross section width, measured at half of the penetration depth, was 2.2 mm. For the structural steel S355, the maximum penetration depth was determined to be 19.5 mm, here with 10 kW laser power, focus position on the material surface, and a comparable welding speed of 2.4 m/min. Therefore, a weld bead width of about 0.75 mm was measured, while the weld cross section width was about 0.55 mm. Finally, first butt welds of 19.5 mm thick S355 samples were generated characterized by smooth welds with a bead width of about 2.5 mm and a weld cross section width of 0.5 mm, respectively.