Isamu Miyamoto

Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses

Thermal conduction model is presented, by which nonlinear absorptivity of ultrashort laser pulses in internal modification of bulk glass is simulated. The simulated nonlinear absorptivity agrees with experimental values with maximum uncertainty of ± 3% in a wide range of laser parameters at 10 ps pulse duration in borosilicate glass. The nonlinear absorptivity increases with increasing energy and repetition rate of the laser pulse, reaching as high as 90%. The increase in the average absorbed laser power is accompanied by the extension of the laser-absorption region toward the laser source. Transient thermal conduction model for three-dimensional heat source shows that laser energy is absorbed by avalanche ionization seeded by thermally excited free-electrons at locations apart from the focus at pulse repetition rates higher than 100 kHz.

Optimization of laser-induced forward transfer process of metal thin films

The optimization of laser-induced forward transfer (LIFT) process using laser ablation of thin film and the evaluation of the dependence in laser spot size on the resolution of deposited material are reported. Ni thin film of several hundred of nanometer thickness, which is deposited on fused silica substrate, was irradiated by single pulse of KrF excimer laser (wavelength: 248 nm, pulse width: 30 ns), and transferred to a Si acceptor substrate. Images of the plume, which were photographed using image intensified CCD camera, indicated the cause of transfer of particles. The deposited material with clear contour was obtained under the condition that thin film was in contact with an acceptor substrate to inhibit the transfer of particles which were generated by laser irradiation. The resolution of a deposited material tended to be improved as the size of laser spot became smaller.

Sample preparation method for glass welding by ultrashort laser pulses yields higher seam strength

Glass welding by ultrashort laser pulses allows joining without the need of an absorber or a preheating and postheating process. However, cracks generated during the welding process substantially impair the joining strength of the welding seams. In this paper a sample preparation method is described that prevents the formation of cracks. The measured joining strength of samples prepared by this method is substantially higher than previously reported values.

Characteristics of laser absorption and welding in FOTURAN glass by ultrashort laser pulses

The nonlinear absorptivity of FOTURAN glass to ultrashort laser pulses is evaluated by experimental measurement and thermal conduction model at different parameters including energy and repetition rate of the laser pulse, translation speed and thermal properties of the sample. The mechanical strength of an embedded laser-melted sample and an overlapped weld sample is determined by a three-point-bending test and a shear test, respectively. The results are related to the average absorbed laser power Wab. We found the mechanical strength of an overlapped weld joint to be as high as that of the base material for low Wab, if the sample pair is pre-bonded to provide optical contact.

Femtosecond laser quenching of the ε phase of iron

The quenching of the e phase of iron, which has not been observed under a conventional shock compression, was attained using a femtosecond laser. The crystalline structure in a recovered iron sample was determined using an electron backscatter diffraction pattern system. The femtosecond laser driven shock may have the potential to quench high-pressure phases of other materials.

Characterizing keyhole plasma light emission and plasma plume scattering for monitoring 20 kW class CO2 laser welding processes

In very deep penetration, the plasma inside the keyhole exhibits complicated behavior and the plasma plume covering outside the keyhole can also affect the observation of keyhole plasma. Measuring the light emission from keyhole plasma is an effective method to monitor laser welding processes in thin sheets; however, in welding thick sheets, several critical phenomena must be observed. In this article, we provide experimental investigations using a 20 kW CO2 laser to characterize the wavelength contents of the keyhole plasma light emission. We also demonstrate that the plasma plume outside the keyhole can scatter the light emission from the keyhole plasma significantly. A scattering model is proposed and verified experimentally. The particles responsible for scattering are also identified.

Crack-free conditions in welding of glass by ultrashort laser pulse

The spatial distribution of the laser energy absorbed by nonlinear absorption process in bulk glass w(z) is determined and thermal cycles due to the successive ultrashort laser pulse (USLP) is simulated using w(z) based on the transient thermal conduction model. The thermal stress produced in internal melting of bulk glass by USLP is qualitatively analyzed based on a simple thermal stress model, and crack-free conditions are studied in glass having large coefficient of thermal expansion. In heating process, cracks are prevented when the laser pulse impinges into glass with temperatures higher than the softening temperature of glass. In cooling process, shrinkage stress is suppressed to prevent cracks, because the embedded molten pool produced by nonlinear absorption process behaves like an elastic body under the compressive stress field unlike the case of CW-laser welding where the molten pool having a free surface produced by linear absorption process is plastically deformed under the compressive stress field.

Development of in-process monitoring system for laser welding

A novel in-process monitoring system employing two detectors set above the work at different aiming angles of 5 and 75 degrees was developed to detect whether or not the CO2 laser welding is of full penetration through the back surface of the steel sheets. The acquired signal involved AC components with frequencies up to approximately 10kHz of the emission of the laser-induced plasma in the plume and in the keyhole. The mean square value of the AC signal obtained by using 75-degree sensor in the full-penetration welding was much larger than that of the partial the full-penetration welding, showing that full penetration welding can be monitored with high accuracy by using 75-degree sensor.A novel in-process monitoring system employing two detectors set above the work at different aiming angles of 5 and 75 degrees was developed to detect whether or not the CO2 laser welding is of full penetration through the back surface of the steel sheets. The acquired signal involved AC components with frequencies up to approximately 10kHz of the emission of the laser-induced plasma in the plume and in the keyhole. The mean square value of the AC signal obtained by using 75-degree sensor in the full-penetration welding was much larger than that of the partial the full-penetration welding, showing that full penetration welding can be monitored with high accuracy by using 75-degree sensor.

Photochemical process of divalent germanium responsible for photorefractive index change in GeO2-SiO2 glasses.

The photoluminescence spectra of the divalent Ge (Ge2+) center in GeO2-SiO2 glasses with different photosensitivities were investigated by means of excitation-emission energy mapping. The ultraviolet light induced photorefractivity has been correlated with the local structure around the Ge2+ centers. The glasses with a larger photorefractivity tended to exhibit a greater band broadening of the singlet-singlet transition on the higher excitation energy side accompanied by an increase in the Stokes shifts. This strongly suggests the existence of highly photosensitive Ge2+ centers with higher excitation energies. It is also found that the introduction of a hydroxyl group or boron species in GeO2-SiO2 glasses under appropriate conditions modifies the local environment of Ge2+ leading to an enhanced photorefractivity.

Beam Absorption Mechanism In Laser Welding

Mechanism of laser beam absorption was analyzed in key hole welding of thin plate and deep penetration welding of thick plate on the basis of the two-dimensional cavity model where beam is absorbed by cavity wall and ionized metal vapor. The beam absorptivity of the cavity wall A and the absorption coefficient of the plasma u are approximately 50 % and 1- 1.5 cm-1, respectively. In key hole welding of thin plate, laser beam is absorbed predominantly by the liquid cavity wall and the cavity shape is self-controlled so as to absorb the energy required for bead formation. In deep penetration welding of thick plate, laser power is absorbed mainly by plasma where the self-controlling is less effective, and hence excess energy absorbed at low welding speeds is wsted to widen the bead with little increase in penetration depth. In vacuum of 10-3-10-4 torr, u decreases down to approximately 0.5 cm-1, providing large penetration depth.