Takayuki Tamaki

Direct Welding between Copper and Glass Substrates with Femtosecond Laser Pulses

We demonstrate direct microwelding between glass and copper substrates by use of femtosecond (fs) pulses. The joint strength is as high as >16 MPa. A scanning electron microscopy cross-sectional image of the sample proves successful joining without voids or cracks. Furthermore, we show that, compared with ns-pulses, the use of fs-pulses can reduce the pulse energy required for welding by two orders of magnitude, leading to the suppressed effect of heat and to the precise control of the welded region.

Ultrashort Laser Welding and Joining

In this chapter, the most recent results and trends in ultrashort laser welding and joining are reported. The review will cover the possibility of joining transparent slabs made of equal or different glasses, as well as welding between glass and silicon. The role of the laser repetition rate will also be discussed.

Ultrafast laser microwelding for transparent and heterogeneous materials

When ultrafast laser pulses are focused inside the interface between a couple of transparent materials, the optical intensity in the focal area can usually become high enough to initiate the filamentary propagation of optical pulses and almost simultaneously the nonlinear absorption occurs in the filamentary area. Due to this absorption of optical energy, both of the materials can locally be melted and the interface is joined after resolidification. The laser microwelding technique based on the nonlinear phenomena has several unique features: (i) the insertion of any intermediate layers is no need for the microwelding, (ii) its possible to weld the materials with different thermal expansion coefficients, (iii) the joint area can also be arbitrarily extended by scanning the filamentary area. We call this powerful technique ultrafast laser microwelding. In this paper, we present the results on ultrafast laser microwelding for transparent materials such as the silica and borosilicate glass, and heterogeneous materials such as glass and metals.

Filamentation in Ultrafast Laser Material Processing

When an ultrashort laser pulse is focused inside the bulk of a transparent material, filamentation occurs as a result of the dynamic balance between the Kerr self-focusing and defocusing effects in the electron plasma, which is generated through the ionization process. The optical intensity in the filamentary volume can become high enough to induce permanent structural modifications. In this chapter, we review the applications of filamentation in the microprocessing of transparent materials and in microwelding.

Dendrite-joining of air-gap-separated PMMA substrates using ultrashort laser pulses

We demonstrate joining polymethyl methacrylate (PMMA) substrates by a dendrite pattern of a quenched melt using ultrashort laser pulses. Laser pulses from a 250-fs fiber laser at a repetition rate of 1 MHz were focused at the interface of the two PMMA substrates with an air gap of approximately 14 μm and direct laser joining was accomplished between two pieces of PMMA. Melted PMMA from the laser-irradiated region spread within a gap between the substrates and dendrite morphology of the melt spread outside the direct laser irradiated area of square spiral contour and increased the joining strength. The joint strength was 11 MPa for tensile and 21 MPa for the shear stress. Ultrashort laser pulses are useful to directly join PMMA substrates using localized melting and resolidification with a gap between the substrates.

Femtosecond laser micromachining and biological therapy

Nonlinear micromachining with femtosecond laser pulses has attracted much attention in microphotonics and biophotonics. In this paper, we demonstrate the fabrication of optical elements in bulk transparent materials, direct welding between transparent substrates, and subcellular ablation in living cells by focusing femtosecond laser pulses.

Filamentation in laser microprocessing and microwelding

Focused femtosecond laser pulse inside the bulk transparent materials leads to filamentary propagation. The optical intensity in the filamentary volume can become high enough to induce permanent structural modifications. The induction of filamentary structural modifications is a versatile technique for the fabrication of three-dimensional photonic devices in transparent materials. In this paper, we present applications of filamentation in microprocessing by demonstrating the fabrication of waveguide devices and diffractive optical elements in transparent materials. Filamentation is applied to micro welding of transparent substrates.

Phase measurement of structural modifications created by femtosecond laser pulses in glass with phase-shifting digital holographic microscopy

Abstract. When ultrashort laser pulses are focused inside a glass at a high repetition rate, structural changes occur because of the heat accumulation from the train of laser pulses. This report describes phase measurement of the structural changes induced inside glass by phase-shifting digital holographic microscopy. Two-dimensional phase distribution across the structural change for static exposure is retrieved. By focusing femtosecond laser pulses at the interface between two glass substrates, melted materials can directly weld glass plates. Quantitative phase measurement of welded glass substrates revealed that the refractive index decreased in the laser-irradiated zone.

Heat transfer analysis of two wavelengths laser microprocessing inside glass

We investigated the relationship between size of melting marks formed inside glass and irradiation time and absorptivity of femtosecond laser beam. For this investigation the absorptivities for static exposure in fs-laser processing (femtosecond laser microprocessing inside glass) was estimated to approximately 4.5 [%]. Also the size of melting marks formed by fs-laser processing was measured with two irradiation times (1/125 [s] and 1/4 [s]). The sizes were much the same. Thus, in this time scale, the size was nearly independent of the irradiation time. Furthermore, luminescence phenomenon that occurred in fs-laser processing was observed. The duration of this luminescence was less than 2/1000 [s]. With the above experimental results, we demonstrated numerical heat transfer analysis during the fs-laser processing. From the experimental and numerical results it thought that the most process in fs-laser processing finishes within 2/1000 [s].

Effects of report-writing on a multi-stage-experience educational program using an autonomous mobile robot

System design and development are fundamental capabilities for an engineer. The Department of Control Engineering at the National Institute of Technology, Nara College, has been offering a course called “Practical System Design,” a unique opportunity for students to experience the entire process of developing an autonomous mobile robot, to fourth-year students for about 20 years. However, in recent years, robot design and construction have become a huge challenge for these students. To address this problem, we have worked on improving the engineering program. First, we have introduced a problem-based learning (PBL) education program in all experimental engineering classes in years 1-3. Also, we require students to submit a written report after each class session. This report is intended to help improve the students schedule management, role-sharing, and work-tracking skills. These reports are reviewed by the relevant course teachers and assessed for logic and quantitative representation. In this paper, we discuss the approach to, achievements of, and future subjects to be taken up by this multi-stage-experience educational program, especially the effect of report-writing.