We describe three specific high power laser systems that are being developed in our laboratory for many applications in high field science, nonlinear optics and material processing. We report on a femtosecond petawatt-class Ti:sapphire chirped-pulse amplification laser system that can produce a pulse energy of 20 J of 40 fs pulse duration, a picosecond high intensity Yb:YAG chirped-pulse amplification laser system that can generate a pulse energy of 100 mJ of 0.5 ps pulse duration, and a nanosecond high repetition rate Nd:YAG laser system that can provide an average power of 360 W with a pulse duration of 30 ns delivered at a 1 kHz repetition rate. We discuss the basic design aspects and present the results from our experimental investigations of these laser systems.

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We observed dynamics of laser ablation in liquids through two high-speed imaging techniques, highspeed pump-and-probe photography and high-speed laser stroboscopic videography, and briefly described both imaging techniques. The target materials are epoxy-resin blocks immersed in a liquid environment, and the images were taken through circular polariscope optics in a bright-field mode. We simultaneously observed both phenomena in gas or liquid phases as well as inside a solid and visually demonstrated the confi ning effects of liquid on laser-induced shock processes. We also described the effects of liquid-layer thickness on the laser-induced shock process and studied the dynamical behavior of laser-induced cavitation bubbles through high-speed videography. The dynamics of a cavitation bubble was recorded successfully with 4-μs time resolution and its growth and shrinkage were analyzed. Secondary shocks are generated when the fi rst bubble collapsed and the magnitude of the shock inside the solid was approximately one order smaller than that of the initial laser-induced shock.

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We have succeeded in quenching metastable high-pressure phases which are not quenchable using conventional compression methods and forming a high density of lattice defects in solids using femtosecond laser-driven shock wave. In-situ X-ray diffraction (XRD) under femtosecond laser-driven shock compression of solids using X-ray Free Electron Laser (XFEL) is one of the most powerful tools to directly capture the behaviour of lattice planes and lattice defects to make clear the mechanism. In this report we review the femtosecond laser-driven shock compression of solids and make brief introduction of experiments performed at XFEL facility in Japan “SACLA.”

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Laser Peening is expected to improve the high cycle fatigue property of metal materials. The fatigue property is improved by competition between positive peening effects and the negative ones that are caused by laser peening. The major positive peening effects include an increase in the hardness of the material surface and compressive residual stress near the surface layer. On the other hand, the primary negative peening effect is an increase in the surface roughness. This paper introduces examples of these peening effects and fatigue property improvement by laser peening under such major fatigue loading conditions as axial loading, rotating bending, and plane bending.

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[in Japanese]

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We found some damage at the forks of low-pressure turbine blades caused by random vibration and steam-flashback vibration at nuclear power plant. In this study, to improve fatigue property, we developed and examined laser peening technology for 12Cr stainless steel and material properties. Laser peening induces compressive residual stress on material surfaces. We fabricated fatigue specimens that simulate the stress concentration zone of the forks and applied laser peening to the specimen surfaces. The residual stress was measured by X-ray diffraction and compressive residual stress was formed on the peened surface. From the fatigue test results, the fatigue strength of the laser peened specimens improved about 40% more than the unpeened specimens.

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Duplex stainless steel (DSS) provides excellent mechanical strength and corrosion resistance and its application has been expanding to diverse industrial fi elds where strong corrosion resistance and mechanical strength are simultaneously desired. For the enhancement of surface hardness and wear resistance of DSS, various surface treatment techniques have been adopted previously, which however produced mostly minor improvement of corrosion resistance or occasional degradation of corrosion behavior. In this study, the results of laser shock peening of DSS are presented in comparison with those of previously attempted methods. It was demonstrated that the surface hardness, wear resistance, and corrosion resistance of DSS can be simultaneously increased by properly applied laser shock peening process. The maximum surface hardness enhancement of 29%, wear volume decrease of 66%, and corrosion rate decrease of 74% were achieved by water immersion type laser shock peening and similar results were also obtained by nozzle type laser shock peening.

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Many fatigue cracks from welded joints have been detected in steel bridges. Among various methods to improve the fatigue strength of welded zones, this paper focuses on laser peening, which introduces compressive residual stresses on surfaces because they effectively enhance the fatigue lives of welded components. Tensile residual stress due to welding is one of the most prominent factors to reduce fatigue lives. Even though structural steels are widely used for bridges and buildings, the effects of laser peening on them remains unestablished, especially on the welded zones. In this paper, we review the results of recent studies related to the effects of laser peening on the welded zones of structural steel.

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We have demonstrated an optical parametric chirped pulse amplifi cation (OPCPA)/Yb:YAG ceramic thin disk hybrid laser system having the hundred mJ pulse energy level with sub-picosecond duration and high temporal contrast. At an input chirped-pulse energy of 3.8 mJ from an OPCPA preamplifer an output energy of 130 mJ has been generated from a multipass laser-diode (LD) pumped Yb:YAG ceramic thin disk amplifi er. A recompressed pulse duration of 450 fs with a contrast level of less than 7.2 × 10^‒9 at 150 ps before the main pulse has been obtained. The contrast level was the highest value achieved in a Yb:YAG chirped pulse amplifi cation (CPA) laser system at the hundred mJ level. This laser system is well-suited to studying laser matter interactions in the laser peening mechanism.

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In order to analyze high strain produced by laser peening during tens of ns, which can’t be performed in so short process, the surface profi le of after laser peening with square spots was compared with that with circle spots, steep sidestep showed high stain and deformation produced in laser peening with square spot. The microstructure transforming in laser peening titanium alloy Ti-6Al-4V in the center of shocked zone and at the edge of square spot was researched in this paper. The results showed that nanometers size crystals were formed at the edge of square spots because of the shearing strain produced during laser peening of titanium alloy Ti-6Al-4V.

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Mechanical surface treatments such as laser-shock peening or deep rolling are known to induce highly strain-hardened nanoscale microstructures as well as deep compressive residual stress layers into surface regions of fan blade titanium alloys such as Ti-6Al-4V.^1,2） The benefit of such treatments is well documented for cyclic loading at room temperature. In addition, it appears that the induced microstructures are stable enough to provide significant lifetime and strength improvements even at temperatures up to 550 ℃ which is far above the operating temperature of such components. This study seeks to provide basic understanding for the strengthening mechanisms of deep rolled/lasershock peened and high temperature fatigued Ti-6Al-4V under stress-control. For this purpose, the stability of near-surface microstructures is investigated by TEM (transmission electron microscopy) after step-wise thermal exposure by in situ heating and after isothermal stress-controlled fatigue up to 550 ℃. Furthermore, for structural assessment of the softening or hardening behaviour, the plastic strain amplitude is recorded as a function of temperature and number of cycles. The resulting stress-life behaviour clearly indicates that laser-shock peening and deep rolling are both extremely efficient methods for fatigue strength enhancement of solution treated and overaged Ti-6Al-4V, even at elevated temperatures up to 550 ℃, despite a very pronounced relaxation of compressive residual stresses.

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Laser shot peening is increasingly used as one of the better alternative methods to improve the surface properties of structural components and for the in-suit preventive maintenance of nuclear reactor componets. This paper attempts to investigate the effect of laser peening with a low energy laser (300 mJ, 1064 nm, 10 ns) on precipitation hardened aluminum alloy Al-6061-T6 with and without sacrifi cial coating on the target surface. The laser peened and unpeened surfaces were characterized using XRD based surface residual stress measurement, SEM, AFM based roughness analysis and micro hardness measurements. The study concluded that low energy laser is enough to impart compressive residual stress on the surface. Micro hardness study conformed the hardening of laser peened surfaces and depth profi le study indicate the presence of strain hardened layer up to 0.7 mm from the top surface.

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This paper describes an oil identifi cation system that was developed to verify food safety and measures. We studied the absorption property of oils by spectrometer. The principle component analysis of the absorption spectrum of each oil identifi ed it if we used fi ve adjacent wavelengths at 1750, 1727, 1713, 1685 and 1657 nm. Our experimental prototype system consisted of fi ve laser diodes (LD) as a light source, one photo diode as a receiver, and a 1 mm-length absorption cell. Each of the fi ve LDs operated one of the fi ve wavelengths in turn. Oil identifi cation, which distinguished between inedible and edible oil, was clearly shown on a two-dimension map. Our results confi rmed the feasibility of a LD-based sensor-type identifi cation system that has many advantages such as compactness, low power and easy to use.

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We performed measurement of laser-shock compressed silicon along [001] orientation using in-situ wide-angle x-ray diffraction technique and line-imaging velocimetry (VISAR). The x-ray diffraction detector fi lm recorded x-rays that are diffracted from multiple lattice planes. These diffracted x-rays indicate uncompressed state and compressed state. VISAR interferometry image shows a specifi c rearsurface velocity change indicating that a shock wave propagate in the silicon target. These data provide information on the crystal structure and stress state under shock compression.

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