Today, fuel-air mixtures in internal combustion engines are ignited by an electric spark discharge. Therefore, we call them “spark ignition engines”. However, torch-hole flame (1794～) or hot-tube (1855～) were the main ignition sources of early internal combustion engines, namely, flames were used. The paper presents the historical changes of the ignition method of engines and the recent enhancement method of ignition including laser ignition.

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To develop a new ignition method, we carried out a series of experiments on ignition with excimer and Nd:YAG lasers. When we use an excimer laser, we can produce radicals in the focus point that advance the mixture’s ignition reaction. When we use a Nd:YAG laser, the focused giant laser pulse induced breakdowns in the mixture. In both cases, the laser ignited the mixture. In this paper, we show some results of our laser ignition studies from the past 20 years at AIST.

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Physical processes in the ignition of chemical combustion by laser-induced breakdown were classified into laser-induced breakdown, triggering of chemical reaction by a laser-produced plasma, and formation of a self-sustained combustion wave, and their characteristics were discussed. From previous studies, the processes of the laser-induced breakdown and the formation of a self-sustained combustion wave have been clarified semi-quantitatively. However, the triggering of chemical reaction by a laser-produced plasma has not yet been understood well, and needs a lot of investigations in future.

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This article presents a brief overview on recent trends in giant pulse lasers for laser plasma ignition for engines and combustors. Giant pulses available from diode-pumped passively Q-switched Nd:YAG lasers having pulse energy exceeding several millijoules with pulse duration in a range from subnanosecond to several nanoseconds have become promising drivers to generate sufficient amount of plasma using nonresonant breakdown for initiation of fuel ignition. VCSELs (Vertical-cavity surfaceemitting diodes) are emerging as key pump sources for giant-pulse solid-state lasers for ignition.

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Practical solid-state lasers for the ignition of internal combustion engines were discussed.　Laser ignition systems require compact size and stable operation over a wide range of temperatures.　In this paper, we summarized our recent progress on laser ignition on VCSEL-pumped solid-state lasers.　We developed a 14-cm3 fiber-coupled VCSEL module as a wavelength-stabilized pump source of a Nd: YAG/Cr:YAG micro-laser and obtained Q-switched output pulse energy > 1 mJ in a temperature range of the VCSEL module from 10 to 80℃.

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Giant pulse Nd:YAG/Cr4+:YAG passively Q-switched microchip laser, developed for laser ignition, generates several MW peak power sub-nanosecond width pulses. However, this output cannot be used, as it is, for wavelength conversion, since the output polarization is not stable. We redesigned the laser by using [110]-cut Cr4+:YAG, instead of the normally used [100]-cut, to obtain stably linearly polarized MW peak power output for efficient wavelength conversion. Using this compact microchip laser, we have achieved efficient second to ninth harmonic generation. In particular, we have designed a palm-top size giant pulse UV laser giving > 4 MW, 150 ps pulse width, 100 Hz output at 266 nm. We have also demonstrated ninth harmonic generation (118 nm) in xenon gas, using the giant pulse microchip laser.

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We developed a new calibration method for laser velocimetry for fl ow measurements to establish the traceability of measurement uncertainty and to reduce calibration uncertainty. In conventional methods, large radial uncertainty of a scattering object is dominant due to the difficulty of accessing the true calibration radius. Our new method successfully solves this difficulty by providing a reliable radial estimate at calibration. It employs a rotating disk and a precision linear stage and utilizes the linear relationship between the true radii and the resulting Doppler frequencies. Uncertainty analysis was performed based on Guide to the Expression of Uncertainty in Measurement (GUM). The traceability chain is established once the elemental traceabilities are obtained. We built prototype calibration setups and conducted three series of calibration experiments. The uncertainty was reduced to around 0.1%, which is suffi ciently small compared to the general limit of ordinary laser velocimetry.

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Ti-indiffused LiNbO3 waveguides offer a variety of applications since they can support both TE and TM modes. In this work we optimized the conditions to establish the fabrication techniques of Ti-indiffused waveguides for telecommunication wavelength in photorefractive damage resistant MgO (5 mol%): LiNbO3. We found that Ti-indiffusion with a slow temperature rise in dry Ar was effective for obtaining a smooth waveguide surface. The effects of Ti film thickness and diffusion conditions on the waveguiding properties were examined. We obtained waveguide of good quality with propagation losses of 3.5 dB/cm (TM) and 1.6 dB/cm (TE) at 1.55-μm wavelengths and 4.7 dB/cm (TM) and 4.9 dB/ cm (TE) at 0.79-μm wavelengths by the diffusion of Ti stripe (250 nm thickness, 8.0 μm width) on the -Z surface at 1135 ℃ for 12 h in wet O2 after heating up at 15 ℃/min (20~750 ℃), 3 ℃/min (750~1135 ℃) in dry Ar.

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