Shinji Motokoshi

High-efficiency and economical solar-energy-pumped laser with Fresnel lens and chromium codoped laser medium

The authors achieved 11%–14% slope efficiency of solar-pumped laser by Cr-codoped Nd:yttrium aluminum garnet ceramic and Fresnel lens focusing from natural sunlight. The laser output of 24.4W was achieved with 1.3m2 Fresnel lens. The maximum output for unit area of sunlight was 18.7W∕m2, which is 2.8 times larger than previous results with mirror collector. The utilization of Cr3+ ion enabled efficient absorption and energy transfer to Nd3+ ion of solar spectrum. The fluorescence yield at 1064nm for various pumping wavelengths was measured both for Crcodoped and nondoped laser media, and 1.8 times enhancement of laser output from sunlight is predicted.

Temperature dependence of laser-induced damage thresholds by short pulse laser

Temperature dependence of laser-induced damage thresholds were measured by Nd:YAG laser (1064-nm wavelength, 4- ns pulse width) and Ti:Sapphire laser (800-nm wavelength, 100-fs, 2-ps, and 200-ps pulse widths) to elucidate the effects of laser-induced damage mechanisms. As experimental samples, SiO2, MgF2, Al2O3, HfO2, ZrO2, and Ta2O5 were prepared by electron evaporation. With longer pulses than few picoseconds, laser-induced damage thresholds were increased with decreasing temperature. Temperature dependence was reversed for shorter pulses than a few picoseconds. The effects of temperature at different pulse width to laser-induced damage mechanisms were considered with separated processes. In the conclusions, a temperature effect to free-electron generations by photoionization and multi photon ionization is negligible. However, the temperature affects to electron multiple (electron avalanche) and critical density. Electron multiple decreased at low temperature and the laser-induced damage thresholds increased. On the other hand, critical density decreased at low temperature and the laser damage thresholds decreased. Influence of electron avalanche is much greater than the impact of critical density. Thus, the trend and the strength of the temperature dependence on laser-induced damage threshold will be decided by electron avalanche.

Investigation of electric field formed in a multilayer mirror under simultaneous irradiation of two wavelengths

Electric field intensity formed in a multilayer mirror has been investigated numerically based on the analytical calculation, for the case where two laser beams with different wavelengths enter the mirror simultaneously. In this case, the intensity changes with time due to the interference between the two waves. By taking into account it, a design method of a multilayer mirror for high power lasers is proposed.

Detection of the laser-damage onset in optical coatings by the photothermal-deflection method

Photo-thermal deflection (PD) method is one of the typical pump-probe techniques for absorption evaluation. In this study, the PD method was used for detection of a signature of laser-induced damage prior to critical destruction. A setup for the PD technique was incorporated into the nanosecond damage testing system and the absorption of some optics was measured in combination with N-on-1 damage test. The absorption decreased with pulse-laser irradiation at first. Then, the absorption increased at some point. Finally, the laser damage was caused and the absorption decreased. The relaxation times of the absorption were also evaluated to reveal the reason for the variation. The measured results indicated that the decrease came from the disposal of initial contamination and the effect might be considered as laser conditioning. On the other hand, the increase of the absorption might be attributed to generation of laser-induced defects (self-trapped excitons).

Dual wavelength mirror with high laser induced damage threshold under simultaneous irradiation of two wavelength

Dual wavelength mirrors with high laser induced damage threshold (LIDT) under simultaneous irradiation of two wavelengths are studied by numerical simulations based on analytical calculations of electric fields.

Database on damage thresholds of picoseconds pulse for HR coatings

Laser-induced damage thresholds of 2.2 picoseconds pulse for HR coatings prepared by Japanese optics makers were measured. The damage thresholds were compared with that of 10 nanoseconds pulse measured at last year. In picoseconds pulse, almost HR coatings had an average threshold of about 5 J/cm2. This meant that the damage thresholds of picoseconds pulse were unconnected to a little defects and/or contaminants in the coatings.

Temperature dependences of fluorescence characteristic for Nd/Cr:YAG materials

Cr3+ co-doped Nd:YAG is very useful for the flashlamp or solar pumped laser through Cr3+ ion has broad absorption bands in the visible region and an efficient energy transfer caused from Cr3+ to Nd3+. But the process of the energy transfer is not understood well. It is also a problem that the absorption at near infrared is appeared with changes of Cr3+ to Cr4+. We investigated these problems by measuring the temperature dependence of fluorescence characteristics (lifetime, excitation fluorescence and absorption) and the change after UV irradiation. The excitation fluorescence spectra at 450 K increased because the Cr3+ absorption depended on temperature. The fluorescence lifetime excited at the Cr3+ absorption bands shortened with increase temperature, that is, the energy transfer time will be short. Solarization was appeared in Nd/Cr:YAG ceramics irradiated by KrF excimer laser pulses at 248 nm.

Pre-evaluation method for the spectroscopic properties of YAG bulk materials by sol-gel synthetic powder

The sol-gel synthetic powder evaluation method is the most effective and the easiest technique for pre-evaluating the spectroscopic properties of luminescent impurity doped YAG. The luminescent elements doped sol-gel powder agrees well with that of a single crystal or ceramic YAG regarding the luminescent spectra, the intensity, and the lifetime, even though the powder sintered at a low temperature of 1100°C in comparison with the YAG crystal melting point of 1970°C, because a good crystallinity of the sol-gel powder is led by the hard agglomerates of very small primary particles. It is considered that the significance of this method is to apply for not only of YAG but also of other oxides, for instance Y2O3, ZrO2, TiO2, and MgAl2O4

Development of the diode-pumped solid state laser and its application to laser fusion and industry

The progress of implosion physics research and relevant technologies enable us to examine technical and economical feasibility, and to plan the realistic strategy to the commercial power plant. The most important key issue for IFE is the driver technology. The development of the laser fusion driver is opening new industrial technologies based on the photon processes and new fields of high energy physics.