Katsuhiro Mikami

Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator

The optical properties, Faraday effect and Verdet constant of ceramic terbium gallium garnet (TGG) have been measured at 1064 nm, and were found to be similar to those of single crystal TGG at room temperature. Observed optical characteristics, laser induced bulk-damage threshold and optical scattering properties of ceramic TGG were compared with those of single crystal TGG. Ceramic TGG is a promising Faraday material for high-average-power YAG lasers, Yb fiber lasers and high-peak power glass lasers for inertial fusion energy drivers.

Temperature dependence of laser-induced damage threshold of optical coatings at different pulse widths

The temperature dependence of the laser-induced damage threshold on optical coatings was studied in detail for laser pulses from 123 K to 473 K at different temperature. For pulses longer than a few picoseconds, the laser-induced damage threshold of coated substrates increased with decreasing temperature. This temperature dependence was reversed for pulses shorter than a few picoseconds. We describe the physics models to explain the observed scaling. The electron avalanche is essential to explain the differences in the temperature dependence.

Temperature dependence of laser-induced damage threshold in silica glass

The laser-induced damage thresholds in silica glasses at different temperature conditions (123 K – 473 K) by Nd:YAG laser fundamental (wavelength 1064 nm) and third harmonic (wavelength 355 nm) 4 ns of pulses were measured. In the results, the damage thresholds increased at low temperature. At 1064 nm, the temperature dependence became strong by the concentration of impurities. However, at 355 nm, the temperature dependences of almost sample were almost the same for different concentration of impurities.

Laser-induced damage thresholds in silica glasses at different temperature

The laser-induced damage thresholds in silica glasses at different temperature conditions (123 K - 473 K) by Nd:YAG laser fundamental (wavelength 1064 nm) and third harmonic (wavelength 355 nm) 4 ns of pulses were measured. In the results, the damage thresholds increased at low temperature. At 1064 nm, the temperature dependence became strong by the concentration of impurities. However, at 355 nm, the temperature dependences of almost sample were almost the same for different concentration of impurities.

Laser-induced damage thresholds of optical coatings at different temperature

Laser-induced damage thresholds for dielectric and metal single-layer coatings at different temperature conditions (123-473 K) were measured by 1064-nm wavelength and 4-ns pulses to elucidate the effects of initial temperature to laser damage mechanisms. SiO2, MgF2, gold, silver and copper single-layer coatings were prepared as experimental samples. In the experimental results, temperature dependence of LIDTs for optical substrates and all dielectric single-layer coatings indicated same trend as that for bulk silica glasses, which increased linearly with decreasing the temperature. However, all metallic coatings had the inverse trend of the dependence for dielectric coatings. The effects of initial temperature to laser damage mechanisms were considered with separated processes from the experimental results. In the conclusions, free-electron generation and electron multiple caused difficultly at low temperature and the laser-induced damage thresholds increased. On the other hand, plasma heating caused easily at low temperature and the laser-damage thresholds decreased.

Temperature dependence of nonlinear optical phenomena in silica glasses

A linear increase of the laser-induced damage thresholds in silica glasses with decreasing the temperature was reported in this conference at last year. Various nonlinear phenomena should be generated in silica glasses besides the damage in high intensity. Temperature dependences of the nonlinear refractive indices and the SBS (stimulated Brillouin scattering) thresholds in silica glasses at temperature 173 K to 473 K were measured with single-mode Q-switched Nd:YAG laser at fundamental wavelength. As the result, the nonlinear refractive indices increased with decreasing temperature. Because the change was not enough to explain the temperature dependence of laser-induced damage thresholds, the temperature dependence of nonlinear refractive indices would be negligible on laser-induced damage thresholds. On the other hand, the SBS thresholds also increased with decreasing temperature. This result means that acoustic phonons arise easily at high temperature. Probably, the SBS phenomenon is one of reasons for temperature dependence of laser-induced damage thresholds.

Laser-induced damage thresholds at different temperature for optical devices

Studies for temperature dependences of laser-induced damage thresholds for optical devise is introduced in this paper. Additionally, the temperature dependence of the laser-induced damage threshold of single-layer optical coatings as resent progress was clarified using Nd:YAG and Ti:sapphire lasers. The wavelengths of the lasers were 1064 nm and 800 nm and the pulse widths were 4 ns, 200 ps, 2 ps, and 100 fs. For pulses longer than a few picoseconds, the laser-induced damage threshold of coated substrates increased with decreasing temperature. This temperature dependence was reversed for pulses shorter than a few picoseconds. A flowchart was presented including the several mechanisms for laser damage mechanism. The differences in the temperature dependence are explained by the flowchart. As one of results in theoretical analysis, the electron resistivity i. e. electron mobility is key point to elucidate the temperature dependence of laser-induced damage threshold.

Recent progresses on insights of laser damage mechanisms and influence of contamination in optics

In high power laser system, laser-induced damage threshold (LIDT) in optical coating is very important for obtaining high performances. The dependence of LIDT on the pulse duration and the repetition rate are well known phenomena. But recently, LIDT was found to have strong temperature dependences in the bulk, surface of substrates, and in coatings. This temperature dependence of LIDT was carefully measured, and the damage formation model was constructed regarding to this temperature dependence. This paper introduces LFEX laser system for First Ignition scheme in the laser fusion. A large-scale pulse compression chamber was designed and constructed, and segmented grating system has been employed for large-scale pulse compressor. This compressor provided good pulse compression performances, but we observed a heavy oil-contamination of optics in this chamber. We have analyzed contaminants and evaluated the effects of the contamination. We also developed new cleaning methods to remove contamination from the coating, and the quantitative analysis of contamination on LIDT was made. We have investigated the characteristics of LIDT in dielectric coatings under the controlled contamination. LIDT of coating drops to 1/2 in the saturated toluene vapor at room temperature.

Influences of oil-contamination on LIDT and optical properties in dielectric coatings

Laser-induced damage threshold (LIDT) in optical coating is very sensitive to organic contaminations accumulated in coating layers during storage and using condition. The sources of contamination are commonly exists, and optical coatings are easily contaminated regardless to the environment pressure, LIDT at ns region decreased largely by contamination, but LIDT at ps seems insensitive. In this study, we have investigated the influence of contamination of optical coating on LIDT and other optical properties. We examined several kinds of coating to clarify the sensitivity to the contamination. Degradations of LIDT were commonly observed in e-beam deposition, IAD and IBS. Some coatings changed spectral characteristics by contamination, and other coatings did not change. Some samples were contaminated as received condition, and some were very clean. Furthermore, we have investigated the characteristics of LIDT in dielectric coatings under the controlled contamination. LIDT of coating drops to 1/2 in the saturated toluene vapor at room temperature.

Temperature Dependences of Laser Induced Plasma Thresholds and Periodic Structures by Nanosecond Infrared Laser for Copper, Iron, and Chrome

The temperature dependences of laser induced plasma thresholds for copper, iron, and chrome with linearly and circularly polarized pulses from a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (wavelength 1064 nm, pulse width 4 ns) were evaluated at different temperatures from 123 to 423 K. The temperature dependence of copper with linearly polarized pulses was different from those of iron and chrome. On the other hand, the dependences of all materials with circularly polarized light showed the same tendency. The geometries of the ablated sites presented interesting tendencies, revealing some aspects of interaction between metal materials and laser that warrant further investigation.