Atsushi Kosuge

Erratum: “Design of Rectangular Transmission Gratings Fabricated in LiNbO3 for High-Power Terahertz-Wave Generation”

Precise parameters are investigated for rectangular transmission gratings fabricated in a LiNbO3 substrate for high-power terahertz-wave generation. Here, a simple setup is considered, where the pumping laser is injected into the grating surface from air or fused silica. The maximum diffraction efficiencies are 0.4 for air and 0.9 for fused silica. The designed gratings have deep grooves with a depth of about 0.5 µm and a width of 0.12–0.14 µm. It was found that the diffraction efficiency is much less sensitive to the groove depth than to the groove width.

Improvement of diffraction efficiency of dielectric transmission gratings using anti-reflection coatings.

A novel method for increasing diffraction efficiency of transmission gratings is proposed. In this method, dielectric multilayers are inserted between a grating region and a substrate. These multilayers work as an anti-reflection coating for the transmission grating. It is presented that a grating with 1740 grooves/mm has the diffraction efficiency over 99% using this anti-reflection coating.

Polarization-selectable cavity locking method for generation of laser Compton scattered γ-rays

Nowadays, generation of energy-tunable, monochromatic γ-rays is needed to establish a nondestructive assay method of nuclear fuel materials. The γ-rays are generated by collision of laser photons stored in a cavity and relativistic electrons. We propose a configuration of an enhancement cavity capable of performing polarization control fabricated by a combination of a four-mirror ring cavity with a small spot inside a cavity and a three-mirror of reflective optics as an image inverter for polarization-selectable γ-rays. The image inverter introduces a phase shift of specific polarization which can be used to generate an error signal to lock an optical cavity at a resonance condition.

Negative effect of crystallization on the mechanism of laser damage in a HfO2/SiO2 multilayer

Elucidation for the mechanism of laser damage is indispensable to realize a high resistance mirror for the ultra-short pulse high intense lasers of the next generation. In this study, the surface and a section of a laser-damaged area of a laser mirror were observed with a laser microscope and a transmission electron microscope (cross-sectional TEM), respectively. A grain boundary of HfO2 microcrystal was observed in the damaged area. This observation, and an evaluation of the mirrors damage resistance showed that the formation of crystals in the multilayer mirror is one of the major determinants of damage resistance.

Present status of laser development for the intense laser-Compton gamma-ray source at JAEA

We report on present status of high average power (~100 W class) and high repetition rate(~80 MHz) short pulse laser development include laser storage cavity for nondestructive detection of isotopes using laser-Compton γ-rays.

Simple synchronization technique of a mode-locked laser for Laser-Compton scattering γ-ray source

We propose a simple and effective synchronization technique between a reference electrical oscillator and a mode-locked laser for a narrowband picosecond Laser-Compton scattering γ-ray source by using a commercial-based 1-chip frequency synthesizer, which is widely used in radio communication. The mode-locked laser has been successfully synchronized in time with a jitter of 180 fs RMS for 10 Hz-100 kHz bandwidth. A good stability of 640 μHz at 80 MHz repetition rate for 10 h operation has also been confirmed. We discuss in detail the design and performance of this technique (in terms of timing jitter, stability, and validity).