Following three different types of high power lasers at Kansai Photon Science Institute are overviewed and controlling
the laser damages in these laser systems are described: (1) PW-class Ti:sapphire laser for high field science, (2) zig-zag
slab Nd:glass laser for x-ray laser pumping, and (3) high-repetition Yb:YAG thin-slab laser for THz generation. Also
reported is the use of plasma mirror for characterization of short-wavelength ultrashort laser pulses. This new method
will be useful to study evolution of plasma formation which leads to laser damages.
We developed high-resistant anti-reflection (AR) coating by using Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> multilayer for Yb:YAG thin disk
amplifier. The AR coating was designed both for 940 nm of pump laser at an incident angle of 30 degrees and for 1030
nm of seed laser at 5 degrees. The Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> multilayer was deposited by using the electron beam evaporation
technique on a fused silica substrate and then the laser induced damage threshold was evaluated. The sample was
irradiated by 1030 nm laser with 520 ps duration delivered from the Yb:YAG thin-disk regenerative amplifier. The
measured damage threshold of the Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> AR coating was 75 J/cm<sup>2</sup>.
Laser crystal integration using a neodymium-doped yttrium vanadate (or orthovanadate) laser crystal, and non-doped yttrium vanadate crystals that function as cold fingers has been demonstrated. In our bonding technique of YVO<sub>4</sub> crystals, a newly developed dry etching process was adopted in the preparation for contact of mechanically polished surfaces. In the successive heat treatment process, temperature optimization was essential to get rid of precipitation of vanadic acid caused by the thermo-chemical reaction in a vacuum furnace. The bonded surface of 5 mm × 6 mm was studied via optical characteristics and magnified inspection. In addition, we also compared the integrated crystal with a normal one in laser output power pumped by a CW laser diode. From these experiments, it was clear that the integrated Nd:YVO<sub>4</sub> laser crystal, securing the well-improved thermal conductivity, can increase laser output power nearly twice that of the conventional single crystal which was cracked in high power laser pumping due to its intrinsic poor thermal conductivity.
Direct bonding without the use of adhesives was demonstrated on Ti:sapphire laser crystals with a bonding surface of 12 mm x 6 mm and the bonded region was evaluated from the macroscopic to the atomic level by three different methods. Wavefront distortion caused by the bonded region of 10 mm x 5 mm was estimated at 0.031 wavelengths (λ) at 633 nm. Micro defect measurements by a laser tomography method showed that the number of micro defects on the bonded region were much smaller than that of the intrinsic ones inside the crystal. From a magnified inspection, atoms in the bonded region were well arranged with the same regularity as inside the crystal. In addition, micro defects 1 nm in size appeared slightly along the bonded interface where the titanium ion concentration was four times higher than other parts of the crystal.
Thermal lens effects on highly pumped Yb doped phosphate glass was measured by a Shack-Hartmann wavefront sensor for the development of compact chirped pulse amplification systems. High energy pump pulses of 1 - 2 Joules were produced by a flashlamp pumped Ti-sapphire laser. The pumping intensity on the Yb:glass surface exceeded 800 kW/cm<SUP>2</SUP>. The pulse energy of 330 mJ from Yb:glass was obtained with 53% slope efficiency with 0.5 Hz reputation. The absorbed pump energy generated the thermal lens effects inside the Yb:glass. The wavefront distortion completely disappeared after 300 ms of pump pulse. Neither heat accumulation nor pumping damage was observed on the Yb:glass.
A high energy flashlamp pumped Ti:sapphire laser has been developed for the pumping source of Yb:glass chirped pulse amplification. The free running oscillator generates 12 Joule/pulse at 793 nm at 1 Hz repetition. The output energy of 6 Joule/pulse at 920 nm has been obtained.