We report the first demonstration of polycrystalline Nd-doped YAG ceramics with almost perfect pore-free structure and Nd-doped YAG single crystal by advanced ceramic processing. The laser conversion efficiency of pore-free polycrystalline Nd:YAG ceramics is extremely high and its optical quality is comparable to that of commercial high quality Nd:YAG single crystal. Moreover, we have succeeded also in fabrication of Nd:YAG single crystal, which enables laser oscillation, by solid-state reaction method. Laser oscillation efficiency was very low when the pores were remained inside single crystal, however the laser oscillation efficiency of pore-free Nd:YAG single crystal was slightly higher than that of polycrystalline Nd:YAG ceramics having grain boundaries. From this fact, it was found that the optical scattering inside the Nd:YAG ceramics occurs mainly at the residual pores and the scattering at the grain boundary is very little. In addition, we confirmed that high concentration Nd:YAG single crystal can be fabricated by sintering method. Applying the above single crystallization technology by sintering method, we have demonstrated the fabrication of layer-by-layer single crystal composite and micron size spherical single crystal.
Since the composite laser media using single crystal such as Nd:YAG and undoped YAG was reported in 1998, the composite with various structures provided to experimental or industrial field. However, conventional bonding condition is not enough for laser application, and fabrication process is extremely complex and long delivery owing to necessity of polishing and diffusion bonding. Ceramic composite laser gain media having layer by layer and clad-core structure were fabricated successfully for the first time by advanced ceramic processing. Advanced ceramic technology provides a direct formation of composite having complex structure without polishing and diffusion bonding. The bonding condition of ceramic composite was confirmed to almost optically perfect, so we could oscillate successfully using composite laser media.
The laser-induced damage threshold of polished fused silica surfaces is much lower than the damage threshod of its bulk. It is well known that contaminations of polished surface are one of the causes of low threshold of laser-induced surface damage. Particularly, polishing contamination such as cerium dioxide (CeO<sub>2</sub>) compound used in optical polishing process is embedded inside the surface layer, and cannot be removed by conventional cleaning. For the enhancement of surface damage resistance, various surface treatments have been applied to the removal of embedded polishing compound. In this paper, we propose a new method using slective chemical removal with high-temperature sulfuric acid (H<sub>2</sub>SO<sub>4</sub>). Sulfuric acid could dissolve only CeO<sub>2</sub> from the fused silica surface. The surface roughness of fused silica treated H<sub>2</sub>SO<sub>4</sub> was kept through the treatment process. At the wavelength of 355 nm, the surface damage threshold was drastically improved to the nearly same as bulk quality. However, the effect of our treatment was not observed at the wavelength of 1064 nm. The comparison with our previous results obtained from other surface treatments will be discussed.
High-power solid-state ultraviolet (UV) lasers by using a have been in high demand because of their convenient operation procedure. An effective technique for UV generation is cascaded sum-frequency generation pumped by the output of near-IR solids-state lasers. The performance of such solid-state UV lasers appears to depend on the ability and reliability of nonlinear optical (NLO) crystals that are employed for laser frequency conversion. Discovery of CsLiB<sub>6</sub>O<sub>10</sub>(CLBO) crystals have enabled the production of such practical high-power all solid-state UV lasers. In 2001, UV output power up to 23.0 W by fourth harmonic generation of Nd:YAG laser was achieved. It is fact that laser-induced damage of NLO crystal is a limiting factor on reliable operation of high-power solid-state UV lasers. Bulk laser-induced damage of NLO crystal is related to the crystal's quality. In this paper, we have investigated the relationship among the bulk laser-induced damage threshold (LIDT), dislocation density and absorption of laser light in CLBO crystals with various crystallinity. The bulk LIDT of CLBO increased with decreasing dislocation density. High-quality crystals with a higher LIDT (15 - 18 GW/cm<sup>2</sup>) have a lower dislocation density of 6.6 x 10<sup>3</sup>/cm<sup>2</sup> than that of conventional CLBO (~15.0 x 10<sup>3</sup>/cm<sup>2</sup>). The relationships between crystal quality and absorption of laser light will be presented.
Silica glasses can be used as optical material in various applications such as deep-ultraviolet lithography and nuclear fusion, because they have no internal absorption and extremely small defects. In these applications, laser-induced bulk damage is an important factor in practical use. The laser-induced damage threshold (LIDT) is expected to depend on the types and production conditions of silica glasses. In this paper, the LIDT of synthetic fused silica which contains 4~1220ppm of OH, 1.7x1018~2x1019 molecules/cm3 of H2, and 100~3700 ppm of fluorine were studied by irradiating the higher harmonics of Nd:YAG laser at 355 and 266 nm with pulse width of 4 ns. Current experimental results show that the improvement of the LIDT with impurity contained silica glass is possible.
The surface-damage resistance of fused silica was enhanced 2.8-fold by removing a subsurface damage. For the conventionally polished fused silica surface, μm-scale subsurface damage and a shallow (20 nm to 100 nm) structurally modified zone produced during grinding and polishing were formed on the top of surface. Several surface etching techniques and super-precise polishing process were used to remove subsurface damage from a fused silica surface. First the conventionally polished surfaces were chemically etched in a buffered HF solution to remove 300μm of surface material, and then super-precise polishing was performed to obtain an optical surface. After that, the polishing compound was removed by using ion-beam etching. The effect of subsurface damage on laser damage resistance was characterized by the measuring of the laser-induced damage threshold (LIDT) for the laser radiations of 1064 nm and 266 nm respectively. For the wavelength of 1064 nm, the effect of the removal of subsurface damage wasn't clearly seen, although the enhancement of surface-damage resistance by the ion-beam etching could be confirmed. However, in the case of 266 nm, enhanced LIDT of 28 J/cm<sup>2</sup> was obtained from the subsurface damage removed surface. The surface LIDT increased by 2.8 times compared to that of conventionally polished fused silica surfaces.
A transparent low refractive index SiO2 film was laminated on a glass substrate with photochemical reaction by an Xe2* excimer lamp in the presence or NF3, 02 and silicon wafer at room temperature. The glass substrate and the silicon wafer were placed in the reaction chamber, which was filled with NF3 and O2 gases in the mixing ratio of 10:1 and under 330 Torr. The Xe2* excimer lamp was, then, irradiated for 20 minutes. The SiO2 film was spontaneously laminated on the glass substrate by repeating an adsorption of SiF4 and a photochemical oxidization with NO2, which was photo-dissociated from a mixed gas of NF3 and O2. The film thickness was 160 nm and the infrared spectrum was measured; the Si-O peaks were depicted at 600, 700, 1100[1/cm-1]. And Si-F peak was observed at 740[1/cm-1]. Then, the refractive index of the SiO2 film was 1.36. After annealing the film for one hour at 200 degree, the refractive index increased to 1.42.
There has been great interest in high-repetition, high-power ultraviolet (UV) source for various applications in semiconductor processing, micro machining, and other fields. Discovery of CsLiB<sub>6</sub>O<sub>10</sub>(CLBO) crystals have enabled the production of such practical high-power all solid-state UV lasers. In 2001, UV output power up to 23.0 W by fourth harmonic generation of Nd:YAG laser was achieved.
In general, one of the limiting factors for the development of high-power solid-state UV lasers is laser-induce damage of NLO crystal due to some kinds of defects inside the materials. Recently, we have succeeded to grow the high crystallinity CLBO with an enhanced bulk laser damage resistance. On these samples, an increase in the surface damage resistance could be expected. Measurement of the surface laser-induced damage threshold (LIDT) on CLBO crystals with various crystallinity was performed by using a 266 nm laser. For the crystal with high damage resistance (15-18 GW/cm<sup>2</sup>), LIDT of as-polished surface was 1.3 times higher than that of crystal with conventional damage resistance (9-12 GW/cm<sup>2</sup>). In addition, polishing compound embedded inside the crystal surface was removed by using an ion-beam etching process. We have observed 1.5 times improved surface LIDT by ion beam etching for both high damage resistance and conventional damage resistance samples. The relationships between vickers hardness and crystal quality will be presented.
Optical characteristics of a new commercially available perfluorinated organic polymer have been measured to prepare a quarterwave AR coating for Nd3+:YAG crystal and other optical materials. Coatings are carried out by dip method from solution at room temperature, they have a refractive index of 1.364. The laser-induced damage threshold at 1064nm, 532nm, 355nm, and 266nm is approximately same values with Teflon AR 2400.
Laser supported propulsion of a micro-airplane with water-covered ablator is demonstrated. The repetitive use of overlay structure is experimentally demonstrated with specially-designed water supply. The various transparent overlay is investigated by the CIP-based hydrodynamic code and experiments by pendulum and semi-conductor load cell. The momentum coupling efficiency of 5000 N-sec/MJ has been achieved by ORION experiments that agree with the simulation code. With the maximum efficiency approximately 10<SUP>5</SUP> N- sec/MJ predicted by the simulation, 30 pulses of MJ laser can give the sound speed to 10tons airplane. The concept can also be used for driving a micro-ship inside human body and a robot under the accidental circumstance of nuclear power reactor in which large amount of neutron source makes electronic device useless.