Ken-ichi Ueda
Institute for Laser Scicnece, Univ. of Electro-Communications
1-5-1 Chofugaoka, Chofu, Tokyo 182-8585 Japan
Tel +81-424-43-5700, FAX +81-424-85-8960
e -mail: ueda@ils.uec.ac.jp

Solid state laser technologies made a significant progress last decade. Major part of this progress is attributed to the laser diode (LD) pumping. At the end of the last century, another kind of progress was achieved by the new solid state laser material, ceramic YAG. In this talk, I introduce the history of creation and development of new concept of solid state lasers.

Fig.1 Conceptual discussion on ideal laser crystals.

In 1997, Prof. Kaminskii and Prof. Bagayev asked me to prepare one or two page viewgraphs for the round-table discussion on the future laser crystals in MPLP’97 in Novosibirsk. I talked about the gap between the ideal laser crystal and the laser and nonlinear optical crystals at present. Many scientists and engineers are satisfied in YAG lasers. But is a YAG crystal is an ideal crystal, or not? This is the point I mentioned.

To my understanding, a YAG crystal is not so ideal at all. It generate high power output with high efficiency. Is it good enough? No. Laser action is one of the functions of laser performance today. We have other important functions on coherent power generation and conversion. We need Q-switching, harmonic generation, paramatric amplification, Raman shifting, power modulation and diffraction by electro-optical and acousto-optical properties. But, every function needs some special character in crystal nature which collide with each other. We have no natural crystal which can satisfy all these requirements.

However, we can develop an ideal crystal artificially, if we apply the real high technology which can control the nano-meter class structure. After the experiments on the Nd:YVO4/S-VAP hybrid laser system, we understood the intrinsic part of the hybrid laser crystals is the cooperative interaction between optical crystals. Another way to develop the artificial crystal, the ceramic technology is available, because the ceramic fabrication is the real crystal growth process under the human assistance.

Another stimulation came from the laser fusion research. Today the feasibility to generate the fusion is clear from the scientific view. For the fusion power plants, we need 5 to 10 Hz repetition rate MJ laser drivers. We have no good solid state lasers now. The thermal conductivity of glass laser is too small and we have no large size single crystal. We need glass-like fabricated crystalline laser pumped by high power LDs.

Ceramic laser is a best candidate today. Two Japanese groups developed ceramic laser by different techniques. Dr. Ikesue demonstrated the first laser oscillation in 1995. But his method, hot press method, is good for microchip but has no scalability. Dr. Yanagitani, Konoshima Chemical company, published the patent on pure chemical method for ceramic YAG laser component. Nanometer size precurser and nano-YAG-crystal grow to micro-crystals with grain size of 10 micron through the solid phase crystal growth. This is a real crystal growth with multi-seeds. Through the sintering process under the vacuum, the ceramic YAG is going to transparent with very low scattering coefficient. As a result, we developed a laser quality Nd-doped ceramic YAG in a traditional manner for ceramic formation.

We demonstrated high efficiency operation in the end-pumping scheme. The optical-optical efficiency was measured to be about 60% in 1% and 2% doping. This is almost the best data for a single crystal and is a clear evidence to show the high quality of ceramic YAG material.

We believed the biggest advantage of ceramic laser to the single crystal laser is the scaling to the large aperture size. So, we developed high power ceramic YAG laser 100 mm long and 3 mm in diameter. We used a unique designed laser head of LD pumping named VPS (Virtual Point Source) pumping for the high power laser experiment. We got 31 W output immediate, but the efficiency was only a half because of the wavefront distortion. This was the first rod in ceramic fabrication. It is not easy to keep the uniform heating during crystal growth. We improved the process and got higher quality ceramic rods step by step. The output power made a rapid progress during last two years 499mW > 31W > 72W > 88W > 128W > 1460W as shown in Fig.2. We try to develop 10kW output in a CW mode until the end of this year.

A ceramic laser is a real revolution in solid state lasers. It has a nature of crystalline laser like large and homogeneously broadened emission cross section, thermal conductivity, mechanical constant. But the fabrication process is really glass-like-fabricated. We already demonstrated a large aperture sample of 1m x 1m in the ceramic forming process. The result is very attractive for the meter-size ceramic lasers for laser fusion driver.

We have a dream for the future production process for mass production of ceramic lasers in a manner of continuous production like steel and glass production. I believe it is quite possible from the view of laser science and engineering.

Fig.2 Ceramic YAG lasers made a rapid progress in these years.