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Ceramic YAGs set to challenge single crystals

17 Jun 2002

Single-crystal growth is an expensive, time-consuming and size-limiting method for making YAG laser rods. Now materials scientists in Japan say that they have come up with a way to mass-produce polycrystalline ceramic rods that maintain high efficiency conversion and good optical characteristics. Michael Hatcher reports.

From Opto & Laser Europe December 2001

Fabricating crystals is no easy business. Making high-quality single crystals for optical applications is even tougher, requiring much skill and time. In fact, single crystals are generally "grown" (rather than manufactured), using the Czochralski method.

Using this technique means that a single crystal's maximum size is limited by the size of the expensive iridium crucible in which it is grown. If, however, YAG crystals could be made using a method that was more amenable to mass manufacture, they would become cheaper to produce. It would also be possible to grow the crystals to a larger size, making them more suitable for high-power industrial applications.At the University of Electro-Communications in Tokyo, Japan, a research team led by Ken-ichi Ueda might just have come up with a way to make this possible. When Ueda rose to speak at the CLEO Focus 2001 meeting on high-power solid-state lasers in Munich, Germany, in June, everybody in the room was expecting him to announce a 128 W YAG laser - a power level far beyond anything achieved with ceramics so far. Instead, Ueda calmly crossed out 128 W on his slide and replaced it with 1.46 kW. His team had recently increased the power of the end-pumped laser more than ten-fold.

The increased output power of Ueda's laser resulted from it depending solely on the input power supplied by laser-diode excitation. "We have improved the quality of our ceramic rods, so increasing the output power is not a problem. How soon we will reach a power of 10 kW is not a scientific or engineering issue. It is simply a question of money," said Ueda.

The idea of ceramic laser rods is nothing new, but according to Jianren Lu of Ueda's group, researchers who had attempted to tackle the problem before had simply failed to find a suitable fabrication technology and had given up trying. As a result, Ueda's group is one of only two worldwide that have actively pursued the generation of ceramic sources in recent years.

The first Nd:YAG ceramic laser was developed by Akio Ikesue and colleagues at Japan's Krosaki Corporation in 1995. Ikesue used a hot-press method to make the ceramic rods, but found that the quality and size of the crystals limited their use to microchip lasers, the output power of which reached only 300 mW.

The problem with making laser rods from ceramic material is that ceramics are polycrystalline and some of their characteristics, such as grain boundaries, pores, composition gradients and lattice imperfections, increase the scattering of light in the rod. This adds to the opacity of the material, making it unsuitable for laser action. The key is to find a manufacturing method in which the crystals that make up the rod are very similar in size and small enough to have little effect on incident light with a wavelength of around 1 µm.

Ueda's breakthrough came when he found a manufacturing method that produced similarly-sized nanoparticles: a vacuum sintering technique. "Now we can make large-scale, high-quality ceramic laser materials, which is not possible with single crystal growth methods," he said. "We use a liquid-phase chemical reaction to produce the nanoparticles for ceramic formation. The nanoparticles are homogenous, so we do not use any pressure. This means that we can scale the process up to make a large sheet - to as much as 1 x 1 m if the thickness is less than 2 cm."

Ueda realized that to make rods large enough to significantly increase the laser power, he needed more room than was available in his laboratory. He therefore decided to team up with Japan's Toshiba, which had the necessary space and equipment to make the bigger rod. The result of the collaboration was the 1.46 kW laser.

The only fault in Ueda's design appeared to be the beam quality, which he quoted as having an M2 value of 28 at the CLEO Focus meeting. Now that the rod quality has been improved, however, Ueda says that beam quality will be solely a function of the optical cavity design. "We do not see any difference [in quality] between ceramic rods and single crystals," he told OLE.

Crystal-manufacturing companies are now beginning to take notice of ceramic-laser developments. Ueda said: "We have finished the optimization of the Nd:YAG ceramic and have already delivered samples to crystal manufacturers in Japan and the US. These companies are interested because they see the ceramic laser as a competitive product. Single crystals are good but the growth process is limiting."

Paul Johnson is director of sales and marketing at VLOC, a II-VI subsidiary in the US that grows various crystals including Nd:YAG. Johnson had just received a ceramic rod from Ueda when OLE contacted him. "There's a lot of testing work to do, but if it is as good as the guys in Japan say, this could be a revolutionary technology," said Johnson. "It could really open up the market [for Nd:YAG lasers]."

For the moment, Johnson is sceptical - but only because he has yet to carry out tests in which VLOC will directly compare a single crystal with a ceramic rod. Johnson will be looking closely at wavefront quality and the distribution of neodymium ions within the ceramic YAG rod. He sees welding and cutting applications as the major market opportunities, should the ceramic measure up to Ueda's claims.Another possibility is that ceramic laser rods could incorporate multiple-laser functionality. "We could achieve Q-switching and Raman-shift generation within the bulk ceramic material. It hasn't been demonstrated yet, but we will try," said Lu. He added that making ceramic YAG crystals is not restricted to neodymium-doped material or YAG crystals: "We can make Er3+, Yb3+, Nd3+, Eu3+, Dy3+ and Cr4+ as well as Y2O3 host crystals."

Ueda sees Nd:Y2O3 and Yb:Y2O3 ceramic laser materials as having an extra advantage over single crystals: "It is very hard to grow a single Y2O3 crystal because its melting temperature is 2430 °C. We think that [for these crystals] ceramic lasers will open up new fields for laser technology." The sintering temperature for Y2O3 is some 700 °C lower than its melting point, meaning that large Y2O3 ceramics could be manufactured using a vacuum sintering method.

One of the advantages of Y2O3 is its thermal conductivity, which is twice that of YAG for ceramic materials. This could make it more appropriate for use in small solid-state sources. With 1.5% neodymium doping, Ueda has seen 160 mW output from 742 mW diode pumping. Lu says that the team now plans to try to make large-scale Nd:Y2O3 lasers, which are impossible to grow as single crystals.

Lu adds that as well as the new possibilities in welding and cutting applications, the ability to make large Nd:YAG optics could be of use in a laser fusion set-up.

Lothar Ackermann is managing director of laser-crystal manufacturer FEE, based in Idar-Oberstein, Germany. He agrees that the advent of ceramic YAGs could revolutionize laser-crystal manufacture - if Ueda's claims of ceramic-rod quality are verified. "If ceramic lasers can do the same job as single-crystal lasers, crystal growth [as a process] will die," he said. Despite this possibility, Ackermann is not especially concerned about the impact that ceramic sources might have on his business. He says that the laser-crystal market is relatively small, so it might not make sense for manufacturing companies to invest heavily in new equipment to make ceramic rods.

For now, the Czochralski technique is the manufacturing method of choice for laser crystals. By next year, it will become clear whether its days are numbered. The idea of making a ceramic laser rod is nothing new. Since the 1960s, it has been speculated that a dense polycrystal of an isotropic, pure material would be optically indistinguishable from a single crystal of the same material. The only problem has been finding a fabrication method. Making ceramic laser rods has five distinct advantages over single-crystal growth:


*Ease of manufacture It takes 4-6 weeks to grow crystals using the Czochralski method, but the ceramic method makes rods in just a few days. The sintering temperature of a ceramic is usually much lower than its melting point.


*Cost-effectiveness Single crystals have to be grown in an expensive iridium crucible. Ceramic rod growth requires no crucible and is also faster. The cost of a single crystal increases dramatically with its size, unlike ceramics.


*Size of rods Single-crystal growth limits crystal size, which in turn limits the potential output power. The maximum crystal size is about 23 cm long, but polycrystalline ceramic YAG rods of twice this length can be made.


* Functionality Ceramic fabrication could enable the incorporation of Q-switching and Raman shifting within the source, which is impossible with a single crystal.


*Mass-production suitability Ceramic rods can be fabricated in a production-line fashion, reducing the time and cost required for single-crystal YAG rod manufacturing.

IDS Imaging Development SystemsBerkeley Nucleonics CorporationHyperion OpticsTRIOPTICS GmbHLASEROPTIK GmbHAlluxaLaCroix Precision Optics
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