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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. 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 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]." 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.
*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.
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