In the optically pumped solid-state laser market, the quest is for a high-efficiency, high-power system with the smallest possible spot size. Researchers at the Advanced Technology R&D Center of Mitsubishi Electric Corp. (Amagasaki, Hyogo prefecture, Japan) have focused in on that magic trinity. Led by Kimiharu Yasui, the group developed a diode-pumped neodymium-doped yttrium aluminum garnet (Nd:YAG) laser that operates at better than 20% efficiency and generates 500 W of power on average (5 kW peak), all with a spot diameter only 50 µm.
"The opinion that high-efficiency, high-output, tight-beam lasers were impossible in this venue could be found even in some scientific papers, so when we announced our accomplishment, there was an undercurrent of disbelief," says Yasui, manager of the Laser and Electronics Applications Technology Department at the center.
Yasui says that when he started working on the project in 1997, the team could not seem to get the laser's efficiency above 18%. Then they began using wedge-shaped lenses to deliver diode-laser-pump light to the Nd:YAG rods. These lenses were key to the development of the new high-efficiency laser.
Yasui and his engineers developed a new software program for their computer that enabled design of the precise angles necessary to get maximum efficiency from the rods. By 1999, although the spot diameter was 600 µm, efficiency had jumped to a record 28%. But the researchers faced a fact of lifereducing the size of the beam spot also reduced system efficiency.
In 2000, the team started working on laser-beam quality. Instead of using a single Nd:YAG rod, they decided to use two, with a rotator between them to reverse the beam's polarization. With this system, they were able to reduce the beam spot size to the desired 50 µm and eventually broke the 20% efficiency barrier.
While claiming that this segment of the market is not his specialty, Montana University physics professor and laser specialist Rufus Cone considers the Mitsubishi laser an important product that represents great progress in efficiency and output. Cone also pointed out that other manufacturers are offering competitive products, however. One example is the diode-pumped Nd:YAG DP-23 laser from TRW (Redondo Beach, CA), which develops 3 kW and cuts both titanium and nickel alloy in tests done at the company's Precision Laser Machining Application Center. TRW does not say, however, what the efficiency of its laser is.
The Advanced Solid State Lasers (ASSL 2000; Davos, Switzerland) also featured papers on high-power diode-pumped lasers from Germany, Japan, and the United States. The consensus seems to be that solid-state lasers such as the one developed by Mitsubishi represent the future of industrial lasers. Yasui says that his group plans to use the technology it developed to have a solid-state diode-pumped x-ray laser ready for semiconductor fabs by 2005.