17 Jun 2002
Japanese scientists have made a diode-pumped Nd:YAG laser that emits more than a kilowatt of optical power.
Exclusive from Opto & Laser Europe magazine
Scientists at the Institute for Laser Science of the Tokyo-based University of Electro-Communications, have produced a 1.46 kW source, a development that could open up a whole new field of solid-state laser technology (see App. Phys. Letts 78 3586).
Ken-Ichi Ueda, director of Tokyo's Institute for Laser Science at the University of Electro-Communications, says that much higher powers are possible: "I believe that we can achieve a 10 kW output in the near future," he remarked.
Ceramic lasers have a combination of some properties of glass and single-crystal sources. Owing to their relatively straightforward manufacture, the ceramic media can easily be made into thin, meter-scale devices at a low cost.
Compared with single-crystal YAGs, the ceramics are much easier to make, less expensive, readily increased in size and suited to mass production. However, until last year, even the most powerful lasers that were based on the material emitted barely a single watt of optical power (see figure).
Ueda's collaborators at Toshiba, Japan, measured the 1.46 kW output of an 8 x 150 mm ceramic rod pumped by 808 nm diodes, because the cavity required was too large for Ueda to determine the laser output in his own laboratory. With 0.6% doping, the laser had a slope efficiency of 36.3%, which is marginally less than a conventional single-crystal YAG.
"I am sure that ceramic [YAG] lasers have the same essential properties as single crystals and can go to the 10 kW class," said Ueda. He added: "It was relatively easy to contain a high concentration of neodymium in the ceramic matrix. For high-power operation, doping at less than 1% is necessary, but a high doping level of 10% could be used to make high-power microchip lasers."
Crucial to the laser's success is the confinement of grain boundaries in the ceramic to less than 1 nm. Ueda's team has developed a technique called "modified urea precipitation" to make the rods. This reduces the effects of scattering that have caused large losses in previous ceramic lasers.
Ueda highlighted that thermal lensing in the ceramic material was equivalent to that seen in single-crystal YAGs, and that the size of the rod could be increased.
Concerns over the beam shape (the quoted M2 value was 28) remain, but Ueda says that although the beam quality is currently a "bit low", it is not a problem in principle and that it will be improved in the future.