The vanadate crystals YVO4 and GdVO4 have been very successful hosts for diode-pumped rare-earth-ion lasers because their stimulated-emission and absorption cross sections are significantly higher than other hosts, such as YAG. Now a collaboration between scientists at Shandong University in Jinan, China, and at Technische Universität Berlin in Germany has grown and tested another promising vanadate, Nd:LuVO4. The results indicate that this material is a good candidate for miniature, diode-pumped lasers.
Other laboratories have investigated Nd:LuVO4, which has even higher absorption and stimulated-emission cross sections than the better-known vanadates. These earlier studies, however, employed tiny, flux-grown crystals and produced only low-power results when pumped with a Ti:sapphire laser. The scientists in the current group grew larger (3 × 3 × 6 mm) crystals by the Czochralski technique and pumped them with a high-power diode laser, producing significantly higher outputs than the early experiments.
Figure 1. When longitudinally pumped with light from a diode-laser array, the Nd:LuVO4 laser produced promising outputs in both CW and Q-switched modes.
In their work, the researchers placed an Nd:LuVO4 crystal with 0.9 percent atomic Nd doping in an 80-mm-long resonator formed by a spherical and a flat mirror (Figure 1). They longitudinally coupled up to 30 W of 807-nm pump light from a fiber-coupled diode-laser array into the resonator through the spherical mirror, which was a maximum reflector at 1.06 µm and a maximum transmitter at 809 nm. Several flat mirrors, with transmission ranging from 5 to 15 percent, coupled the output from the other end of the resonator. An acousto-optic Q-switch was present for the pulsed lasing experiments, and removed for continuous-wave experiments. With the Q-switch removed, the resonator was shortened to 60 mm.
When pumped with 25 W of 807-nm power, the CW configuration produced 12.55 W of output at 1.06 µm, for a conversion efficiency of 50.2 percent. The corresponding slope efficiency was 52.3 percent. The results were essentially the same with the 5, 10 and 15 percent output couplers. When the pump power was higher than 25 W, the output diminished because of overheating and thermal focusing of the laser crystal.
Figure 2. The Czochralski-grown Nd:LuVO4 crystal is a good candidate for efficient, diode-pumped microlasers.
When the investigators added the Q-switch to the resonator, they observed an average 1.06-µm power of 5.42 W at 40 kHz from 18 W of pump power. They produced up to 138 µJ per pulse at 25 kHz, in 8.5-ns pulses with peak powers of 16.2 kW.
A drawback of the most common vanadate, YVO4, is its low thermal conductivity -- because it is almost the same as the YAG's, GdVO4 is often preferred in higher-power lasers. Precise measurements of the thermal conductivity of LuVO4 have yet to be reported.
MORE FROM PHOTONICS MEDIA