Yb:Y2O3 Ceramic Laser Generates 4.2 W
Ceramic lasers may have a dramatic effect on the solid-state laser marketplace because, among other advantages, they promise inexpensive mass production. Several ceramic ion-host combinations have been investigated, including Nd:YAG, Yb:YAG, Nd:Y2O3, and Yb:Y2O3, and average powers in excess of a kilowatt have been achieved with ceramic Nd:YAG.
For generation of short pulses, the larger bandwidth of ytterbium gives it an advantage over neodymium as the lasing ion, and Y2O3 has a thermal conductivity twice that of YAG. Thus, Yb:Y2O3 has the potential to become an important laser material but to date, output powers of these lasers have been less than 1 W. Now, a collaboration of researchers from Nanyang Technological University in Singapore, the University of Electro-Communication in Chofushi, Japan, and Konoshima Chemical Co. Ltd. in Takuma, Japan, has demonstrated a 4.2-W Yb:Y2O3 laser.
Figure 1. The end-pumped, Yb:Y2O3 ceramic laser generated up to 4.2 W, which the researchers believe is the highest yet reported for such a laser.
The laser's experimental layout is shown in Figure 1. The fiber-pigtailed diode-laser bar supplied up to 20 W of pump power at 940 nm. The resonator's 1-m-curvature back mirror was coated for maximum reflectivity between 1030 and 1100 nm and for minimum reflectivity at the 940-nm pump wavelength. The Yb:Y2O3 gain medium was a 3-mm cube, with 8 percent atomic ytterbium doping and optical surfaces antireflection-coated from 1030 to 1090 nm. Several different mirrors, all with a 50-mm curvature but with 1064-nm reflectivities between 90 and 98 percent, served as the output coupler.
The highest output power, 4.2 W, was achieved with a 96 percent output coupler and with an input pump power of 19.5 W. The corresponding slope efficiency was 29 percent, and a beam-quality measurement yielded an M2 of ~1.6 (Figure 2). The researchers believe that this is the highest power yet reported for an Yb:Y2O3 ceramic laser. They estimate the pump-beam diameter in the ceramic at ~200 µm, which implies a pump-power density of at least 28 kW/cm2. They believe that their laser has potential for higher powers, but they were reluctant to pump harder for fear of damaging the ceramic.
Figure 2. The quality of the output beam was measured to be ~1.6.
The researchers also investigated the performance of the laser with different output couplers. The optimum coupler was a 96 percent reflecting mirror, and they observed a wavelength shift as the output coupling varied and a similar shift with changes in pump power. A laser's wavelength can shift because the laser shifts to a different transition or because the lasing action shifts to a slightly different wavelength within the same transition.
A crystalline Yb:YAG laser, for example, will shift from 1030 nm to 1048 nm when the output coupling is changed, corresponding to a shift to a different transition. In the case of the Yb:Y2O3 laser, however, the scientists found that the shift accompanying a change in output-coupler reflectivity corresponded to a slightly different wavelength from the same transition.
In general, a laser will not lase at its fluorescence peak but at its peak intracavity gain wavelength -- that is, at the wavelength where its net gain is greatest. In a quasi-three-level system such as Yb:Y2O3 with a complex reabsorption spectrum, the wavelength of greatest net gain is a complex function of output coupling, pump power, spatial overlap between the pump beam and the intracavity laser beam, and other parameters, so that the lasing wavelength is difficult to predict or explain.
In the case of the ceramic Yb:Y2O3 laser, the somewhat disordered, ceramic nature of the host material may cause strong inhomogeneous broadening, the researchers suggest, which in turn decreases mode competition and leads to greater wavelength-dependence on the reabsorption spectrum. The assumption of strong inhomogeneous broadening is questionable, however, because other experts have calculated that less than 0.3 percent of the ytterbium ions are close enough to the ceramic boundaries to cause inhomogeneous broadening.
What is clear is that further work will be necessary before the extent of inhomogeneous broadening or the spectral behavior in ceramic lasers is well understood, and researcher Jian Kong of Nanyang Technological University said that such work will be a priority in the coming months.
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