Stoichiometric Crystal Promises Laser Uses
Daniel S. Burgess
A team of researchers from Germany and Spain is investigating the properties of a stoichiometric crystal for use in face-cooled lasers, such as thin-disc and microchip designs. The crystal, KYb(WO4)2 -- or KYbW -- has demonstrated 20 mW of 1068-nm CW laser output at room temperature and has displayed a quantum defect as low as 1.6 percent, suggesting that it may find application in high-power CW and mode-locked lasers and amplifiers for materials processing and spectroscopy.
The stoichiometric crystal KYb(WO4)2 (top) is an ideal material for use in face-cooled microchip and thin-disc lasers. In recent tests, a 125-µm-thick sample, shown mounted with water cooling (bottom), produced up to 20 mW of 1068-nm radiation when pumped with a CW Ti:sapphire laser operating at 1025 nm. Courtesy of Vision Crystal Technology.
Peter Klopp of the Max Born Institut in Berlin explained that thin-disc lasers have an advantage over traditional solid-state lasers because their thin gain media minimize thermal effects on their beam parameters. Thin-disc lasers display highly efficient heat removal because the heat generated in optically pumping them does not travel far through the medium to get to the sink. This results in low thermal lensing effects, because the thermal gradient is parallel to the propagation of the laser beam in the active medium.
To absorb the pump radiation in such thin crystals, however, the laser materials must display short absorption lengths. It is here that stoichiometric crystals, and KYbW in particular, excel. Because of the high active ion concentration in stoichiometric materials (equivalent to doping at 100 percent), the crystals display high pump absorption and high laser gain per unit length. KYbW, for example, has a pump absorption length of approximately 13 µm for wavelengths near 981 nm. Moreover, the crystal quality usually is better than in their nonstoichiometric isostructural analogs, and their thermal conductivity does not suffer from the presence of the active ions, Klopp said.
In a demonstration of KYbW's application for CW lasing, the researchers pumped a 125-µm-thick platelet of the crystal with a Ti:sapphire laser operating at 1025 nm, which yielded a maximum output of 20 mW. They coated one side of the KYbW with a high-reflection coating for both the pump and 1068-nm output wavelengths, and coated the other side with a broadband antireflection coating. A water-cooled, gold-coated copper holder served as the laser mount. Because the folding mirror used in the setup had a transmission of 16 percent at the pump wavelength, the pump power incident on the crystal was limited to 220 mW.
To enable KYbW laser operation at high pump powers and with pump wavelengths near the absorption maximum, the researchers want to use thinner media, probably resulting in an ultrathin-disc laser. An active medium thickness of 10 to 20 µm would maximize the heat removal and also would enable them to fully exploit KYbW's short pump absorption length, Klopp said.
It is very difficult to manufacture and handle ultrathin crystal discs without cracking them, however. Moreover, the necessary highly reflective coating on one side causes tensions that lead to a deformation of the crystal and that cannot be fully compensated by the design of the laser resonator.
The researchers are investigating means to counter the deformation, mating a thin layer of the active crystal to a thick stabilizing layer of inactive material. Diffusion bonding, sputtering, epitaxial or hydrothermal processes may enable the fabrication of such composite structures in KYbW and potassium yttrium tungstate, Klopp said.
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