The very high-energy Neodymium (Nd)-glass lasers used for inertial confinement fusion in NIF, employ large plates of nonlinear crystals as electro-optic switches and frequency converters in the laser beam array. The crystals substantially increase the laser's energy output. The inertial confinement fusion lasers under construction in the U.S. and France have an aperture of approximately 40 cm x 40 cm. They require three different types of single crystal boules with linear dimensions in the range of 50-100 cm per side.
Potassium dihydrogenphosphate, KH2PO4, (KDP) and its analogs are the only nonlinear crystals currently used for these applications due to their unique physical properties. These properties include transparency over a wide region of the optical spectrum, resistance to damage by laser radiation, and relatively high nonlinear efficiency, in combination with reproducible growth to large size and easy finishing.
The main limitation in the growth of such large crystals by traditional techniques is a relatively slow growth rate of 0.5 - 1 mm/day. One to two year growth cycles for sizable KDP crystals that are produced by traditional means are normal. Traditional methods face a number of considerable challenges that include construction and reliable performance of the crystallizer, overcoming an inherently high risk of failure, and protecting against crystal defects during the long growing periods. The process typically results in low yields and high costs of the final crystals. These reasons stimulated development of new techniques to accelerate growth without sacrificing optical quality of large KDP crystals.
Research on the rapid growth of KDP crystals began in the early 1980s when the world's largest laser of that time, Nova, which had 10 laser beams, was being built at LLNL. New, rapid-growth techniques explored radical modification of standard crystallization equipment. It was thought that the simple crystallizers which were widely used in industry and scientific research would not be suitable for rapid growth for several reasons. First was the limitation in the mass of crystals grown from a closed volume of solution using a temperature reduction method. Second, it seemed impossible to achieve the hydrodynamic conditions and control of supersaturated solution concentrations needed to avoid defect formation and spontaneous nucleation. Alternative designs were proposed, but they could not solve these problems. As a result, the 27 cm x 27 cm plates of KDP crystals that were used in the Nova project were grown by traditional techniques.
Research, which began at Moscow State University in Russia, demonstrated that KDP crystals could be grown in standard crystallizers using an approach that was fundamentally similar to the traditional solution-growth process without observing spontaneous nucleation or visible crystal defects. This approach shortened the growth time by one to two orders of magnitude from conventional methods. The process was further refined and scaled-up for production of crystals for Nova's successor, the lasers in NIF and for the Laser Megajoule Project (LMJ) in France. The NIF laser system with its 192 beams will produce 1.8 MJ, which is 40 times the energy of Nova. The NIF lasers have an aperture of approximately 41 cm x 41 cm and require about 600 crystal plates of both KDP and its analog DKDP, a mono deuterated KDP analog. The crystals are used in Pockels cells, and also in flat lenses of two types. Type I using KDP. and Type II using DKDP. These two types are used for frequency conversion, i.e. shifting the laser beam frequency from infrared to the higher energy ultraviolet region.
