Seamless and scalable aperture enlargement of LBO crystals

Problem Statement: Based on the design of the large-aperture high-energy lasers developed at the National Ignition Facility, most planned laser fusion facilities will need to operate at frequency converted wavelengths of light that are shorter than are produced directly by foreseeable lasers. These light sources will also have to operate with improved efficiency and at much higher average powers and repetition rates than are currently used. As compared to currently used frequency conversion crystals in fusion experiments (Potassium Dihydrogen Phosphate), the nonlinear crystal Lithium Triborate (LBO) has higher intrinsic efficiency, much wider angular acceptance, and an unparalleled laser-induced damage threshold. Lithium Triborate would enable much more compact and efficient designs and would save hundreds of millions of dollars in each laser fusion facility. However, Lithium Triborate isnĺt currently available in large enough aperture sizes for such high energy lasers. Growing sufficiently large single-crystalline Lithium Triborate boules is an extremely difficult, expensive, and lengthy process with a myriad of severe risks. Larger boules also tend to be higher absorbing and less uniform than smaller boules. An optically-seamless and infinitely-scalable process to increase the aperture of Lithium Triborate crystals would have an enormous impact on the viability of laser driven fusion for commercial energy production. Objective: To develop an optically seamless and infinitely scalable fabrication process to increase the optical apertures of Lithium Triborate crystals used for nonlinear frequency conversion. In Phase I, pairs of Lithium Triborate crystals were bonded. The crystals were cut for second harmonic generation. The fundamental issue of the angular mismatch in crystal orientation across the bond has been assessed. Orientation errors in bonding pairs of crystals were mitigated by innovative improvements to the crystal fabrication process, which were tested for manufacturability and scalability. In Phase II, 4-part and 9-part bonded Lithium Triborate composite crystals will be demonstrated, and then, tested in an operational high energy laser system. High energy free-space communications and directed energy weapons are also looking for large aperture Lithium Triborate crystals for frequency conversion applications. Future laser fusion power plants will make use of these large aperture crystals for highly-efficient high repetition rate laser drivers. Beyond Lithium Triborate, this methodology of seamlessly increasing the aperture of single crystalline materials provides a path for making optimized materials for high energy laser systems. System designers will be able to choose materials based on their desired characteristics rather than aperture availability. In doing so, they will be able to achieve ground-breaking performance from more compact and efficient systems.