Cubic alkaline-earth fluorides, specifically CaF2 and SrF2, have long been recognized as good laser host candidate materials for high-power amplifiers. Their low linear and nonlinear refractive indices, negative thermo-optic coefficient, high thermal conductivity, low intrinsic optical loss over a broadband spectrum and ability to incorporate rare-earth ions with low quantum defects are just a few of their attractive properties. Traditionally grown from the melt up to foot-size dimensions, these materials can also be advantageously prepared in the form of transparent ceramics to improve on gain uniformity by eliminating dopant segregation and stress-induced birefringence. Transparent ceramic gain media offer additional benefits in terms of enhanced thermal shock resistance due to their fine microstructure and process scalability. The fabrication of transparent ceramics with controlled microstructures is usually achieved by sintering fine powders below their melting point. However, due to the high surface area of these powders, oxide contamination can lead to scattering loss in the sintered parts. In an attempt to circumvent this limitation, this work investigates the fabrication of transparent ceramics by fusion casting and their ceramization via hot forging. Specifically, we have evaluated the processing conditions for fusion casting of Nd:SrF2 and shown how reducing the cooling rate from 25°C/hr to 1.5°C/hr can increase optical transmission up to the Fresnel limit in the near-IR. Similarly, we have delineated the conditions under which a randomized microstructure can be obtained by uniaxial deformation of CaF2 single crystals and Nd:SrF2 fusion cast ceramics, yielding a 30% increase in fracture toughness from 1.7 to 2.2 MPa*m1/2 for CaF2. While further optimization can still be achieved, our best samples exhibit inline scattering losses of 0.002 cm-1 at 1.5 μm compared to 0.25 cm-1 at 1.06 μm for ceramics fabricated via hot-pressing. This two-step approach has the potential to provide a cost-effective pathway to large, low-loss alkaline-earth fluoride laser gain media.
| Identifer | oai:union.ndltd.org:ucf.edu/oai:stars.library.ucf.edu:etd2020-2535 |
| Date | 01 January 2023 |
| Creators | Cintron, Adrian |
| Publisher | STARS |
| Source Sets | University of Central Florida |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Electronic Theses and Dissertations, 2020- |
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