John Q. Dumm

Co-casting and optical characteristics of transparent segmented composite Er:YAG laser ceramics

A novel colloidal co-casting process was developed to fabricate laser quality, multisegment composite ceramic laser gain materials. The approach was demonstrated for a three segment transparent composite rod 62 mm long by 3 mm diameter consisting of undoped yttrium aluminus garnet (YAG), 0.25% Er:YAG, and 0.5% Er:YAG. The Er concentration profile in the composite has steep, controllable gradients at the segment interfaces, while maintaining constant dopant concentrations within each segment. The composite rod has 84% transmittance at 1645 nm (the lasing wavelength) with a scatter loss of 0.4% cm −1 . Laser operation of such a composite Er:YAG ceramic rod was demonstrated for the first time, with nearly equivalent lasing behavior to an Er:YAG single crystal rod.

Processing technology, laser, optical and thermal properties of ceramic laser gain materials

Recently there has been increasing interest in high quality ceramic laser gain materials, particularly for high-energy lasers, due to the successful application of high-volume advanced ceramics consolidation techniques to transparent oxide gain materials. In this paper, a brief comparison of manufacturing techniques is presented, including an overview of the co-precipitation process and the solid-state reaction process. Merits and risks of each will be presented from a processing viewpoint. Ceramic Nd:YAG in particular shows promise for high power laser design. The program reported here is also compiling a definitive database to compare ceramic and single crystal Nd:YAG materials. Uniform doping levels of up to 9 at% Nd3+ have been reported by Konoshima Chemical Co. in ceramic Nd:YAG, and studied by the US Army Research Laboratory and the US Air Force Research Laboratory. All ceramic Nd:YAG materials studied to date have exhibited similar, if not identical, spectroscopic parameters to those measured for single crystal samples. Thermal properties, laser damage thresholds and refractive indices for a range of temperatures and wavelengths are reported. Diode-pumped free running laser experiment results with highly concentrated (up to 8 at% Nd3+) ceramics and their comparison with our modeling results are presented. High pulse repetition frequency actively (AO) Q-switched laser experiments are in progress. While there are still challenges in the manufacturing of ceramic laser gain materials, and the benefits of the application of ceramic technology to laser material are yet to be fully realized, ceramic Nd:YAG shows promise and could provide new options to the laser design engineer.

Comparison of Optical, Mechanical and Thermo-Optical Properties of Oxide Polycrystalline Laser Gain Materials with Single Crystals

Comparisons of the spectroscopic, mechanical, thermo-optic, and laser performance properties between single crystal and ceramic oxide gain materials will be presented. Inaccuracies and myths regarding these ceramics will be dispelled by presentation of statistically-significant data.

Electron microscopy of compound oxide laser materials

Oxide single crystals, such as yttrium aluminum garnet (YAG) and yttrium orthovanadate (YVO4), are important host crystals for solid-state laser applications. These crystals are often grown by the Czochralski process and are doped with neodymium during growth. The microstructure of the resultant crystal affects the overall laser performance and it is necessary to be able to characterize grown-in defects in the material. Scanning electron microscopy has been used to examine the fracture surfaces of YAG and has shown the presence of microscopic voids, which act as stress concentrators and in some cases appear to be the cause of fracture. Transmission electron microscopy (TEM) has been used to characterize various defects in both YAG and YVO4 crystals. The defects found depend on the growth conditions, specifically the Nd concentration in the crystal and the position within the boule. One of the most common defects identified in both materials were microscopic spherical particles. In YAG these particles appeared to be located primarily in the core regions and analysis of high resolution images indicate that they are due to regions that are both compositionally and orientationally different from the matrix phase. Direct observation of dislocations in YVO4 was made using TEM. In YAG only indirect evidence for dislocations could be found from the observation of river marks on fracture surfaces.