YAG Solid State
Breakthroughs at Raytheon
Raytheon has long been a leader in optical materials research and development, with multiple patents involving multi-spectral zinc sulfide (ZnS), Raytran zinc selenide, and aluminum oxynitride (ALON), just to name a few. More recently, the focus has shifted to next-generation optical materials that will enable further system capabilities and higher performance. For example, as missile domes and windows and as laser gain media. Yttrium aluminum garnet (Y3Al5O12, or YAG) is a laser gain host material widely used for solid state lasers. When doped with rare earth elements such as neodymium (Nd), ytterbium (Yb) or erbium (Er), and pumped with light from an external source, energy is transferred to these atoms and then released to emit laser light at a different wavelength. While single crystals have traditionally been used, polycrystalline or ceramic YAG is gaining momentum as preferred for high-power solid state lasers because of several advantages and capabilities afforded by ceramics.
As the size and design complexity requirements become more stringent for high-power solid state laser systems, optically transparent ceramic laser gain materials are replacing single crystals. Most single crystal YAG is grown according to the Czochralski method, in which a seed crystal is slowly pulled and rotated inside an Iridium crucible with molten YAG to form a crystal boule. Large crystals are difficult to fabricate because the growing process introduces significant stress and other index distortions. Due to the mass, the maximum size of the boule without fracture is also limited.
Ceramics offer competitive advantages in overcoming these shortcomings. Ceramic material can be fabricated faster and in larger sizes with more uniform optical properties and doping. Ceramic parts can be made as large as the size of the furnace hot zone, eliminating the need for optical diffusion bonding as commonly practiced with single crystal material. Ceramic YAG can also be doped at higher concentrations than single crystal in many cases. The complex, net shape capability also contributes to the cost competitiveness of ceramic materials. Ceramic material can be produced in several days with the appropriate furnace, while growing a crystal boule requires several weeks and expensive iridium crucibles at very high temperatures. Ceramic sintering temperatures are well below the melting temperatures required for single crystals. Ceramic processing is also amenable for producing complex monolithic structures in which graded doping profiles and dopant type can be tailored. Ceramic laser gain material has already demonstrated equivalent or superior performance to single crystal. In fact, all of the properties critical to the laser performance — such as propagation loss, thermal birefringence, dopant absorption and emission characteristics, and refractive index — have been proven identical to those of single crystal laser media.
Fabrication of Laser Ceramics
Optical ceramic YAG materials are fabricated from a starting nanopowder material which is consolidated into a desired shape and then heat-treated at temperatures well below the melting point for pore removal and densification. The fabrication process flowchart is shown in Figure 1.
The extensive powder characterization step is critical in understanding and predicting how the powder will behave during consolidation and densification. It is also a screening method to evaluate chemical and crystalline phase purity information of the powder to see if it will produce optically superior ceramic. The average particle size of the YAG powder is in the nanometer range, which makes them prone to agglomeration due to their large surface area (see Figure 2). A deagglomeration step is therefore necessary to break up the powder aggregates before consolidating it into a green body. This step allows for uniform pore distribution and optimal green density, both of which aid in uniform sintering. The green ceramic body is fabricated using the uniaxial and isostatic pressing apparatus on the die filled powder. As seen in Figure 3, the green body has approximately 50 percent porosity contained within, which renders the sample opaque.
Sintering followed by hot isostatic pressing (HIP) is the method of densification for optical ceramic YAG fabrication at Raytheon. Following sintering, the ceramic still has some residual porosity left in the part. The HIP takes these remaining pores out of the ceramic by using both temperature and pressure. The resulting ceramic is optically transparent if the starting powder was of high chemical and phase purity. The sample then needs to be polished on major faces before it can be further analyzed for optical transmission, absorption, lasing characterization, and other microstructural examination. State-of-the-art ceramic processing ensures refractive index and dopant uniformity of the final ceramic as well as complete elimination of porosity and controlled grain growth.
It is critical that the starting nanopowder is the highest quality possible in terms of its chemical purity, sinterability, and stoichiometry. If the powder contains chemical impurities, especially those that absorb around the wavelengths of interest for the application, not only will they degrade the laser power output, they also will contribute to significant heating of the laser gain medium. Another source of loss in laser ceramic YAG comes from scattering. Scatter centers can originate from poor sinterability and off-stoichiometry of the starting powder. Sinterability attributes to the affinity of the powder to coalesce and form ceramic and effectively diffuse and eliminate pores along the grain boundaries, while undergoing heat treatment. Pores that remain in the final ceramic, whose refractive index is hugely different from that of YAG, act as scatter centers. Stoichiometry refers to the molar or atomic ratio of the YAG compound, Y3Al5O12, or more specifically, 3 to 5 molar ratio of yttrium to aluminum. When the YAG powder composition deviates from this stoichiometric ratio by more than 1 part in 1,000, the ceramics will contain second phase inclusions that have different indices of refraction than YAG and also cause optical scatter. Because the high-quality starting nanopowder is exceptionally critical to the overall optical properties of the final ceramic (the other factor being the optimum ceramic processing technique), Raytheon has established strategic relationships with several nanopowder suppliers.
Raytheon Ceramic YAG
The most basic differences between single crystal and ceramic material are the presence of grain boundaries and the random orientation of individual grains in ceramics. Figure 4 shows the scanning electron micrograph of Raytheon laser ceramic YAG. Cyrstallographically, YAG is a cubic material and therefore optically isotropic. The average grain size of Raytheon ceramic YAG is less than 1.5 microns — particles coalesce and the grains grow during densification. Fracture strength of polycrystalline ceramic materials tends to be greater than the corresponding single crystal, primarily because the residual flaw size scales with grain size. The ring-on-ring biaxial flexure fracture strength test was carried out on 25 mm diameter disk samples of Raytheon ceramic YAG and the result was compared against that of single crystal YAG samples. Fracture toughness was also measured by Vickers indentation method. As can be seen from Table 1, Raytheon ceramic materials are as much as 50 percent stronger than the single crystal samples and the fracture toughness proved to be exceptionally higher — 100 percent increase over single crystal.
This research has established Raytheon as the leading domestic source of optically transparent laser ceramic YAG with quality that matches the lasing performance of the leading international supplier. The material quality has been verified through optical transmission and direct laser efficiency comparisons of Raytheon's ceramic Nd:YAG and that from the other leading supplier. Specifically, the slope efficiency, which is the key measure of lasing performance, was measured as 54 percent for Raytheon's material and 52 percent for the other supplier.
This accomplishment was made possible through 30-plus years of experience in the development of optical ceramic materials at Raytheon. In addition to this significant milestone, Raytheon has also demonstrated appreciable lasing in Yb- and Er-doped ceramic YAG materials. Scale up was dem¬onstrated with Nd:YAG and Yb:YAG in dimensions of 90 mm diameter by 5 mm thickness and 125 x 35 x 2.5 mm, respectively. Ceramic laser gain materials are a key enabler in advancing the next generation of high-power solid state lasers. Raytheon's goal is to become the domestic supplier of high-quality laser ceramics to all bona fide high-power solid state laser system developers.
Jean Huie Imholt; Richard Gentilman