The United States had been at the forefront of technology, including crystal growth, from the mid 1900's until several
years ago. The growth of crystalline materials is generally
capital-intensive and low profit, with the value-added
fabrication and thin film coating steps comprising the majority of the cost of the final optic. With the continuous
improvements realized by scientists in foreign countries, many U.S. companies with crystal growth facilities are opting
to procure material from outside the U.S. to boost profits. Compounded with Federal procurement regulations, the end
result is that it has become difficult, if not impossible, to procure some mission-critical materials from U.S. sources,
putting numerous DoD programs in potential jeopardy. In addition, there is a limited amount of research currently
underway on new materials state-side. If the current trends hold, DoD programs will be at the mercy of foreign
companies to supply crystalline materials which are mission critical to the DoD.
We have performed a laser evaluation of the Yb3+-doped Y2O3 ceramic laser material. The Q-CW output power in excess of 30 W was obtained with the slope efficiency of 47.3%. This output power is, to the best of our knowledge, the highest reported so far for diode-pumped Yb3+-doped Y2O3 laser.
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.
Cerium has been doped in both LiCAF and LiSAF. However, each host has some intrinsic problems resulting in scatter and thermal expansion coefficient issues, respectively. In order to optimize the laser host, crystals were grown on various compositions of Ce,Na:LiSrxCa1-xAlF6. The materials produced were characterized using a variety of methods, including differential thermal analysis, glow-discharge mass spectrometery, electron microprobe, and various microscopic techniques. Samples were fabricated to characterize the absorption and emission properties for each crystal composition. Finally, laser threshold and efficiency were characterized for the various samples. Through an analysis and comparison of these critical aspects, an optimized, commercially-viable crystal can be identified to be incorporated into a tunable, solid-state UV laser system. In the case of the selected crystal system, the crystal growth characteristics of the various crystals are very similar. In the case of the selected crystal system, the crystal growth characteristics of the various crystals are very similar. Absorption and emission values for the strontium-rich compositions are greater than those for the calcium-rich compositions. Lasing was achieved in all of the samples with different output coupler transmissions. The Sr/Ca=35/65 composition showed the highest output powers. Taking into consideration all three aspects of this resaerch, there exists an intermediate composition close to Sr/Ca=35/65, that would be an optimized host as a novel UV laser host.
The development of solid state tunable lasers in the ultraviolet region would have a wide range of applications, including atmospheric remote sensing, atmospheric spectroscopy and pollution monitoring. The first work with Ce:LiCAF was published in 1993, with subsequent reports on favorable results with Ce:LiSAF. While the original quality of Ce:LiSAF is superior, it suffers from solarization. In contrast, Ce:LiCAF shows minimal solarization and higher efficiencies, even with the typically high-scatter materials available. This presentation will summarize the crystal growth and material characterization of a series of colquiriite crystals with various Sr/Ca ratios. Based on these results, in addition to laser measurements, an optimum composition of Ce:LiSrxCa1-xAlF6 will be determined.
The search for an efficient solid state laser with tunable emission in the ultraviolet wavelength region has resulted in the growth and development of cerium doped colquiriite crystals, such as LiCaAlF6 (LiCAF) and LiSrAlF6 (LiSAF). Results from preliminary research at Lawrence Livermore National Laboratory indicate that Ce:LiSAF is the preferred material of the colquiriite hosts, since it shows higher gains than Ce:LiCAF. Although the growth of chromium doped LiSAF has been developed to produce long boules of near-scatter-free material, the doping of LiSAF with cerium introduces different variables into the growth of high optical quality crystals. The main crystal growth issue for Ce:LiSAF is the charge compensation mandated by the substitution of the trivalent cerium ion for divalent strontium, which is located in the only site large enough to support the cerium ion. Initial growth runs produced opaque, heavily cracked crystals with less than 10 mm of cleaner, single crystal material. The addition of charge compensating ions into the starting charge and their effect in attaining higher doped and less-stressed material is discussed. The selection of growth parameters needed to produce higher-doped, near-scatter-free Ce:LiSAF will also be described.
The ultraviolet laser properties of LiSrAlF6 doped with trivalent cerium (Ce:LiSAF) and LiCaAlF6 (Ce:LiCAF) are reported. The polarized absorption, emission and excited state absorption cross sections for both Ce:LiSAF and Ce:LiCAF were determined for the 266 nm pump and 290 nm laser emission wavelengths. The single exponential emission lifetime was measured to be 28 +/- 2 and 25 +/- 2 ns for Ce:LiSAF and Ce:LiCAF, respectively. The gain was found to be highest when the probe was polarized parallel to the optic axis of the crystal due to the anisotropic nature of the excited state absorption. The excited state absorption is ascribed to a transition from the 5d orbital of Ce3+ to the conduction band of the host. Laser slope efficiencies of up to 29% and 21% were measured for Ce:LiSAF and Ce:LiCAF when the 266 nm pulsed pump beam was polarized parallel to the optic or c- axis. These are the highest laser slope efficiencies yet observed for any known solid-state ultraviolet laser material. When the pump beam was polarized perpendicular to the optic axis of the crystals, the gain and laser efficiencies dropped by up to an order of magnitude. The magnitude of pump-induced solarization was found to be highly variable for different crystals, and to have insignificant impact on the laser performance when favorable crystals were utilized.
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