Ionizing-Radiation-Induced Color Centers in YAG, Nd:YAG, and Cr:Nd:YAG: Developing and Analyzing a Radiation-Hard Laser Gain Medium
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Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.Abstract
This report presents results from a series of experiments in which YAG samples (undoped, as well as doped with Nd and Cr3+) were exposed to ionizing radiation (gamma rays and UV). These experiments were performed for the purpose of investigating the various phtodarkening processes taking place in these materials in response to the ionizing radiation. The purpose of this investigation was to establish whether and how co-doping YAG with Cr3+ improves the material's resistance to photodarkening due to the ionizing radiation. The experiments tracked time-resolved transmittance of the samples at 1064 nm in immediate response a pulsed exposure to the ionizing radiation, as well as steady-state spectrally-resolved changed in the samples' optical absorption after the irradiation. The investigation revealed a number of photodarkening processes occurring in the samples in response to the ionizing radiation, both transient and permanent. It was further revealed that inclusion of Cr3+ ions in YAG significantly reduces these photodarkening processes, improving the material's radiation resistance. It was observed that materials containing at least 0.5 at% Cr3+ are essentially radiation-hardened, resisting both transient and steady-state changes in transmittance observed in the materials not containing Cr3+.Type
textElectronic Dissertation
Degree Name
Ph.D.Degree Level
doctoralDegree Program
Optical SciencesGraduate College
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University of ArizonaCollections
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Investigation of Ionizing-Radiation-Induced Photodarkening in Rare-Earth-Doped Optical Fiber Amplifier MaterialsMotivated by an increasing demand for functionality and reliability of systems operating in harsh, ionizing-radiation environments, the core of the present research is an investigation of the response of rare-earth-doped, aluminosilicate fibers to ionizing radiation. These rare-earth-doped fibers, consisting of fibers doped with ions of erbium (Er³⁺) and ytterbium (Yb³⁺) designed for use in amplifier systems, reveal average specific losses in response to ⁶⁰Co gamma radiation to be in the range of 0.0285 - 0.193 dB/(m•krad(Si)) at wavelengths from 1300 nm to 1400 nm. An ionizing dose rate dependence was identified in which high dose rates of approximately 40 rad(Si)/s invariably lead to higher induced losses than lower dose rates of approximately 14 rad(Si)/s, indicating the possibility of complex radiation-related phenomena underlying the observed absorption. Data clearly show that Er³⁺-doped fibers are more sensitive to ionizing-radiation in comparison to Yb³⁺-doped fibers, while Er³⁺/Yb³⁺ co-doped fibers are found to be the least sensitive to radiation of all the fibers examined. Evidence of color center formation associated with the dopant aluminum is found in results of visible spectroscopy conducted on gamma-irradiated preform samples and on fibers flown in low-Earth orbit. Near infrared spectroscopic data is consistent with absorption derived from this dopant as well, with the interpretation of band-tailing from the visible portion of the spectrum. Evidence of the formation of a defect intrinsic to the silicate host matrix, the Non-Bridging Oxygen Hole Center (NBOHC), is also found following ionizing radiation of the optical fiber preforms. Since the observed ionizing-radiation-induced absorption is concentrated in the visible portion of the spectrum, the performance of actively operated rare-earth-doped amplifiers is largely impacted by the pump wavelength, which is located at higher energies within the near-infrared portion of the spectrum and therefore closer to the visible portion of the spectrum than the lasing wavelength. Experimental results stemming from rare-earth-doped amplifiers operated under ionizing radiation substantiate the importance of the pumping wavelength, and suggest the presence of cascaded pump photon absorption processes. Based on these results, pumping at longer wavelengths is advised to reduce the effect of color center absorption on this crucial aspect of active fiber amplifier operation.
