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Characterization of Light Scattering in Transparent Polycrystalline Laser Ceramics


Limitations in single-crystal growth technology have led to the development of transparent

Polycrystalline Laser Materials (PLMs) as viable alternatives towards fabricating large

dimension laser gain media having engineered dopant profiles to improve the thermo-optic

properties and continue power scaling of solid-state laser systems operating in the 1 - 5 μm

wavelength range. Bulk scattering due to non-uniform refractive index distribution is the

primary loss mechanism in the PLMs, resulting in the degradation of laser performance and

lower output power. The objective of this dissertation was to formulate a methodology for fast,

reliable and accurate identification and characterization of bulk scatter in Erbium (Er) and

Neodymium (Nd) doped YAG (Yttrium Aluminum Garnet - Y3Al5O12) and Yttria (Yttrium

Oxide - Y2O3) PLMs. Scanning Electron Microscopy, Transmission Electron Microscopy, and Confocal Laser Scanning Microscopy were used for material characterization and investigation

of PLM microstructures at various stages of fabrication, thus identifying the source of refractive index inhomogeneities, which result in bulk scattering. Three optical characterization

techniques, Transmitted Beam Wavefront Profiling (TBWP), Angle Resolved Scatter (ARS)

measurements, and Schlieren Imaging, to identify bulk scatter in PLMs, are developed. TBWP

was able to directly and quickly image the distortions introduced to a propagating laser beam

caused by the presence of bulk scattering in the PLMs. ARS was able to map the distribution of

the scattered intensity in space, and was found to be very sensitive for comparing samples. A

modified white light Schlieren imaging setup utilizing variable focusing capability, demonstrated

high sensitivity for directly imaging local spatial variations in refractive index across and

through the entire PLM regardless of dimensions. Finally, by comparing laser performance of

0.9% Nd:YAG single crystal, high quality and low quality PLMs, with results from TBWP and

Schlieren Images, the utility of characterization techniques towards identifying bulk scatter and

assessing the optical quality are validated. The methods developed are applicable towards the

characterization of any transparent material exhibiting bulk scatter.

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