Development of Trivalent Ytterbium Doped Fluorapatites for Diode-Pumped Laser Applications

One of the major motivators of this work is the Mercury Project, which is a 1 kW scalable diode-pumped solid-state laser system under development at Lawrence Livermore National Laboratory (LLNL). Major goals include 100 J pulses, 10% wallplug efficiency, 10 Hz repetition rate, and a 5 times diffraction limited beam. To achieve these goals the Mercury laser incorporates ytterbium doped Sr{sub 5}(PO{sub 4}){sub 3}F (S-FAP) as the amplifier gain medium. The primary focus of this thesis is a full understanding of the properties of this material which are necessary for proper design and modeling of the system. Ytterbium doped fluorapatites, which were previously investigated at LLNL, were found to be ideal candidate materials for a high power amplifier systems providing high absorption and emission cross sections, long radiative lifetimes, and high efficiency. A family of barium substituted S-FAP crystals were grown in an effort to modify the pump and emission bandwidths for application to broadband diode pumping and short pulse generation. Crystals of Yb{sup 3+}:Sr{sub 5-x}Ba{sub x}(PO{sub 4}){sub 3}F where x < 1 showed homogeneous lines offering 8.4 nm (1.8 times enhancement) of absorption bandwidth and 6.9 nm (1.4 times enhancement) of emission bandwidth. The gain saturation fluence of Yb:S-FAP was measured to be 3.2 J/cm{sup 2} using a pump-probe experiment where the probe laser was a high intensity Q-switched master oscillator power amplifier system. The extraction data was successfully fit to a homogeneous extraction model. The crystal quality of Czochralski grown Yb:S-FAP crystals, which have been plagued by many defects such as cracking, cloudiness, bubble core, slip dislocations, and anomalous absorption, was investigated interferometrically and quantified by means of Power Spectral Density (PSD) plots. The very best crystals grown to date were found to have adequate crystal quality for use in the Mercury laser system. In addition to phase distortions which are fixed by material growth, thermal loading of the S-FAP media also leads to distortions due to thermal expansion, {alpha}, temperature dependent refractive index, {partial_derivative}n/{partial_derivative}T, and stress optic effects. The stress optic coefficients necessary for modeling thermal distortions in Yb:S-FAP slab amplifiers were measured giving q{sub 33} = 0.308 x 10{sup -12} Pa{sup -1}, and q{sub 31} = 0.936 x 10{sup -12} Pa{sup -1}. Nonlinear optical losses due to high intensity laser interaction with S-FAP were evaluated including Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering. The SRS gain coefficient was measured to be 1.3 cm/GW. The SRS losses in the Mercury amplifier system were successfully modeled and shown to be an issue for high-energy short pulse operation. Countermeasures including the addition of bandwidth to the extraction beam and wedging of amplifier surfaces would allow operation of the Mercury laser at 100 J and 2 ns output below SRS threshold. A simple model of SBS losses in the Mercury laser system shows SBS will also be a problem, however suppression is possible with the introduction of moderate bandwidth (relative to the SRS case). Finally, a Q-switched Yb:S-FAP oscillator was developed which operates three-level at 985 nm with a 21% slope efficiency. Frequency conversion of the 985 nm light to the 2nd harmonic at 492.5 nm was achieved with a 31% conversion efficiency. A diode pumped, doubled Yb:S-FAP laser at 492.5 nm would make a compact efficient blue laser source.