Joel A. Speth

Laser demonstration of Yb/sub 3/Al/sub 5/O/sub 12/ (YbAG) and materials properties of highly doped Yb:YAG

We have demonstrated the first stoichiometric Yb/sup 3+/ laser based on Yb/sub 3/A/sub 5/O/sub 12/ (YbAG). The laser operated in pulsed mode with a highest possible duty cycle of 85%. A slope efficiency of 27%, with respect to absorbed energy, was measured and the free-running lasing wavelength was 1048 nm for a 10% duty cycle. In a systematic analysis, measurements of spectroscopic and materials properties of (Yb/sub x/Y/sub 1-x/)/sub 3/Al/sub 5/O/sub 12/ for nominal x values of 0.05, 0.1, 0.15, 0.18, 0.25, 0.5, and 1 are reported. We also present a formalism to calculate the intrinsic fluorescence quantum efficiency (free of radiation trapping) and the fraction of reabsorbed light, based on measurements of the bulk and intrinsic emission lifetimes and the fractional thermal loading. Our best YbAG sample has an intrinsic lifetime of 0.664 ms at 94% quantum efficiency and a thermal conductivity at room temperature of 0.072 W/(cm-K).

Ultraviolet laser emission properties of Ce 3+ -doped LiSrAlF 6 and LiCaAlF 6

The UV laser properties of trivalent cerium-doped LiSrAlF6 (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 optical axis of the crystal, owing to the anisotropic nature of the excited-state absorption. The excited-state absorption was ascribed to a transition from the 5d orbital of Ce3+ to the conduction band of the host. Laser slope efficiencies as high as 29% and 21%, respectively, were measured for Ce:LiSAF and Ce:LiCAF when the 266-nm pulsed pump beam was polarized parallel to the optical or the c axis. These are the highest laser slope efficiencies yet observed to our knowledge for any known solid-state UV laser material. When the pump beam was polarized perpendicular to the optical axis of the crystals, the gain and the laser efficiencies dropped by as much as an order of magnitude. The magnitude of pump-induced solarization was found to be highly variable for different crystals and to have an insignificant effect on the laser performance when favorable crystals were used.

115-W Tm:YAG diode-pumped solid-state laser

A compact diode-pumped Tm:YAG laser capable of generating greater than 100 W of CW power at 2 /spl mu/m has been demonstrated. A scalable diode end-pumping architecture is used in which 805-nm radiation, coupled to the wing of the Tm/sup 3+3/H/sub 6/-/sup 3/H/sub 4/ absorption feature, is delivered to the end of the laser rod via a lens duct. To facilitate thermal management, undoped YAG end caps are diffusion bonded to the central doped portion of the laser rod. For 2% and 4% Tm-doped rods of the same length, the lower doping level results in higher power, indicating that cross relaxation is still efficient while offering lower thermal stress and reduced absorption at the laser wavelength. Output powers for various output coupler reflectivities are compared to the predictions of a quasi-three-level model. Thermal lensing, cavity stability, and stress-induced birefringence measurements are described. The beam quality was analyzed with the 2% Tm-doped rod and a flat output coupler, yielding M/sup 2/ values of 14-23.

Induced optical absorption in gamma, neutron and ultraviolet irradiated fused quartz and silica

Abstract We have investigated the effects of ultraviolet (4.7 eV), gamma (∼ 1 MeV), and neutron (∼ 1 MeV) irradiations on the optical properties of SiO 2 glass samples. Samples from various sources were studied, including synthetic fused silicas and natural fused quartz. Several relationships among the different types of ionizing radiation were examined. For example, both UV light and gamma rays convert the germanium impurity to the B 1 absorption band in fused quartz samples. On the other hand, only neutrons are capable of inducing the oxygen deficient centers (ODCs) with the ∼krad-level doses employed here; the ODCs are produced by way of direct knock-on collisions. The ODCs generated by the neutrons can be converted into E′ centers afterwards with γ-rays. Transient grating and pump-probe optical experiments show that only a small fraction of the induced defect absorption remains permanently, while nearly all recover to the original condition after a UV pulse. Finally, thermal annealing experiments indicate that the radiation-induced defects can be annealed away at temperatures > 350°C. We compare the impacts of gamma, neutron, and ultraviolet radiation in terms of the mechanism by which defects are generated.

Analysis of an intracavity-doubled diode-pumped Q-switched Nd:YAG laser producing more than 100 W of power at 0.532 µm

A diode-pumped Nd:YAG laser was frequency doubled to 0.532 microm with an intracavity KTiOPO(4) crystal in a V-cavity arrangement, achieving an output power of 140 W. Acousto-optic Q switching was employed at repetition rates of 10-30 kHz, and the beam quality was assessed at M(2) approximately 50. It was deduced on the basis of our model that the strength of the nonlinear frequency conversion is the main parameter determining the pulse width.

Fused Silica Final Optics for Inertial Fusion Energy: Radiation Studies and System-Level Analysis

The survivability of the final optic, which must sit in the line of sight of high-energy neutrons and gamma rays, is a key issue for any laser-driven inertial fusion energy (IFE) concept. Previous work has concentrated on the use of reflective optics. Here, we introduce and analyze the use of a transmissive final optic for the IFE application. Our experimental work has been conducted at a range of doses and dose rates, including those comparable to the conditions at the IFE final optic. The experimental work, in conjunction with detailed analysis, suggests that a thin, fused silica Fresnel lens may be an attractive option when used at a wavelength of 351 nm. Our measurements and molecular dynamics simulations provide convincing evidence that the radiation damage, which leads to optical absorption, not only saturates but that a “radiation annealing” effect is observed. A system-level description is provided, including Fresnel lens and phase plate designs.

Ce : LiSrAlF6 laser performance with antisolarant pump beam

Abstract We have explored the impact of 266 nm pump-induced solarization on the 290 nm laser performance of Ce: LiSrAlF 6 crystals. Among the issues considered are the incorporation of codopants (e.g. Na + , Mg 2+ , Zn 2+ ), and the use of an additional 532 nm beam to rapidly destroy the interfering color centers. The solarization mechanism has been unraveled and found to involve two-photon creation of color centers (via the 4f→ 5d→ conduction band pathway of Ce 3+ ), followed by the one-photon bleaching of the color centers. Ce : LiSrAlF 6 (Ce : LiSAF) laser slope efficiencies as high as 47% can be achieved with the simultaneous introduction of the 266 nm pump and 532 nm bleaching beams; 33% with the 266 nm beam only.

115 W Tm:YAG CW diode-pumped solid-state laser

High-power lasers operating at wavelengths around 2 µm are useful for a wide variety of applications ranging from laser surgery, material processing and coherent lidar, to serving as a pump source for mid-IR optical parametric oscillators.

Penetrating radiation impact on NIF final optic components

The principal technical goal of the National Ignition Facility (NIF) is to achieve thermonuclear ignition in a laboratory environment via inertial confinement fusion. This will enable NIF to service the DOE stockpile stewardship management program, inertial fusion energy goals, and advance scientific frontiers. All of these applications will make use of the extreme conditions that the facility will create in the target chamber. In the case of a projected 20 MJ yield scenario, NIF will produce approximately 1019 neutrons with a characteristic DT fusion 14 MeV energy per neutron. There will also be a substantial amount of high energy x-rays as well as solid, liquid and gaseous target debris produced with directly or indirectly by the inertial confinement fusion process. A critical design issue is the protection of the final optical components as well as sophisticated target diagnostics in such a harsh environment.

Neutron and gamma irradiated optical property changes for the final optics of the National Ignition Facility

Based on studies we have performed with several radiation sources such as pulsed nuclear reactors, we have been able to construct a physical picture and measure quantitative parameters necessary to model the radiation-induced losses expected for fused silica and fused quartz National Ignition Facility (NIF) target area. It is important to note that these surrogate radiation sources do not have identical temporal and spectral characteristics to NIF, therefore caution is in order since the results obtained to date must be extrapolated somewhat to predict NIF performance.