The Mercury laser uses ytterbium-doped strontium fluorapatite (Yb:S-FAP) crystals as the gain medium with a nominal
clear aperture of 4 x 6 cm. Recent damage test data have indicated the existence of bulk precursors in Yb:S-FAP that
initiate damage starting at approximately 10 J/cm2 at 9 ns under 1064 nm irradiation. In this paper, we report on
preliminary results on bulk damage studies on Yb:S-FAP crystals.
Antireflection (AR) coatings typically damage at the interface between the substrate and coating. Therefore the substrate finishing technology can have an impact on the laser resistance of the coating. For this study, AR coatings were deposited on Yb:S-FAP [Yb3+:Sr5(PO4)3F] crystals that received a final polish by both conventional pitch lap finishing as well as magnetorheological finishing (MRF). SEM images of the damage morphology reveals laser damage originates at scratches and at substrate coating interfacial absorbing defects. Previous damage stability tests on multilayer mirror coatings and bare surfaces revealed damage growth can occur at fluences below the initiation fluence. The results from this study suggest the opposite trend for AR coatings. Investigation of unstable HR and uncoated surface damage morphologies reveals significant radial cracking that is not apparent with AR damage due to AR delamination from the coated surface with few apparent cracks at the damage boundary. Damage stability tests show that coated Yb:S-FAP crystals can operate at 1057 nm at fluences around 20 J/cm2 at 10 ns; almost twice the initiation damage threshold.
Space debris constitutes a significant hazard to low earth orbit satellites and particularly to manned spacecraft. A quite small velocity decrease from vaporization impulses is enough to lower the perigee of the debris sufficiently for atmospheric drag to de-orbit the debris. A short pulse (picosecond) laser version of the Orion concept can accomplish this task in several years of operation. The "Mercury" short pulse Yb:S-FAP laser being developed at LLNL for laser fusion is appropriate for this task.
We report initial operation of the Mercury laser with seven 4 x 6 cm S-FAP amplifier slabs pumped by four 80 kW diode arrays. The system produced up to 33.5 J single shot, 23.5 J at 5 Hz, and 10 J at 10 Hz for 20 minute runs at 1047 nm. During the initial campaign, more than 2.8 x 104 shots were accumulated on the system. The beam quality of the system was measured to be 2.8 x 6.3 times diffraction limited at 110 W of output, with 96% of the energy in a 5X diffraction limited spot. Static wavefront glass plates were used to correct for the low order distortions in the slabs due to fabrication and thermal loading. Scaling of crystal grown has begun with the first full size slab produced from large diameter growth. Using an energetics optimization code we find the beam aperture is scalable up to 20 x 30 cm and 4.2 kJ.
A scaleable diode end-pumping technology for high-average- power slab and rod lasers has been under development for the past several years at Lawrence Livermore National Laboratory (LLNL). This technology has particular application to high average power Yb:YAG lasers that utilize a rod configured gain element. Previously, this rod configured approach has achieved average output powers in a single 5 cm long by 2 mm diameter Yb:YAG rod of 430 W cw and 280 W q-switched. High beam quality (M2 equals 2.4) q-switched operation has also been demonstrated at over 180 W of average output power. More recently, using a dual rod configuration consisting of two, 5 cm long by 2 mm diameter laser rods with birefringence compensation, we have achieved 1080 W of cw output with an M2 value of 13.5 at an optical-to-optical conversion efficiency of 27.5%2. With the same dual rod laser operated in a q-switched mode, we have also demonstrated 532 W of average power with an M2 less than 2.5 at 17% optical-to-optical conversion efficiency. These q-switched results were obtained at a 10 kHz repetition rate and resulted in 77 nsec pulse durations. These improved levels of operational performance have been achieved as a result of technology advancements made in several areas that will be covered in this manuscript. These enhancements to our architecture include: (1) Hollow lens ducts that enable the use of advanced cavity architectures permitting birefringence compensation and the ability to run in large aperture-filling near-diffraction-limited modes. (2) Compound laser rods with flanged-nonabsorbing-endcaps fabricated by diffusion bonding. (3) Techniques for suppressing amplified spontaneous emission (ASE) and parasitics in the polished barrel rods.
We have begun building the 'Mercury' laser system as the first in a series of new generation diode-pumped solid-state lasers for inertial fusion research. Mercury will integrate three key technologies: diodes, crystals, and gas cooling, within a unique laser architecture that is scalable to kilojoule energy levels for fusion energy applications. The primary performance goals include 10 percent electrical efficiencies at 10 Hz and 100J with a 2-10 ns pulse length at 1.047 micrometers wavelength. When completed, Mercury will allow rep-rated target experiments with multiple target chambers for high energy density physics research.
We present the energy, propagation, and thermal modeling for a diode-pumped solid-state laser called Mercury being designed and built at LLNL using Yb:S-FAP [i.e., Yb3+-doped Sr5(PO4)3F crystals] for the gain medium. This laser is intended to produce 100 J pulses at 1 to 10 ns at 10 Hz with an electrical efficiency of approximately 10%. Our modeling indicates that the laser will be able to meet its performance goals.
The combination of our unique capabilities at LLNL in diode development, crystal growth, and system designs have allowed us to pursue a variety of research areas of interest to the commercial, medical, and defense industries. We have developed a flexible diode pumping technology which utilizes low cost silicon microchannel coolers to enable high average power diode operation and a shaped cylindrical microlens technology which allows the radiance conditioning of large two- dimensional laser diode arrays. The flexibility that this diode technology has brought to pump power generation in both average power and radiance have broadly expanded the number of ion-host combinations that can be efficiently excited and used in diode pumped solid state lasers. Some of our laser developments are briefly discussed.
We report on the design and successful fabrication of a dichroic multilayer stack using a procedure that allowed shifting from high reflectance to high transmittance within 89 nm and surviving high laser fluences. A design approach based on quarter-wave thick layers allowed the multilayer stack to be optically tuned in the last layers of the stack. In our case, this necessitated removing the samples from the coating chamber for a transmittance scan prior to depositing the last layers. This procedure is not commonly practiced due to thermal stress- induced failures in an oxide multilayer. However, D. J. Smith and co-workers reported that reactive e-beam evaporated hafnia from a Hf source produced laser-resistant coatings that had less coating stress compared to coatings evaporated from a HfO2 source. Tuned dichroic coatings were made that had high transmittance at 941 nm and high reflectance at 1030 nm. The coating was exposed for 5 minutes to a 100 kW/cm2 1064 nm (180-ns pulsewidth, 10.7 kHz) laser beam and survived without microscopic damage. The same coating survived a 140 kW/cm2 of laser intensity without catastrophic damage before optical tuning was performed.
To provide high-energy, high-power beams at short wavelengths for inertial-confinement-fusion experiments the authors rountinely converted the 1.053-micrometers output of the Nova, Nd:phosphate-glass, laser system to its third-harmonic wavelength. We describe performance and conversion efficiency modeling of the 3 X 3 arrays potassium-dihydrogen-phosphate crystal plates used for type II/type II phase-matched harmonic conversion of Nova 0.74-m diameter beams, and an alternate type I/type II phase-matching configuration that improves the third-harmonic conversion efficiency. These arrays provide energy conversion of up to 65% and intensity conversion to 70%.