Mike C. Nostrand

New laser crystals based on KPb2Cl5 for IR region

Abstract The KPb2Cl5 single crystals, doped by different Rare Earth ions were grown. The RE segregation coefficient was found to decrease from 1.0 to 0.15 in the set from Nd to Yb. Spectroscopic properties and luminescence decay were studied, stimulated emission was demonstrated at 1.06 μm (Nd3+) and at 2.43 μm (Dy3+).

Techniques for qualitative and quantitative measurement of aspects of laser-induced damage important for laser beam propagation

Characterizing laser-induced damage in optical materials is important for laser design and operation. Previous methods of evaluating optical materials damage resistance to high-power laser irradiation have typically suffered from shot-to-shot uncertainties in laser energy output and/or have insufficient sensitivity. More importantly, such methods do not address the aspects of laser-induced damage important to laser beam propagation, namely the amount of light scattered by the damage. We present a method for the quantitative correlation of material modification on the surface or in the bulk of optical materials to laser parameters, which deconvolutes the effects of laser output instability. In image analysis, two images, one a fluence spatial profile and the other a visible light scatter image of the damage, are directly compared to extract scatter as a function of fluence. An automated microscope is used to record the location and number of bulk damage sites and determine a calibration factor between the scatter signal observed and damage density. We illustrate the method with a determination of both bulk damage density as a function of laser fluence and of a representative size distribution in a DKDP crystal. Our method is capable of determining damage densities with an absolute uncertainty of ±0.3 pinpoint damage sites per cubic millimetre (pp mm−3) in the range 1–100 pp mm−3 with our minimum detectable density being 0.01 pp mm−3. We also determined the pps produced by laser pulses of 351 nm, 3 ns light to have a mean diameter of 5.5 ± 2.5 µm (1/e2).

Room-temperature laser action at 4.3–4.4 µm in CaGa 2 S 4 :Dy 3+

We report room-temperature mid-IR laser operation in a new low-phonon-frequency nonhygroscopic host crystal, calcium thiogallate (CaGa(2)S(4)) . Laser action at 4.314.38 mum on the Dy(3+)H(11/2)(6)?H(13/2)(6) transition occurred with a maximum slope efficiency of 1.6%.

Damage mechanisms avoided or managed for NIF large optics

Abstract After every other failure mode has been considered, in the end, the high-performance limit of all lasers is set by optical damage. The demands of inertial confinement fusion (ICF) pushed lasers designed as ICF drivers into this limit from their very earliest days. The first ICF lasers were small, and their pulses were short. Their goal was to provide as much power to the target as possible. Typically, they faced damage due to high intensity on their optics. As requests for higher laser energy, longer pulse lengths, and better symmetry appeared, new kinds of damage also emerged, some of them anticipated and others unexpected. This paper will discuss the various types of damage to large optics that had to be considered, avoided to the extent possible, or otherwise managed as the National Ignition Facility (NIF) laser was designed, fabricated, and brought into operation. It has been possible for NIF to meet its requirements because of the experience gained in previous ICF systems and because NIF designers have continued to be able to avoid or manage new damage situations as they have appeared.

Engineered defects for investigation of laser-induced damage of fused silica at 355 nm

Embedded gold and mechanical deformation in silica were used to investigate initiation of laser-induced damage at 355 nm (7.6 ns). The nanoparticle-covered surfaces were coated with between 0 and 500 nm of SiO2 by e-beam deposition. The threshold for observable damage and initiation site morphology for these engineered surfaces was determined. The gold nanoparticle coated surfaces with 500 nm SiO2 coating exhibited pinpoint damage threshold of <0.7 J/cm2 determined by light scattering and Nomarski microscopy. The gold nanoparticle coated surfaces with the 100 nm SiO2 coatings exhibited what nominally appeared to be film exfoliation damage threshold of 19 J/cm2 via light scattering and Nomarski microscopy. With atomic force microscopy pinholes could be detected at fluences greater than 7 J/cm2 and blisters at fluences greater than 3 J/cm2 on the 100-nm-coated surfaces. A series of mechanical indents and scratches were made in the fused silica substrates using a non-indentor. Plastic deformation without cracking led to damage thresholds of approximately 25 J/cm2, whereas indents and scratches with cracking led to damage thresholds of only approximately 5 J/cm2. Particularly illuminating was the deterministic damage of scratches at the deepest end of the scratch, as if the scratch acted as a waveguide.

Optics Recycle Loop Strategy for NIF Operations Above UV Laser-Induced Damage Threshold

Abstract The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) houses the world’s largest laser system, composed of 192 individual, 40-cm-aperture beamlines. The NIF laser routinely operates at ultraviolet (UV) fluences above 8 J/cm2, more than twice the (3ω only) damage threshold of commercially available UV-grade fused silica. NIF is able to maintain such high fluence operation by using an optics recycling loop strategy. Successful operation of the loop relies on a number of technologies specifically developed for NIF. One of the most important is the capability developed by LLNL and their vendors for producing highly damage-resistant optics. Other technologies developed for the optics recycle loop raise the operating point of NIF by keeping damage growth in check. LLNL has demonstrated the capability to sustain UV fused silica optic recycling rates of up to 40 optics per week. The optics are ready for reinstallation after a 3-week trip through a recycle loop where the damage state of each optic is assessed and repaired. The impact of the optics recycle loop has been profound, allowing the experimental program to routinely employ energies and fluences that would otherwise have been unachievable. Without the recycle loop, it is likely that the NIF fluence would need to be kept below the UV threshold for damage growth, ~4 J/cm2, thus keeping the energy delivered to the target significantly below 1 MJ. With the recycle loop implemented during the National Ignition Campaign, NIF can routinely deliver >1.8 MJ on target, an increase in operational capability of more than 100%. In this review, the enabling technological advances, optical performance, and operational capability implications of the optics recycle loop are discussed.

New low-phonon frequency crystals based on rare-earth-doped double halogenides for multiwavelength diode-pumped solid state lasers

Energy (frequency) of phonons is the main parameter which determines ratio between probabilities of radiative and radiationless transitions in luminescence. Single crystals of double chlorides KPb2C15 and bromides KPb2Br5 , which are formed by heavy ions and have a low energy phonon spectrum (h? <200 and 150 cm-1, respectively), of optical quality were obtained using the Bridgmen-Stockbarger technique. We studied the optical spectra and luminescence kinetics of RE —doped crystals (RE= Pr3+, Nd3+, Tb3+, Ho3+, Er3+ etc). Intensity parameters were determined by the Judd-Ofelt method, radiative and non-radiative transition probabilities were calculated. It was shown that low multiphonon relaxation rate in these crystals together with high values of radiative probabilities leads to the evidence of high intensity luminescence in spectral domain from 360 to 9000 nm. These features make these crystals promising for practical applications as active media for UV, VIS and mid-IR solid state lasers and amplifiers with laser diode pumping.

Spectroscopic study of neodymium-doped potassium-lead double chloride Nd3+:KPb2Cl5 crystals

Optical spectra, radiative and nonradiative transition intensities, and luminescence kinetics of neodymium-doped potassium-lead double chloride crystals Nd3+:KPb2Cl5, (Nd3+:KPC) are investigated. Crystals were grown by the Stockbarger-Bridgman technique. Experimental studies of absorption and luminescence spectra are performed, intensity parameters are obtained by the Judd-Ofelt method, radiative transition probabilities and branching ratios are calculated, and nonradiative transition probabilities are estimated. Luminescence kinetics of 2K13/2, 2P3/2, and 4D3/2 radiative levels of neodymium under selective excitation in the 355-nm region are studied.

The National Ignition Facility 2007 laser performance status

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a 192-beam 3.6 MJ neodymium glass laser that is frequency converted to 351nm light. It has been designed to support high energy density science (HEDS), including the demonstration of fusion ignition through Inertial Confinement. To meet this goal, laser design criteria include the ability to generate pulses of up to 1.8-MJ total energy at 351nm, with peak power of 500 TW and precisely-controlled temporal pulse shapes spanning two orders of magnitude. The focal spot fluence distribution of these pulses is conditioned, through a combination of special optics in the 1ω (1053 nm) portion of the laser (continuous phase plates), smoothing by spectral dispersion (SSD), and the overlapping of multiple beams with orthogonal polarization (polarization smoothing). In 2006 and 2007, a series of measurements were performed on the NIF laser, at both 1ω and 3ω (351 nm). When scaled to full 192-beam operation, these results lend confidence to the claim that NIF will meet its laser performance design criteria and that it will be able to simultaneously deliver the temporal pulse shaping, focal spot conditioning, peak power, shot-to-shot reproducibility, and power balance requirements of indirect-drive fusion ignition campaigns. We discuss the plans and status of NIFs commissioning, and the nature and results of these measurement campaigns.

Spectroscopic properties of TR3+ -doped double chloride crystals

We studied the optical spectra and luminescence kinetics of double chloride Kpb2Cl5:TR3+ crystals as a new luminescent material promising for UV, VIS and mid-IR lasers, pumped with laser diodes. Intensity parameters were determined by the Judd-Ofelt method, radiative and non- radiative transition probabilities were calculated. It is shown that low multiphoton relaxation rate in the se crystal together with high values of radiative probabilities leads to the evidence of high intensive luminescence in spectral domain from 360 to 5000 nm. These features make these crystals promising for practical applications as active media for UV, VIS and mid-IR solid state lasers and amplifiers.