Revisiting an airgap split-optics mitigation for beam filamentation in high power lasers.

We extend the split-optic approach for mitigating filamentation in a thick optical component previously proposed for small beams to conditions relevant to high-power lasers. The split-optic approach divides a thick optic into two thinner optics separated by an airgap to reduce filamentation through diffraction management. Our numerical study focuses on filamentation of a flat-top beam with intensity modulation noise sources passing through a split-optic system. The improvement in the distance to collapse in glass is shown to be potentially substantial (>30%), yet has limited increase with the airgap size, unlike the common understanding when considering a collapse of a whole beam or a sole perturbation on a beam. The improvement in the collapse distance in glass asymptotes to an upper bound value that depends mainly on the beam mean intensity and its contrast for any airgap size above some value that depends mainly on the shortest spatial periods comprising the excitation noise source. Examining the difference in the simulation results for a periodic versus a randomly generated perturbation source-term suggests that the observed effect is governed by the statistical interference dynamics of the beam while propagating through the airgap.

Enhancement of laser material drilling using high-impulse multi-laser melt ejection.

Laser drilling and cutting of materials is well established commercially, although its throughput and efficiency limit applications. This work describes a novel approach to improve laser drilling rates and reduce laser system energy demands by using a gated continuous wave (CW) laser to create a shallow melt pool and a UV ps-pulsed laser to impulsively expel the melt efficiency and effectively. Here, we provide a broad parametric study of this approach applied to common metals, describing the role of fluence, power, spot size, pulse-length, sample thickness, and material properties. One to two order-of-magnitude increases in the average removal rate and efficiency over the CW laser or pulsed-laser alone are demonstrated for samples of Al and stainless steel for samples as thick as 3 mm and for holes with aspect ratios greater than 10:1. Similar enhancements were also seen with carbon fiber composites. The efficiency of this approach exceeds published values for the drilling of these materials in terms of energy to remove a given volume of material. Multi-laser material removal rates, high-speed imaging of ejecta, and multi-physics hydrodynamic simulations of the melt ejection process are used to help clarify the physics of melt ejection leading to these enhancements. Our study suggests that these high-impulse multi-laser enhancements are due to both laser-induced surface wave instabilities and cavitation of the melt for shallow holes and melt cavitation and ejection for deeper channels.

Revisiting beam filamentation formation conditions in high power lasers.

The Bespalov-Talanov gain (BT-gain) and IL-rule (i.e., the product of input intensity and self-focusing length is constant) expressions are examined and generalized for filamentation under realistic conditions associated with high power lasers: filamentation seeded by both amplitude and phase perturbations on a large, flat-top beam, and the impact of cross-phase modulation from unconverted light in UV frequency-converted lasers. The validity of these models is examined with NLSE numerical calculations, which show that there are parameters beyond the commonly-used IL rule, such as the perturbation amplitude and period content. The BT-gain model presents a fair description of the tendency of spatial periods to filament, but not of the quantitative self-focusing length. Spatial filtering of short periods is shown to suppress filamentation, due to both, the removal of the more prone to filament periods, as well as the reduction of the spatial intensity amplitude root-mean-square. At the edge of a top hat beam we find that the IL product reduces in the roll-off regions, even though the self-focusing length increases. When adding a co-propagating harmonic, we find that the cross-phase modulation (XPM) could enhance or inhibit the filamentation formation, depending on the perturbation period.

Gigashot optical degradation in silica optics at 351 nm.

As applications of lasers demand higher average powers, higher repetition rates, and longer operation times, optics will need to perform well under unprecedented conditions. We investigate the optical degradation of fused silica surfaces at 351 nm for up to 10(9) pulses with pulse fluences up to 12 J/cm(2). The central result is that the transmission loss from defect generation is a function of the pulse intensity, I(p), and total integrated fluence, φ(T), and is influenced by oxygen partial pressure. In 10(-6) Torr vacuum, at low I(p), a transmission loss is observed that increases monotonically as a function of number of pulses. As the pulse intensity increases above 13 MW/cm(2), the observed transmission losses decrease, and are not measureable for 130 MW/cm(2). A physical model which supports the experimental data is presented to describe the suppression of transmission loss at high pulse intensity. Similar phenomena are observed in anti-reflective sol-gel coated optics. Absorption, not scattering, is the primary mechanism leading to transmission loss. In 2.5 Torr air, no transmission loss was detected under any pulse intensity used. We find that the absorption layer that leads to transmission loss is less than 1 nm in thickness, and results from a laser-activated chemical process involving photo-reduction of silica within a few monolayers of the surface. The competition between photo-reduction and photo-oxidation explains the measured data: transmission loss is reduced when either the light intensity or the O(2) concentration is high. We expect processes similar to these to occur in other optical materials for high average power applications.

Model laser damage precursors for high quality optical materials.

Surface damage is known to occur at fluences well below the intrinsic limit of the fused silica. A native surface precursor can absorb sub band-gap light and initiate a process which leads to catastrophic damage many micrometers deep with prominent fracture networks. Previously, the absorption front model of damage initiation has been proposed to explain how this nano-scale absorption can lead to macro-scale damage. However, model precursor systems designed to study initiation experimentally have not been able to clearly reproduce these damage events. In our study, we create artificial absorbers on fused silica substrates to investigate precursor properties critical for native surface damage initiation. Thin optically absorbing films of different materials were deposited on silica surfaces and then damage tested and characterized. We demonstrated that strong interfacial adhesion strength between absorbers and silica is crucial for the launch of an absorption front and subsequent damage initiation. Simulations using the absorption-front model are performed and agree qualitatively with experimental results.

Extracting the distribution of laser damage precursors on fused silica surfaces for 351 nm, 3 ns laser pulses at high fluences (20-150 J/cm2).

Surface laser damage limits the lifetime of optics for systems guiding high fluence pulses, particularly damage in silica optics used for inertial confinement fusion-class lasers (nanosecond-scale high energy pulses at 355 nm/3.5 eV). The density of damage precursors at low fluence has been measured using large beams (1-3 cm); higher fluences cannot be measured easily since the high density of resulting damage initiation sites results in clustering. We developed automated experiments and analysis that allow us to damage test thousands of sites with small beams (10-30 µm), and automatically image the test sites to determine if laser damage occurred. We developed an analysis method that provides a rigorous connection between these small beam damage test results of damage probability versus laser pulse energy and the large beam damage results of damage precursor densities versus fluence. We find that for uncoated and coated fused silica samples, the distribution of precursors nearly flattens at very high fluences, up to 150 J/cm2, providing important constraints on the physical distribution and nature of these precursors.