Christopher W. Carr

Growth behavior of laser-induced damage on fused silica optics under UV, ns laser irradiation

The growth behavior of laser-induced damage sites is affected by a large number of laser parameters as well as site morphology. Here we investigate the effects of pulse duration on the growth rate of damage sites located on the exit surface of fused silica optics. Results demonstrate a significant dependence of the growth parameters on laser pulse duration at 351 nm from 1 ns to 15 ns, including the observation of a dominant exponential versus linear, multiple-shot growth behavior for long and short pulses, respectively. These salient behaviors are tied to the damage morphology and suggest a shift in the fundamental growth mechanisms for pulses in the 1-5 ns range.

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).

Effect of temporal pulse shape on optical damage

The conditions under which optical materials are susceptible to laser-induced damage is a topic which has been the subject of considerable study. Laser parameters such as wavelength and temporal pulse duration have been studied extensively. Until this work the effect of temporal pulse shape has not been considered. The authors present here data from a simple single-parameter model and a supporting experiment which predicts that a flat-in-time pulse will produce damage at approximately 80% of the fluence of a Gaussian pulse of the same duration.

System for evaluation of laser-induced damage performance of optical materials for large aperture lasers

The evaluation of optical components in various laser systems, with regard to their resistance to laser-induced damage, has often relied on measuring damage threshold fluences. For large-aperture laser systems a small amount of damage in optics does not impede performance. This necessitates the development of damage testing instrumentation that can directly provide information regarding beam obscuration. The number and size of damage scattering sites for a specific laser fluence, wavelength, and pulse duration determine overall beam losses due to damage. We present a design for rapid quantitative characterization of bulk damage performance of optical materials for use in large-aperture laser systems.

Probability of growth of small damage sites on the exit surface of fused silica optics

Growth of laser damage on fused silica optical components depends on several key parameters including laser fluence, wavelength, pulse duration, and site size. Here we investigate the growth behavior of small damage sites on the exit surface of SiO₂ optics under exposure to tightly controlled laser pulses. Results demonstrate that the onset of damage growth is not governed by a threshold, but is probabilistic in nature and depends both on the current size of a damage site and the laser fluence to which it is exposed. We also develop models for use in growth prediction. In addition, we show that laser exposure history also influences the behavior of individual sites.

Complex morphology of laser-induced bulk damage in K2H(2−x)DxPO4 crystals

We present a detailed study of the morphology of laser-induced bulk damage in K2H(2−x)DxPO4 crystals. We see three distinct regions of the internal damage sites: a rubble filled core, a shell that has probably been melted and compacted, and a larger outer region of slightly modified shocked material. The nature of these regions is important for understanding the impact of laser-induced damage on light scattering, which can be significant for high-power laser operation. We propose a simple model to explain the morphology and light scattering we observe.We present a detailed study of the morphology of laser-induced bulk damage in K2H(2−x)DxPO4 crystals. We see three distinct regions of the internal damage sites: a rubble filled core, a shell that has probably been melted and compacted, and a larger outer region of slightly modified shocked material. The nature of these regions is important for understanding the impact of laser-induced damage on light scattering, which can be significant for high-power laser operation. We propose a simple model to explain the morphology and light scattering we observe.

The effect of laser pulse shape and duration on the size at which damage sites initiate and the implications to subsequent repair

Growth of laser damage on SiO(2) optical components used in high power lasers can be reduced or eliminated by pre-exposure to pulses of a few hundred ps in duration. Such pre-exposure would cause weak locations on the optics surface to self-identify by initiating very small damage sites. The sites which initiate will be only a few microns in diameter and will have a very low probability of growing even without any further treatment. Repairing damage sites when small is important because both laser mitigation and acid etching are very successful in preventing such small sites from growing.

The effect of laser pulse duration on laser-induced damage in KDP and SiO2

We examine the effect of pulse duration on both density and morphology of laser-induced damage in KDP and SiO2. In both materials the density of damage sites scales with pulse duration to the ~ 0.4 power for 351-nm pulses between 1 and 10 ns. In SiO2 three types of damage sites are observed. The sizes of the largest of these sites as well as the size of KDP damage sites scale approximately linearly with pulse duration. Similarities of damage in very different materials points to properties of laser-induced damage which are material independent and give insight to the underlying physics of laser-induced damage.

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.

Pulse length dependence of laser conditioning and bulk damage in KD2PO4

An experimental technique has been developed to measure the damage density ρ(Φ) variation with fluence from scatter maps of bulk damage sites in plates of KD2PO4 (DKDP) crystals combined with calibrated images of the damaging beams spatial profile. Unconditioned bulk damage in tripler-cut DKDP crystals has been studied using 351 nm (3ω) light at pulse lengths of 0.055, 0.091, 0.30, 0.86, 2.6, and 10 ns. It is found that there is less scatter due to damage at fixed fluence for longer pulse lengths. The results also show that for all the pulse lengths the scatter due to damage is a strong function of the damaging fluence. It is determined that the pulse length scaling for bulk damage scatter in unconditioned DKDP material varies as τ0.24±0.05 over two orders of magnitude of pulse lengths. The effectiveness of 3ω laser conditioning at pulse lengths of 0.055, 0.096, 0.30, 0.86, 3.5, and 23 ns is analyzed in term of damage density ρ(Φ) at 3ω, 2.6 ns. The 860 ps conditioning to a peak irradiance of 7 GW/cm2 had the best performance under 3ω, 2.6 ns testing. It is shown that the optimal conditioning pulse length appears to lies in the range from 0.3 to 1 ns with a low sensitivity of 0.5 J/cm2/ns to the exact pulse length.