Quantification of laser-induced damage growth using fractal analysis

Laser-induced damage growth of fused silica optics near growth threshold at 351 nm

Laser-induced damage growth on the exit surface of fused silica optics triggered by nanosecond pulses at 351 nm is widely described with exponential dynamics. In this Letter, a particular experimental setup allowed us to study damage growth with a large beam and fluences near damage growth threshold for a high number of shots. This allowed us to observe and characterize a regime with a slow and linear growth dynamic not documented in the literature and yet fundamental for the operation of high-power laser installations.

Optical model-based calibration of gray levels for laser damage size assessment

Fused silica is prone to damage under ultraviolet laser irradiation. Because they are key components to achieve fusion on high energy laser facilities, final fused silica optics are analyzed after each laser shot. The quantification of damage sites is limited by the image resolution. Measurements of scattered light by damage sites allow for sub-pixel detection and growth monitoring after a calibration step based on time-consuming measurements at laser facilities. It is proven herein that modeling laser damage size monitoring based on light scattering is efficient to link gray levels to damage diameters, thereby avoiding any experimental calibration based on reference optics at the facility.

Estimating and monitoring laser-induced damage size on glass windows with a deep-learning-based pipeline

Laser-induced damage is a major issue in high power laser facilities such as the Laser MégaJoule (LMJ) and National Ignition Facility (NIF) since they lower the efficiency of optical components and may even require their replacement. This problem occurs mainly in the final stages of the laser beamlines and in particular in the glass windows through which laser beams enter the central vacuum chamber. Monitoring such damage sites in high energy laser facilities is, therefore, of major importance. However, the automatic monitoring of damage sites is challenging due to the small size of damage sites and to the low-resolution images provided by the onsite camera used to monitor their occurrence. A systematic approach based on a deep learning computer vision pipeline is introduced to estimate the dimensions of damage sites of the glass windows of the LMJ facility. The ability of the pipeline to specialize in the estimation of damage sites of a size less than the repair threshold is demonstrated by showing its higher efficiency than classical machine learning approaches in the specific case of damage site images. In addition, its performances on three datasets are evaluated to show both robustness and accuracy.

Laser-pulse-induced temperature, thermal stress, and crater morphology effect during multipulse nanosecond laser manufacturing

We construct a numerical model for multipulse laser drilling. It is found that the previous laser-pulse-induced temperature accumulation, thermal stress occurrence, and crater morphology change promote subsequent pulse laser drilling. Among them, previous laser-pulse-induced temperature accumulation contributes significantly to the drilled crater depth when the workpiece temperature is higher than its melting point just before the subsequent laser pulse irradiation, especially in a short pulse interval condition. The crater morphology change becomes the main contributor when the workpiece temperature decreases below the melting point, often in a long pulse interval condition. Besides, the previous occurrence of laser-pulse-induced thermal stress always has had little influence on the drilled crater. This work can be a theoretical reference, especially for multipulse laser manufacturing.

Parametric study of laser-induced damage growth in fused silica optics with large beams at 351 nm. Part 2: fractal analysis

The impact of laser fluence and pulse duration on both the rate and probability of growth of laser-induced damage sites has been reported and analyzed statistically in a companion paper. In this paper, we report and analyze the volume morphology of damage sites during the growth process in fused silica optical components, at 351 nm, under various laser fluence and pulse durations. Fractal analysis has been used to quantify the bulk damage morphology. A clear link between the damage morphology and laser pulse duration has been observed. The results from fractal analysis allows for a better understanding of the results from the stochastic approach developed in our companion paper. More specifically, fractal analysis shows how the laser parameters such as fluence and pulse duration impact the phenomenology and the dynamics of the growth process.

Parametric study of laser-induced damage growth in fused silica optics with large beams at 351 nm. Part 1: stochastic approach

Both the rate and probability of the growth of laser-induced damage sites in fused silica depend on several parameters. In this two-part paper, we investigate the impact of the laser parameters on damage growth. In Part I, we present statistical measurements of damage growth at different energy densities, pulse durations, and initial damage sizes. In Part II, we use fractal analysis to quantify the evolution of the damage morphology as a function of the laser energy density and pulse duration. Damage initiation is performed using phase masks. These phase masks allow for the initiation of evenly spaced damage sites that can then be exposed to the same laser beam, and, therefore, the same pulse duration. This configuration allowed the study of damage growth in a large population of more than 5000 damage sites. The results clearly indicate that both the probability and the rate at which a damage site will grow strongly depend on the laser pulse duration. These differences can be explained by hypotheses that we have developed from an observation of the bulk damage morphology. Such observations will be presented in detail in the second part of this article.

Investigation of stress wave and damage morphology growth generated by laser-induced damage on rear surface of fused silica

In this study, stress wave and damage morphology on the rear surface of silica during laser induced damage with cumulative UV pump shots is investigated. Time resolved imaging system, along with stress induced birefringence detecting, is used to demonstrate the development of stress wave. The properties of three types of stress waves, namely, compressive wave (P-wave), shear wave (S-wave), and Rayleigh wave (R-wave) are recorded and studied during the damage process. The experimental and simulated results indicated that R-wave exhibits the highest intensity among all the three stress waves. Because the R-wave is mainly localized at the region near the surface, it is responsible for the mechanical damage along the surface; in addition, it rapidly increases the damage diameter, which was observed from the front view of the damage.

Large optics metrology for high-power lasers

The Laser MégaJoule (LMJ) is a high-power laser dedicated to laser-plasma experiments. At the beginning of the project in the mid-1990s, an optical metrology laboratory was created at CEA to help accomplish all the steps in the construction of this laser. This paper proposes an overview of the capabilities of this metrology laboratory in four main fields: surface imperfections, photometry, laser damage measurement, and wavefront measurement. The specificities for high-power laser optics in each domain are highlighted as well as the specific features that make our instruments unique.

Multi-wavelength growth of nanosecond laser-induced surface damage on fused silica gratings

The nanosecond laser-induced damage growth phenomenon on the exit surface of fused silica grating is investigated at 1064 nm and 355 nm separately and also simultaneously. Experiments are first carried out on damage sites on a plane fused silica sample showing two different morphologies, and a damage type is selected for ensuring the repeatability of the subsequent tests. Comparing the mono-wavelength growth results on a grating and a plane fused silica sample, the periodic surface structure is found to be an aggravating factor for damage growth. This is highly supported by calculations of the enhancement of the optical electric field intensity thanks to Finite-Difference Time-Domain simulations. Finally, the mono-wavelength results enable us to quantify a coupling occurring in the multi-wavelength configuration, which could originate from the heating of the plasma (more likely produced in the ultraviolet) preferentially by the infrared pulse. This study provides interesting results about the involvement of the surface topography in damage growth, and paves the way towards the comprehension of this phenomenon at high-energy nanosecond laser facilities where fused silica gratings are simultaneously irradiated at several wavelengths.