Dam-Bé Douti

Quantitative phase imaging applied to laser damage detection and analysis

We investigate phase imaging as a measurement method for laser damage detection and analysis of laser-induced modification of optical materials. Experiments have been conducted with a wavefront sensor based on lateral shearing interferometry associated with a high-magnification optical microscope. The system has been used for the in-line observation of optical thin films and bulk samples, laser irradiated in two different conditions: 500 fs pulses at 343 and 1030 nm, and millisecond to second irradiation with a CO2 laser at 10.6 μm. We investigate the measurement of the laser-induced damage threshold of optical material by detection and phase changes and show that the technique realizes high sensitivity with different optical path measurements lower than 1 nm. Additionally, the quantitative information on the refractive index or surface modification of the samples under test that is provided by the system has been compared to classical metrology instruments used for laser damage or laser ablation characterization (an atomic force microscope, a differential interference contrast microscope, and an optical surface profiler). An accurate in-line measurement of the morphology of laser-ablated sites, from few nanometers to hundred microns in depth, is shown.

Laser-induced damage of optical thin films submitted to 343, 515, and 1030 nm multiple subpicosecond pulses

Abstract. Optical materials submitted to multiple subpicosecond irradiations are known to exhibit a decrease of the laser-induced damage threshold (LIDT) with the applied number of pulses, an effect referred to as “fatigue” or “incubation.” In this work, we experimentally investigate this effect for the case of optical thin films submitted to multiple exposures with 500 fs pulses at different wavelengths: 1030, 515, and 343 nm. Niobia, hafnia, and silica films made with dense coating techniques (magnetron sputtering, ion-assisted deposition, and reactive low voltage ion plating) are studied, as well as the surface of a fused silica substrate. These samples have been exposed to different pulse numbers (from 1 to 100,000) at a low-frequency repetition rate (less than 1 kHz) and the LIDT has been measured. The results reveal the differences between materials and for the various wavelengths such as the decrease rate of the LIDT or the stabilization level that is reached after multiple exposures. All the results evidence the role of native- and laser-induced defects that we discuss on the basis of published works on the subject.

Correlation of Optical Properties and Laser Damage Resistance for Ion Beam Sputtered Al2O3/AlF3 and Al2O3/SiO2 Mixture Coatings

Oxifluoride and oxide mixture thin films with different ratios are produced by ion beam sputtering. The optical properties and the composition of Al2O3/AlF3 and Al2O3/SiO2 mixture coatings are correlated with the femtosecond laser damage resistance.

Laser damage measurement techniques for the femtosecond regime

Laser-induced damage is defined as any permanent laser-induced change in the characteristics of a sample. This change can be observed by many different inspection techniques, with different sensitivity, depending on the intended objectives and available techniques. The damage threshold definition and measurement are therefore very subjective and related to the detection method. The choice and implementation of a damage test system is then a critical issue on any experiment. In this work we present some implementation of detection techniques for laser damage metrology in the sub-picosecond regime. Different damage testing methods that have been applied will be discussed in view of their potential applications for testing functional optical components or to study physical process in the femtosecond regime, particularly the role of defects: optical microscopy, phase microscopy and time-resolved microscopy.

Analysis of energy deposition and damage mechanisms in single layers of HfO2 and Nb2O5 submitted to 500fs pulses

Laser Induced Damage Thresholds and morphologies of damage sites on thin films samples irradiated by sub-ps pulses are studied based on experimental and numerical studies. Experiments are conducted with 500fs pulses at 1030nm and 343nm and the irradiated sites are analyzed with phase imaging, AFM and SEM. The results are compared to simulations of energy deposition in the films based on the Single Rate Equation taking account transient optical properties of the films. Results suggest that a critical absorbed energy as a damage criterion give consistent results both with the measured LIDT and the observed damage morphologies.

UV to IR laser damage of Magnetron Sputtering films submitted to multiple sub-picosecond pulses

We report on the laser-induced damage threshold at 500fs of optical films made by Magnetron Sputtering and submitted to single and multiple irradiations at different harmonics of an Ytterbium laser (1030nm, 515nm and 343nm). Single layers of SiO2, HfO2, and Nb2O5 as bare fused silica samples are under investigation.

In-line quantitative phase imaging for damage detection and analysis

We investigate quantitative phase imaging as a measurement method for laser damage detection and analysis of laser induced modification of optical materials. Experiments have been conducted with a wavefront sensor based on lateral shearing interferometry technique associated to a high magnification optical microscope. The system has been used for in situ observation of optical thin films and bulk samples irradiated by 500fs pulses. It is shown that the technique realizes high sensitivity, convenient use and can provide quantitative information on the refractive index or surface modification of the samples under test.