Christophe Hecquet

Removal of scratches on fused silica optics by using a CO 2 laser

We investigate the efficiency of local CO₂laser processing of scratches on silica optics in order to enhance the nanosecond UV-laser damage resistance. The surface deformations induced by the process have been measured for different CO₂laser parameters and then the pulse duration and the beam diameter have been chosen accordingly to limit those deformations below 1 µm. From the study of the laser damage resistance as a function of different material modifications we identify a range of optimal radiation parameters allowing a complete elimination of scratches associated with a high threshold of laser damage. Calculation of the temperature of silica using a two-dimensional axi-symmetric code was compared with experiment, supporting an optimization of the laser parameter as a function of the maximal dimensions of scratches that could be removed by this process.

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.

Resonant metamaterial absorbers for infrared spectral filtering: quasimodal analysis, design, fabrication, and characterization

We present a modal analysis of metal–insulator–metal (MIM)-based metamaterials in the far infrared region. These structures can be used as resonant reflection bandcut spectral filters that are independent of the polarization and direction of incidence. We show that this resonant reflection dip is due to the excitation of quasimodes (modes associated with a complex frequency) leading to quasi-total absorption. We have fabricated large area samples made of chromium nanorod gratings on top of Si/Cr layers deposited on silicon substrate. Measurements by Fourier transform spectrophotometry show good agreement with finite element simulations. A quasimodal expansion method is applied to obtain a minimal resonant model that fits well full wave simulations and that highlights excitation conditions of the modes.

All dielectric broadband mirror for Fabry-Perot interferometer

We present our first results on Fabry-Perot plates manufacturing with a broadband dielectric mirror on the front side and an antireflective coating on the rear face. Physical thicknesses of both coatings are optimized to reduce stress deformations.

Characterization of scratches on fused silica optics and a way to remove them

Various scratches on fused silica optics after polishing have been characterized with confocal microscopy and then tested with nanosecond UV laser. Scratches are identified as a major contributor to laser damage even if they are only a few micrometers wide. We propose a process in order to remove these scratches whose depth ranges from 2 to 16 μm. We use a CO2 laser to heat the scratched areas at high temperature which will heal fractures due to viscous flow. The characterizations were completed by laser damage tests that finally prove the effectiveness of the repair. We conclude also that this repair process proves to be fast, localized to the scratch and clean because no debris are generated.

Multispectral filters assemblies for earth remote sensing imagers

With the aim to expand its capability to offer state-of-the-art space qualified multispectral optical filters assemblies, Sodern continues its effort to evaluate and incorporate new technologies in its designs. For many years, Sodern has been developing complex optical devices incorporating spectral filters.

Optical filters for UV to near IR space applications

We present hereafter the results on the fabrication of complex optical filters within the Institut Fresnel in close collaboration with CILAS. Bandpass optical filters dedicated to astronomy and space applications, with central wavelengths ranging from ultraviolet to near infrared, were deposited on both sides of glass substrates with performances in very good congruence with theoretical designs. For these applications, the required functions are particularly complex as they must present a very narrow bandwidth as well as a high level of rejection over a broad spectral range. In addition to those severe optical performances, insensitivity to environmental conditions is necessary. For this purpose, robust solutions with particularly stable performances have to be proposed.

Analysis of energy deposition and damage mechanisms in single layer optical thin films irradiated by IR and UV femtosecond pulses

We report on the sub-picosecond laser-induced damage of optical thin films of different thickness made by Magnetron sputtering, Ion assisted deposition and Ion plating, and submitted to single irradiation of the first and the third harmonics of an Ytterbium laser (1030 and 343nm). Using a single rate equation approach for free electron excitation coupled with calculation of the spatial and temporal distribution of the electric field, we investigate numerically the spatial density distribution of the absorbed energy and evaluate its capacity to describe damage phenomena especially damage threshold and morphologies (damage diameter, ablation deepness). Laser-induced damage thresholds are compared for different film thicknesses and different irradiation conditions.

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.