Mireille Commandre

Measurements of absorption losses in TiO 2 films by a collinear photothermal deflection technique

We review the principle of photothermal deflection for measuring absorption losses in TiO(2)films. A collinear arrangement gives the best sensitivity for the detection of losses in a low absorbing film deposited on a transparent substrate. The nineteen TiO(2) films produced by six different processes (electron beam evaporation, ion assisted deposition, ion beam sputtering, ion plating, ...), discussed at the 1986 Optical Society of America annual meeting, are measured by this technique. The extinction coefficients of the different films do not show obvious correlation with the deposition method. An important fact is that we have detected a variation in absorption as a function of time on some layers. This absorption shift is connected with the illumination conditions of the sample under study (wavelength: 600 nm; incident power: 400 W/cm(2)). Experimental results over time are given. The evolution of the photothermal signal is different from one sample to another. This phenomenon is partially reversible and depends on moisture degree of atmosphere.

Localized pulsed laser interaction with sub-micronic gold particles embedded in silica: a method for investigating laser damage initiation

Laser damage phenomena in fused silica are currently under study because of numerous related high power laser applications. Nanosized defects are believed to be responsible for some laser damage initiation. In order to predict and to quantify this initiation process, engineered submicronic gold defects were embedded in silica. The study of these samples by localized pulsed irradiation of isolated gold particles coupled with Nomarski, atomic force and photothermal microscope observations permits us to discriminate between two distinct stages of material modification: one detectable at the surface and the second in the neighbourhood of the embedded particle. Comparison between the observations and simulations results in good agreement if we assume that inclusion melting initiates the damage.

Simulation of the degradation of optical glass substrates caused by UV irradiation while coating

The radiation induced degradation of optical glass substrates during thin film deposition, which has been shown previously, is studied using simulation. Influence of UV irradiation by spectral lamps and excimer laser on fused silica and borosilicate glasses is investigated. Photothermal deflection is used to measure the increase in BK7 absorption versus time during UV irradiation. We also study means permitting a curing of UV induced absorption: first thermal annealing, and second focusing a visible laser beam onto the irradiated area. The results show that UV radiation can be responsible for an important part of substrate absorption increase while coating.

LIDT improvement of multilayer coatings by accurate analysis of fabrication steps

We present Laser Induced Damage Threshold (LIDT) results on multilayer components with different optical functions for near infrared applications. In this paper, we investigate the different fabrication steps of such functions. In particular, we show experimental results on surface preparation (polishing, cleaning,) and on deposition techniques (Ion beam assisted process) related to the materials involved in coatings. Laser damage tests are performed at 1064-nm with a 5-ns pulse Nd:Yag laser and experiments are made at surfaces of optical components using a 12 μm diameter focused beam. Accurate damage probability curves are plotted thanks to a reliable statistical measurement of laser damage. Use of a statistical model permits to deduce the densities of laser damage precursors. A systematic analysis of all the steps involved in fabrication allows then to build multilayer components with high laser induced damage resistance.

Laser damage investigation in nonlinear crystals : Study of KTiOPO4 (KTP) and RbTiOPO4 (RTP) crystals

High power or miniaturized laser systems are limited by the laser damage resistance of optical components, particularly of nonlinear crystals. The laser damage of optical components depends on many factors such as wavelength, frequency, pulse duration, spot-size,... Moreover, in nonlinear crystals, the anisotropy of physical parameters may cause anisotropy of the Laser Induced Damage Threshold (LIDT). Thus, the LIDT may depend on polarization or propagation direction of the laser beam. The aim of this paper is to discuss the laser damage results of two nonlinear crystals: KTiOPO4 (KTP) and RbTiOPO4 (RTP). In general, due to its higher effective electro-optic coefficients, RTP is more used for electrooptic applications, whereas KTP is popular for second harmonic generation. Laser damage tests in KTP and RTP reveal that for both crystals the LIDT depends on the polarization. The laser damage tests were carried out at 1064nm with a nanosecond Nd:YAG laser. The tests were performed with the polarization and the propagation direction of the light along a principal crystal axis, and all configurations were tested with a parallel beam (waist diameter 75μm). As they belong to the same crystal family, RTP and KTP crystals have similar nonlinear optical properties. This work also reveals that the laser resistance of KTP and RTP is very close. Functional laser damage tests of RTP for Pockels cell applications and SHG-cut KTP were performed too. We also discuss the influence of second harmonic generation on the LIDT.

Highlighting of local inhomogeneity in excimer conditioning of KDP

In order to increase the laser induced damage threshold of KDP crystal, a well-known solution consists in a laser conditioning process. In our case, the irradiation of the crystal is performed with an excimer laser XeF (λ = 351 nm, 16 ns). The improvements in laser damage thresholds measured at CEA/CESTA laboratory (Lutin, Yag facility 2.5 ns, parallel beam) and at CEA/Ripault laboratory (Excimer facility 16 ns, focused beams) are different. A possible reason to explain this difference is the depth of focus between both facilities. In order to minimize the influence of limited depth of focus, a solution consits in a multi-plane conditioning process. By means of a local study, it is possible to exhibit with a high accuracy the Laser Induced Damage Threshold (LIDT) in different planes along sample irradiation axis (z-axis). The laser damage threshold is measured locally (8 μm) at 355 nm with a Nd:Yag (pulse duration 7 ns) at Fresnel Institute Marseille. Using the local LIDT measurements, the purpose of this paper is to highlight the depth of focus in the excimer conditioning process. We demonstrate that it is possible to exhibit a local increase in the conditioning gain till a maximum value, measured with the excimer laser.

Photothermal analysis of submicrometric scale defects in laser damage studies

Laser-induced damage has long been widely acknowledged as a localized phenomenon associated with the presence of defects such as nodules, scratches, fractures, polishing or cleaning residues, impurities, contaminants, metal or dielectric inclusions, etc. Destructive investigations in ultra pure fused silica have led to the conclusion that defects, typically a few nanometers in size, were responsible for laser damage initiation. The understanding of damage phenomena requires the development of more sophisticated, non destructive tools with both high spatial resolution and high sensitivity, to detect defects as small as possible. Photothermal microscopy has been widely employed to characterize optical absorption, thermal properties of optical materials and for mapping defects. This technique has been coupled and compared with scattering mapping for studying laser damage processes before and after irradiation. Furthermore the capability of collinear photothermal deflection to reach sub-micrometric resolution by reduction of the pump beam diameter has been theoretically explored and experimentally demonstrated on specially prepared absorbing targets. A photothermal microscope based on photothermal deflection of the transmitted beam and well-suited for multi-scale studies of absorbing defects in thin films has been coupled with an experimental set-up allowing damage threshold measurement at the same wavelength. We present an overview of these developments in the field of photothermal microscopy and scattering mapping in connection with laser damage.

Front and back illumination of coated substrates for in-depth localization of absorption

We experimentally demonstrate that part of the absorptance of coated multicomponent glass substrates results from a photoinduced absorptance of the substrate during the evaporation process. This in- crease in the glass substrate absorptance is due to a solarization pro- cess by UV radiation, which is produced in the coating plant when elec- trons emitted by the electron gun interact with the material to be evaporated. Thus, we can experimentally show the dependence of the multicomponent glass substrate absorptance on the evaporation time. An original method of front and back illumination of samples is presented that enables the separation of each part of the in-depth absorptance using photothermal measurements: bulk and interface absorption of the thin film and substrate photoinduced absorption. This new method over- comes the difficulties presented by other measurement techniques and caused by the dependence of substrate and interface absorptances on the evaporation time. Absorption losses in different single-layer films are balanced.

Toward an absolute measurement of LIDT

The improvement of LIDT value of optical components had lead to develop a lot of experimental setups and procedures of test around the world. In this context it is often very difficult to make accurate comparisons of laser damage threshold values between the different apparatus. The differences are due to the procedure of test, the spatial and temporal beam variations, the laser damage criterion and others. A specific laser damage testing apparatus, with an accurate damage initiation detection and allowing a real time acquisition of the different shot parameters, lead to exhibit the influence of each parameter on the damage process. Laser beam profiling is performed in real time and give access to the fluence for one pixel (0.2μm2), therefore an effective fluence or a pixel fluence can be calculated in order to reach an “absolute” threshold value. The metrology developed will be detailed and some results obtained on silica and BK7 at 1064nm and 355nm with different procedures of test will be presented and discussed to illustrate the aim of this study.

Absorptance measurements of optical coatings: a round robin

An international round robin study was conducted on the absorption measurement of laser-quality coatings. Sets of optically coated samples were made by a reactive DC magnetron sputtering and an ion beam sputtering deposition process. The sample set included a high reflector at 514 nm and a high reflector for the near infrared (1030 to 1318 nm), single layers of silicon dioxide, tantalum pentoxide, and hafnium dioxide. For calibration purposes, a sample metalized with hafnium and an uncoated, superpolished fused silica substrate were also included. The set was sent to laboratory groups for absorptance measurement of these coatings. Whenever possible, each group was to measure a common, central area and another area specifically assigned to the respective group. Specific test protocols were also suggested in regards to the laser exposure time, power density, and surface preparation.