Damage induced by nanosecond laser in optical materials can often be attributed to the presence of laser damage precursor in the material. The presence of these precursors within dielectric optics can be successfully described by so called distributed defect ensembles. The physical parameters of these precursor presence models can be deduced by fitting experimental laser damage probability data. For a degenerate defect ensemble these parameters are the precursor threshold and the precursor density in the sample. To deduce precursor densities correctly it is essential to consider the real shape of laser beam that often deviates from Gaussian or hat-top models. To address these issues we discuss a new fitting procedure that minimizes significant errors in the deduced model parameters using experimental beam profile images. We suggest two methods: Defining a Gaussian replacement beam or using a numerical approximation of the surface over threshold (SOT) of the real beam. Both methods are discussed at the example of a degenerate damage precursor population but apply to any type of damage precursor population.
In the last decades, the resistance to high-power laser flux was largely improved in most of optical components insofar as 1-on-1 measurements are concerned. Another challenge lies in improving their resistance to multiple laser shots for highpower laser applications. Indeed, in multi-pulse irradiation, a decrease of the laser-induced damage threshold with increasing number of pulse was observed in various optical materials as in glasses, crystals, and thin-films. This effect, commonly denominated "fatigue" effect, is a limiting factor in many applications where optics have to be long-lifetime, as for example for space applications. Representing the laser damage probability as a function of pulse number for a given fluence allows to distinguish statistical pseudo-fatigue and fatigue which is due to cumulative material modifications. Investigating on the fatigue effects in the bulk of synthetic fused silica (Suprasil 1®) for different wavelengths, we evidenced that the fatigue effect was due to statistical pseudo-fatigue when irradiated at 1064 nm while the fatigue effect at 355 nm came from cumulative material modifications. The current work is dedicated a more detailed study of fatigue effects in Suprasil 1®, testing the influence of the beam size on the fatigue effects. Moreover, an estimation of the lifetime of the created defects is performed using a destructive technique.
LIDT specification of optical components is now widely required to perform laser systems where high power and/or long
term life are the critical points. Depending on the application, the system has to resist to single shot with the highest
possible intensity as in the NIF and Megajoule projects. Otherwise a compromise on LIDT value is necessary to permit a
great shot number (109 shots) for example for spatial applications.
In order to characterize the LIDT and long term life of components, specific laser induced damage setups are used. The
test configuration is close to the final application (wavelength, pulse duration, beam shape), but most often, parameters
as spatial and temporal beam profiles are different. Otherwise, different test procedures as raster scan method or statistic
procedure are used to determine the behavior of material, which induces additional differences between the obtained
LIDT values. For practical reasons (cost and size of sample) the beam size is most often smaller than in the final
application. Because of these parameters misfit, differences are highlighted between the setups of different laboratories
even using the same ISO norm.
In this context differences with the final system has to be expected in terms of LIDT. A good knowledge of the influence
of the different parameters on the final result is required to reduce the LIDT uncertainty. In this paper, to illustrate the
purpose, we will present examples of significant influence induced by test sampling, beam diameter, spatial and temporal
profile applied to non-linear crystal and silica.
In the past decades, efforts have been concentrated on reaching more laser resistant multilayers optical components in the
Infrared (IR) range. New designs and materials have been investigated and among them binary or ternary oxide mixtures
have revealed to be very profitable to improve the laser damage resistance of the coatings in the IR. The physical
characteristics of such mixed materials are indeed tunable and the deposition process associated allows to obtain
multilayers with smoother interfaces, which reduces considerably the damage threshold of the component.
The present work is focused on the study of pure materials and their binary oxide mixtures in the UV range, using
S-on-1
testing, for two different laser beam sizes. Samples resistance to multipulse irradiation is then compared for both beam
sizes, extracting the data with a thermal model assuming nanometric inclusions. The fatigue effects of the set of sample
have also been investigated, showing no clear trend of fatigue for all tested components.
Multipulse laser-induced damage is an important topic for many applications of nonlinear crystals. We studied multipulse damage in X-cut KTiOPO4. Using a 6-ns Nd:YAG laser with a weakly focused beam, a fatigue phenomenon was observed. We addressed whether this phenomenon necessarily implies material modifications. Two possible models were tested, both of them predicting increasing damage probability with increasing pulse number while all material properties are kept constant. The first model, pulse energy fluctuations and depointing, increases the probed volume during multiple pulse experiments. The probability to cause damage thus increases with increasing pulse number; however, this effect is too small to explain the observed fatigue. The second model assumes a constant single-shot damage probability p1, so a multipulse experiment can be described by statistically independent resampling of the material. Very good agreement was found between the 2000-on-1 volume damage data and this statistical multipulse model. Additionally, the spot size dependency of the damage probability is well described by a precursor presence model. Supposing that laser damage precursors are transient, the presented data explain the experimental results without supposing material modifications.
In nanosecond laser damage investigations, the specific defect density in the optical component or thin film plays the key
role in triggering optical breakdown. UV irradiation can induce additional defects in optical materials before the
damaging event takes place. This increased defect density can even be the main cause for UV laser damage as shown
before in fused silica.
Moving on to oxide thin films, this contribution will present studies on SiO2, Al2O3, and
HfO2 ion beam sputtered
coatings. Pure material single layers as well as composite material single layers comprised of two oxides have been
investigated concerning their tendency to generate additional defects resulting from UV laser irradiation. Within this
work, tests at 355 nm and 266 nm have been performed and are compared.
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