Laser damage measurements with multiple pulses at constant fluence (S-on-1 measurements) are of high practical importance for design and validation of high-power photonic instruments. Using nanosecond lasers, it was recognized long ago that single-pulse laser damage is linked to fabrication-related defects. Models describing the laser damage probability as the probability of encounter between the high fluence region of the laser beam and the fabrication-related defects are thus widely used. Nanosecond S-on-1 tests often reveal the “fatigue effect,” i.e., a decrease of the laser damage threshold with increasing pulse number. Most authors attribute this effect to cumulative material modifications operated by the incubation pulses. We discuss the different situations that are observed upon nanosecond S-on-1 measurements that are reported in literature and speak in particular about the defects involved in the laser damage mechanism. These defects may be fabrication related or laser induced, stable or evolutive, cumulative or of short lifetime.
Laser damage measurements with multiple pulses at constant fluence (S-on-1 measurements) are of high practical importance for design and validation of high power photonic instruments. Mimicking the usual operation conditions, they allow observing possible modifications of the laser damage behavior during operation. In fact, nanosecond S-on-1 tests often reveal the “fatigue effect”, i.e. a decrease of the laser damage threshold with increasing pulse number. When irradiating with ultraviolet wavelengths, the fatigue effect is caused by cumulative material modifications. Systematic improvement of the concerned optical materials can only be achieved if the material modifications operated by the laser irradiation are identified. In this presentation we will show our latest results on the material modifications observed by photoluminescence in the bulk of fused silica. Causing the modifications and pumping the photoluminescence at 266 nm, modifications in the color center concentrations can be observed before the occurrence of damage. These observations can thus help to predict imminent fatigue laser damage under certain irradiation conditions. The lifetime and the nature of the observed modifications differ for low OH and high-OH silica types. Although bulk fatigue damage is only limiting at 266 nm, we also made first investigations using 355 nm as modification wavelength. However, the lifetime of the modifications causing the reduced laser damage threshold is much longer than the lifetime of the modified color centers, indicating that the observed modifications only accompany the initial stage of the problematic and still unknown modifications that weaken the damage threshold.
Laser damage measurements with multiple pulses at constant fluence (S-on-1 measurements) are of high practical importance for design and validation of high power photonic instruments. Using nanosecond lasers, it has been recognized long ago that single pulse laser damage is linked to fabrication related defects. Models describing the laser damage probability as the probability of encounter between the high fluence region of the laser beam and the fabrication related defects are thus widely used to analyze the measurements. Nanosecond S-on-1 tests often reveal the “fatigue effect”, i.e. a decrease of the laser damage threshold with increasing pulse number. Most authors attribute this effect to cumulative material modifications operated by the first pulses. In this paper we discuss the different situations that are observed upon nanosecond S-on-1 measurements of several different materials using different wavelengths and speak in particular about the defects involved in the laser damage mechanism. These defects may be fabrication-related or laser-induced, stable or evolutive, cumulative or of short lifetime. We will show that the type of defect that is dominating an S-on-1 experiment depends on the wavelength and the material under test and give examples from measurements of nonlinear optical crystals, fused silica and oxide mixture coatings.
Fatigue effects in fused silica have been largely studied in the past years, as this phenomenon is directly linked to the lifetime of high power photonic materials. Indeed, in the UV regime, we observe a decrease of the LIDT when the number of laser shots increases and this has been attributed to laser-induced material modifications. Under 266 nm laser irradiation, with nanosecond pulses of constant fluence, we observed that the photoluminescence is modified until damage occurs. High-OH fused silicas like Suprasil, “UV fused silica” or Herasil® show NBOHC (Non-Bridging Oxygen Hole Center) luminescence at 664 nm (1.87 eV) whereas low-OH fused silica like Infrasil shows ODC (Oxygen- Deficient Center) luminescence at 404 nm (3.07 eV). We found that the laser-induced density of NBOHCs increased until bulk damage occurred while the ODC’s density decreased. We propose a new representation of the experimental Son- 1 breakdown data which allows predicting the occurrence of material breakdown consuming fewer sample surface and saving time compared to the classic representation Nd (Number of shots before damage) versus F (Fluence). The link between LIF and the modifications leading to breakdown is however modified if a break is used during the irradiation.
A fatigue effect is often observed under multiple laser irradiations, overall in UV. This decrease of LIDT, is a critical
parameter for laser sources with high repetition rates and with a need of long-term life, as in spatial applications at
355nm. A challenge is also to replace excimer lasers by solid laser sources, this challenge requires to improve drastically
the lifetime of optical materials at 266nm. Main applications of these sources are devoted to material surface nanostructuration,
spectroscopy and medical surgeries. In this work we focus on the understanding of the laser matter
interaction at 266nm in silica in order to predict the lifetime of components and study parameters links to these lifetimes
to give keys of improvement for material suppliers. In order to study the mechanism involved in the case of multiple
irradiations, an interesting approach is to involve the evolution of fluorescence, in order to observe the first stages of
material changes just before breakdown. We will show that it is sometime possible to estimate the lifetime of component
only with the fluorescence measurement, saving time and materials. Moreover, the data from the diagnostics give
relevant informations to highlight “defects” induced by multiple laser irradiations.
The fatigue effect under multiple UV-laser irradiations can be attributed in many materials to a local material modification induced by the subsequent nanosecond laser pulses. Non-destructive investigations before breakdown are essential tools to study the mechanisms involved in the material modification process. In this work, we discuss the possibility to highlight the first stage of the material changes in UV-irradiated silica. Laser-induced material “defects” are studied by local in situ fluorescence measurements.
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<i> Gaussian replacement beam </i>or using a <i>numerical approximation of the surface over threshold (SOT)</i> 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.