Guillaume Duchateau

Coupling statistics and heat transfer to study laser-induced crystal damage by nanosecond pulses

By coupling statistics and heat transfer, we investigate numerically laser-induced crystal damage by multi-gigawatt nanosecond pulses. Our model is based on the heating of nanometric absorbing defects that may cooperate when sufficiently aggregated. In that configuration, they induce locally a strong increase of temperature that may lead to a subsequent damage. This approach allows to predict cluster size distribution and damage probabilities as a function of the laser fluence. By studying the influence of the pulse duration onto the laser-induced damage threshold, we have established scaling laws that link the critical laser fluence to its pulse duration tau. In particular, this approach provides an explanation to the deviation from the standard tau(1/2) scaling law that has been recently observed in laser-induced damage experiments with KH(2)PO(4) (KDP) crystals [J.J. Adams et al., Proc. of SPIE 5991, 5991R-1 (2005)]. In the present paper, despite the 3D problem is tackled, we focus our attention on a 1D modeling of thermal diffusion that is shown to provide more reliable predictions than the 3D one. These results indicate that absorbers involved in KDP damage may be associated with a collection of planar defects. First general comparisons with some experimental facts have been performed.

Simple models for laser-induced damage and conditioning of potassium dihydrogen phosphate crystals by nanosecond pulses

When potassium dihydrogen phosphate crystals (KH(2)PO(4) or KDP) are illuminated by multi-gigawatt nanosecond pulses, damages may appear in the crystal bulk. One can increase damage resistance through a conditioning that consists in carrying out a laser pre-exposure of the crystal. The present paper addresses the modeling of laser-induced damage and conditioning of KDP crystals. The method is based on heating a distribution of defects, the cooperation of which may lead to a dramatic temperature rise. In a previous investigation [Opt. Express 15, 4557-4576 (2007)], calculations were performed for cases where the heat diffusion was permitted in one and three spatial dimensions, corresponding respectively to planar and point defects. For the sake of completeness, the present study involves the 2D heat diffusion that is associated with linear defects. A comparison to experimental data leads to the conclusion that 1D calculations are the most appropriate for describing the laser-induced damage in KDP. Within this framework, the evolution of the damage density is given as a function of the laser energy density and an in-depth analysis of the results is provided based on simple analytical expressions that can be used for experimental design. Regarding the conditioning, assuming that it is due to a decrease in the defect absorption efficiency, two scenarios associated with various defect natures are proposed and these account for certain of the observed experimental facts. For instance, in order to improve the crystal resistance to damage, one needs to use a conditioning pulse duration shorter than the testing pulse. Also, a conditioning scenario based on the migration of point (atomic-size) defects allows the reproduction of a logarithmic-like evolution of the conditioning gain with respect to the number of laser pre-exposures. Moreover, this study aims at refining the knowledge regarding the precursor defects responsible for the laser-induced damage in KDP crystals. Within the presented modeling, the best candidate permitting the reproduction of major experimental facts is comprised of a collection of one-hundred-nanometer structural defects associated with point defects as for instance cracks and couples of oxygen interstitials and vacancies.

Laser-induced damage of KDP crystals by 1ω nanosecond pulses: influence of crystal orientation

We investigate the influence of THG-cut KDP crystal orientation on laser damage at 1064 nm under nanosecond pulses. Since laser damage is now assumed to initiate on precursor defects, this study makes a connection between these nanodefects (throughout a mesoscopic description) and the influence of their orientation on laser damage. Some investigations have already been carried out in various crystals and particularly for KDP, indicating propagation direction and polarization dependences. We performed experiments for two orthogonal positions of the crystal and results clearly indicate that KDP crystal laser damage depends on its orientation. We carried out further investigations on the effect of the polarization orientation, by rotating the crystal around the propagation axis. We then obtained the evolution of the damage probability as a function of the rotation angle. To account for these experimental res ts, we propose a laser damage model based on ellipsoid-shaped defects. This modeling is a refined implementation of the DMT model (Drude Mie Thermal) [Dyan et al., J. Opt. Soc. Am. B 25, 1087-1095 (2008)], by introducing absorption efficiency calculations for an ellipsoidal geometry. Modeling simulations are in good agreement with experimental results.

A KDP equation of state for laser-induced damage applications

High power lasers such as NIF in the USA or LMJ in France are being developed for inertial confinement fusion applications. However, the performance of the optics is limited by laser-induced damage (LID), which occurs, for instance, in the potassium dihydrogen phosphate (KH2PO4 or KDP) crystals utilized for frequency conversion. An accurate equation of state (EOS) is required to explain the LID process and to predict damage size. For the design of such EOS, a pulsed electron beam was used to generate a quasi-plane stress wave of 0.7 GPa in KDP. The sample response was deduced from photonic Doppler velocimetry. Equations of state and deviatoric stress components are designed and compared to experimental data. They are used in laser-induced bulk damage simulations, showing that strength may play a significant role.

Pump-pump experiment in KH2PO4 crystals: Coupling two different wavelengths to identify the laser-induced damage mechanisms in the nanosecond regime

Laser-induced damage experiments based on an original pump-pump set-up have been carried out in the nanosecond regime for KH2PO4 (KDP) crystal. The impact of a simultaneous mixing of 351 nm and 1064 nm pulses has been experimentally studied and compared to a model based on heat transfer, the Mie theory and a Drude model. This study sheds light on the physical processes implied in the KDP laser damage. In particular, a three-photon ionization mechanism is shown to be responsible for laser damage in KDP.

Model for nanosecond laser induced damage in potassium titanyl phosphate crystals

A model for nanosecond laser induced damage in the bulk of potassium titanyl phosphate nonlinear optical crystals is presented. In a first step, laser-induced damage precursors are produced by multiphoton absorption. In a second step, the damage precursors are activated. Damage occurs if the precursor activation rate exceeds a critical value. Basic considerations allow evaluating the parameters of the model. The validity of the model is discussed by comparing it to several experimental observations, in particular, the decrease of the laser damage threshold during second harmonic generation of 1064u2009nm pulses.

Catastrophic nanosecond laser induced damage in the bulk of potassium titanyl phosphate crystals

Due to its high effective nonlinearity and the possibility to produce periodically poled crystals, potassium titanyl phosphate (KTiOPO4, KTP) is still one of the economically important nonlinear optical materials. In this overview article, we present a large study on catastrophic nanosecond laser induced damage in this material and the very similar RbTiOPO4 (RTP). Several different systematic studies are included: multiple pulse laser damage, multi-wavelength laser damage in KTP, damage resistance anisotropy, and variations of the laser damage thresholds for RTP crystals of different qualities. All measurements were carried out in comparable experimental conditions using a 1064u2009nm Q-switched laser and some were repeated at 532u2009nm. After summarizing the experimental results, we detail the proposed model for laser damage in this material and discuss the experimental results in this context. According to the model, nanosecond laser damage is caused by light-induced generation of transient laser-damage precur...

Toward a better understanding of multi-wavelength effects on KDP crystals

Laser damage in KDP crystals has been studied since several years and more accurately with emergence of projects like LMJ (Laser MégaJoule, in France) or NIF (National Ignition Facility, in US). Laser damage tests are essentially performed at 351-nm wavelength (3ω), with regards to their optical behaviours on forementioned facilities. But only few data are available at 1064 nm (1ω) and at 532 nm (2ω), and even with wavelength-mixing more representative of operational conditions of KDP crystals. So in a first approach, we tried to carry out an identity chart of the crystal by performing mono-wavelength tests at 1ω, 2ω and 3ω. Then, a campaign of combination of multi-wavelength (typically 3ω and 1ω) tests has been started with several temporal delays between 3ω and 1ω pulses. These first results lead us to improve pre-existing modelling codes developed by CEA, which have proved their robustness to 3ω -experiment results. Foremost interests consist in implementing wavelength dependency and energy deposition mechanism as a consequence of our first observations on KDP.

Modeling of laser-induced damage in KDP crystals by nanosecond pulses : a preliminary hydrodynamic study

The aim of this preliminary study is to provide a simple model for estimating the laser-induced damage formation in potassium dihydrogen phosphate crystals (KH2PO4 or KDP) irradiated by nanosecond laser pulses operating at 351nm. In our modelling approach, a damaged site is assumed to be induced from a nanometric existing defect, i.e. a precursor defect. It makes it possible to absorb an important part of the incident laser energy which results in a damage formation by some processes which combine heating and hydrodynamic processes. In our model, the main expected features of the damage scenario are accounted for: the defect-assisted laser absorption and subsequent plasma formation and evolution, the plasma absorption, heat transfer and hydrodynamic processes via a simple Equation Of State (EOS). In these calculations, a crystal zone is assumed to damage since it undergoes high enough density variations. Calculations shows that a nanometric precursor defect can effectively lead to damaged site of several tens of micrometers in size as observed experimentally. Also, we demonstrate the reliability of the long-standing assumption regarding the precursor defect size. Furthermore, a particular morphology of the damaged site exhibiting various regions is obtained. These estimates have now to be confirmed especially by improving the EOS and by introducing an elasto-plastic behavior.

Identification of the laser-induced damage mechanisms in KDP by coupling 355nm and 1064nm nanosecond pulses

Nanosecond Laser-Induced Damage (LID) in potassium dihydrogen phosphate (KH2PO4 or KDP) remains an issue for light-frequency converters in large-aperture lasers such as NIF (National Ignition Facility, in USA) and LMJ (Laser MegaJoule, in France). In the final optic assembly, converters are simultaneously illuminated by multiple wavelengths during the frequency conversion. In this configuration, the damage resistance of the KDP crystals becomes a crucial problem and has to be improved. In this study, we propose a refined investigation about the LID mechanisms involved in the case of a multiple wavelengths combination. Experiments based on an original pump-pump set-up have been carried out in the nanosecond regime on a KDP crystal. In particular, the impact of a simultaneous mixing of 355 nm and 1064 nm pulses has been experimentally studied and compared to a model based on heat transfer, the Mie theory and a Drude model. This study sheds light on the physical processes implied in the KDP laser damage. In particular, a three-photon ionization mechanism is shown to be responsible for laser damage in KDP.