In order to characterize the effect of thermal annealing on laser damage resistance of KDP,
several combinations of laser conditioning and thermal annealing were applied to two SHG KDP
samples. One sample was tested at 3ω, 16ns and the other one at 3ω, 2.5ns. Results show that
whereas thermal annealing improves laser damage for a 16ns pulse, no effect can be measured at a
pulse length of 2.5ns. Combining laser conditioning and thermal annealing has a stronger effect
on laser damage resistance than laser conditioning alone, even for a 2.5ns pulse length for which
thermal annealing was found to have little or no influence. It was also found that for a short pulse
length maximum gain was obtained when thermal annealing was applied after laser conditioning.
Potassium dihydrogen phosphate (KH2PO4 or short KDP) is one of the major nonlinear optical crystals for frequency
conversion and electro-optic switching in high power lasers. In particular, this material has been chosen for the
frequency converters of the Laser Mega Joule in France and the National Ignition Facility in the US. These laser work
close to the damage threshold of the crystals and large efforts have been provided to improve the laser induced damage
threshold for KDP at different wavelength.
We present in this paper first results of a new setup dedicated to the correlation of non destructive luminescence
spectroscopy and destructive laser damage tests. We concentrate on the differences between conventionally grown KDP
and KDP-crystals that have been produced by the rapid growth method that has been developed in the last years
especially for the large laser installations LMJ and NIF. Different photoluminescence spectra are obtained from
conventionally and rapidly grown KDP for both pump configurations: (i) pulsed pumping by the forth harmonic of a
Nd:YAG laser at 266nm, and (ii) continuous pumping using a frequency doubled Argon ion laser at 244nm.
In this paper, the nature of the crystalline phases observed at the surface damage sites resulting from laser
irradiation is investigated by X-ray diffraction. The results are compared against new data on thermal decomposition of
KDP salt. The damage sites consist of polycrystalline KDP and partially dehydrated phases. The comparison with the
thermal decomposition study allows to assign a temperature range to the overall temperature reached by the surface
during the damaging process. Finally, the difference between surface damage and bulk damage is discussed.
In this paper, we present various laser conditioning experiments which have been performed with KDP SHG and DKDP THG samples. The different conditioning facilities used delivered laser pulses at 351 nm in the nanosecond (from 3 to 12 ns) or in the sub-ns (600 ps) regime. Finally, the efficiency of the various conditioning protocols was compared: 526 nm-6 ns and 351 nm-3 ns damage tests were performed respectively on SHG and THG samples. The results show that laser-conditioning SHG KDP samples at 351 nm either with ns or sub-ns pulses allows reducing the laser damage density so that it becomes consistent with the specification of high power lasers. They also confirm that conditioning THG DKDP samples at 351 nm using sub-ns pulses is more efficient than using ns pulses.
For large aperture solid state lasers, the laser resistance of the optical component remains an important limitation
for the performances and the maintenance costs. Since decades, laser induced damage has been intensively
studied in order to understand and control the origin of the phenomenon. LID measurements are commonly
performed with table top lasers whose characteristics change from one to another and, sometimes, the scaling
laws do not permit to explain the experimental differences. For example, we have previously demonstrated that,
in KH2PO4 (KDP) crystals, the laser beam size can influence strongly the determination of the damage probability.
Here, we present a systematic study realized on KDP crystal to quantify the influence of the beam size
on the LIDT (Laser Induced Damage Threshold) measurement at 355 nm. The use of an unique Gaussian beam
ranged from micronic to sub-millimetric sizes permits to highlight different types of laser-damage precursor.
LIDT measurements realized with beams of small (lower than 100 microns at 1/e2)or large (upper than 400 microns at
1/e2)dimensions give information about the behavior of material regarding precursor defects.
This study is concerned with the identification of the defects responsible for laser damage observed on
KDP/DKDP frequency triplers used in high power lasers. We reported at BDS 2005 a non destructive high energy X-ray
topographic setup able to characterize lattice imperfections in optics. Results obtained using this technique on KDP and
DKDP crystals are reported and discussed. The influence of each type of defect, observed or likely to exist in optics, is
discussed in light of damage mechanisms recently published. Finally, an experimental setup presumably able to reveal
those defects is proposed.
In this paper, we present different procedures of laser conditioning realized on KDP doubler crystals. First, components
are treated either with an excimer laser (SOCRATE facility, 351 nm, 12 ns) or a Nd: YAG laser (MISTRAL facility,
355 nm, 7 ns). Then damage tests are performed at 2ω (532 nm - 5 ns BLANCO facility) and 3ω (355 nm - 2.5ns
LUTIN facility) in order to estimate the conditioning gain for these two wavelengths.
For the best procedures, results show that it is possible to increase laser damage threshold at 532 nm so that it becomes
compatible with the nominal specifications of the LMJ. Moreover, tests realized at 355 nm highlight also an
encouraging improvement for the laser conditioning of tripler crystals.
In this paper we examine how optical techniques can be used for impurities and defects detection in KH2PO4 (KDP)
components. This is important in so far as some of these defects are responsible for a weaker than expected laser-induced
threshold in these materials. Photothermal deflection, polariscopy, fluorescence and photoexcitation are
investigated with the aim of localizing and identifying the laser-induced damage precursors. Impurities concentration
is measured directly by ICP-AES and Fe is accordingly checked to be at the origin of a higher absorption in the
prismatic sectors of rapidly grown KDP crystals. We also exhibit a fluorescence signal in the near-ultraviolet range
by pumping at 248 nm; in rapidly grown crystals, in the same way as iron, the incorporation rate of the fluorescent
centers is shown to depend on the growth sector.
A thermal model is considered to better understand Laser-Induced Damage and conditioning mechanism in
KH2PO4 (KDP) and D2xKH2(1-x)PO4(DKDP) crystals. We mainly focus on two points, the probed volume of
the laser beam and the optimization of the conditioning process. Our predictions are in agreement with recent
experimental data.
KDP crystals are currently used for frequency conversion and Pockels cells in large aperture laser systems such as the LMJ and NIF. These different functions are obtained by cutting the KDP crystals with different orientations. We show by measuring the LIDT with three different facilities, that the cut angle plays a key role in the damage mechanism. Consistently with the three measurement set-ups, we demonstrate that the doublers have a weaker LIDT value than the triplers. The z-cut KDP samples have a LIDT higher than both the doublers and the triplers. These results are analyzed in terms of probed volumes and pulse duration.
X-ray diffraction is a non destructive technique used in order to characterize defects in the single crystal. Unfortunately, this analysis can not be performed throughout the whole volume on thick KH2PO4 (KDP) crystals used in the high power lasers systems like NIF and LMJ, these crystals having a thickness close to 10 mm. Considering the usual energy range radiation used for X-ray diffraction and topography (20-30 keV), the beam is rapidly absorbed by the material. However, this problem can be solved by the use of high energy X-ray radiation in order to analyse the complete volume of crystal. The principle of this device will be exposed and preliminary results are shown along with corresponding optical measurements.
At very high powers the energy for a single shot in the LIL/LMJ laser is today limited among others by the robustness of the KDP-based components used for frequency conversion. Subsequently it is vitally important to improve as much as possible the Laser Induced Damage Threshold (LIDT) of these components to make possible even more powerful shots. The exceptionally large aperture of such lasers (40*40 cm2) required the development of rapid growth methods. Investigations are under way to improve the damage resistance of such materials by implementing more efficient conditioning procedures. In this work we focus on composition heterogeneities induced by the rapid growth method in KDP crystals and we examine the impact on the laser-damage resistance. Two LIDT measurement facilities are used to investigate KDP triplers robustness. Spatially resolved LIDT measurements at 355 nm show that the LID resistance is significantly lower in some regions. The efficiency of the excimer conditioning in the different regions is also addressed.
The high-power Laser MegaJoule (LMJ) for inertial confinement fusion experiments that is currently under construction at CEA-CESTA in France will require a high number of large aperture Pockels cells and frequency converters made of potassium dihydrogen phosphate (KDP) and DKDP (Deuterated KDP). These optical components will be operated several times a year at fluences close to their Laser Induced Damage Threshold (LIDT) which may reduce significantly their lifetime and increase substantially the maintenance costs of the LMJ. In a global effort to reduce these costs we have designed the SOCRATE facility as a complete system for materials characterization, LIDT measurement and optics conditioning by laser to increase their lifetime. In this paper we examine the relevance of adapting the laser conditioning process to the bulk KDP quality. First the existence of heterogeneities in large KDP crystals is stressed; next the LIDTs in the different parts of the crystals using focused or collimated beams are compared. Finally we focus on the efficiency of the excimer conditioning process in the different growth sectors of KDP samples and demonstrate that for the current conditioning process the efficiency depends only weakly on the original material heterogeneities.
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.
We investigated the crystals with different non-destructive optical diagnostics during the conditioning of KDP with an excimer laser at 351 nm. We measured in the same time the luminescence, the absorption and bulk scattering of the material. These observations pointed out the defects within the crystal. We demonstrated a correlation of the optical signal intensity with the laser damage threshold of KDP.
We perform thne conditioning of various KDP crystals with a XeF excimer laser working at 351 nm. We determine the maximum available excimer laser fluence for conditioning without damage initiation within the crystal. We demonstrate enhancement of the damage resistance with the increase of the cumulative excimer laser fluence. Using the conditioning parameters we show that the damage resistance is also dependent on the crystalline orientation of the KDP samples.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.