The dynamics of electrons and holes in potassium dihydrogen phosphate ( KH2PO4 or KDP) crystals and its
deuterated analog (KH2PO4 or DKDP) induced by femtosecond laser pulses is investigated at λ = 800nm. To
do so, experiments based on a femtosecond time-resolved interferometry technique have been carried out. It
is shown that two relaxation dynamics exist in KDP and DKDP crystals. In particular, it appears that one
dynamics is associated with the migration of proton/deuteron in the crystalline lattice. Both of the dynamics
correspond to physical mechanisms for which the multiphoton order required to promote valence electrons to
the conduction band is lower than the one of a defect-free crystal. These results suggest the presence of states
located in the band gap that may be due to the presence of defects existing before any laser illumination or
created in the course of interaction. In order to interpret the experiments, a model based on a system of rate
equations has been developed. Modeling results are in good agreement with the experimental data, and allow
one to obtain fundamental physical parameters governing the
laser-matter interaction as multiphoton absorption
cross sections, capture cross sections, recombination times, and so forth. Finally, it will be shown how these
results can be used to the understanding of laser-induced damage by nanosecond pulses in inertial confinement
fusion class laser aperture.
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.
By coupling statistics and heat transfer, we investigate numerically laser-induced KDP crystal damage by multi-gigawatt nanosecond pulses. Our model is based on the heating of nanometric absorbing defects that may cooperate when they are sufficiently aggregated. In such a case, they induce locally a strong increase of temperature that may lead to a subsequent damage. Statistics is used to evaluate the initial defect cluster size distribution. When the crystal is illuminated, by considering in addition heat transfer processes, this approach allows to predict damage probabilities and the evolution of the damaged sites density as a function of the laser fluence. We show that the scaling law exponent, linking the critical laser fluence to its pulse duration, takes a value close to 0.3 departing from the standard 0.5 value that is in a good agreement with recents experiments. Furthermore, these results indicate that absorbers involved in KDP damage may be associated with a collection of planar defects.
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.
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.
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.
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.