This paper reports a study on correlation between stress field generated by extensive crystalline defects such as
dislocation or growth boundaries and laser damage. It is found that stress fields decrease laser damage resistance. This result is compatible with the hypothesis that laser damage precursors consist of clusters of punctual defects.
Indeed, such defects are affected by stress fields as their concentration varies in order to minimize the free energy of the crystal. Chemical analysis carried out on one of the crystal tend to show that the punctual defects
involved are intrinsic rather than extrinsic.
Previous work on KDP has shown that thermal annealing could improve laser damage resistance of KDP optics at 3w. However, the improvement varies with the pulse length: whereas a strong improvement was observed at 16ns, no improvement at all was observed for a pulse length of 2.5ns. Whatever the pulse length, though, combinations of laser conditioning and thermal annealing led to better results than laser conditioning alone. The goal of this study is to verify if these results also hold for DKDP. A major difference is that, due to quadratic to monoclinic high temperature transition, the annealing temperature considered for KDP cannot be applied to KDP. This paper reports the temperature range considered for DKDP as well the modifications brought by thermal annealing on laser damage resistance at 12ns and 2.5ns.
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.
As laser conditioning ever increases the performance of KDP optics with respect to laser induced bulk damage so that it can meet high-power laser specifications, it is expected that surface damage may become the next threat that must be dealt with. This paper presents new data on surface damage initiation and growth at 3w. A surface damage mitigation process based on the ball-dimpling method is presented along with the first results on the behavior of this process with respect to laser irradiation.
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.
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.
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 KH<sub>2</sub>PO<sub>4</sub> (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.