Investigation of laser annealing parameters for optimal laser-damage performance in deuterated potassium dihydrogen phosphate

Performance characterization of freeform finished surfaces of potassium dihydrogen phosphate using fluid jet polishing with a nonaqueous slurry

Potassium dihydrogen phosphate (KDP) and its deuterated analog (DKDP) are unique nonlinear optical materials for high power laser systems. They are used widely for frequency conversion and polarization control by virtue of the ability to grow optical-quality crystals at apertures suitable for fusion-class laser systems. Existing methods for freeform figuring of KDP/DKDP optics do not produce surfaces with sufficient laser-induced-damage thresholds (LIDT's) for operation in the ultraviolet portion of high-peak-power laser systems. In this work, we investigate fluid jet polishing (FJP) using a nonaqueous slurry as a sub-aperture finishing method for producing freeform KDP surfaces. This method was used to selectively polish surface areas to different depths on the same substrate with removals ranging from 0.16 μm to 5.13 μm. The finished surfaces demonstrated a slight increase in roughness as the removal depth increased along with a small number of fracture pits. Laser damage testing with 351 nm, 1 ns pulses demonstrated excellent surface damage thresholds, with the highest values in areas devoid of fracture pits. This work demonstrates, for the first time, a method that enables fabrication of a waveplate that provides tailored polarization randomization that can be scaled to meter-sized optics. Furthermore, this method is based on FJP technology that incorporates a nonaqueous slurry specially designed for use with KDP. This novel nonaqueous FJP process can be also used for figuring other types of materials that exhibit similar challenging inherent properties such as softness, brittleness, water-solubility, and temperature sensitivity.

Progress on deuterated potassium dihydrogen phosphate (DKDP) crystals for high power laser system application

In this review, we introduce the progress in the growth of large-aperture DKDP crystals and some aspects of crystal quality including determination of deuterium content, homogeneity of deuterium distribution, residual strains, nonlinear absorption, and laser-induced damage resistance for its application in high power laser system. Large-aperture high-quality DKDP crystal with deuteration level of 70% has been successfully grown by the traditional method, which can fabricate the large single-crystal optics with the size exceeding 400 mm. Neutron diffraction technique is an efficient method to research the deuterium content and 3D residual strains in single crystals. More efforts have been paid in the processes of purity of raw materials, continuous filtration technology, thermal annealing and laser conditioning for increasing the laser-induced damage threshold (LIDT) and these processes enable the currently grown crystals to meet the specifications of the laser system for inertial confinement fusion (ICF), although the laser damage mechanism and laser conditioning mechanism are still not well understood. The advancements on growth of large-aperture high-quality DKDP crystal would support the development of ICF in China.

Effect of laser pulse duration and fluence on DKDP crystal laser conditioning

The impact of laser conditioning (LC) fluence and pulse duration on nanosecond (ns) laser damage performance of deuterated potassium dihydrogen phosphate (DKDP) crystal is studied. The result shows that higher LC fluence leads to a better damage resistance. In general, the sub-nanosecond LC effect is better than the nanosecond LC. However, in the range of 0.3 ns to 0.8 ns, the pulse duration has no obvious impact on the LC effect. An ultra-fast process characterization technology is employed to demonstrate that the cleaning effect of the protuberance defects on the surface is one of sub-ns LC mechanism. Eventually, a couple of optimized LC parameters that doubled the maximum damage threshold of DKDP crystal is proposed.

Influences of surface defects on the laser-induced damage performances of KDP crystal

When potassium dihydrogen phosphate (KDP) crystals are exposed to high-energy laser irradiation, the pre-existing surface defects may act as damage precursors and will reduce the lifespan of the crystal components. Although it has been found that different kinds of surface defects exhibit distinct damage characteristics, the influence of surface defects on the laser-induced damage performance of KDP crystal is not yet clear. In this paper, KDP surface defects have been characterized by multiple measuring methods and classified into five categories according to their structure features. Laser-induced damage tests were then carried out to investigate the laser-induced damage thresholds of different kinds of KDP surface defects as well as the evolution of the morphology of damage sites. The results of the experiment indicate that the damage thresholds of cracks, fracture pits, and surface protuberances are between 6 and 11  J/cm² (355 nm, 3 ns, similarly hereinafter), which are much lower than the thresholds of plastic scratches, discontinuous scratches, and a defect-free KDP surface. In addition, it has been found that fluorescence enhancement is just a necessary condition for reduction of damage thresholds. Finally, reasons for the formation of the most threatening KDP surface defects have been analyzed and corresponding suppression measures have been proposed for increasing the surface damage thresholds of the crystal components.

Influence of surface cracks on laser-induced damage resistance of brittle KH₂PO₄ crystal

Single point diamond turning (SPDT) currently is the leading finishing method for achieving ultra-smooth surface on brittle KH(2)PO(4) crystal. In this work, the light intensification modulated by surface cracks introduced by SPDT cutting is numerically simulated using finite-difference time-domain algorithm. The results indicate that the light intensification caused by surface cracks is wavelength, crack geometry and position dependent. Under the irradiation of 355 nm laser, lateral cracks on front surfaces and conical cracks on both front and rear surfaces can produce light intensification as high as hundreds of times, which is sufficient to trigger avalanche ionization and finally lower the laser damage resistance of crystal components. Furthermore, we experimentally tested the laser-induced damage thresholds (LIDTs) on both crack-free and flawed crystal surfaces. The results imply that brittle fracture with a series of surface cracks is the dominant source of laser damage initiation in crystal components. Due to the negative effect of surface cracks, the LIDT on KDP crystal surface could be sharply reduced from 7.85J/cm(2) to 2.33J/cm(2) (355 nm, 6.4 ns). In addition, the experiment of laser-induced damage growth is performed and the damage growth behavior agrees well with the simulation results of light intensification caused by surface cracks with increasing crack depths.

Fabrication of spherical mitigation pit on KH2PO4 crystal by micro-milling and modeling of its induced light intensification

Micro-machining is the most promising method for KH(2)PO(4) crystal to mitigate the surface damage growth in high power laser system. In this work, spherical mitigation pit is fabricated by micro-milling with an efficient machining procedure. The light intensification caused by rear surface features before and after mitigation is numerically modeled based on the finite-difference time-domain method. The results indicate that the occurrence of total internal reflections should be responsible for the largest light intensification inside the crystal. For spherical pits after mitigation, the light intensification can be greatly alleviated by preventing the occurrence of total internal reflections. The light intensification caused by spherical mitigation pit is strongly dependent on the width-depth ratio and it is suggested that the width-depth ratio of spherical mitigation pit must be devised to be larger than 5.0 to achieve the minimal light intensification for the mitigation of surface damage growth. Laser damage tests for KH(2)PO(4) crystal validate that the laser damage resistance of initially damaged surface can be retrieved to near the level of ideal surface by replacing initial damage site with predesigned mitigation pit.

Dynamics of transient absorption in bulk DKDP crystals following laser energy deposition

The transient changes in the optical properties of bulk DKDP material arising from its exposure to high temperatures and pressures associated with localized laser energy deposition are investigated. Two methods for initiation of laser-induced breakdown are used, intrinsic, involving relatively large energy deposition brought about by focusing of the laser beam to high intensities, and extrinsic, arising from more localized deposition due to the presence of pre-existing absorbing damage initiating defects. Each method leads to a very different volume of material being affected, which provides for different material thermal relaxation times to help better understand the processes involved.

Probability of growth of small damage sites on the exit surface of fused silica optics

Growth of laser damage on fused silica optical components depends on several key parameters including laser fluence, wavelength, pulse duration, and site size. Here we investigate the growth behavior of small damage sites on the exit surface of SiO₂ optics under exposure to tightly controlled laser pulses. Results demonstrate that the onset of damage growth is not governed by a threshold, but is probabilistic in nature and depends both on the current size of a damage site and the laser fluence to which it is exposed. We also develop models for use in growth prediction. In addition, we show that laser exposure history also influences the behavior of individual sites.

The effect of laser pulse shape and duration on the size at which damage sites initiate and the implications to subsequent repair

Growth of laser damage on SiO(2) optical components used in high power lasers can be reduced or eliminated by pre-exposure to pulses of a few hundred ps in duration. Such pre-exposure would cause weak locations on the optics surface to self-identify by initiating very small damage sites. The sites which initiate will be only a few microns in diameter and will have a very low probability of growing even without any further treatment. Repairing damage sites when small is important because both laser mitigation and acid etching are very successful in preventing such small sites from growing.

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.

SOCRATE: an optical bench dedicated to the understanding and improvement of a laser conditioning process

We present an automatic excimer laser bench (SOCRATE) allowing for the treatment of optical components by laser conditioning. This apparatus, developed at the Commissariat a l'Energie Atomique-Le Ripault, has been designed to add to this conditioning process an in situ, accurate laser-induced damage threshold (LIDT) measurement and different nondestructive optical techniques for the characterization of the component during treatment. Through different examples, we demonstrate the importance of these characterizations to improve the understanding of the laser conditioning. The role of an in situ adapted metrology associated in real time with a laser conditioning bench offers new opportunities to analyze laser-induced damage mechanisms and subsequently to increase the LIDT of optical components.