Laser-induced Bulk Damage Research Articles

Enhancement of the CsB3O5(CBO) crystal quality by fast cooling after crystal growth

CsB3O5 (CBO) is an excellent nonlinear optical crystal and can be grown by top-seeded solution growth (TSSG) from self-flux solutions. However, there are scattering centers in the as-grown crystal, which decrease the bulk laser-induced damage threshold (LIDT) of the crystal. Postgrowth quenching and vapor transport equilibration (VTE) can reduce the scattering centers effectively. Based on our previous hypothesis and analysis of retrograde solid solution at high temperature in CBO crystal, we adopt selective quenching to 500°C—fast cooling. Our investigations showed that, after the cooling process, the scattering centers and optical loss of CBO were decreased. It was found that with the increase in cooling rate, the LIDT of CBO crystals was increased. The highest LIDT of fast cooling CBO was 1.5 times that of conventional cooling process; so the crystal quality was improved.

Predicting laser-induced bulk damage and conditioning for deuterated potassium dihydrogen phosphate crystals using an absorption distribution model

We present an empirical model that describes the experimentally observed laser-induced bulk damage and conditioning behavior in deuterated potassium dihydrogen phosphate (DKDP) crystals. The model expands on an existing nanoabsorber precursor model and the multistep absorption mechanism to include two populations of absorbing defects, one with linear absorption and another with nonlinear absorption. We show that this model connects previously uncorrelated small-beam damage initiation probability data to large-beam damage density measurements over a range of nanosecond pulse widths. In addition, this work predicts the damage behavior of laser-conditioned DKDP.

Laser-induced damage investigation at 1064 nmin KTiOPO_4 crystals and its analogy with RbTiOPO_4

Bulk laser-induced damage at 1064 nm has been investigated in KTiOPO4 (KTP) and RbTiOPO4 (RTP) crystals with a nanosecond pulsed Nd:YAG laser. Both crystals belong to the same family. Throughout this study, their comparison shows a very similar laser-damage behavior. The evolution of the damage resistance under a high number of shots per site (10,000 shots) reveals a fatigue effect of KTP and RTP crystals. In addition, S-on-1 damage probability curves have been measured in both crystals for all combinations of polarization and propagation direction aligned with the principal axes of the crystals. The results show an influence of the polarization on the laser-induced damage threshold (LIDT), with a significantly higher threshold along the z axis, whereas no effect of the propagation direction has been observed. This LIDT anisotropy is discussed with regard to the crystallographic structure.

Accurate measurements of laser-induced bulk damage density

The laser damage community has long been searching for the reproducibility of damage measurements in the nanosecond (ns) regime. Here we show that laser-induced bulk damage density of frequency conversion crystals can be measured with high accuracy and repeatability in the range from 2 to 20 ns. The rasterscan test procedure (Lamaignère et al 2007 Rev. Sci. Instrum. 78 103105), previously developed in order to determine laser damage density of large aperture UV fused silica optics, has been adapted to bulk damage measurement. The large volume scanned during tests permits us to measure very low damage densities. For smaller optical components, small volumes are tested using the normalized 1/1 test procedure. Whatever test procedures, accuracy and repeatability are obtained by means of a suitable data reduction. For comparison between different procedures, the classical damage probability plot has to be converted in terms of damage density. The consideration of error bars on damage site distributions is compulsory to compare experimental data. A special emphasis is put on damage detection tools. When tests are carried out on diverse facilities, pulse duration, spatial distribution and beam overlap are the key parameters which are to be taken into account to compare experimental data. We describe the equipment, test procedures and data analysis to perform these damage tests with small beams (Gaussian beams, about 1 mm @1/e, and top-hat beams). Other tests are realized with larger beams (cm sized) which are compared with small beam results. The consistency of all the results gives confidence in the measurements. Reproducibility of measures is discussed in connection with current theoretical understanding of laser damage.

Effect of water impurity in CsLiB_6O_10 crystals on bulk laser-induced damage threshold and transmittance in the ultraviolet region

We investigate the effect of water impurity in a CsLiB(6)O(10) (CLBO) crystal on the ultraviolet properties of the bulk laser-induced damage threshold (LIDT) and transmittance. The water impurity was eliminated by heating the CLBO sample with dimensions of 5 mm x 5 mm x 10 mm at 150 degrees C in an ambient atmosphere and subsequently in a dry atmosphere. The bulk LIDT of the sample after heat treatment improved by about 1.6-fold compared with that before heat treatment.

UV laser-induced damage tolerance measurements of CsB 3O 5 crystals and its application for UV light generation

Cesium triborate, CsB 3O 5 (CBO), is an excellent nonlinear optical crystal for ultraviolet light generation. CBO crystals can be grown by top seeded solution growth from self flux solutions. The growth of large-size crystal is difficult because of the multi-nucleation and dissolution of seed during growth. These problems have been avoided and large size crystals (159.2 g) of size 50 × 45 × 45 mm 3 ( a × b × c) have been grown. The bulk laser-induced damage threshold (LIDT) of CBO was determined using an 1-on-1 technique by the third-harmonic (355 nm) light of longitudinal single-mode Q-switch Nd:YAG laser. The bulk LIDT was found to be two times that of fused quartz for a 6 ns pulse. The as-grown CBO crystal has been used to generate the highest ever reported third-harmonic power of 103 W for the case of a single-pass frequency-conversion system using a high-power Nd:YAG laser.

Differentiation of defect populations responsible for bulk laser-induced damage in potassium dihydrogen phosphate crystals

The defects responsible for damage in KDP and DKDP crystals are investigated by analyzing the density of damage sites and their individual morphology under different irradiation wavelengths and fluences. The experimental results indicate the presence of two defect species initiating damage over different wavelength ranges. Disparities in the morphology of damage sites initiated are also observed over each wavelength range.

Complex morphology of laser-induced bulk damage in K2H(2−x)DxPO4 crystals

We present a detailed study of the morphology of laser-induced bulk damage in K2H(2−x)DxPO4 crystals. We see three distinct regions of the internal damage sites: a rubble filled core, a shell that has probably been melted and compacted, and a larger outer region of slightly modified shocked material. The nature of these regions is important for understanding the impact of laser-induced damage on light scattering, which can be significant for high-power laser operation. We propose a simple model to explain the morphology and light scattering we observe.

Effect of multiple wavelengths on laser-induced damage in KH(_2−x)D_xPO_4 crystals

Laser-induced damage is a key factor that constrains the ways in which optical materials are used in high-power laser systems. We study the size and density of bulk laser-induced damage sites formed during frequency tripling in a DKDP crystal. The characteristics of the damage sites formed during tripling, for which 1053, 526, and 351 nm light is simultaneously present, are compared to those of damage sites formed by 351 nm light alone. The fluence of each wavelength is calculated as a function of depth with a full 4D(x,y,z,t) frequency conversion code and compared with measured damage density and size distributions. The density of damage is found to be governed predominantly by 351 nm light with some lesser, though nonnegligible, contribution from 526 nm light. The 1053 nm light does not appear to contribute to the damage density. The morphology of the damage sites, however, is seen to be relatively insensitive to wavelength and to depend only on total fluence of all wavelengths present. The strong and negligible wavelength dependences of initiation density and damage morphology, respectively, indicate that the dominant energy deposition mechanism varies during the damaging pulse.

Dependences of Laser-Induced Bulk Damage Threshold and Crack Patterns in Several Nonlinear Crystals on Irradiation Direction

The dependence of the bulk damage threshold of nonlinear crystals, such as KDP, KTP, LBO, BBO, CLBO, LiNbO3, and LiTaO3 on laser irradiation direction and their polarization under a single-shot operation have been investigated. We found that the intrinsic bulk-damage threshold for nonlinear crystals was mainly caused by dielectric breakdown from self-focusing with nonlinear refractive index. LBO single crystals have the highest value studied among nonlinear crystals. For KDP, BBO, and CLBO crystals, the damage threshold when an incident laser was propagated along the <001> axis is approximately twofold that in the {100} or {010} plane at a 1.064 µm wavelength. The damage crack pattern is also suggested to be in the mechanically weak direction due to inner pressure caused a laser-induced plasma in the crystal as a result of the laser breakdown due to electron avalanche breakdown.

Decomposition of KH2PO4 crystals during laser-induced breakdown

The structure of KH2PO4 single crystals (so-called KDP) irradiated with ∼3ns, 355 nm laser pulses with fluences above the laser-induced breakdown threshold is studied by a combination of Raman scattering, photoluminescence, and soft x-ray absorption spectroscopies. We compare spectra from the as-grown material, surface and bulk laser-induced damage sites, as well as from KPO3 references. Results show that irradiation with fluences above the laser-induced breakdown threshold leads to decomposition of KDP at surface damage sites but not at bulk damage sites. New spectroscopic features are attributed to dehydration products. For the laser irradiation conditions used in this study, the decomposed near-surface layer absorbs photons at ∼3.4eV (364 nm). These results may explain the recently reported fact that surface laser damage sites in KDP crystals tend to grow with subsequent exposure to high-power laser pulses, while bulk damage sites do not.

Growth of CsLiB 6O 10 crystals with high laser-damage tolerance

CsLiB 6O 10 crystals were grown from high-quality single-phase sintered powder by the solution-stirring top-seeded solution growth (SS-TSSG) technique. The sintered powder was obtained by the mixing-in-aqueous-solution technique. The grown crystals possessed an approximately 2.3-fold higher bulk laser-induced damage threshold (LIDT) for ultraviolet laser than those grown by the conventional method and fused quartz. In addition, the higher-LIDT crystals exhibited slow dissolution in a dilute water solution. We report that a moderate growth rate is easily achievable by the SS-TSSG method due to the homogenization of the supersaturation distribution. We also discuss the importance of the growth from single-phase sintered powder obtained by the mixing-in-aqueous-solution technique.

Anisotropy of Laser-Induced Bulk Damage of Single Crystals

The regularities of laser-induced damage of anisotropic materials, such as LiNbO3 and KDP dielectric single crystals, are experimentally studied. It is revealed that the shape of laser-induced damage in the dielectric crystals depends on the elastic symmetry of crystal and the propagation direction of the laser beam. When the beam propagates along the optic axis of crystals, the figures of the laser damage are six-path stars for LiNbO3 and four-path ones for KDP crystals. For the direction parallel to X and Y axes in KDP crystal, the damage has initially cross-like configuration, with further splitting of Z-oriented crack into two cracks in the process of damage evolution, leading to transformation of orthogonal-type damage to a hexagonal-type one.

Laser-Induced Bulk Damage of Various Types of Silica Glasses at 266 nm

Laser-induced bulk damage of various types of silica glasses at 266 nm with pulse widths of 4 ns and 30 ps was measured. At a single pulse, the laser-induced damage thresholds (LIDTs) of fused quartz (FQ) produced by melting natural quartz powder were greater than those of synthetic fused silica (SFS), and a SFS containing 40 ppm of OH had the highest LIDT among the SFSs. The LDITs of SFSs containing no OH were less than those of the other samples. At 12000 pulses, the difference in LIDT among all samples became small.

Laser-Induced Bulk Damage of Various Types of Silica Glasses at 532 and 355 nm

Laser-induced bulk-damage threshold (LIDT) of various types of silica glasses at 532 and 355 nm caused by lasers with pulse widths of ≈5 ns (refered as nanosecond laser [NSL]) and 30 ps (refered as picosecond laser [PSL]) were observed. The LIDTs of fused quartzes (FQs) produced by melting natural quartz powder were less than those of synthetic fused silicas (SFSs) at both wavelengths. At 355 nm, the LIDT of SFS containing 1000 ppm of Cl was almost the same as that of FQ at 1 pulse, and that at 12000 pulses was between those of SFSs and FQs. The variation of LIDT among samples for PSL is less than that for NSL.

Crystal Growth Technique for High-Quality CsLiB6O10 (CLBO) Crystal for Use in High-Power UV Generation.

A nonlinear optical crystal CsLiB6O10 (CLBO) is suitable for high-power solid-state ultraviolet (UV) lasers. One of the factors limiting the increasing of UV power is the formation of laser-induced damage. To obtain high-quality CLBO, we have developed a new growth apparatus based on an effective solution stirring technique. The grown crystals have a higher bulk laser-induced damage threshold than that of conventional crystals. They also possess sufficient optical uniformity for practical use in industrial fields. Furthermore, these high-quality CLBO are clearly superior in terms of dislocation density, Vickers hardness, and self-heating by UV absorption. Using these crystals, 266-nm UV power of 20 W operating over 100 hours was achieved.

Recent development of nonlinear optical borate crystals for UV generation

Abstract Recent development of high-power solid-state UV radiation by nonlinear optical (NLO) borate crystals is reviewed. The performance of such UV light sources has rapidly improved in the past five years because of the superior NLO properties of CsLiB6O10 (CLBO) crystals. The performance of such UV light sources also depends on the reliability of the NLO crystals. The relation between the bulk laser-induced damage threshold (LIDT), dislocation density and UV absorption of CsLiB6O10 (CLBO) was investigated. A newly developed synthesis process allows the growth of CLBO crystals with LIDT 2.5-fold higher than those grown by the conventional top-seeded solution growth (TSSG) technique. High-quality CLBO possesses lower dislocation density and smaller absorption of UV light ( λ=266 nm ) than conventional CLBO. Reduction of the dislocation density can suppress absorption of UV light that helps to enhance the resistance of CLBO to laser-induced damages, to alleviate thermal dephasing during high-power generation of UV light and thus strengthen the reliability of CLBO for UV light generation.

Investigation of bulk laser damage in KDP crystal as a function of laser irradiation direction, polarization, and wavelength

Bulk laser-induced damage in KDP crystal was measured using a single-shot 1-ns pulse Nd:YAG laser in a transverse and longitudinal single mode. It is found that the damage threshold of KDP single crystal depends on the laser irradiation direction, polarization direction and laser wavelength. The damage threshold in the direction of c axis is about two times higher than that of in the a(b) axis at 0.532 and 1.064 μm wavelength. This result is consistent with the mechanical strength tests for various directions of KDP crystal.

Laser-induced bulk damage in various types of vitreous silica at 1064, 532, 355, and 266 nm: evidence of different damage mechanisms between 266-nm and longer wavelengths

Laser-induced-damage thresholds (LIDT's) with various types of vitreous silica at 1064, 532, 355, and 266 nm are investigated. At 1064 nm no difference in the LIDT was observed in any sample. At 1064-355 nm the wavelength dependence of the LIDT of synthetic fused silica (SFS) can be described well by the relation I(th) = 1.45lambda(0.43), where I(th) is the LIDT in J/cm2 and lambda is the wavelength in nanometers. At 266 nm, however, LIDT's were smaller than half of the calculated value from the relation above. This difference can be explained by the damage mechanism; at 266 nm two-photon absorption-induced defects lower the LIDT as in the case of KrF-excimer-laser-induced defects, whereas at longer wavelengths the two-photon process does not occur. LIDT's of fused quartz (FQ) at 532 and 355 nm and that of SFS containing approximately 1000 ppm of Cl and no OH at 355 nm were a little lower than those of the other SFS's. This lower LIDT may be related to the absorption of metallic impurities in FQ and dissolved Cl2 molecules in SFS. At 266 nm, on the other hand, LIDT's of FQ's were higher than those of most SFS's.

Laser-Induced Bulk Damage of Various Types of Silica Glasses with Fundamental and Higher Harmonics of Nd:YAG Laser

The laser-induced damage threshold (LIDT) of various types of silica glass at 1064, 532, 355, and 266 nm was investigated. At 1064 nm, no difference in the LIDT was observed for all samples. In the range of 1064–355 nm, LIDTs of synthetic fused silica (SFS) were proportional to λ0.5, where λ is the wavelength. The LIDTs at 532 and 355 nm of fused quartz (FQ) and SFS containing 1000 ppm of Cl and no OH at 355 nm were slightly less than those of other SFSs. The mechanism of bulk damage at the 266 nm beam must be different from that at a longer wavelength, because the LIDT is considerably smaller than that of the value extrapolated from the λ0.5 dependence of SFS.