Bulk Damage Threshold Research Articles

The influence of laser irradiation conditions on the bulk damage of JGS1 fused silica

The fused silica is an important optical material in the large laser devices, which are often subject to laser induced damage in the laser systems. In this work we studied nanosecond laser induced damage in JGS1 fused silica at the three harmonic wavelengths of the 1064 nm, 355 nm and 248 nm. Under 1064nm/10ns, 355nm/10ns and 248nm/22.8ns laser irradiation, the bulk damage threshold is 150 J/cm2, 15 J/cm2 and 7 J/cm2, respectively. The results showed that the weakest point of bulk damage threshold among the three tested wavelengths is 248 nm. And the damage threshold decreases sharply with the decrease of wavelength.

Influence of bulk defects on bulk damage performance of fused silica optics at 355 nm nanosecond pulse laser

We demonstrate the effects of typical bulk defects in fused silica on the bulk damage threshold under nanosecond UV pulse in this study. A new test method is proposed to accurately evaluate laser induced bulk damage performance. The bulk bubble, hydroxyl, metal impurity, and weak absorption of the 355 nm laser are respectively characterized. The effects of bulk defects on bulk damage performance are analyzed statistically based on the correlation principle. For synthetic fused silica, metal impurities and hydroxyl have a weak correlation coefficient with the bulk damage threshold, while there is strong correlation between weak UV absorption and the bulk damage threshold. The influence of bulk damage threshold on surface damage performance is also discussed.

Laser-writing inside uniaxially birefringent crystals: fine morphology of ultrashort pulse-induced changes in lithium niobate.

This work presents a detailed analysis of the morphology of femtosecond laser-induced changes in bulk lithium niobate (LiNbO3) - one of the most common host materials in photonics - using second-harmonic generation microscopy and scanning electron microscopy. It is shown that focused linearly polarized near-infrared pulses can produce two or three distinct axially separated regions of modified material, depending on whether the pulse propagation is along or perpendicular to the optical axis. When laser writing in LiNbO3 is conducted in multi-shot irradiation mode and the focused light intensity is kept near the bulk damage threshold, periodic planar nanostructures aligned perpendicular to the laser polarization are produced inside the focal volume. These results provide a new perspective to laser writing in crystalline materials, including the fabrication of passive and active waveguides, photonic crystals, and optical data storage devices.

Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator

The optical properties, Faraday effect and Verdet constant of ceramic terbium gallium garnet (TGG) have been measured at 1064 nm, and were found to be similar to those of single crystal TGG at room temperature. Observed optical characteristics, laser induced bulk-damage threshold and optical scattering properties of ceramic TGG were compared with those of single crystal TGG. Ceramic TGG is a promising Faraday material for high-average-power YAG lasers, Yb fiber lasers and high-peak power glass lasers for inertial fusion energy drivers.

The effect of material, thermal and laser conditioning on the damage threshold of type II tripler‐cut DKDP crystals

AbstractBy using the traditional temperature‐reduction method, KD2PO4 (DKDP) crystals were grown from 85%‐deuterated aqueous solution synthesized by different kind of KH2PO4 (KDP) raw materials. An experimental technique was employed to measure the variation of bulk damage threshold in plates of DKDP crystals. Bulk laser damage in unconditioned, thermal‐conditioned and laser‐conditioned type II tripler‐cut DKDP crystals has been studied using 1064 nm (ω) and 355 nm (3ω) laser at pulse lengths of 5ns, 5ns respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Femtosecond laser-induced damage of gold films

Single- and multi-shot ablation thresholds of gold films in the thickness range of 31–1400 nm were determined employing a Ti:sapphire laser delivering pulses of 28 fs duration, 793 nm center wavelength at 1 kHz repetition rate. The gold layers were deposited on BK7 glass by an electron beam evaporation process and characterized by atomic force microscopy and ellipsometry. A linear dependence of the ablation threshold fluence F th on the layer thickness d was found for d ≤ 180 nm. If a film thickness of about 180 nm was reached, the damage threshold remained constant at its bulk value. For different numbers of pulses per spot ( N-on-1), bulk damage thresholds of ∼0.7 J cm −2 (1-on-1), 0.5 J cm −2 (10-on-1), 0.4 J cm −2 (100-on-1), 0.25 J cm −2 (1000-on-1), and 0.2 J cm −2 (10000-on-1) were obtained experimentally indicating an incubation behavior. A characteristic layer thickness of L c ≈ 180 nm can be defined which is a measure for the heat penetration depth within the electron gas before electron–phonon relaxation occurs. L c is by more than an order of magnitude larger than the optical absorption length of α −1 ≈ 12 nm at 793 nm wavelength.

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.

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.

Bridgman growth and characterization of nonlinear optical single crystals Ca 4GdO(BO 3) 3

Nonlinear optical single crystals Ca 4GdO(BO 3) 3 (GdCOB) have been grown by the vertical Bridgman method. Bubble-free, twin-free and crack-free GdCOB single crystals 25 mm in diameter and 65 mm in length were obtained. The average thermal expansion coefficients along a-, b- and c-axes from 293 to 1273 K are 10.35×10 −6 K −1, 7.78×10 −6 K −1 and 13.10×10 −6 K −1, respectively. The bulk damage threshold is 130 GW cm −2 for a single pulse of a mode-locked Nd:YAG laser with 35 ps pulse duration. The effective nonlinear coefficient ( d eff) in the phase matching directions of ( θ, φ)=(66.8°, 132.6°) is 1.51 pm V −1.

Thermal and Nonlinear Optical Properties of Ca4YO(BO3)3

Ca4YO(BO3)3 (YCOB) crystals have been grown using the vertical Bridgman method. The thermal properties of YCOB were measured for the first time to our knowledge. The specific heat is 729.7 J/kg K at 373K. The average thermal expansion coefficients along the a, b and c axes are 9.9 × 10-6 /K, 8.2 × 10-6 /K and 12.8 × 10-6 /K, respectively, in the temperature range of 293-1173 K. The thermal conductivities along the a, b and c axes are 1.83 W/mK, 1.72 W/mK and 2.17 W/mK at 373 K. The anisotropy in the measured thermal conductivities of YCOB is consistent with the experimental results of the thermal expansion. The SHG of a Nd: YAG laser was compared with that of a KDP crystal. The effective nonlinear coefficients (deff) of YCOB in type I phase matching directions of (θ, ϕ) = (66.3°, 143.5°) and (65.9°, 36.5°) were estimated to be 1.45 pm/V and 0.91 pm/V, respectively. The bulk damage threshold was observed as 85 GW/cm2 for single pulse of a Nd:YAG laser with 10 ns pulseduration.

Growth of ultra-violet grade CaF 2 crystals and their application for excimer laser optics

A crystal growth method for growing UV grade CaF 2 crystals from fluorspar has being developed using purified Bulgarian fluorspar mixed with 2% PbF 2. Excimer laser damage characteristics of the grown crystals were performed in a specially designed laboratory system into a high pressure camera. It was found that the fluorine mixture does not change, neither on the outer surface of the crystals nor in the bulk even after 3500 h of contact; no color centers were observed on the windows of the XeCl laser and that there are no remarkable differences between the surface and the bulk damage thresholds. Thus, the grown crystals of CaF 2 are suitable for manufacturing windows for excimer lasers and for other optical elements used in UV and vacuum UV spectra regions.

Single shot bulk damage threshold and conversion efficiency measurements on flux grown KTiOPO 4 (KTP)

The single shot bulk laser damage threshold was measured on thirteen different flux grown KTP crystals. The measurements were made with the KTP oriented for type II phase matched second harmonic generation (SHG) at 1064 nm. The width of the O-switched Nd:YAG laser pulses was 8.5 ns. The crystals were grown under different experimental conditions. Some of the crystals were measured topographically in order to find possible differences in the damage thresholds between different growth sectors and to check the crystals for optical homogeneity. The measured average threshold for dielectric breakdown of KTP is 31.5±4GW/cm 2. Topographic measurements showed, that the samples have a good optical homogeneity with the exception of trivalent cation doped KTP. The presence of Sc 3+ and Cr 3+ lowers the damage threshold of KTP and areas with preferential chromium incorporation in KTP show a lowered damage threshold in comparison with areas containing less chromium. The conversion efficiency for type II phase matching of the 1064 nm beam of a Nd:YAG laser has a maximum value of 55%. The measured d eff was 3.1 pm/V.

Laser-induced damage in beta-barium metaborate

Laser-induced damage in beta-barium metaborate (BBO) has been investigated at the wavelengths of 1064, 532, and 355 nm. The single-shot bulk damage threshold of BBO is 50 GW/cm2 at 1064 nm, 48 GW/cm2 at 532 nm, and 25 GW/cm2 at 355 nm. Damage by multiple pulse irradiation has also been studied and the probabilistic nature of the damage is discussed. Brief comments are also made on the morphology of the damage.

Changes in the bulk damage threshold of optical glasses as a result of repeated irradiation with laser pulses

Experiments demonstrated that interaction of laser radiation of subthreshold intensity and nanosecond duration with optical glasses free of absorbing microinhomogeneities (K8, KU1, LK6, etc.) induced residual changes in the optical properties, which reduced the bulk damage threshold for the subsequent (repeated) action of laser pulses. Residual changes in the nonlinear refractive index induced by subthreshold radiation of wavelengths 0.53 and 1.06 μm (i.e., under conditions such that hv < Eg/2, where Eg is the ionization potential of the glass matrix) were observed for the first time.

Influence of Ca and Pb impurities on the bulk optical strength of ultrapure NaCl and KCl crystals

A significant increase (by a factor of about 3) of the bulk damage threshold in the case of interaction of CO2 laser radiation pulses with ultrapure NaCl and KCl crystals grown in a reactive atmosphere was observed on introduction of divalent metal ions Ca and Pb in concentrations of 10−5–10−6 mol/mol. Impurities were introduced in concentrations of 10−8–10−3 and 2×10−7–10−4 mol/mol into the melts of KCl and NaCl, respectively. The concentration of other impurities (including OH) did not exceed ~10−6 mol/mol. A physical model was developed to account for the observed dependence on the basis of an analogy between a system of colloidal particles and F centers in a crystal and a liquid–vapor system.

Carbon dioxide laser polishing of fused silica surfaces for increased laser-damage resistance at 1064 nm

Mechanically polished fused silica surfaces were heated with continuous-wave CO(2) laser radiation. Laser-damage thresholds of the surfaces were measured with 1064-nm 9-nsec pulses focused to small spots and with large-spot, 1064-nm, 1-nsec irradiation. A sharp transition from laser-damage-prone to highly laser-damage-resistant took place over a small range in CO(2) laser power. The transition to high damage resistance occurred at a silica surface temperature where material softening began to take place as evidenced by the onset of residual strain in the CO(2) laser-processed part. The small-spot damage measurements show that some CO(2) laser-treated surfaces have a local damage threshold as high as the bulk damage threshold of SiO(2). On some CO(2) laser-treated surfaces, large-spot damage thresholds were increased by a factor of 3-4 over thresholds of the original mechanically polished surface. These treated parts show no obvious change in surface appearance as seen in bright-field, Nomarski, or total internal reflection microscopy. They also show little change in transmissive figure. Further, antireflection films deposited on CO(2) laser-treated surfaces have thresholds greater than the thresholds of antireflection films on mechanically polished surfaces.

The effect of baking and pulsed laser irradiation on the bulk laser damage threshold of potassium dihydrogen phosphate crystals

We increased the bulk laser damage threshold of potassium dihydrogen phosphate crystals by as much as a factor of 5 by first baking the crystals at 140 °C for 24 h and then irradiating them with laser pulses of increasing fluence. The combination of baking and subthreshold laser irradiation was more effective in improving bulk damage thresholds than either process alone. The combined process was effective for all laser pulse durations from 1 to 20 ns.

Optical strength of YAG and LiNbO3crystals irradiated with giant pulses from a YAG:Er3+laser (λ= 2.94μ)

Measurements were made of the bulk damage threshold Id of yttrium aluminum garnet (YAG) crystals, either free of deliberately introduced impurities or activated with Er3+ ions in different concentrations (up to 80 wt.%), and also of LiNbO3 crystals. This threshold was determined using giant pulses from a YAG:Er3+ laser emitting at λ = 2.94μ; the energy per pulse was 70 mj and the pulse duration was 120nsec. In the case of YAG crystals it was found that Id varied considerably from sample to sample in the range Id = (1.5–18) × 1010 W/cm2, which was not associated with the presence of Er3+ but was clearly due to the presence of some accidental impurities. A considerable increase in Id was observed for YAG:Er3+ samples characterized by low damage thresholds when these samples were annealed in air. In the case of LiNbO3, a typical value of the damage threshold was Id ≈ 1011 W/cm2. The results obtained indicated a sufficiently high optical strength of YAG:Er3+ and LiNbO3 crystals, used respectively as the active and modulating elements in YAG:Er3+ lasers.

Influence of optical absorption on the laser damage threshold of glasses

It was shown experimentally that the bulk damage threshold of K8 optical glasses containing color centers is proportional to α–0.5 (α is the absorption coefficient). This result may be attributed to nonlinearity of α. A consequence of this assumption is the dependence (Wth is the damage threshold energy density and τp is the radiation pulse duration).

Mechanism of bulk laser damage to CdS-type semiconductors

An investigation was made of the photoconductivity of pure CdS crystals at the moment of appearance of bulk damage and of the kinetics of this damage. An analysis of the experimental data revealed direct multiplication of carriers as a result of bulk damage, dependence of the bulk damage threshold on the amplitude of the optical wave field, and the instantaneous nature of the damage itself. This evidence shows that electron avalanche ionization is the dominant bulk damage mechanism in the case of pure CdS-type semiconductors subjected to nanosecond laser radiation pulses.