Two localized CO2 laser treatment methods for mitigation of UV damage growth in fused silica

Two methods：high-power,short-time,single-shot irradiation（Method A） and low-power,long-time,multi-shot irradiation（Method B） are investigated to mitigate the UV damage growth in fused silica by using a 10.6-μm CO2 laser.To verify the mitigation effect of the two methods,the laser induced damage thresholds（LIDTs） of the mitigated sites are tested with a 355-nm,6.4-ns Nd：YAG laser,and the light modulation of the mitigation sites are tested with a 351-nm continuous Nd：YLF laser.The mitigated damaged sites treated with the two methods have almost the same LIDTs,which can recover to the level of pristine material.Compared with Method A,Method B produces mitigated sites with low crater depth and weak light modulation.In addition,there is no raised rim or re-deposited debris formed around the crater edge for Method B.Theoretical calculation is utilized to evaluate the central temperature of the CO2 laser beam irradiated zone and the radius of the crater.It is indicated that the calculated results are consistent with the experimental results.     

Mitigation of laser damage growth in fused silica by using a non-evaporative technique

A non-evaporative technique is used to mitigate damage sites with lateral sizes in a range from 50 μm to 400 μm and depths smaller than 100 μm.The influence of the pulse frequency of a CO 2 laser on the mitigation effect is studied.It is found that a more symmetrical and smooth mitigation crater can be obtained by increasing the laser pulse frequency form 0.1 to 20 kHz.Furthermore,the sizes of laser-affected and distorted zones decrease with the increase of the laser pulse frequency,leading to less degradation of the wave-front quality of the conditioned sample.The energy density of the CO 2 laser beam is introduced for selecting the mitigation parameters.The damage sites can be successfully mitigated by increasing the energy density in a ramped way.Finally,the laser-induced damage threshold(LIDT) of the mitigated site is tested using 355 nm laser beam with a small spot(0.23 mm 2) and a large spot(3.14 mm 2),separately.It is shown that the non-evaporative mitigation technique is a successful method to stop damage re-initiation since the average LIDTs of mitigated sites tested with small or large laser spots are higher than that of pristine material.     

Influence of secondary treatment with CO_2 laser irradiation for mitigation site on fused silica surface

The ablation debris and raised rim, as well as residual stress and deep crater will be formed during the mitigation of damage site with a CO2 laser irradiation on fused silica surface, which greatly affects the laser damage resistance of optics. In this study, the experimental study combined with numerical simulation is utilized to investigate the effect of the secondary treatment on a mitigated site by CO_2 laser irradiation. The results indicate that the ablation debris and the raised rim can be completely eliminated and the depth of crater can be reduced. Notable results show that the residual stress of the mitigation site after treatment will reduce two-thirds of the original stress. Finally, the elimination and the controlling mechanism of secondary treatment on the debris and raised rim, as well as the reasons for changing the profile and stress are analyzed. The results can provide a reference for the optimization treatment of mitigation sites by CO_2 laser secondary treatment.     

激光诱导熔石英表面损伤修复中的气泡形成和控制研究

基于熔石英材料在CO_2激光作用下的温度分布和结构参数变化的计算结果,对熔石英损伤修复中的气泡形成和控制进行了研究.针对损伤尺寸介于150—250μm之间的损伤点,提出了一种能够有效控制气泡形成的长时间低温预热修复方法.基于低温下熔石英材料结构弛豫时间常数较长的特点,该方法在不引起熔石英材料结构发生显著变化的同时,能够解吸附表面和裂纹处所附着的气体和杂质,可有效降低裂纹闭合过程中气泡形成的概率.实验结果表明,长时间低温预热修复方法的成功修复概率可达到98%.     

熔石英表面损伤修复点的损伤概率

针对3个光斑尺寸的CO2激光辐照尺寸小于400 m损伤点后得到的修复点,从4个方面对损伤修复点经355 nm激光辐照后的损伤概率分别进行研究。结果表明,根据修复光斑尺寸或损伤点尺寸分别讨论修复点的损伤概率,更能反映修复点的抗激光损伤能力。通过分别讨论可以看出,采用小光斑CO2激光辐照损伤点或者修复尺寸小于200 m的损伤点,修复成功率超过90%。同时,可以通过损伤修复点的激光损伤阈值都服从正态分布的规律来预测修复点的损伤概率。     

Incident laser modulation of a repaired damage site with a rim in fused silica rear subsurface

Local CO2 laser treatment has proved to be an effective method to prevent the 351-nm laser-induced damage sites in a fused silica surface from exponentially growing, which is responsible for limiting the lifetime of optics in high fluence laser systems. However, the CO2 laser induced ablation crater is often surrounded by a raised rim at the edge, which can also result in the intensification of transmitted ultraviolet light that may damage the downstream optics. In this work, the three-dimensional finite-difference time-domain method is developed to simulate the distribution of electrical field intensity in the vicinity of the CO2 laser mitigated damage site located in the exit subsurface of fused silica. The simulated results show that the repaired damage sites with raised rims cause more notable modulation to the incident laser than those without rims. Specifically, we present a theoretical model of using dimpled patterning to control the rim structure around the edge of repaired damage sites to avoid damage to downstream optics. The calculated results accord well with previous experimental results and the underlying physical mechanism is analysed in detail.     

CO2激光扫描辐照修复熔石英表面损伤

采用振镜系统结合10.6 m的CO2激光,对熔石英表面损伤点进行辐照,成功地修复了直径500 m的大尺寸损伤点,损伤修复后表面无烧蚀残留,损伤内部完全愈合,无气泡和残余裂纹。和定点修复方式相比,这种修复具有修复尺度大、面形影响区域小、应力分布范围小的优势。80%扫描辐照修复的损伤点的初始损伤阈值恢复甚至超过了基底的初始损伤阈值。     

Study on incident laser modulation using surface micro-defects on KH₂PO₄ crystal

KH2PO4 crystal is a crucial optical component of inertial confinement fusion.Modulation of an incident laser by surface micro-defects will induce the growth of surface damage,which largely restricts the enhancement of the laser induced damage threshold.The modulation of an incident laser by using different kinds of surface defects are simulated by employing the three-dimensional finite-difference time-domain method.The results indicate that after the modulation of surface defects,the light intensity distribution inside the crystal is badly distorted,with the light intensity enhanced symmetrically.The relations between modulation properties and defect geometries(e.g.,width,morphology,and depth of defects) are quite different for different defects.The modulation action is most obvious when the width of surface defects reaches 1.064 μm.For defects with smooth morphology,such as spherical pits,the degree of modulation is the smallest and the light intensity distribution seems relatively uniform.The degree of modulation increases rapidly with the increase of the depth of surface defects and becomes stable when the depth reaches a critical value.The critical depth is 1.064 μm for cuboid pits and radial cracks,while for ellipsoidal pits the value depends on both the width and the length of the defects.     

Localized CO2 laser treatment and post-heating process to reduce the growth coef f icient of fused silica surface damage

The lifetime of optical components in high-fluence ultraviolet (UV) laser applications is typically limited by laser-initiated damage and its subsequent growth. Using 10.6-μ m CO 2 laser pulses, we successfully mitigate 355-nm laser induced damage sites on fused silica surface with dimensions less than 200 μ m. The damage threshold increases and the damage growth mitigates. However, the growth coefficients of new damage on the CO 2 laser processed area are higher than those of the original sample. The damage grows with crack propagation for residual stress after CO 2 laser irradiation. Furthermore, post-heating is beneficial to the release of residual stress and slows down the damage growth.     

10BaF₂:NaF,Na₃AlF₆/TiO₂ composite as a novel visible-light-driven photocatalyst based on upconversion emission

A rare-earth free upconversion luminescent material, 10BaF 2 :NaF, Na 3 AlF 6 , is synthesized by a hydrothermal method. The study of fluorescent spectrum indicates that it can convert visible light (550 nm–610 nm) into ultraviolet light (290 nm–350 nm), and two emission peaks at 304 nm and 324 nm are observed under the excitation of 583 nm at room temperature. Subsequently, 10BaF 2 :NaF, Na 3 AlF 6 /TiO 2 composite photocatalyst is prepared and its catalytic activity is evaluated by the photocatalytic reduction of CO 2 under visible light irradiation (λ > 515 nm). The results show that 10BaF 2 :NaF, Na 3 AlF 6 /TiO 2 is a more effective photocatalyst for CO 2 reduction than pure TiO 2 , their corresponding methanol yields are 179 and 0 μmol/g-cat under the same conditions. Additionally, the mechanism of photocatalytic reduction of CO 2 on 10BaF 2 :NaF, Na 3 AlF 6 /TiO 2 is proposed.     

熔石英损伤修复坑下游光场调制的数值模拟与实验研究

系统研究了熔石英激光损伤修复后的形貌特征,根据测量数据建立了典型的损伤修复坑三维模型,利用标量衍射理论并结合快速傅里叶变换算法研究了修复坑在351 nm激光辐照下游光场调制的分布规律.研究表明,修复坑引起的光场调制会使得下游不同距离位置处出现环形光场增强区和轴上位置光场增强点;环形光场增强区位置距离修复元件较近,其环形调制极大值主要受修复坑深度的影响,且随修复坑深度的增大而逐渐增加;轴上位置光场增强点位置距离修复元件较远,其轴上调制极大值主要受修复坑边缘凸起高度的影响,且随凸起高度的增大而快速增加;环形调制极大值或轴上调制极大值增大的同时,其分布位置与修复元件之间的距离均会逐渐减小.实验验证表明,利用三维修复坑模型得到的下游光场调制数值模拟结果与实验测量结果具有较好的一致性.本研究结果对控制熔石英元件损伤修复形貌特征以抑制调制增强效应给出了具体的控制方向,对修复工艺的改进与完善提供了非常有意义的参考.     

Diode-pumped laser and self-frequency-doubling properties of a Nd³⁺:Na₃La₉O₃(BO₃)₈ crystal

We report efficient, diode-pumped, self-frequency doubling (SFD) in the newly developed laser crystal Nd³⁺:Na₃La₉O₃(BO₃)₈ (Nd:NLBO). More than 730?mW of fundamental output power at 1072?nm was achieved with a slope efficiency of 16.2%. With incident pump power of 8?W, 29?mW of green cw laser emission at 536?nm was observed with proper phase matching. This initial performance and the good optical properties of the crystalline host are encouraging for the development of a high power diode-pumped SFD visible light laser source.     

Effect of Laser Irradiation on Optical Properties and Damage Threshold of Nd:Glass

Journal of Russian Laser Research - We investigate the effects of 10.6 μm CO2 laser irradiation and 572 nm dye laser irradiation on optical properties and laser damage threshold of Nd:glass....     

增强掺钕磷酸盐玻璃激光破坏强度的研究

用ＣＯ２，Ｎｄ：ＹＡＧ和Ｃｕ蒸气激光分别对三种掺钕磷酸盐玻璃进行激光预辐照处理，研究了不同的激光波长的辐照条件对磷酸盐玻璃１．０６μｍ激光破坏强度的影响。     

Surface damage morphology investigations of silicon under millisecond laser irradiation

The surface damage morphologies of single crystal silicon induced by 1064 nm millisecond Nd:YAG laser are investigated. After irradiation, the damage morphologies of silicon are inspected by optical microscope (OM) and atomic force microscope (AFM). The plasma emission spectra of the damaged region are detected by the spectrometer. It is shown that surface oxidation and nitridation have occurred during the interaction of millisecond laser with silicon. In addition, the damage morphologies induced by 2 ms and 10 ns pulse width laser are compared. The damage morphology obtained by 2 ms laser is an evident crater. Three types of damage morphologies are formed at different laser energy densities. The circular concentric ripples are found surrounding the rim of the crater. The spacing of the ripples is 15 ± 5 μm. Two types of cracks are observed: linear crack and circular crack. The linear crack is observed in the center of the damaged region which propagates to the periphery of the damaged region. The circular crack is located at the rim of the crater. The damage morphology induced by 10 ns laser is surface layer damage. The periodic linear waves are generated due to the interference between the incident beam and the scattered beam. The spacing of the ripples is 1.54 μm which is close to the incident laser wavelength 1.064 μm. The linear crack is located at the center of the damaged region. Furthermore, for the same laser energy density, the dimension of the damaged region and the crater depth induced by 2 ms laser are greater than that of 10 ns laser. It indicates that the damage mechanism under millisecond pulse laser irradiation is strongly different from the case of nanosecond pulse laser.     

Nd:YVO₄ amplifier for ultrafast low-power lasers

An Nd:YVO₄ amplifier consisting of two modules end pumped at 808?nm at 30?W total absorbed power has been designed for efficient, diffraction-limited amplification of ultrafast pulses from low-power seeders. We investigated amplification with a 50?mW, 7?ps Nd:YVO₄ oscillator, a 2?mW, 15?ps Yb fiber laser, and a 30?mW, 300?fs Nd:glass laser. Output power as high as 9.5?W with 8?ps pulses was achieved with the 250?MHz vanadate seeder, whereas the 20?MHz fiber laser was amplified to 6?W. The femtosecond seeder allowed extracting Fourier-limited 4?ps pulses at 7?W output power. To our knowledge, these are the shortest pulses from any Nd:YVO₄ laser device with at least 7?W output power. This suggests a novel approach to exploit the gain bandwidth of vanadate amplifiers with high output power levels. Such amplifier technology promises to offer an interesting alternative to high-power thin disk oscillators at few picoseconds duration, as well as to regenerative amplifiers with low-repetition-rate fiber seeders.     

Generation of all-solid-state, high-power continuous-wave 213-nm light based on sum-frequency mixing in CsLiB6O10

Achievement of more than 100 mW of pure continuous-wave deep-ultraviolet radiation at 213 nm has been demonstrated in an efficient all-solid-state laser system that uses two Brewster-cut CsLiB6O10 (CLBO) crystals. The first crystal is used for 266-nm generation by external resonant doubling of 532-nm radiation from a frequency-doubled Nd:YVO4 laser. Subsequent sum-frequency mixing is performed in a second CLBO crystal placed in a diode-pumped Nd:YAG laser cavity to mix the single-pass 266-nm output with circulating 1064-nm light.     

Enhanced photosensitivity in germanosilicate fibers exposed to CO2 laser radiation

We report a novel method to increase the UV photosensitivity of GeO(2): SiO(2) optical fibers based on exposure to CO(2) laser irradiation before grating writing. Fibers treated with a CO(2) laser can produce gratings with refractive-index modulation two times greater and a Bragg wavelength that can be 2 nm longer than those of untreated fibers. Experiments on GeO(2): SiO(2) preform samples treated with a CO(2) laser in a way similar to the fibers showed a marked increase of the 242-nm absorption band, which is associated with an increase of germanium oxygen-deficient centers and is believed to be responsible for the higher photorefractive response.     

Crater geometry characterization of Al targets irradiated by single pulse and pulse trains of Nd:YAG laser in ambient air and water

High intensities laser pulses are capable to generate a crater when irradiating metal targets. In such condition, after each irradiation significant ablation occurs on the target surface and as a result a crater is formed. The crater characterization is very important specifically for some applications such as micromachining. In this paper, the crater formation in metal targets was studied experimentally. The planar aluminum 5052 targets were irradiated by frequency doubled (532 nm), Q-switched Nd:YAG (∼6 ns) laser beam in ambient air and distilled water. A crater was produced after each irradiation and it was characterized by an optical microscope. Different laser intensities as well as pulse trains were applied for crater formation. The effects of laser characteristics in crater geometry were examined. The depth of the craters was measured by optical microscope and the diameter (width) was characterized by processing of the crater image. The results were explained in terms of ablation threshold and plasma shielding. The results show that the crater geometry extremely depends on the laser pulse intensity, the number of laser pulses, and ambient.     

Laser-conditioning mechanism of ZrO_2/SiO_2 HR films with fitting damage probability curves of laser-induced damage

<正>The method of fitting damage probability curves of laser-induced damage is introduced to investigate the laser-conditioning mechanism of ZrO_2/SiO_2 high reflection(HR) films.The laser-induced damage thresholds(LIDTs) of the sample are tested before and after the laser-conditioning scanning process.The parameters of the defects are obtained through the fitting process of the damage probability curve.It can be concluded that the roles of laser conditioning include two aspects:removing defects with lower threshold and producing new defects with higher threshold.The effect of laser conditioning is dependent on the competition of these two aspects.