Absorption modification by laser irradiation in DKDP crystals

Laser-induced modification at 355 nm of deuterated potassium dihydrogen phosphate(DKDP) crystals following exposure to nanosecond(ns) and sub-ns laser irradiation is investigated in order to probe the absorption mechanism in damage initiation. Laser damage resistance is greatly improved by sub-ns laser conditioning,whereas only a little improvement occurred after ns laser conditioning at the same laser fluence. Moreover, scattering and transmittance variations after the two types of laser conditioning indicate similar reduction of linear absorption. However, by contrast, large differences on nonlinear absorption modification are discovered using Z-scan measurement. This characteristic absorption modification by laser irradiation provides evidence that a nonlinear absorption mechanism plays a key role in damage initiation at 355 nm.     

大口径氘化磷酸二氢钾晶体离线亚纳秒激光预处理技术

大口径氘化磷酸二氢钾(DKDP)晶体抗激光损伤性能偏低严重地制约着大型高功率激光装置输出水平.本研究利用离线亚纳秒激光预处理技术有效地提升了大口径DKDP晶体抗激光损伤性能.实际使用情况表明,采用离线亚纳秒激光预处理后,DKDP晶体在9 J/cm²激光通量辐照下的表面平均损伤密度得到大幅下降,由未处理前的5.02 pp/cm²(1pp表示1个百分点)降至0.55 pp/cm²,降幅为一个数量级.同时,激光预处理对晶体损伤尺寸具有一定的抑制作用,预处理后晶体损伤点尺寸分布曲线向尺寸减小的方向平移,尺寸分布峰值由预处理前的25μm降至预处理后的18—20μm.     

Wavelength dependence of laser-induced damage: determining the damage initiation mechanisms

A novel experimental approach is employed to understand the mechanisms of laser induced damage. Using an OPO (optical parametric oscillator) laser, we have measured the damage thresholds of deuterated potassium dihydrogen phosphate (DKDP) from the near ultraviolet into the visible. Distinct steps, whose width is of the order of k(B)T, are observed in the damage threshold at photon energies associated with the number of photons (3-->2 or 4-->3) needed to promote a ground state electron across the energy gap. The wavelength dependence of the damage threshold suggests that a primary mechanism for damage initiation in DKDP is a multiphoton process in which the order is reduced through excited defect state absorption.     

Expedited laser damage profiling of KDxH(2-x)PO4 with respect to crystal growth parameters

We investigate the laser-induced damage resistance at 355 nm in deuterated potassium dihydrogen phosphate (DKDP) crystals grown with varying growth parameters, including speed of growth and temperature. The aim is to explore a new expedited method to study the growth parameters affecting the laser-induced damage resistance in DKDP material to obtain crystals with enhanced performance.     

Femtosecond pulse parametric amplification at narrowband high power gas laser pumping

In ultrashort pulse amplification a narrowband gas pump pulse laser has been used for the first time. An all-stage optical parametric chirped pulse amplifier (OPCPA) was driven by a single-shot iodine photodissociation laser. For the first time a broadband amplification was achieved in potassium dihydrogen phosphate crystal at 800 nm seeding. Ti:sapphire laser pulses stretched from 12.5 fs to 250 ps were amplified and compressed to 27 fs at a 0.5 TW output power. The results suggest using narrowband high power gas lasers as OPCPA drivers to generate petawatt beams.     

热退火对KDP晶体损伤阈值的影响（英）

研究了不同退火温度下磷酸二氢钾（KDP）晶体的透过光谱和损伤阈值的变化。发现热退火对晶体的透过光谱没有影响,退火温度分别为140 ℃和160 ℃时晶体的损伤阈值没有明显变化。但是在150 ℃下,晶体的损伤阈值提高了约1.4倍。实验证明150 ℃下的热退火对提高晶体的损伤阈值效果最好。     

Characterization of biaxial crystals for tangentially phase-matched frequency conversion

Ultraviolet radiation has been generated by tangentially phase-matched sum-frequency mixing in biaxial L-arginine phosphate (LAP) crystal for the first time using Nd:YAG output at 1064 nm and Rh 6G dye laser output at 560 nm as the two input sources. Characterization has also been made of such a cheap, biaxial crystal for its possible use in devices for tangentially phase-matched short wavelength generation. If the crystal is of proper cut, thickness and quality so that its maximum capability can be exploited it can replace the potassium dihydrogen phosphate (KDP) group of crystals for various applications.     

Raman gains of ADP and KDP crystals

In this paper, the Raman gain coefficients of ammonium dihydrogen phosphate(ADP) and potassium dihydrogen phosphate(KDP) crystals are measured. By using a pump source of a 30-ps, 532-nm laser, the gain coefficients of ADP and KDP are 1.22 cm/GW, and 0.91 cm/GW, respectively. While for a 20-ps, 355-nm pump laser, the gain coefficients of these two crystals are similar, which are 1.95 cm/GW for ADP and 1.86 for KDP. The present results indicate that for ultra-violet frequency conversion, the problem of stimulated Raman scattering for ADP crystal will not be more serious than that for KDP crystal. Considering other advantages such the larger nonlinear optical coefficient, higher laser damage threshold,and lower noncritical phase-matching temperature, it can be anticipated that ADP will be a powerful competitor to KDP in large aperture, high energy third-harmonic generation or fourth-harmonic generation applications.     

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

Laser annealing via preexposure to laser pulses at sub-damage-threshold fluences is known to improve the resistance of KDP crystals to laser-induced damage. Using a specific damage-testing method, we investigate the laser annealing process as a function of fluence and number of preexposure pulses (at 355 nm, 2.5 ns). Our aim is to reveal the key laser parameters in order to devise a practical and efficient protocol for optimizing performance of the material for operation in laser systems in the near UV. Results suggest that a near twofold improvement to the laser-damage performance can be achieved with a limited number of preexposure pulses.     

Effect of S Substitution for P Point Defects in KDP Crystals： First-Principles Study

The electronic structure and geometric distribution of phosphor replaced by sulfur in potassium dihydrogen phosphate （KDP） are investigated by first-principles calculations. The point defect narrows down the energy gap to about 4.9eV, corresponding to a two-photon absorption of 355nm after correction. This can explain the decrease of the laser damage resistance in KDP crystals. Moreover, the defects twist the crystal structure and weaken bonds, especially the O-H bonds, so these bonds may be the first sites to crack under laser irradiation.     

大口径DKDP晶体受激拉曼散射增益系数的测量（英）

大口径KDP/DKDP晶体在强紫外光辐照下产生横向受激拉曼散射效应（TSRS）, 受激放大的拉曼散射光将导致激光能量损失甚至激光损伤, 测量DKDP晶体TSRS增益系数对设置激光装置的运行区间以确保晶体的安全使用非常重要。采用高精度光谱仪探测大口径DKDP晶体（氘含量65%）在351 nm激光辐照下的横向拉曼散射信号, 得到了拉曼散射光的增长曲线, 拟合得到的拉曼增益系数为0.109 cm/GW。同时, 实验结果表明晶体体损伤不影响TSRS增长行为, 表明晶体体损伤对拉曼增益系数测量结果的影响可以忽略。     

Molecular Dynamics Study on Microstructure of Potassium Dihydrogen Phosphates Solution

用分子动力学模拟方法研究了不同温度下磷酸二氢钾(KDP)水溶液的内能和微结构. 水分子被看作简单点电荷模型,磷酸二氢根被看作七节点模型. 另外系统地研究了溶液的内能和径向分布函数. 在饱和温度附近,局域粒子数密度有很大的波动;饱和溶液的比热要比不饱和溶液的比热高表明溶液在低于饱和温度的过程中经历了结晶过程;水中的氧原子跟磷酸二氢根中的氢原子的径向分布函数表明二者之间形成很强的氢键结构;在KDP水溶液中,钾阳离子跟磷酸二氢根中的氧原子有很强的相互作用,并且在饱和溶液中形成的离子对数量更多.     

HfO2/SiO2多层高反膜激光预处理技术

采用不同的光斑移动距离,对电子束蒸发制备的HfO2/SiO2多层高反膜进行了单步及多步预处理。结果表明:为了使薄膜不产生损伤,预处理最高能量密度最好不超过薄膜零几率损伤阈值的90%; 相同预处理效率下进行的单步预处理对提高光学薄膜抗激光损伤阈值的效果比多步预处理好;对HfO2/SiO2高反膜进行98.4%能量覆盖的两步预处理后薄膜损伤阈值提高81%;控制薄膜的缺陷源,初始物质应采用金属Hf。     

On the phenomenological theory of the linear electrooptical effect in uniaxial crystals

The linear electrooptical effect in uniaxial crystals is considered with the aid of direct tensor methods. Covariant expressions for finding biradials and ray refractive indices are developed. A general expression is derived relating the nonlinear optical polarizability parameters and the electrooptical properties of a crystal. The results are applied to cuts of uniaxial ammonium dihydrogen phosphate (ADP), potassium dihydrogen phosphate (PDP), and quartz crystals often used for light modulation. The induced phase differences between the light waves propagating in such crystals are found for the field orientations considered.     

Laser-induced damage studies in GaAs

Laser-induced damage studies have been carried out on monocrystal GaAs at 1.06 μm wavelength as a function of pulse repetition rate in the nanosecond regime. The single shot observed laser damage threshold is 0.9 J/cm². It has been found that the damage threshold decreases when the sample is irradiated with large number of pulses. However, the above effect is observed only when the repetition rate is higher than 1 Hz. Various laser damage mechanism theories have been discussed to explain the results.     

Ultrashort laser pulse bioeffects and safety.

Recent studies of retinal damage due to ultrashort laser pulses have shown that less energy is required for retinal damage for pulses shorter than 1 ns than that for longer pulses. It has also been shown that more energy is required for near-infrared (NIR) wavelengths than in the visible because the light focuses behind the retina, requiring more energy to produce a damaging fluence on the retina. We review the progress made in determining the trends in retinal damage from laser pulses of 1 ns to 100 fs in the visible and NIR wavelength regimes. We have determined the most likely damage mechanism(s) operative in this pulse width regime.     

HPLC测定奥扎格雷钠的有关物质

建立高效液相色谱法测定奥扎格雷钠的有关物质.以十八烷基硅烷键合硅胶为填充剂,以磷酸二氢钾溶液-甲醇(75 ∶ 25,V/V)为流动相;检测波长为272nm.奥扎格雷钠检出限约为0.12ng,测定下限约为0.4ng,主成分峰与其有关物质能完全分离.本方法简便、准确、专属性好,可用于奥扎格雷钠有关物质的测定.     

Electronic Structure and Optical Property Calculation of an Oxygen Vacancy in NH_4H_2PO_4 Crystals

The electronic structure of perfect ammonium dihydrogen phosphate(ADP) and defective ADP with an oxygen(O) vacancy are calculated by screened-exchange hybrid density functional HSE06. The optimized structural parameters of the defective ADP crystal are analyzed. The PO_4 tetrahedron with an O vacancy is distorted and its symmetry is broken. The band gap of the defective ADP with an O vacancy is about 1.5 eV lower than the perfect ADP, which is due to the new O vacancy defect states near the valence band maximum. Moreover, more peaks appear in the low-energy region(lower than 6 eV) in the curves of the linear optical properties for the defective ADP. The results indicate that the O vacancy will significantly influence the laser damage performance of ADP crystals.     

Red picosecond pulses generated by frequency doubling a Raman amplified widely tunable 1.3 microm fiber ring laser

We generated 0.66 microm picosecond pulses by second-harmonic generation of the Raman amplified output of a 1.3 microm actively mode-locked fiber ring laser in a periodically poled potassium titanyl phosphate (PPKTP) waveguide. The ring laser produced 9 ps pulses at a 20 GHz repetition frequency, was tunable over 1284-1330 nm, and was based on a semiconductor optical amplifier and a Mach-Zehnder amplitude modulator. The Raman amplifier served both to amplify the ring laser and to compress the pulses as solitons. The spectral flexibility of the amplifiers and the modulator should enable similar configurations to be made at other wavelengths and facilitate efficient frequency doubling in waveguides to other visible wavelengths.     

Analysis of thresholds of optical damage to the potassium and sodium chloride surface by femtosecond laser pulses

Comparative analysis of optical damage thresholds of potassium and sodium chloride surface by pulses with durations of 80 and 40 fs has been carried out. The critical radiation power in a pulse at the optical damage threshold for the ion crystals studied has been determined. An estimate of the optical damage thresholds of the sodium chloride surface by laser pulses with a duration of 20 fs is presented.