Fabrication and laser performance of highly transparent Nd:YAG ceramics from well-dispersed Nd:Y<Subscript>2</Subscript>O<Subscript>3</Subscript> nanopowders by freeze-drying

Well-dispersed Nd:Y₂O₃ powders with uniform particle size of about 60 nm were synthesized from freeze-dried precursors. Highly transparent 2 at.% Nd:YAG ceramics were fabricated from the as-synthesized Nd:Y₂O₃ powders and commercial Al₂O₃ powders by vacuum sintering at 1,750 °C for 5 h. Phase evolution, microstructures, and spectroscopic properties of the Nd:YAG transparent ceramics were investigated. Freeze-drying played an important role in the synthesis of high-quality Nd:Y₂O₃ nanosized powders, which were essential for the fabrication of highly transparent Nd:YAG ceramics. Optical transmittance of a 3-mm thick sample reached 82% in the wavelength range of 200–900 nm. 5.23 W output power was obtained with 14.3 W diode laser pumping, giving a slope efficiency of 36.5%.     

Al<Subscript>2</Subscript>O<Subscript>3</Subscript> plasma production during pulsed laser deposition

A Nd:YAG laser operating in second harmonic (532 nm), 3 ns pulse duration, 150 mJ pulse energy, and 10 Hz repetition rate, is employed to irradiate Al₂O₃ target placed in high vacuum. The produced plasma is investigated by an ion collector used in time-of-flight configuration and by a mass quadrupole spectrometer, in order to determine the equivalent plasma temperature and the atomic and molecular composition. Pulsed laser deposition technique has been used to produce thin films on different substrates placed close to the target. Different surface analyses, such as energy dispersive X-ray fluorescence (EDXRF), X-ray photoelectron spectroscopy (XPS) and surface profilometry are employed to characterize the produced films. Measurements of ablation yield, plasma equivalent temperature, acceleration voltage and characterization of grown thin films are presented and discussed.     

脉冲激光辐照液滴锡靶等离子体极紫外辐射的实验研究

采用波长13.5 nm的极紫外光作为曝光光源的极紫外光刻技术是最有潜力的下一代光刻技术之一, 它是半导体制造实现10 nm及以下节点的关键技术. 获得极紫外辐射的方法中, 激光等离子体光源凭借转换效率高、收集角度大、碎屑产量低等优点而被认为是最有前途的极紫外光源. 本文开展了脉冲TEA-CO₂激光和Nd:YAG激光辐照液滴锡靶产生极紫外辐射的实验, 对极紫外辐射的谱线结构以及辐射的时空分布特性进行了研究.实验发现: 与TEA-CO₂激光相比, 较高功率密度的Nd:YAG激光激发的极紫外辐射谱存在明显的蓝移; 并且激光等离子体光源可以认为是点状光源, 其极紫外辐射强度随空间角度变化近似满足Lambertian分布.     

Laser-diode-pumped Nd:YVO<Subscript>4</Subscript>/Nd:YAG MOPA burst-mode laser

We report a high-repetition-rate, high-peak-power laser diode (LD) pumped burst-mode 1064 nm laser from a Nd:YVO₄/Nd:YAG master oscillator power amplifier. 10–100 kHz pulse burst in a duration up to 2 ms is achieved in LD end-pumped Nd:YVO₄ acousto-optically Q-switched laser. After amplification with LD side-pumped Nd:YAG rod amplifiers, the single pulse energy reaches 73 mJ in 10 kHz pulse burst laser with a peak power of 7.8 MW.     

Effect of prepulse laser wavelength on EUV emission from CO2 reheated laser-produced Sn plasma

We investigated the role of prepulse laser wavelength on extreme-ultraviolet (EUV) emission and ionic debris generation. A 6 ns Nd:YAG laser operating at 266 nm was used to generate a pre-plasma that was then reheated by a 35 ns CO₂ laser pumping pulse at 10.6 μm. At an ideal delay time, improvement in EUV conversion efficiency (CE) of up to 30 % was seen compared to the CE from the pumping pulse alone. It was also shown that the most probable Sn ion kinetic energies were reduced significantly with the use of a prepulse, however, ion fluence increased. These results were compared to those obtained using a 1064 nm prepulse.     

Spectroscopic studies of laser-produced plasmas formed in CO and CO2 using 193, 266, 355, 532 and 1064 nm laser radiation

The wavelength dependence of laser-produced breakdown in air, CO and CO₂ has been studied using the four Nd:YAG harmonics (266 nm, 355 nm, 532 nm and 1064 nm) and the ArF-excimer laser (193 nm). Breakdown thresholds at these wavelengths are reported for air, CO and CO₂. A significant reduction in the breakdown thresholds for both CO and CO₂ is apparent when comparing 193 nm with the four Nd:YAG harmonics. This reduction is attributed to the resonance-enhanced two-photon ionization of metastable carbon atoms generated in the laser focus at the ArF-laser wavelength. In addition to reporting breakdown thresholds, the laser-produced plasmas in CO and CO₂ are characterized in terms of plasma temperatures and electron densities which are measured by time-resolved emission spectroscopy. Electron densities range from 9 × 10¹⁷ cm^–3 to 1 × 10₁₇ cm^–3. Excitation temperatures range from 22 000 K at 0.2 µs to 11 000 K at 2 µs. Ionization temperatures range from 22 000 K at 0.1 µs to 16 000 K at 2 µs. Evidence is presented to indicate that, like ArF-laser-produced plasmas, Nd:YAG-laser-produced plasmas formed in CO and CO₂ are in or near a state of Local Thermodynamic Equilibrium (LTE) soon after their formation.     

An Intracavity-Triggered Passively-Switched Nd:YVO<Subscript>4</Subscript> Laser

We demonstrate an intracavity-triggered passively Q-switched Nd:YVO₄ laser within a diode-end-pumped configuration. We employ a Cr⁴⁺:YAG saturable absorber as the passive Q switch and an Nd:LiYF₄ (YLF) laser as the laser triggering of the Q-switched laser. Since we use the same Cr⁴⁺:YAG crystal and output coupler with the Nd:YVO₄ laser, the Cr⁴⁺:YAG Q switch is triggered inside the Nd:YLF laser cavity. As a result, the timing jitter in standard deviation of Nd:YVO₄ laser can be reduced to 16 ns.     

Performance comparisons of passively Q-switched intracavity-frequency-doubled Nd:Gd<Subscript>0.19</Subscript>Y<Subscript>0.81</Subscript>VO<Subscript>4</Subscript> and Nd:Gd<Subscript>0.83</Subscript>Y<Subscript>0.17</Subscript>VO<Subscript>4</Subscript> lasers at 671 nm

Performance comparisons of laser-diode pumped passively Q-switched intracavity-frequency-doubled Nd:Gd_0.19Y_0.81VO₄ and Nd:Gd_0.83Y_0.17VO₄ lasers at 671 nm are demonstrated for the first time to our knowledge. KTP crystal is used as the frequency doubling material and V:YAG crystal as the saturable absorber with initial transmission of 89%. The dependences of average output power, pulse width, pulse repetition rate, single-pulse energy and peak power on incident pump power are measured and contrasted. The experimental results show that, Nd:Gd_0.83Y_0.17VO₄ laser has more excellent properties than Nd:Gd_0.19Y_0.81VO₄ laser at 671 nm.     

Stoichiometric transfer by infrared pulsed laser deposition of y-doped Bi–Sr–Ca–Cu–O investigated using time-resolved optical emission spectroscopy

Pulsed laser deposition of Bi₂Sr2Ca_1–x YxCu₂O_8+δ (Bi-22Y2) with x = 0, 0.30, and 0.49 on an MgO (100) substrate was conducted using a Q-switched 1064 nm Nd:YAG laser The laser-produced plasma (LPP) emission was collected during the deposition. Time-resolved optical emission spectroscopy reveals that the plasma plume consists of neutral atoms and ions. SEM images indicate that clusters of correct stoichiometry arrive on the substrate surface. Our result confirms that IR PLD transfers material stoichiometrically.     

Diode-pumped Q-switched Nd:Y<Subscript>0.5</Subscript>Lu<Subscript>0.5</Subscript>VO<Subscript>4</Subscript> laser double-modulated with an acousto-optic modulator and Cr<Superscript>4+</Superscript>:YAG saturable absorber

A diode pumped Nd:Y_0.5Lu_0.5VO₄ pulse laser modulated with an acousto-optic (AO) Q-switcher and Cr⁴⁺:YAG saturable absorber is first demonstrated in this paper. The laser is shown to generate shorter pulse width than AO Q-switched laser alone, and have a higher peak power and single pulse energy than passively Q-switched laser with only Cr⁴⁺:YAG. A laser pulse width of 6.16 ns and a peak power of about 43.83 kW are achieved at the incident pumping power of 14.09 W.     

Passive Q-switching of a laser-diode-pumped intracavity-frequency-doubling Nd:Nd<Superscript>3+</Superscript>:NaY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>/KTP laser with Cr<Superscript>4+</Superscript>:YAG saturable absorber

Nd³⁺:NaY(WO₄)₂, known as Nd:NYW, is a new type crystal. By using laser-diode as pump source, a passive Q-switching of intracavity-frequency-doubling Nd:NYW/KTP laser has been realized with Cr⁴⁺:YAG saturable absorber. The dependence of pulse repetition rate, pulse energy, pulse width, and peak power on incident pump power for different small-signal transmissions of Cr⁴⁺:YAG are measured. The coupled rate equations are used to simulate the Q-switched process of laser, and the numerical solutions agree with the experimental results.     

Precision continuous-wave cavity ringdown spectroscopy of CO<Subscript>2</Subscript> at 1064 nm

A high performance continuous-wave cavity ringdown spectrometer using a rapidly swept cavity has been applied to investigate weak absorption of CO₂ line near 1064 nm. In the experimental setup, offset locking of the Nd:YAG laser to other iodine-stabilized Nd:YAG laser provides a frequency accuracy better than 20 kHz. The ringdown time was obtained by fitting the experimental ringdown data to a theoretically obtained ringdown curve. The absorption coefficient and absolute center wavelength are determined with accuracy better than two orders of magnitude with respect to the previous observations. Our experimental setup yields high performance in a relatively simple, low cost, and compact system that is amenable to chemical analysis of trace gases in medicine, agriculture, industry, and the environment.     

Patterning and morphology of nonlinear optical Gd<Subscript>x</Subscript>Bi<Subscript>1-x</Subscript>BO<Subscript>3</Subscript> crystals in CuO-doped glass by YAG laser irradiation

Dots and lines consisting of nonlinear optical Gd_xBi_1-xBO₃ crystals were patterned on the surface of CuO-doped Gd₂O₃-Bi₂O₃-B₂O₃ glass by heat-assisted (200 °C) Nd:YAG laser irradiations with a wavelength of λ=1064 nm, where the laser energy absorbed by Cu²⁺ is converted to the local heating of the surrounding Cu²⁺. The surface morphology and orientation of crystals in the patterned lines were clarified from confocal scanning laser microscope observations and polarized micro-Raman scattering spectra. Crystal lines with periodic bumps (i.e., ladder-shape like lines) were patterned by laser irradiations with a power of 0.79 W and a scanning speed of 60 μm/s, and the orientation of Gd_xBi_1-xBO₃ crystals in the lines was proposed. The present study demonstrates that the combination of Cu²⁺ and continuous wave Nd:YAG laser with λ=1064 nm is effective in inducting crystallization of oxide glasses. The mechanism of laser-induced crystallization in glass has also been discussed. PACS 61.43.Fs; 42.70.Mp; 68.35.Bs; 78.30.-j; 79.20.Ds     

Dynamics of ions produced by laser ablation of ceramic Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Al at 193 nm

We study the dynamics of ions produced upon ablation of Al and ceramic Al₂O₃ targets using nanosecond laser pulses at 193 nm (6.4 eV) as a function of the laser fluence from threshold up to 12 J cm⁻². An electrical (Langmuir) probe located at 40 mm from the target surface has been used for determining the ion yield and calculating the kinetic energy distributions. The results for both targets show the existence of a significant amount of ions having kinetic energies >200 eV (≈20% around threshold fluence), and kinetic energies are up to >1.5 keV. The results are related with the existence of direct photonionization processes caused by the photon energy of the laser being higher than the ionization potential of Al (5.98 eV). Comparison of the ion yield when ablating the two types of targets for fluences above threshold to data reported in the literature suggests that the magnitude of the yield and its threshold are parameters depending on the thermal properties of the target rather than on the laser wavelength. Around threshold, the different behavior of ion yield when ablating Al and Al₂O₃ targets suggests that the threshold for neutral aluminium and ion species in the case of ablation of the Al₂O₃ target must be similar.     

1 mJ, 500 kHz Nd:YAG/Nd:YVO<Subscript>4</Subscript> MOPA laser with a Nd:YAG cavity-dumping seed laser

We report a high-repetition-rate, high-pulse-energy master oscillator power amplifier (MOPA) laser system, in which the seed laser from the Nd:YAG rod-based oscillator cavity dumped at 500 kHz, was scaled up consecutively by a four-stage Nd:YVO₄ preamplifier and a two-stage Nd:YAG zigzag slab main amplifier. The laser pulsed output with the average power of 510 W was achieved, with the efficiency extraction of 26.6% at the main amplifier stage and the single-pulse energy of 1.02 mJ.     

A Novel 1,066 nm Nd:Gd<Subscript>0<Emphasis Type="Italic">.</Emphasis>69</Subscript>Y<Subscript>0<Emphasis Type="Italic">.</Emphasis>3</Subscript>NbO<Subscript>4</Subscript> Passively <Emphasis Type="Italic">Q</Emphasis>-Switched Pulse-Burst Laser

We demonstrate for the first time a Cr⁴⁺:YAG passively Q-switched 1066 nm pulse-burst laser under 879 nm direct pump with a novel Nd:Gd_0.69Y_0.3NbO₄ crystal. The output laser characteristics with different pump repetition rates and different Cr⁴⁺:YAG initial transmission are studied. Without the Cr⁴⁺:YAG, we obtain a maximum output energy of 2.55 mJ at an absorbed pump energy of 5.79 mJ with the highest 48% slope efficiency. The pulse-burst laser contains a maximum of 7 pulses for a Cr⁴⁺:YAG initial transmission of 55% and a pump repetition rate of 1 kHz. The single-pulse energy and narrowest pulse width reach 160 μJ and 5.5 ns at 38.2 kHz, with a peak power of 32 kW.     

Lasers for materials processing: specifications and trends

An overview is given of the types of lasers dominating the field of laser materials processing. The most prominent lasers in this field are the CO₂ and the Nd: YAG laser. The domain of CO₂ lasers is applications which demand high laser powers (up to 30 kW are available at present), whereas the domain of Nd:YAG lasers is micro-machining applications. In the kilowatt range of laser output power, the two types of lasers are in competition. New diffusion-cooled CO₂ laser systems are capable of output laser powers of several kilowatts, with good beam qualities, while still being quite compact. The output power and beam quality of Nd:YAG lasers has been improved in recent years, so that Nd:YAG lasers are now an alternative to CO₂ lasers even in the kilowatt range. This is especially true for applications that demand optical fibre transmission of the laser beam, which is possible with Nd:YAG laser light but not with the longerwavelength light emitted by CO₂ lasers. The main problem in solid-state lasers such as Nd:YAG is the thermal lensing effect and damage due to thermal stresses. In order to reduce thermal loading, cooling has to be enhanced. Several alternative geometries have been proposed to reduce thermal loading and, by this, thermal lensing effects. There are now slab and tube geometries which allow much higher output powers than the conventionally used laser rods. A very new scheme proposes a thin slab whose cooled side is also used as one of the laser mirrors, so that thermal gradients occur mainly in the direction of the beam propagation and not perpendicular to it, as is the case in the other geometries. As well as CO₂ and Nd:YAG lasers, semiconductor laser diodes are very promising for direct use of the emitted light or as pump sources for Nd:YAG and other solid-state lasers. When packaging together thousands of single laser diodes, output powers of several kilowatts can be realized. Major problems are collimation of the highly divergent laser beams and cooling of the laser diode bars.     

Passively Q-switched mode-locking in a diode-pumped Nd:LuVO<Subscript>4</Subscript> laser with V:YAG

A passively Q-switched and mode-locked Nd:LuVO₄ laser with V:YAG at 1.34 μm was successfully demonstrated. Comparisons between c-cut and a-cut Nd:LuVO₄ lasers were experimentally made. The maximum average output power of 170 mW, the highest Q-switched pulse energy of 4.5 μJ were obtained in c-cut Nd:LuVO₄ laser. The duration of mode-locked pulse was estimated to be less than 540 ps with repetition rate of 110 MHz.     

Timing-jitter reduced high-repetition-rate Q-switched Nd:YVO<Subscript>4</Subscript>/Cr<Superscript>4+</Superscript>:YAG micro laser with low pump power

We report a high repetition rate Q-switched Nd:YVO₄/Cr⁴⁺:YAG micro laser with small pump power. Unwanted defects in pulse train, which are inherently large in passively Q-switched laser, was simply minimized by controlling temperature of Nd:YVO₄/Cr⁴⁺:YAG medium. When T ₀ = 90% Cr⁴⁺:YAG and R _OC = 90% output coupler were used, Q-switched Nd:YVO₄/Cr⁴⁺:YAG micro laser showed the optimum output; maximum output power of 58 mW, optical-to-optical efficiency of 9.1%, repetition rate of 1.1 MHz, and pulse width of 57 ns were achieved with 640 mW pumping. MHz-order repetition rate in Nd:YVO₄/Cr⁴⁺:YAG Q-switched laser with low pumping (<1 W) is the highest value to the best of our knowledge.     

Non-critical phase-matched second harmonic generation in Gd<Subscript>x</Subscript>Y<Subscript>1-x</Subscript>COB

We have characterized non-critical phase-matching (NCPM) for both Type I and Type II second harmonic generation (SHG) in y-cut Gd_xY_1-xCOB using a nanosecond optical parametric oscillator (OPO). The variation of the NCPM wavelength with temperature was investigated for different values of the compositional parameter x. Efficient SHG of 1064 nm was achieved by choosing the suitable compositional parameter x=0.28 and by tuning the temperature of the crystal to 52 °C. Using a 25-mm-long Gd_0.28Y_0.72COB crystal, conversion efficiencies of 41 and 43% were obtained respectively from a mode-locked Nd:YAG and a Q-switched Nd:YAG laser. PACS 42.25.Lc; 42.65.Ky; 42.70.Mp; 42.79.Nv