Diode-end-pumped Nd:CaYAlO4 mode locked laser

A diode pumped passively mode locked Nd:CaYAlO₄ (Nd:CYA) laser is demonstrated for the first time to the authors' knowledge. Using a V-shaped cavity and a semiconductor saturable absorbing mirror (SESAM), self-started mode locking of the laser is experimentally achieved. The mode-locked pulses are as short as 3.9 ps at a central wavelength of 1080.2 nm. The mode-locked laser produced a maximum average output power of 2.25 W with a slope efficiency of 23.2%.     

The dynamics of a mode locked and frequency-doubled laser

The theory for the transient build-up of mode locked pulses in a mode-locked and frequency-doubled laser is presented. The time required for the mode-locked pulsewidth to reach a steady-state value is computed. It is found that steady-state is reached faster in the presence of internal frequency doubling because of the broadening effect of the doubling crystal. The effects of different conversion efficiencies and modulation depths on the mode-locked pulsewidth are investigated and the results are graphically presented.     

A parametric study of negative feedback Nd:YAG laser

The output pulse parameters of a mode-locked Nd:YAG laser with a passive negative feedback element were studied experimentally. The pulse evolution within the train was experimentally recorded using a modified second harmonic generation autocorrelator. By comparing the laser operation with and without an acousto-optic modulator, we found that with the later there is a significant increase in the mode locking probability and the pulse acquires a temporally Gaussian symmetric pulse shape. Further with the active modulator there is a relaxation in alignment tolerances and increase in the range of permissible dye concentrations for stable mode locking. It was also observed that the pulse width of the negative feedback laser depends on the saturation intensity of the mode locking dye and reduces for a dye with higher saturation intensity. The pulse width was also found to reduce linearly as the initial transmission of the dye is reduced.     

The influence of laser power and repetition rate on oxygen and nitrogen insertion into titanium using pulsed Nd:YAG laser irradiation

Oxygen and nitrogen insertion in a titanium substrate is performed in air using a Q-switched Nd-YAG laser. This process modifies the surface by the formation of specific layers on the substrate. These layers show different properties, largely influenced by the insertion of elements in the layers. The treatment conditions, especially the laser parameters (fluence and repetition rate), must be known and controlled. Using nuclear analysis, we demonstrate that oxygen insertion is mainly influenced by repetition rate, and that nitrogen insertion is controlled by laser fluence. The physical phenomena involved in the oxygen and nitrogen insertion are discussed.     

Passive mode locking of a Nd：YAG laser with co－doped Nd, Cr：YAG as saturable absorber

We demonstrate the characteristics of relatively low saturation intensity using co-doped Nd, Cr:YAG as saturable absorber for passively mode locking the Nd:YAG laser. The difference of the saturation intensity between Q-switched and mode-locked operation in co-doped Nd, Cr:YAG was only one to two orders of magnitude, while Cr:YAG was generally reported at a difference of five orders of magnitude. More than 80% mode locking modulation depth was achieved at an incident pump power of 4.4W, corresponding to an intracavity intensity of 6×10^4W/cm^2, using a 68cm long plano-concave cavity.     

CW mode locked Nd:YVO4 laser pumped by 20-W laser diode bar

The efficient cw mode locking (cw-ML) regime was demonstrated in Nd:YVO₄ laser by means of saturable absorber mirror (SAM). The 0.3-at.% Nd³⁺ doped 10-mm-long YVO₄ crystal end pumped by 20-W diode module with a beam shaper was applied as a gain medium located in the close vicinity to the rear flat mirror of the first arm of Z-type resonator of 316 cm total length with two curved mirrors of 100-cm curvature radii. The SAM of 2%-saturable absorptance and saturation fluence of 50 μJ/cm² was mounted at the opposite end of a resonator. The developed “dynamically stable” cavity design mitigates detrimental role of thermal aberration in gain medium, enforcing clean perfect mode locking even for the highest pump densities. The cw-ML pulses with 47.5 MHz repetition rate and pulse durations in the range of 15–20 ps were observed for a wide range of pump powers and output coupler losses. In the best case, for 32% of output coupler transmission, up to 6.2 W of average power with near 35% slope efficiency was achieved. The thresholds for Q-switched ML, cw-ML regimes were 2.67 W and 6.13 W of pump power, respectively. For the maximum pump power of 20 W we obtained 133 nJ of pulse energy with 16-ps pulse duration, resulting in a peak power higher than 8 kW. The threshold energy density at SAM giving the QML regime was estimated to be about 30 μJ/cm², threshold of cw-ML regime was 220 μJ/cm².     

一种连续Nd：YAG激光锁模的新技术

本文首次报道了在连续运转的Ｎｄ：ＹＡＧ激光器上通过移动腔镜实现锁模的实验研究。获得平均宽度为６０ｐｓ的锁模脉冲序列。文中提出了实验结果的初步解释，表明这种锁模现象是一种增强的自锁模。     

用涨落模型解释有关主被动锁模Nd:YAG激光器的若干问题

应用锁模理论的涨落模型分析研究了已报道的若干主被动锁模Nd:YAG激光器的实验现象.对锁模的阈值行为,锁模几率随调制器调制功率的变化规律和系统的稳定性随可饱和吸收染料的相对浓度的变化规律等实验结果从理论上进行了比较成功的解释.     

Moving perforation of rocks using long pulse Nd:YAG laser

Laser perforating is a new method in oil and gas wells where researchers look for an alternative to explosive methods. One of the important problems with this method is the generation of uniform and cylindrical holes at a selected pitch for enhancing the permeability of rocks. In non-moving laser perforation, the nozzle of the laser and the rock do not approach each other and due to laser convergence in a point, uniform and cylindrical holes are not created. For this reason, moving laser perforation is suggested in this research. One of the important parameters in moving laser perforation is the power of the laser that can be perforated at a specific rate. In this article we predicted the laser power for a definite rate of perforation (ROP) and then the accuracy of this prediction was evaluated to support the experiments. A pulsed Nd: YAG laser, with a pulse energy around 5.5 J, pulse repetition rate of 30 Hz and pulse duration of 2 ms were used for rock perforation in this study. The results shows that the presented relation for perforation could reliably be used in practice. Furthermore, by knowing the rate of perforation, the required laser power for consistent drilling could be calculated.     

Laser welding of glasses using a nanosecond pulsed Nd:YAG laser

This work reports on laser welding of two 1 mm thickness borosilicate glasses through the irradiation with a nanosecond pulsed laser, as a novel alternative to the use of ultrashort pulsed lasers for welding of transparent materials. Two different methodologies were investigated and compared in terms of interface quality. In a first approach, the glasses were joined without any absorbing intermediate layer. However, the bond interface possesses defects. To improve the resulting bond interface, the use of a titanium ultrathin intermediate layer was proposed to weld the glasses substrates, acting as a sealant between them. The laser parameters were optimized to achieve the best joining conditions of the Ti film. The use of the Ti layer gives rise to a bond interface more homogeneous and free of damages. As a further step, thin glasses of 86 µm thickness, of great technological value, were joined through the Ti film as well. The joined interfaces were inspected through optical microscopy and scanning electron microscopy (SEM) while the bond quality was evaluated by Scanning Acoustic Microscopy (SAM).     

High repetition rate laser pulses amplified by Nd/Cr:YAG ceramic amplifier under CW arc-lamp-light pumping

Laser pulses with high repetitive rate generated from a Q-switch microchip Nd:YAG oscillator were amplified by Active mirror composed of Nd/Cr:YAG ceramic pumped by CW arc-lamp. The laser pulses were amplified, and saturation of averaged output laser power was observed. Repetitive ratio of injected laser pulses was changed from 20 to 100 kHz. Averaged output laser power and gain were measured, and gain coefficient and power-extraction efficiency were evaluated. Output laser power was calculated and compared to experimentally measured one, and the calculated results are consistent with the experimental ones. For amplification of laser pulses with high repetitive ratio, this laser material can realize very high laser gain with low pumping power density and high optical-optical conversion efficiency under CW-lamp-light pumping.     

Multi-wavelength operation of LD side-pumped Nd:YAG laser

A dual-wavelength laser at 1064 nm and 1319 nm is obtained by a single Nd:YAG crystal rod. On the basis of 1064 nm and 1319 nm dual-wavelength laser installation, the second harmonic waves at 532 nm and 660 nm can be achieved by using non-linear frequency conversion technology. When 1064 nm and 1319 nm lasers oscillate simultaneously, the maximum output power is 30.5 W and 8.78 W, respectively. When the 1319 nm laser is restrained, we obtain a 35.6 W maximum output power at 1064 nm and by contrary 11.2 W at 1319 nm. The maximum output powers of 532 nm and 660 nm lasers are 5.34 W and 1.353 W when oscillating simultaneously. With one of them restrained, the maximum output power is 6.72 W at 532 nm and 1.90 W at 660 nm. The optimum repetition rate of the acousto-optic Q-switch is 10.5 KHz and 20.5 KHz for 532 nm and 660 nm lasers, respectively. The optical-to-optical conversion efficiency from the fundamental waves to the harmonic waves is 17.5% and 15.4%. The instability is less than 2%.     

Frequency-tripled Nd:YAG laser ablation in laser structuring process

With the increasing demand for finer lines/spaces on PCB boards, a new technology—laser structuring—has emerged in recent years. In this research, the frequency-tripled Nd:YAG laser is selected as the laser source in laser structuring; this laser is often used in miniaturization machining. This paper describes in detail the processing parameters’ influences, such as laser power, numbers of repetition, repetition rate and bite size, on laser structuring results. From the research results, it can be concluded that the line width and depth are increased with increases in the laser power and numbers of repetition. Repetition rate, bite size and velocity are related to one another. When the bite size is fixed, the velocity increases with the repetition rate and the depth of the line is decreased at the same time. When the repetition rate is fixed, velocity increases with the bite size.     

可控制变频率微片激光器

 用20 W光纤耦合LD作为泵浦光源，对Nd^{3+sup>:YVO_{4/sub>微片进行了增益开关实验，获得了可控制变重复频率（1 Hz～25 kHz）的调Q激光输出。输出脉冲激光的宽度为16 ns，输出的峰值功率在几W。从速率方程出发进行了增益开关理论研究，通过数值解，分析了输出激光特性：当泵浦电流高于产生单脉冲的电流时，增加泵浦电流将在一个泵浦脉宽中出现多个激光脉冲输出；增加泵浦脉宽，将在一个泵浦脉宽中出现多个激光脉冲输出，泵浦脉宽越大，子脉冲个数越多；当增加重复频率时，输出与泵浦激光重复频率完全一致的激光脉冲，但也将在一个泵浦脉宽中出现多个激光脉冲输出。}}     

Accuracy control of three-dimensional Nd:YAG laser shaping by ablation

Improving the dimensional accuracy along the optical axis without decreasing the materials removing rate is a key issue in three-dimensional laser shaping. This paper presents a concept for performing three-dimensional laser shaping by directly using machining laser as the photo source of the non-contacting measuring device. Due to the high power measuring photo source and a 1.06 μm bandpass filter, the interference caused by the emission light of ablated surface can be effectively avoided, the delay time is not needed to be inserted between the laser pulse and the measurement. So the measurement will not decrease the material removal rate and productivity. By using this system, the shaping accuracy of 30 μm can be achieved at the removing rate of about 4.0×10⁻² mm³/sec for Si₃N₄ ceramic, both are much better than the results obtained before.     

High repetition rate,high peak power acousto-optical Q-switched 946 nm Nd:YAG laser

A diode-end-pumped high repetition rate, high peak power acousto-optical (AO) Q-switched 946 nm Nd:YAG laser was demonstrated in this paper. In our experiments, a 20 mm miniature acousto-optical Q-switch was employed in a 45 mm linear laser cavity for generating the short laser pulse. At a repetition rate of 10 kHz, a maximum average output power of 2.9 W was achieved with a pulse width of 24.4 ns, giving a peak power of 11.9 kW. To the best of our knowledge, this is the highest peak power 946 nm Nd:YAG laser at high repetition rate operation. Moreover, pulse train with good stability was also obtained at the repetition rate of 50 kHz. At an incident pump power of 22.3 W, up to an average output power of 3.5 W pulsed 946 nm laser was generated at 50 kHz with a pulse width of 69 ns, corresponding to an optical conversion efficiency of 15.7% and an average slope efficiency of 24.1%, respectively.     

Pulsed laser deposition process of PLZT thin films using an infrared Nd:YAG laser

Pulsed laser depositions of PLZT thin films were performed using an Nd:YAG (1064 nm) laser. The growths took place in vacuum or in an oxygen background. Room temperature and 500 °C were the used substrate temperatures. The X-ray diffraction analysis revealed a preferential crystallographic orientation in the films grown at room temperature in vacuum. Such result is discussed. The velocity distribution functions of the species in the plasma plume were obtained from a time of flight study using optical emission spectroscopy. The maximums of these distributions functions fall around 10⁶ cm/s, equivalent to an energy range of 18-344 eV. Ionic species of heavy elements (like lead) achieved higher velocities than other lighter species. This result is linked to the creation of an accelerating spatial charge and to the thermal nature of the target material extraction that allows some elements to be released first than others. Chemical state variations of the elements present in the films were analyzed. Under these different growing conditions, lead chemical states varied the most.     

Removal of graffiti from the mortar by using Q-switched Nd:YAG laser

This paper presents part of the larger study on microstructural features of mortars and it's effects on laser cleaning process. It focuses on the influence of surface roughness, porosity and moisture content of mortars on the removal of graffiti by Nd:YAG laser. The properties of this laser are as follows: wavelength (λ) 1.06 μm, energy: 500 mJ per pulse, pulse duration: 10 ns. The investigation shows that the variation of laser fluence with the number of pulses required for the laser cleaning can be divided into two zones, namely effective zone and ineffective zone. There is a linear relationship observed between number of pulses required for laser cleaning and the laser fluence in the effective zone, while the number of pulses required for the laser cleaning is almost constant even though the laser fluence increases in the ineffective zone. Moreover, surface roughness, porosity and moisture content of mortar samples have influence on the laser cleaning process. The effect of these parameters become however negligible at the high level of laser fluence. The number of pulses required for the laser cleaning is low for smooth surface or less porous mortar. Furthermore, the wetness of the samples facilitates the cleaning process.     

Gas protection optimization during Nd:YAG laser welding

Many laser processes, such as welding or surface treatments are associated with an undesired phenomenon, which is oxidation. The solution commonly employed to solve this problem approaches the shielding gas and/or the shielding gas device. What we propose in this paper is a methodology with the goal to optimize the protection gas device design as well as the gas flow in the case of laser welding and surface treatments. The pressure created by the gas flow on the sample surface is recorded and analysed together with the operating parameters influence in order to reach the objectives. The nozzle system designed and presented below assures the protection against material oxidation using minimal gas flow rates and increases the welding penetration in the case of high-power Nd:YAG laser welding.     

激光脉冲重复频率对冲量耦合系数的影响

 设计了触发间隔从100 ms到5 ms可调的双脉冲触发器驱动TEA CO₂激光器高压开关，用以模拟激光器不同的输出脉冲重复频率。用该装置精确控制的不同激光脉冲重复频率下，对抛物面型光船模型进行了大气模式激光推进中冲量耦合系数的实验研究。发现冲量耦合系数随着重复频率的增加而下降，而单位时间内光船获得的冲量耦合系数的增量则随着重复频率的增加而增大。初步分析认为，这是由于飞行器内的空气未得到充分的补充造成的。