Yunxiang Pan

Millisecond laser machining of transparent materials assisted by a nanosecond laser with different delays.

A millisecond laser combined with a nanosecond laser was applied to machining transparent materials. The influences of delay between the two laser pulses on processing efficiencies and modified sizes were studied. In addition, a laser-supported combustion wave (LSCW) was captured during laser irradiation. An optimal delay corresponding to the highest processing efficiency was found for cone-shaped cavities. The modified size as well as the lifetime and intensity of the LSCW increased with the delay decreasing. Thermal cooperation effects of defects, overlapping effects of small modified sites, and thermal radiation from LSCW result in all the phenomena.

Effect of inclusion matrix model on temperature and thermal stress fields of K9-glass damaged by long-pulse laser

Abstract. A model containing an inclusion matrix heated by a millisecond laser is proposed to calculate temperature and thermal stress fields of K9-glass using a finite element method. First, the evolution of temperature and thermal stress fields is analyzed. Results show that both the upper and lower surfaces are damaged. K9-glass is primarily damaged by the combination of radial and axial stresses. Calculated damage morphology is mainly determined by radial stress. Then damage morphology evolution with the increase of the incident laser energy is investigated, which shows that damage area spreads inward from both the front and rear surfaces. Finally, experimental results of long-pulse laser damage of K9-glass are analyzed. The comparison of numerical results with experimental observations shows a good correlation in damage morphology, which indicates that the built inclusion matrix model is applicable to long-pulse laser damage in K9-glass.

Millisecond laser machining of transparent materials assisted by nanosecond laser

A new form of double pulse composed of a nanosecond laser and a millisecond laser is proposed for laser machining transparent materials. To evaluate its advantages and disadvantages, experimental investigations are carried out and the corresponding results are compared with those of single millisecond laser. The mechanism is discussed from two aspects: material defects and effects of modifications induced by nanosecond laser on thermal stress field during millisecond laser irradiation. It is shown that the modifications of the sample generated by nanosecond laser improves the processing efficiency of subsequent millisecond laser, while limits the eventual size of modified region.

Laser-induced damage threshold of silicon under combined millisecond and nanosecond laser irradiation

The laser–silicon interaction process was investigated with the superposed radiation of two pulsed Nd:YAG lasers. A pulse duration of 1 millisecond (ms) was superposed by 7 nanosecond (ns) pulses, creating a combined pulse laser (CPL). The time-resolved surface temperature of silicon was measured by an infrared radiation pyrometer. The melting thresholds of silicon were attained for a single ms laser and a CPL by infrared radiometry and time-resolved reflectance. The concept of threshold boundary was proposed, and a fitted curve of threshold boundary was obtained. An axisymmetric model was established for laser heating of silicon. The transient temperature fields were obtained for single ms laser and CPL irradiation using finite element analysis. The numerical results were validated experimentally, and an obvious decrease in melting threshold was found under CPL irradiation. That is attributed to pre-heating by the ms laser and the surface damage caused by the ns laser.

Time-resolved temperature measurement and numerical simulation of superposed pulsed Nd:YAG laser irradiated silicon

Time-resolved surface temperature of single crystal silicon was measured by an infrared radiation pyrometer. The silicon sample was irradiated by two pulsed Nd:YAG lasers with pulse duration of 1ms superposed by 7ns pulses, referred to as combined pulse laser (CPL). The change of the damage radius with the millisecond (ms) laser energy density was studied, and then compared with that of single ms laser irradiation. An axisymmetric numerical model was established for calculation of the temperature field distribution while silicon was irradiated by single ms laser and CPL, respectively. Compared with experimental results, the CPL-silicon damage mechanism was discussed.

Effect of focus position of ns pulse laser on damage characteristics of K9 glass

Laser-induced damage of optical glasses has been investigated for more than fifty years. Due to the residual scratches, inclusions and other forms of defects at surfaces of optical glasses after the processes of grinding and polishing, it is well known that the sample surface can be damaged more easily than bulk. In order to get the relationship between the damage threshold and the location of the laser spot, we carried out damage experiments on K9 glasses with a 7ns pulse laser. Since ns pulse laser-induced damage of optical glasses always accompanies with the generation of the plasma, a optical microscope connected with a CCD camera was used to observe the plasma flash, which can provide a real time detection of damage sites. The laser pulse was first focused into the bulk, then the spot was moved toward the direction of incident laser beam step by step until the beam was completely focused in ambient air. Damage threshold curves were measured for each focus position, and low thresholds and high thresholds were extracted from those curves. Finally, the relationship between damage thresholds and focus position was analyzed.

Analysis of temperature and thermal stress fields of K9 glass damaged by 1064nm nanosecond pulse laser

There are residual scratches, inclusions and other forms of defects at surfaces of optical materials after the processes of grinding and polishing, which could either enhance the local electric field or increase the absorption rate of the material. As a result, the laser-induced damage threshold at the surface of the material is reduced greatly. In order to study underlying mechanisms and process of short pulsed laser-induced damage to K9 glass, a spatial axisymmetric model where the K9 glass was irradiated by a laser whose wavelength and pulse width are respectively 1064nm and 10ns was established. Taking into account the fact that the surface of the K9 glass is more likely to be damaged, 2μm-thick layers whose absorption coefficients are larger than bulk were set at both the input and output surfaces in the model. In addition, the model assumed that once the calculated tensile/compressive stress was greater than the tensile/compressive strength of K9 glass, the local absorption coefficient increased. The finite element method(FEM) was applied to calculate the temperature and thermal stress fields in the K9 glass. Results show that only the temperature of a small part of interacted region exceeds the melting point, while most of the damage pit is generated by thermal stress. The simulated damage morphology and the size of the damage region are consistent with those reported in literatures, which indicates that the model built in our work is reasonable.

Through-hole energy-density threshold of silicon induced by combined millisecond and nanosecond pulsed laser

We report herein the experimental investigation of the through-hole energy-density threshold of silicon irradiated by a double-pulse laser. The double pulse consists of a 1 ms pulse and a time-delayed 5 ns pulse and is referred to as a combined-pulse laser (CPL). A modified level-set method is used to calculate the process of millisecond laser drilling, and we study how the time delay affects the CPL. The results show that the through-hole energy-density threshold decreases with increasing delay time between the CPL pulses. In addition, the energy density of the nanosecond pulse strongly affects the through-hole energy-density threshold. We also consider the thickness and the doping concentration of the silicon wafers. Compared with the results for single-ms-pulse irradiation, the CPL produces a better through-hole energy-density threshold because the surface ablation caused by the nanosecond pulse increases the energy absorbed by the silicon wafer from the millisecond pulse.We report herein the experimental investigation of the through-hole energy-density threshold of silicon irradiated by a double-pulse laser. The double pulse consists of a 1 ms pulse and a time-delayed 5 ns pulse and is referred to as a combined-pulse laser (CPL). A modified level-set method is used to calculate the process of millisecond laser drilling, and we study how the time delay affects the CPL. The results show that the through-hole energy-density threshold decreases with increasing delay time between the CPL pulses. In addition, the energy density of the nanosecond pulse strongly affects the through-hole energy-density threshold. We also consider the thickness and the doping concentration of the silicon wafers. Compared with the results for single-ms-pulse irradiation, the CPL produces a better through-hole energy-density threshold because the surface ablation caused by the nanosecond pulse increases the energy absorbed by the silicon wafer from the millisecond pulse.

Interaction between a bubble and a metal target for underwater laser propulsion

Optical beam deflection and high-speed photographic methods are employed to investigate the interaction mechanism between a laser-induced bubble and a metal target for underwater laser propulsion. A preliminary theory is proposed to reveal the step increases of the kinetic energy transferred to the target during the process of increasing the incident laser energy. This theory also helps to explain the increasing coupling efficiency with incident laser energy for underwater laser propulsion.

Dynamics study of a laser-induced bubble on a finite metallic surface in water

To investigate the dynamics of a bubble induced on a finite rigid boundary in water, a simple experimental method based on laser beam transmission probe is developed to measure the time dependence of the bubble’s radius on a finite metallic surface under different incident laser energies, and a numerical method is employed to simulate the bubble’s first collapse. A correction factor based on the Raleigh collapse time formula is proposed to describe the collapse time of the bubble induced on a finite rigid boundary. The experimental and simulation results show that the correction factor is slightly different for the bubble’s first and subsequent two oscillations, and its detailed expression is obtained from the experimental and simulation results. The experimental results show that the conversion efficiency of the incident laser energy into bubble energy increases with the former, and the ratio of the energy left for subsequent bubble oscillation increases with the number of bubble oscillation.