To compare the processing efficiency and quality of 20- to 1000-Hz pulsed laser and continuous laser ablating single-crystal germanium wafers, experiments and numerical simulations were performed. The experiments were conducted by varying the duty cycle and repetition frequency of a pulsed laser to ablate single-crystal germanium with the same total laser energy and irradiation time of 100 ms, and comparing the temperature-rise profile during ablation and the damage morphology after ablation. The temperature-rise curves during the ablation and the damage morphologies after the ablation were compared. Numerical simulations were performed to compute the dislocation field of single-crystal germanium ablated by laser with different parameters to compare the size of the heat-affected zone (HAZ) formed on the sample surface after the laser ablation with different parameters. The results show that the sample surface has the largest ablated pore size and the smallest HAZ after ablation at a laser repetition frequency of 20 Hz and a duty cycle of 5%; the smallest pore size and the largest HAZ after ablation at a laser repetition frequency of 1000 Hz and a duty cycle of 50%, and the continuous laser results are in the middle.
Carbon fiber composite has been widely used in structural engineering applications, and research on the mechanical properties under various conditions is attractive. The study experimentally investigated the damage effect of CW laser ( λ=1064nm ) on carbon fiber composite strip under preloading. The effects of laser power and preloading on fracture morphology, temperature of the center of the irradiation area and fracture time were investigated. The experimental results show that when the preloading was lower than the fracture threshold, the laser irradiating could cause matrix damaging, fracturing with fluffy wire drawing and burning through for laser power density of 236.17W/cm2, 407.39W/cm2, 634.39W/cm2, respectively. The maximum temperature of the spot center increased with the increase of laser power, with rising and falling edges varying in a stepwise way. Under the laser irradiation with the same power density, the higher value of preloading corresponded to the less fracture time. However, when the preloading was 20% over the fracture threshold, the influence of the laser power density was significantly reduced. Similarly, when the laser power density was 634.39W/cm2, the fracture time was less influenced by the value of
A microphone was used to study the characteristics of acoustic emission for millisecond laser-induced damage of BK7 glass. Different temporal and spectral distributions were obtained for the front and rear surface damage processes. The front surface damage was associated with the strong thermal melting process, and the size of the conical molten pit was strongly linked to the intense oscillating period of the acoustic signal. The effects of the detection distance and angle on the spectrum below 5 kHz, which were induced during the front surface damage process, were found to be associated with the distribution of the ejection expelled out of the molten pit. The results indicate that the detection of acoustic emission can be used as a real-time online method to obtain information on the millisecond laser-induced damage.
In this paper, the mechanical erosion effect during continuous-wave (CW) laser ablation of GFRP (Glass Fiber Reinforced Polymer) was studied. It happened under the condition of subsonic tangential airflow. In order to know the effect of mechanical erosion in the ablation process. An experimental device was designed to collect mechanical erosion products. The total mass loss of GFRP in the process of laser ablation and mass loss caused by mechanical erosion were measured by experiments. Then the percentage of mechanical erosion was calculated. The experiments were made at different airflow velocities and different laser power densities. Spot center temperature was measured during the experiment. The results show that the percentage of mass loss caused by mechanical erosion will increase and tend to be constant with the increase of airflow velocity. The percentage of mass loss caused by mechanical erosion will increase with the increase of laser power density. The percentage of mass loss caused by mechanical erosion will increase and tend to be constant with the increase of laser irradiation time. The mechanical erosion effect of GFRP is related to its laminated structure. The laminated structure can affect the mechanical erosion mechanism of the material surface. It will cause the regular change of the surface temperature of the material.
A 3D mathematical model was established for the investigation of the thermomechanical behavior of aluminum alloys (Al-7075) under the combined action of tensile loading and laser irradiations. The transient temperature fields and stress-strain field was obtained by using the finite element method. The Johnson-Cook’s constitutive equation is implemented in the FE model to study the failure behaviour of alloy. The effects of various pre-loading and laser power densities on the failure time, temperature distribution and the deformation behavior of aluminum alloys are analyzed. The results indicate the significant reduction in failure time for higher laser power densities and for high preloading values, which implies that preloading may contribute a significant role in the failure of the material at elevated temperature. The numerical result agrees well with our previous experiment results, concluding that the numerical model is reasonable.
The CCD is widely used to detect laser signal in many industrial vision and automation systems. When the charge coupled device(CCD) is irradiated by intense laser, the image quality may decrease and reversible dazzling effects occur such as single pixel saturation, crosstalk, and full saturation. Understanding the laser dazzling phenomena, on the one hand, it can help to choose the suitable laser sources for the measuring systems, on the other hand, it can optimize the designs of the CCD structures. In order to effectively utilize the dazzling effects, the laser saturation threshold of the CCD must be known. Due to the different internal structure and working mode of CCD, the corresponding laser saturation threshold is also different. In this paper, the dazzling effects on the array CCD induced by a 532nm wavelength pulsed laser is investigated by finite element method. A physical model is established base on drift-diffusion equation according to the working process of CCD and the principle of laser dazzling effects. The working state of CCD under different laser power, charge density, electron concentration and potential curve are presented. This model is found to be useful for the analysis of laser saturation threshold.
Laser-induced periodic surface structure (LIPSS) is a universal phenomenon which occurs for both continuous laser and pulsed laser. Recently, most studies are focus on LIPSS irradiated by fs laser. However, LIPSS irradiated by continuous laser still need to be carefully studied. Here, We study LIPSS in silicon wafer irradiated by continuous laser for different duration time and power. For the same power, we can observe the evolution process of LIPSS for different time. It is surprising that the evolution process of LIPSS seems to be layered, which occurs for different power. The inner layer occurs at first, then the outer layer occurs. Our study can be used to control the formation of LIPSS.
A real-time method based on laser scattering technology was used to detect the interaction process of GaAs with a 1080 nm laser. The detector collected the scattered laser beam from the GaAs wafer. The main scattering sources were back surface at first, later turn into front surface and vapor, so scattering signal contained much information of the interaction process. The surface morphologies of GaAs with different irradiation times were observed using an optical microscope to confirm occurrence of various phenomena. The proposed method is shown to be effective for the real-time detection of GaAs. By choosing a proper wavelength, the scattering technology can be promoted in detection of thicker GaAs wafer or other materials.
Thermomechanical behaviour of a glass/epoxy composite plate under local laser irradiation is investigated. Physico-chemical transformations and gas transport in a matrix and fibers are describe by Arrhenius and Darcy's law. The changes of material thermal properties are expressed in terms of the volume fractions of fiber, resin, gas and char. At the same time, we take into account the effects of pore pressure and elevating temperature on thermal stresses and strains. It is established that transverse stress, radius stress and interlayer shear caused by local heating and pore pressure are causes of delamination and cracking of composite plates under laser heating. And interlayer shear can lead failure of composite fast.
To investigate the heat accumulation effect of the laser pulse train, a two dimensional finite element calculation model is established to calculate the temperature field of Aluminum target based on the Fourier heat transfer theory. Take account of the influence of the repetition rate of laser, four different repetition rates (5 kHz, 10 kHz, 50 kHz and 1 MHz) pulse laser train and continuous wave (CW) laser are analyzed in this study. The results indicate that under the same average power and irradiation time, the peak temperature and accumulative temperature increase with the decrease of the repetition rate. With the increase of the repetition rate, the heat accumulation effect is more closer to the CW laser. The heat accumulation effect of pulse laser train with lower repetitive rate is better.
The interaction of CW fiber laser and monocrystal silicon <100> is investigated experimentally and numerically. In the experiment, the damage morphologies are detected by a CCD and an optical microscope. The damaged silicon appears an evident molten pool within the laser spot and several cracks on the surface and slip damage, which indicate that the damage mechanism includes melting and thermal stress damage. The damage morphologies show two types of cracks including radial crack and circumferential crack. Otherwise, an obvious central hillock is found in the molten pool, which may be produced by the fluctuation of the thermal-stress filed and resolidification of the central molten silicon after irradiation. In the numerical simulation, a two-dimensional axisymmetric physical model is established based on the thermo elastic-plastic and classical heat transfer theory and Von Mises yield criterion. The simulation results indicate that the temperature and the stress in the irradiation center are always the highest on the specific condition, which may contribute to the occurrence of the central hillock. The gradient of hoop stress is bigger than the radial stress, thus, it can be inferred that the appearances of the radial cracks in the experiment were closely related to the hoop stress.
KEYWORDS: Silicon, Semiconductor lasers, 3D modeling, Thermal modeling, Systems modeling, Finite element methods, PIN photodiodes, Photodiodes, Crystals, Temperature metrology
A 3D numerical model has been built to investigate anisotropic thermal stresses of (110) silicon induced by millisecond laser. The 12 slip systems resolved shear stress field of the silicon was obtained by using the FEM. The excess resolved shear stress field is identified. comparing to the experiment of the millisecond irradiating (110) PIN photodiode, we conclude that the thermal slips are introduced duo to the anisotropic thermal stresses of silicon surpassed the critical yield stress and brittle cracks are introduced due to the initiation points offered by the thermal slips which will reduce the fracture strength greatly. These thermal slips and brittle cracks increase the dark current of the photodiode greatly.
The photoelectric parameters degradation of Si-based PIN photodiodes irradiated by 1064 nm millisecond Nd:YAG laser has been measured. The samples were the commercial silicon PIN photodiodes BPW34 with plastic package. The applied laser fluence levels range from 20J/cm2 to 1400J/cm2. Surface damage morphology, dark current and sensitivity were investigated for the irradiated photodiodes. It has been shown that the dark current was the first and the most sensitive degradation parameter, and we believe that the dislocation introduced by the tangential component of thermal stress in the [111] and [110] direction was the main reason. The sensitivity decrease until the dark current reach to μA magnitude and the surface have melted seriously, the finite element method was used to calculation the dopant redistribution process. It shows that the degradation of sensitivity depends greatly on the process under various applied laser fluencies.
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