Hong Duc Doan

Laser Processing by Using Fluidic Laser Beam Shaper

Laser beam shaping techniques are important to optimize a large number of laser-material processing applications and laser-material interaction studies. The authors have developed a novel fluidic laser beam shaper (FLBS) with merits such as flexiblility, versatility and low cost. This work presents a fundamentally new approach for laser beam shaping by using FLBS. A Gaussian beam profile is transformed to a flat top beam and annular beam profile in the focal plane. The shaped laser beam is used for laser drilling to investigate the influence of the laser intensity profile in laser processing. The paper concludes with suggestions for future research and potential applications for further the work.Copyright

Long-Term Energy Accumulation in Underground Hot Water Tanks: Fluid Convective Behaviour and Its Influence on the Thermal Losses

Long-term energy storage has great potential to decrease the consumption in buildings, particularly through saving summer excesses to cover winter demands, thus equilibrating the system in both seasons. This chapter deals with hot water storage over 6 months period for an example storage facility in Sofia. Using exact building and weather data in a mathematical model, taking into account fundamental thermodynamical relations, we show amount of energy stored, impact of convection on the thermal losses and we make reference to the building requirements and possible improvements.

Nanosecond time-resolved visualization of short pulse laser ablation processes for various laser beam profiles

Thermal and fluid behavior are governing factors for the via processing of semiconducting substrates. In order to investigate the fundamentals, nanosecond (ns) time-resolved visualization of ns-pulse Nd:YAG laser ablation of SiC substrate has been performed for various conditions. Time-resolved visualizations have been conducted by using a ultra high speed camera with the frame rate of 200 million frames/second for the first time to reveal the ablation process in a single event with the temporal resolution of 5 ns. Both self-emitting light from the ablation plume and the shadow image due to the refractive index profile are visualized. The results show unique features for each spatial beam profiles (Gaussian, quasi-flat top, annular) with different beam energy. Combined with the previous experimental observations, the results help modeling the thermo-fluid numerical modeling of the ablation process.

Fundamental Research on Via Processing by Using Short Pulse Laser in SiC

Laser drilling of silicon carbide (SiC) wafer in air (dry ablation) and underwater by using ns pulsed infrared (1064 nm) Nd: YAG laser is investigated. In order to suggest optimal parameters of via processing in SiC wafer, the effects of pulse number, laser fluence, water film thickness, and focus position are evaluated. As compared with dry ablation vias, decreasing etching rate, increasing via diameter, and generation of cracks in high-energy regime are observed in liquid-assisted processing. However, it is found that it can create vias without debris, HAZ, cracks. Also, optimal parameter set for infrared pulse laser processing under water is found to be the laser fluence of less than 10 J/cm2 and water thickness of 1mm.© 2013 ASME

Numerical Calculation of the Pulse Compression of Ultrashort Pulse Laser Light by Means of the Thermoreflectance of SiC Substrate

A numerical model of the thermoreflectance of doped SiC substrate and typical numerical results are presented. The model considers the temporal response of the electron temperature and the number density of the electronic carriers. Calculated results show steep increase of electron temperature and the resulting increase of reflectivity. As a result, the reflected laser pulse by the substrate is compressed in time domain by means of the temporal response of the thermoreflectance characteristics of SiC substrate. Thermal analysis of the electrons reveals the interesting feature of the thermoreflectance response as a function of pulse intensity, pulse width, or doping concentration. The technique can be used for the compression of ultrashort pulse laser light.Copyright

Fluidic Laser Beam Shaper by Using Thermal Lens Effect

Laser measurement and laser processing techniques have been gaining strong attention from various applications [1,2]. This research aims at the development of a fluidic laser beam shaper, and in order to fulfill the objective, characteristics of the thermal lens effect are studied. This phenomenon has the optical property of a concave lens since the refractive index distribution on the optical axis is formed when the liquid is irradiated. One reason for the refractive index distribution in the liquid is the temperature distribution in the liquid when it is irradiated. In this research, effects of the pump power and propagation distance of the probe beam to probe beam profile are investigated experimentally and theoretically, in order to develop fluidic laser beam shaper. It is indicated that, by controlling some parameters in thermal lens system as pump power (in the regime of linear optics) and absorption coefficient, input Gaussian beam can be converted into flat-top beam profile. The relationship among the distance to obtain a flat-top beam, pump power and absorption coefficient is investigated to show the flexibility of fluidic laser beam shaper in many fields of laser application.© 2011 ASME

Investigation on the interactions among light, material and temperature field in transient lens effect, transmission characteristics in 1D temperature field

Recent advances in measurement techniques using the photothermal effect have drawn more and more attention to the application in various fields. Especially, laser measurements are becoming increasingly important for the thermal design of electronic devices because these measurements can be applied to non-invasive and non-destructive measurement [1, 2]. We focus on the transient lens effect, which is a well-known photo-thermal phenomenon. As this effect is caused by the change of thermal properties or other physical properties, the measurements using this effect have the possibility to measure various materials information. This phenomenon has the optical property of a concave lens since the refractive index distribution on the optical axis is formed when the liquid is irradiated. One reason for the refractive index distribution in the liquid is the temperature distribution in the liquid when it is irradiated. In this research we investigate the interaction between refractive index and temperature distribution on the Transient lens effect, in order to develop fluidic optical devices

Investigation on the interaction among light, material and temperature field in the transient lens effect

Recent advances in measurement techniques using the photo-thermal effect have drawn more and more attention to the application in various fields. Especially, laser measurements are becoming increasingly important for the thermal design of electronic devices because these measurements can be applied to non-invasive and non-destructive measurement [1, 2]. We focus on the transient lens effect, which is a well-known photo-thermal phenomenon. As this effect is caused by the change of thermal properties or other physical properties, the measurements using this effect have the possibility to measure various materials information. This phenomenon has the optical property of a concave lens since the refractive index distribution on the optical axis is formed when the liquid is irradiated. One reason for the refractive index distribution in the liquid is the temperature distribution in the liquid when it is irradiated. In this research we investigate the interaction between refractive index and temperature distribution on the Transient lens effect, in order to develop fluidic optical devices

Investigation on the Interactions Among Light, Material and Temperature Field in Transient Lens Effect

Recent advances in measurement techniques using the photo-thermal effect has draw more and more attention to the application in various fields. Especially, laser measurements are becoming increasingly important for the thermal design of electronic devices because these measurements can be applied to non-invasive and non-destructive measurement [1, 2]. We focus on the transient lens effect, which is a well-known photo-thermal phenomenon. As this effect is caused by the change of thermal properties or other physical properties, the measurements using this effect have the possibility to measure various materials information. This phenomenon has the optical property of a concave lens since the refractive index distribution on the optical axis is formed when the liquid is irradiated. One reason for the refractive index distribution in the liquid is the temperature distribution in the liquid when it is irradiated. So that, in this research we investigate the interaction between refractive index and temperature distribution on the Transient lens effect, in order to develop fluidic optical devices.© 2009 ASME