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The temperature-dependent optical properties of materials, such as refractive index and reflectivity, can be used for remote sensing of temperatures using a laser beam. Wireless sensing mechanism eliminates the need for external electrical contacts to the sensor, typically required in electrical property-based sensors. Such contacts tend to melt at elevated temperatures. This paper investigates the optical response of silicon carbide to temperature changes, indicating the potential for realizing silicon carbide-based high temperature wireless sensors by using a helium?neon laser of wavelength 632.8?nm. The laser power reflected by single crystal 6H?silicon carbide substrates has been measured at different temperatures up to 750??C. The reflected powers exhibited oscillatory patterns caused by multiple beam interference for which the substrate itself acts as a Fabry?P?rot etalon. These results were used to calculate the refractive index, reflectivity and thermo-optic coefficients of silicon carbide as a function of temperature. Two samples with different dopant profiles were investigated to examine the effects of dopants on the optical response of silicon carbide substrates. With proper modification of the microstructure in the substrate, the temperature dependence of reflectivity can be used directly to measure temperature remotely.

Drilling of microvias with Nd : YAG laser of wavelength 1.06 µm is studied for polymer materials used in high density electronic packaging applications. Generally thermal ablation is the dominant mechanism for such polymer materials in the nanosecond and microsecond laser pulse regimes. A transient thermal model is developed to describe the drilling process. Since a fraction of the laser energy is absorbed inside the polymer layer, volumetric heating occurs and an overheated small metastable region can form inside the layer. Thus the drilling mechanism involves thermal phase change, chemical decomposition and possibly explosion due to rapid thermal expansion and vaporization inside the polymer material. The volumetric heating can cause large thermal stresses inside the material, resulting in convex forms at the free surface of the substrate. Experimental results show the formation of such convex surfaces during laser irradiation. These surfaces eventually rupture and the material is removed explosively due to high internal pressures.

A plume consisting of vapour and ionized particles from the workpiece is commonly formed during various types of laser materials processing. The process parameters such as the laser power, spot diameter, scanning speed, material properties and shielding gas affect the properties of the vapour-plasma plume. A mathematical model is presented in this paper to predict the plasma properties, such as its temperature and absorption coefficient, and the partitioning of laser energy between the plasma and workpiece for different process parameters. The effect of plasma on the surface temperature of the liquid metal and the vaporization rate are modelled using the Stefan condition at the liquid-vapour interface. A new experimental technique named as the pinhole experiment is presented in this paper to measure the partitioning of laser energy between the plasma and the workpiece.

A mathematical model is developed to relate the weld depth and width to the laser parameters. The model is based on the Stefan condition for quasi-steady state laser welding. Simple expressions are obtained for the shapes of the solid - liquid and liquid - vapour interfaces. They are found as functions of the laser irradiance, welding speed, absorptivity and thermal diffusivity. Simple expressions for the temperature distributions in the liquid and solid phases are also presented. The results for the weld depths and widths and the temperature distributions in the workpiece are illustrated for various laser intensities and welding speeds. The predictions of this simple model are found to compare well with experimental data and other previous models.

An n-type 4H-SiC substrate is doped with gallium using a laser doping technique and its optical response is investigated at the mid-wave infrared (MWIR) wavelength 4.21 μm as a function of the dopant concentration. The dopant creates a p-type energy level of 0.3 eV, which is the energy of a photon corresponding to the MWIR wavelength 4.21 μm. Therefore, Ga-doped SiC can be used as an uncooled MWIR detector because an optical signal was obtained at this wavelength when the sample was at room temperature. The energy level of the Ga dopant in the substrate was confirmed by optical absorption spectroscopy. Secondary ion mass spectroscopy (SIMS) of the doped samples revealed an enhancement in the solid solubility of Ga in the substrate when doping is carried out by increasing the number of laser scans. A higher dopant concentration increases the number of holes in the dopant energy level, enabling photoexcitation of more electrons from the valence band by the incident MWIR photons. The detector performance improves as the dopant concentration increases from 1.15 × 1019 to 6.25 × 1020 cm−3. The detectivity of the optical photodetector is found to be 1.07 × 1010 cm Hz1/2 W−1 for the case of doping with four laser passes.

Surface radiation represents an important mechanism for heat loss at high temperatures. Thermal control may require improved heat dissipation of highly emitting surfaces in order to keep the maximum temperature below a certain critical value in high-temperature turbine systems. Emissivity allows determining the surface temperature based on thermal spectra measurement or thermal imaging of the turbine blades. In this study, the emissivities of different coating samples including the metal substrate have been measured over a wavelength range 0.4–1.08 µm in the temperature range 400–1150 °C and high values of emissivities are observed. The data are also compared with the theoretical values of emissivity. The comparison between the theory and experiment are, however, poor because the experimental data are obtained at high temperatures, while the theoretical values are calculated using the values of refraction and absorption indices at room temperature in the Fresnel reflection formula. The optical constants of the samples are computed by the Lorentz elastically bound electron theory of insulator and the Drude free-electron theory of metals.

Laser beam shaping allows redistribution of laser power and phase across the cross-section of the beam for drilling perfectly cylindrical holes. The main purpose of this paper is to show that pitchfork beams improve the microvia quality significantly. An optical system, which comprises three lenses, is designed using conventional diffraction analysis and ray tracing technique to transform a Gaussian beam into a pitchfork beam. The first two lenses are the phase elements through which a Gaussian laser beam is transformed into a super Gaussian beam. The ray tracing technique of geometrical optics is used to design these phase elements. The third lens is the transform element which produces a pitchfork profile at the focal plane due to the diffraction effect. Another aspect of this paper is to demonstrate a pinhole scanning power meter to measure the laser beam profile at the focal plane where the irradiance is very high. This measurement technique is used to verify the existence of the pitchfork beam at the high irradiance plane.

During laser materials processing, the melt pool can form a convex or concave surface depending on the processing conditions and surface tension of the liquid metal. The curved surface can act as a lens to refract the laser rays. Temperature distributions below the substrate's surface are calculated for various values of the attenuation coefficient and radii of curvature of the free surface of the melt pool. For convex spherical curvature and a large penetration depth of the laser beam, the subsurface temperatures are found to be extremely high, exceeding the surface temperature severalfold. This is because of self-focusing, that is further focusing of the laser beam, that causes high subsurface laser irradiance.