Diffuse speckle contrast analysis (DSCA) is a noninvasive optical technique capable of monitoring deep tissue blood flow. However, a detailed study of the speckle contrast model for DSCA has yet to be presented. We deduced the theoretical relationship between speckle contrast and exposure time and further simplified it to a linear approximation model. The feasibility of this linear model was validated by the liquid phantoms which demonstrated that the slope of this linear approximation was able to rapidly determine the Brownian diffusion coefficient of the turbid media at multiple distances using multiexposure speckle imaging. Furthermore, we have theoretically quantified the influence of optical property on the measurements of the Brownian diffusion coefficient which was a consequence of the fact that the slope of this linear approximation was demonstrated to be equal to the inverse of correlation time of the speckle.
In order to simplify computation time for multi-exposure speckle imaging, we recently presented a new indicator of blood flow, i.e. the slope of the inverse square of the contrast values versus camera exposure time (kslope). In this paper we simulate a sequence of correlated dynamic speckle images to test the viability of kslope as an indicator of flow velocity. We find that the presence of static scattering doesn’t influence the linear relationship between the slope and flow velocity. We also show that the normalization can be performed to obtain equivalence relation between relative slope values and relative flow velocity. The computation can be greatly simplified for multi-exposure speckle imaging. This new indicator kslope can play an important role in quantitatively assessing tissue blood flow velocity.
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.
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.
Laser processing as laser drilling, laser welding and laser cutting, etc. is rather important in modern manufacture, and the interaction of laser and matter is a complex phenomenon which should be detailed studied in order to increase the manufacture efficiency and quality. In this paper, a two-dimensional transient numerical model was developed to study the temperature field and molten pool size during pulsed laser keyhole drilling. The volume-of-fluid method was employed to track free surfaces, and melting and evaporation enthalpy, recoil pressure, surface tension, and energy loss due to evaporating materials were considered in this model. Besides, the enthalpy-porosity technique was also applied to account for the latent heat during melting and solidification. Temperature fields and melt pool size were numerically simulated via finite element method. Moreover, the effectiveness of the developed computational procedure had been confirmed by experiments.
In order to simulate the interaction between high power laser and carbon fiber polymer composites, a 1D finite element model has been developed to research the decomposition process of composite pyrolysis. Mass and energy balance equations and gas convection have been solved in the model. The results of the computer simulation show a narrow reaction region formed in the composite with a huge pore pressure.
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.
The process of long pulse laser(1ms) interaction with the aluminum plate was analyzed using Mach-Zehnder interferometer in this paper. A continuous semiconductor laser with about 50mW power and 532nm wavelength was used to detect the flame which induced by long pulse laser interaction with the aluminum plate. A high speed camera was used to capture the interferograms. The exposure time of the high speed camera is about 2 microseconds. And the frame rate is 2130fps. The high-speed camera and the long pulse laser pulse was synchronously controlled by the four-channel digital delay (Stanford Research Systems DG535).The FFT(Fast Fourier transform ) analysis is applied to extract the phase of the interferograms. The results provide an understanding of the process of long pulse laser drilling of the Al target.