Deformation of rotating liquid surfaces by laser heating is experimentally and theoretically studied. A horizontal plate containing the liquid sample (crude oil with highly temperature-dependent viscosity and surface tension) turns around a vertical axis fixed in the inertial reference frame at an angular speed continuously variable between 0 and 60 rpm. The liquid surface then adopts a classic parabolic profile which works as a convergent mirror for incoming light beams. The liquid surface is smooth and quite insensitive to external vibrations due to its high viscosity. In the next step a CW, CO<sub>2</sub> laser beam parallel to the rotation axis and also at rest in the inertial frame impinges the rotating liquid surface at a given distance from the rotation axis. While the liquid turns, a circular groove is thus ploughed in the liquid surface as a result of the surface tension temperature dependence. The resulting surface profile adopts an axisymmetric stationary shape after a transient heating stage. Its 3D shape at varying turning speed and laser power is experimentally studied by classic fringe-projection techniques. In spite of the low light-reflectivity of the liquid sample, which impedes its practical application in image-forming instruments, the device is useful to build up prototypes of rotating mirrors and to investigate the optical effect of axisymmetric perturbations in their surface profiles.
Emulsions of heavy oils in water are prepared to allow transportation of highly viscous oils over long distances. Their stability is currently assured by addition of surfactants, which cover the suspended oil particles with an electrically charged protective layer. The oil refractive index, which is related to the oil electrical properties, is therefore an important parameter in emulsion stability theory. In addition, as oils extracted from different wells have different refractive index, knowledge of the latter helps to identify oil samples. Basic principles and operation of an optoelectronic setup allowing real-time measurement of the real part of the refractive index (n) of heavy oil samples as a function of temperature (T) in a wide temperature range are presented. The setup consists of a CW laser beam which locally heats the oil sample (so inducing a time-growing temperature gradient and local deformation of the liquid surface) and an optoelectronic system which records as a function of heating time (t) the time-varying divergences of light beams reflected by and transmitted through the sample. As the latter cited are mathematically related to the
refractive index value, function n(t) is thus experimentally determined. The sample temperature (T) is simultaneously recorded as a function of heating time (t) by means of a thermographic camera, thus obtaining function T(t). Combining both plots [n(t),T(t)] the function n(T) is determined in a few minutes in the whole temperature range.
Performance of an optoelectronic setup allowing heating of a remote engineering structure and subsequent analysis of the resulting deformation is described. The structure is heated by a CO<sub>2</sub> laser beam. This gives rise to a thermal stress in the heated region and to a global deformation of the structure. The latter cited is also illuminated by a HeNe laser beam. The light intensity distribution in the backscattered HeNe light wave is digitally recorded and processed by convenient software, which yields the structure deformation and allows the remote observer to get information about its thermal and mechanical properties. A typical application of the setup, consisting in remote damping of free vibrations of a cantilever bar, is presented.
Regions of the human body placed in the neighborhood of arteries or veins are periodically deflected by the passage of dilatation pulses and by body movements due to breathing. Remote, non-invasive measurement of the deflection is performed by means of optical techniques. The skin in the studied reign of the body is illuminated by a laser beam. The backscattered speckle pattern in recorded by a TV camera. The digitized images are numerically processed in order to determine the contrast of the light intensity distribution in each frame, which is a decreasing function of the instantaneous angular velocity of the reflected beam. The plot of time-integrated contrast vs. recording time is shown to closely resemble direct records of the angular deflection of the illuminated region. Also the breathing pattern is plotted in the same graphs.
A light beam structured as a hollow cone is obtained by oblique illumination of a cylindrical surface with a laser beam. The cylindrical surface may be a metallic needle or the core of an optical fiber. In the first case a speckle pattern appears around the light cone. The direction and mean length of light speckles are studied in the text. In the second case the light intensity distribution around the light cone presents bright and dark regions resulting from interference between light rays reflected from and passing through the fiber core. The hollow light cone is transformed into a hollow light cylinder (HLC) by means of a lens whose focus coincides with the cone vertex. The HLC is used to explore the cylindrical layer placed in the neighborhood of the internal wall of a combustion chamber model. Due to the large diameter of the model a Fresnel lens is used to transform the light cone into a light cylinder. This introduces additional optical noise in the system. The HLC is collected again by another Fresnel lens at the model output. A photodetector placed at the focus of the collected beam produces an electrical output showing pulses whenever residual particles pass through the explored region. A quantitative experimental analysis of the performance of the combustion chamber is thus performed.
Optical methods are used to study the following problems in Fluid Mechanics: (1) propagation of solitary waves in water channels; (2) thermoconvective flow of petroleum-in-water emulsions; (3) flow distribution in combustion chambers; and (4) propagation of blood in human veins.
The surface of a heavy hydrocarbon liquid film is deformed due to inhomogeneous heating with a Gaussian laser beam. The surface profile and the liquid velocity distribution are calculated as functions of time. The liquid surface behaves as an aspherical mirror whose focal properties are controlled by varying the laser beam intensity. This phenomenon allows obtention of self and cross correlations of low-power laser beams. Optical properties and applications of this laser-controlled adaptive mirror are discussed.
The deformation of a vein due to heart beats is measured by sticking a small mirror to the skin and recording the deviation of a laser beam incident upon the mirror as a function of time. As an example of possible clinical applications of this method the curves (vein deformation vs. time) corresponding to an individual executing a physical effort of increasing intensity are presented. In another experiment the skin is directly illuminated without interposition of a mirror. The reflected speckle pattern is deformed when the pulse is passing through. It is shown that the contrast of the recorded speckle pattern is a minimum at the instants of time when the vein deformation speed is a maximum. This allows remote measuring of heart beats frequency by a fully non-invasive technique.
Blood clots are illuminated with a focused HeNe laser beam at different power levels. The time-varying reflected speckle pattern (which is recorded with a video camera and filed in a computer memory) rapidly changes in the early heating stage and then remains progressively at rest. The difference between pairs of time-consecutive images is measured by adding the squared differences of intensities at corresponding pixels. This allows one to define a speed of variation D(t) of the reflected image. It is shown that the time-law D(t) is similar to the function describing the derivative of sample temperature with respect to heating time (t). This supports the hypothesis that the observed effect is due to thermal dilatation of biological material forming the clot.
An optical fiber is illuminated at a point (V) by a collimated laser beam forming an angle ((theta) ) with the fiber axis. A light cone (divergence 2(theta) , vertex V, axis coincident with the fiber) is thus generated. The 3D curve (C) resulting from intersection of the light cone with a given opaque surface is recorded by a CCD camera coaxial with the optical fiber. Numerical processing of the curve image allows determination of the polar coordinates (with origin at point V) of each point of curve (C). This device allows internal topographic inspection of concave surfaces. So it may be considered as the counterpart in polar coordinates of classic methods using lateral projection of a light plane to determine contour lines of convex surfaces in cartesian orthogonal coordinates. Applications of this method to inspection of the human bucal cavity shows strong influence of laser light diffusion within human tissue in the resolution of resulting images.
Spatial characteristics of a thermal lens induced by a Gaussian beam in an absorbing liquid are studied using a nonlinear geometrical optics approximation. We show that the thermal lens in the vicinity of the sample cell is a focal point of light and a ring-of-light foci located in a plane displaced with respect to the focal point. The distance between the focal point and the ring-of-light foci decreases when the light power is increased. Experiments performed on an ethanol solution of iodine confirm the existence of this microscopic structure. The distance between the focal point and the ring of foci was measured for samples of different absorption coefficients. A good agreement between theory and experiment was obtained.
Theoretical and experimental studies of the structure of the thermal lens generated in a liquid by a Gaussian electromagnetic field are performed. It is shown that the light induced thermal lens can be considered a focal source of light plus a ring of light sources located on different planes.