Alexander P. Kubyshkin

Photothermal measurement of bulk and surface absorption of transparent infrared optical elements

Photothermal probe beam deflection spectroscopy is used to separately measure bulk and surface absorption of transparent IR optical elements. Different approaches of this technique are considered. One approach consists in variation of the laser beam heat modulation frequency in the condition that the heating beam component is transverse to the path of the probe beam. Another approach consists in varying the heat and probe cross section measuring point position within the tested optical sample. It is theoretically and experimentally shown that in the second case, the spatial resolution and measurement locality are determined by the size of laser beam spots and the thermal diffusion length in a sample. The relative contribution of surface and bulk absorption to summary absorption can be derived from the ratio of the probe beam deflection signal, measured both near the surface and within the sample volume far from the surface, to thermal wavelength. Absolute calibration of absorption measurements can be carried out using laser calorimetry. A comparison between deflection and calorimetry data on volume absorption in a number of ZnSe laser optical elements is made and shows a good correlation.

Thermal wave characterization of coatings on substrate

The problem of determining coating thickness and thermophysical characteristics using photothermal IR-radiometry is investigated theoretically and experimentally. The information about coating characteristics is contained in frequency dependence of surface temperature oscillation amplitude and phase. The optimization of thermal wave excitation modes (continuous wave and transient) has been done for different coating parameters. In experiments the characteristics of ceramic coatings on metal substrates were determined as well as their changes under technological processes.

Opto-acoustic spectroscopy diagnostics of bubble formation in blood during laser TMR

The mechanism of channel punching in the heart muscle (myocardium) by a pulse of high-power CO2 laser was studied. Water, PMMA and biotissue of animals were used as the models. Diagnostics of the punching process was performed by the methods of registration of sound vibrations in the air and back scattering of laser radiation. It has been revealed that at channel punching in various media, sound vibrations are excited in the spectrum region from 1 to 3.5 kHz, which are due to turbulent pulsation of vapor- drop flow ejected from the channel. The spectrum of acoustic vibrations depends on positions of the channel bottom and is sensitive to the boundary between the myocardial tissue and blood. The measurement of sound vibrations spectra can serve as the basis for diagnostics of the process of channel punching in myocardium.

Laser opto-acoustic profilometry of solid surfaces

A laser opto-acoustic system for measuring 3-D objects surface topology has been described. The main principle of operation consists in excitation of a short acoustic pulse by irradiation of surface under test by short laser pulse and in measuring the time of acoustic pulse traveling to the detector. A laser pulse with nanosecond scale duration and liquid immersion medium with high enough sound speed was used to increase the measurement precision. The examples of system applications including aircraft turbine blade measuring have been presented.

Overheating of metals under transparent films by pulsed laser beam

The paper presents the results of theoretical and experimental investigation on the process of metal boiling initiated by pulsed laser beam, with a transparent layer on the metal surface that provides the impedance acoustic load. The theoretical analysis shows that with the squeezed surface, when the energy thresholds of metal boiling essentially depend on laser pulse shape and duration, considerable overheating of metal is possible in the course of laser pulse. In the experiments on mercury, nonlinear variations in the photoacoustic signal were registered that are due to vapor pressure contribution under the excess of boiling thresholds. The experimental values of mercury boiling thresholds under pulse laser heating agree well with theoretical estimates.

Acoustic spectroscopy diagnostics for laser transmyocardial revascularization

The mechanism of channel punching in the heart muscle (myocardium) by a pulse of high-power CO2 laser was studied. Water, PMMA and biotissue of animals were used as the models. Diagnostics of the punching process was performed by the methods of registration of sound vibrations in the air and back scattering of laser radiation. It has been revealed that at channel punching in various media, sound vibrations are excited in the spectrum region from 1 to 3.5 kHz, which are due to turbulent pulsation of vapor-drop flow ejected from the channel. The spectrum of acoustic vibrations depends on positions of the channel bottom and is sensitive to the boundary between the myocardial tissue and blood. The measurement of sound vibrations spectra can serve as the basis for diagnostics of the process of channel punching in myocardium.

Detection of laser-generated Rayleigh waves by recording their IR radiation

The possibility of remote detection of ultrasonic waves by measuring the IR radiation from a surface, has been shown experimentally. This effect is based on the adiabatic propagation of the surface waves. Rayleigh waves were excited by a pulsed Nd:YAG laser on the surface of a steel sample at temperatures close to 1000 degrees C. The surface temperature was estimated independently by the direct measurement of the surface vibration using a laser heterodyne interferometer. Good correlation of the time-of- flight data of the Rayleigh wave propagation, as well as in evaluated and measured temperature deviation was observed. The possibility of application of this laser ultrasonic technique of material properties evaluation at elevated temperatures is discussed.

Decay dynamics of superheated states of liquid metals

An analysis is made of a kinetic model for the decay of the superheated state of liquid metals formed under the action of a nanosecond laser pulse. It is shown that the temperature varies exponentially with time and the characteristic relaxation time is determined.

Investigation of weld seam structure by laser ultrasonic and thermal-wave methods

Laser opto-acoustic and thermal-wave techniques have been applied for investigation of metals structure modification after laser and electric-arc welding. The changes in granular structure of steels due to welding have been considered. Anisotropy of thermal-conductivity due to thermal-induced stress in the heat-affected zone of laser weld seam has been obtained.

Laser excitation of thermal waves for remote control of Si wafers in plasma-etching process

The method of the thickness and temperature field measurement in Si wafers is discussed. Laser excitation of thermal waves and their diffusion in depth and lateral directions are used. Thermal wave is detected by IR radiometry. The pulsed laser illumination of wafers under testing are discussed. The investigation of depth and lateral diffusion of heat processes to determine the thickness and thermal diffusivity separately. The accuracy, reliability and resolution of the method proposed are discussed.