Janez Mozina

Optodynamic characterization of the shock waves after laser-induced breakdown in water

Plasma and a cavitation bubble develop at the site of laser-induced breakdown in water. Their formation and the propagation of the shock wave were monitored by a beam-deflection probe and an arm-compensated interferometer. The interferometer part of the setup was used to determine the relative position of the laser-induced breakdown. The time-of-flight data from the breakdown site to the probe beam yielded the velocity, and from the velocity the shock-wave pressure amplitudes were calculated. Two regions were found where the pressure decays with different exponents, pointing to a strong attenuation mechanism in the initial phase of the shock-wave propagation.

A beam-deflection probe as a method for optodynamic measurements of cavitation bubble oscillations

High-intensity light from a laser pulse can produce laser-induced breakdown in a liquid followed by a shock wave and the growth of a cavitation bubble. When the bubble reaches its maximum radius, the pressure of the surrounding liquid causes it to collapse; this results in bubble oscillations. The cavitation bubbles oscillations and the corresponding shock waves were measured from the deflections of a laser beam. These deflections were detected using a fast quadrant photodiode, built into the optical probe. The precise relative-positioning system and the small diameter of the beams waist made it possible to detect and analyse the signals from the shock wave and the cavitation bubble. Here, we have demonstrated that a method based on a beam-deflection probe can be used to measure the fast phenomena that follow immediately after laser-induced breakdown as well as the whole dynamics of the bubble oscillations, which corresponds to a three-orders-of-magnitude larger time scale.

Initial healing rates as predictive factors of venous ulcer healing : The use of a laser-based three-dimensional ulcer measurement

There is a need for practical methods to predict the healing time of venous leg ulcers. In a prospective cohort study of 81 patients with venous leg ulcers, we used a recently described laser‐based three‐dimensional measurement of the ulcers at days 0 and 28 to estimate the predictive power of horizontal (HIHR) and vertical initial healing rates (VIHR) for wound healing by week 24. The rates were calculated by Gilmans equation [(A1−A2)/((p1+p2)/2)(0–4)] and by its modification [(V1−V2)/((A1+A2)/2)(0–4)], respectively. The influence of risk factors on both the initial healing rates was also studied. The HIHR and VIHR are important predictors of healing at 24 weeks. They are not influenced by age, ulcer duration, initial ulcer area, and insufficient sapheno‐femoral junction, and/or calf perforating veins. Together with ulcer duration, they are independent predictors of the 24‐week healing (the area under ROC curve equals to 0.9). VIHR gives us additional information and significantly improves the prediction of 24‐week healing.

Measurement of the bending vibration frequencies of a rotating turbo wheel using an optical fibre reflective sensor

An optical fibre reflective sensor was used to analyse the vibrations of a rotating turbo wheel up to 20 400 rpm. The measured signal required correction because of the natural unevenness of the turbo wheel and because of the variable deflection. Because the turbo wheel was rotating the signal became distorted and so we used a special method to extract the frequencies of the vibrations from the power spectra. The analysis showed increased intensity of the first three natural frequencies with an increased speed of rotation. The experimental results match very well with those obtained by numerical computation.

Dual-mode single-crystal photoelastic modulator and possible applications

A new type of acousto-optic device based on a LiTaO(3) crystal is presented. A harmonic voltage with a proper frequency applied to the piezoelectric LiTaO(3) crystal generates mechanical oscillations in the material. Due to photoelasticity, an artificial modulated birefringence is induced by this oscillation. By using a properly adjusted polarizer and analyzer, the transmission of trough-going polarized light can be modulated. By simultaneous excitation of two modes, an advanced optical response can be achieved. For the applications presented here, the first shear eigenmode must have exactly three times the frequency of the first longitudinal eigenmode.

Optoacoustic studies of Er:YAG laser ablation in hard dental tissue

Optoacoustic measurements were carried out in order to obtain better understanding of the ablation mechanisms during the illumination of hard dental tissue by Er:YAG laser radiation. A broadband microphone was used to detect laser generated acoustic waves in the ambient air. Correlation analysis of the laser pulse spikes and the response of the optoacoustic probe indicates that each laser spike ablates the hard dental tissue independently of other spikes. This is in agreement with the model of ablation by means of micro explosions. The optoacoustic signal is observed to be approximately linearly related to the ablation efficiency, and is thus demonstrated to be a good measure of the ablation efficiency. The experiments also show a significant difference in optoacoustic signals obtained during ablation in caries, enamel, and dentin.

Three-dimensional laser based measurement of human foot during walking

Barefoot walking demonstrated a significantly earlier phase shift in sagittal plane ankle and knee kinematics when compared to the VIB and SS (Table 1; Figure 1). There were no temporal differences evident at the hip. All sagittal waveforms showed a high degree of similarity evidenced by CCCs greater than r 0.95. No phase shifts were observed between SS and VIB at these joints in this plane of motion (Table 1). The similarity between waveforms in the frontal plane was particularly weak (r 0.84). At the ankle, although a pronounced phase shift was observed between BF vs SS and BF vs VIB, the 95% CI prevented this from being significant. At the knee however; these were significant.

An analytical model for time-dependent deformation of a circular plate illuminated by a laser pulse

An analytical model is developed which describes the deformation process in a moderately thick plate following the absorption of a laser pulse. The model is based on the temperature gradient mechanism of the three-dimensional Green function of temperature in the plate which is heated with a Gaussian source at its surface. Therefore, the results of the presented model are valid for times longer than the laser pulse.

Interaction thresholds in Er:YAG laser ablation of organic tissue

Because of their unique properties with regard to the absorption in organic tissue, pulsed Er:YAG lasers are of interest for various applications in medicine, such as dentistry, dermatology, and cosmetic surgery. The relatively low thermal side effects, and surgical precision of erbium medical lasers have been attributed to the micro-explosive nature of their interaction with organic tissue. In this paper, we report on preliminary results of our study of the thresholds for tissue ablation, using an opto-acoustic technique. Two laser energy thresholds for the interaction are observed. The lower energy threshold is attributed to surface water vaporization, and the higher energy threshold to explosive ablation of thin tissue layers.

Optoacoustic effects during Er:YAG laser ablation in hard dental tissue

Optoacoustic method is a very useful tool for studying laser induced processes in hard dental tissues. In principle, the method can also be used for on-line monitoring of laser drilling. Our study, however, shows that at high laser energies the optoacoustic energy is not proportional to the volume of the ablated hard dental tissue. In addition, the optoacoustic signal depends critically on the presence of water on the tooth surface. These observations must be taken into account when attempting to use the optoacoustic method for on-line monitoring of the laser drilling process.