Janez Možina

Investigation of a cavitation bubble between a rigid boundary and a free surface

When a high-intensity laser pulse is focused into a liquid the energy is converted into mechanical energy via an optodynamic process. The conversion starts with plasma formation; this is followed by shock-wave propagation and the expansion of a cavitation bubble. A cavitation bubble developed near boundaries results in an asymmetrical collapse, with the generation of a liquid jet during the bubble’s rebound. In the case of a free surface this liquid jet is directed away from the surface and the oscillation times are prolonged. On the other hand, in the case of a rigid boundary, the liquid jet is directed toward the boundary and the oscillation times are shortened. We present measurements of a cavitation bubble oscillating between a free surface and a rigid boundary using deflections of a laser beam as the optical probe. Shadow photography was used simultaneously as a comparison during the experiments. With the beam-deflection probe we also measured the shortening of the oscillation times near a free surface as well as the prolongation of oscillation times near a rigid boundary. In order to explain this shortening of the cavitation-bubble oscillation times near a free surface, Rayleigh’s model was extended and compared with our experimental results.When a high-intensity laser pulse is focused into a liquid the energy is converted into mechanical energy via an optodynamic process. The conversion starts with plasma formation; this is followed by shock-wave propagation and the expansion of a cavitation bubble. A cavitation bubble developed near boundaries results in an asymmetrical collapse, with the generation of a liquid jet during the bubble’s rebound. In the case of a free surface this liquid jet is directed away from the surface and the oscillation times are prolonged. On the other hand, in the case of a rigid boundary, the liquid jet is directed toward the boundary and the oscillation times are shortened. We present measurements of a cavitation bubble oscillating between a free surface and a rigid boundary using deflections of a laser beam as the optical probe. Shadow photography was used simultaneously as a comparison during the experiments. With the beam-deflection probe we also measured the shortening of the oscillation times near a free surfa...

Quadrature phase-shift error analysis using a homodyne laser interferometer

The influence of quadrature phase shift on the measured displacement error was experimentally investigated using a two-detector polarizing homodyne laser interferometer with a quadrature detection system. Common nonlinearities, including the phase-shift error, were determined and effectively corrected by a robust data-processing algorithm. The measured phase-shift error perfectly agrees with the theoretically determined phase-shift error region. This error is systematic, periodic and severely asymmetrical around the nominal displacement value. The main results presented in this paper can also be used to assess and correct the detector errors of other interferometric and non-interferometric displacement-measuring devices based on phase-quadrature detection.

A study of blast waveforms detected simultaneously by a microphone and a laser probe during laser ablation

We examine blast waves generated in air during irradiation of absorbing samples with Nd: YAG laser pulses of fluences exceeding the ablation threshold. Blast waves were detected simultaneously by a wideband microphone and a laser beam deflection probe. By a comparative analysis of both signals in the time and frequency domain we investigate characteristic features of their nonlinear waveform evolution. To explain the observed phenomena we employ the weak shock solution of the point explosion model.

Measurement of venous leg ulcers with a laser-based three-dimensional method : Comparison to computer planimetry with photography

A lack of reproducible and practical methods to assess venous leg ulcer healing is a major problem encountered by investigators evaluating various treatments. We aimed to compare a new laser‐based three‐dimensional (3D) measuring device with computer planimetry with photography for the assessment of venous leg ulcers, and to estimate the reliability of measurements by the methods. Sixty measurements of perimeter and area of 15 venous leg ulcers, <10 cm in diameter (eight patients; six females; mean age 71 years; range 52–90 years), were made with both methods. Two independent investigators performed the measurements at the first visit and 2–4 weeks later. The precision and accuracy of the methods were determined and compared. The accuracies for computer planimetry with photography in comparison with the laser‐based 3D measuring method were 8.4% for perimeter and 16.0% for area measurements. The precisions of ulcer area and perimeter measurements did not differ significantly between the two methods (p=0.993 and 0.201, respectively). The main advantage of the laser‐based measuring method is the 3D ulcer measurement with a precision of 7.5%, which also takes into account distortions created by the limb convexity. The system is accurate, inexpensive, user‐friendly, and appropriate for everyday practice.

Enhanced ellipse fitting in a two-detector homodyne quadrature laser interferometer

The choice of fitting methods for elliptically scattered data obtained with displacement-measuring homodyne quadrature laser interferometers significantly influences the accuracy of the interferometer. This is especially important when the data contain a lot of noise or provide only a segment of the ellipse. The ellipse parameters extracted by the fitting are used either to correct the data or the basic arctangent phase-unwrapping function in order to enhance the accuracy of the measured displacement by reducing the common nonlinearities. We propose the use of linear, ellipse-specific, least-squares fitting that is further bias-corrected using a linear algorithm. This stable fitting method provides a good balance between the accuracy of the fit and the computational efficiency, and never returns corrupt, non-ellipse parameters. It is therefore applicable for an online, uniform fringe subdivision when there is a demand for sub-nanometric resolution. An experimental confirmation of the improvement over traditional fitting methods was carried out with a single-pass, two-detector homodyne quadrature laser interferometer. We were able to operate the interferometer with nanometric accuracy, provided the data draw out at least a quarter-arc of an ellipse.

High-speed measurement of foot shape based on multiple-laser-plane triangulation

Optical three-dimensional shape measurement of live objects is becoming an important developing and research tool because of its nonintrusive nature and high measuring speed. The current methods are reaching truly high speed in one view configuration, but in the case of the entire object shape measurement, they are limited due to mutual interference between multiple measuring modules. The proposed method overcomes this limitation by using a laser multiple-line triangulation technique, where each of several measuring modules uses a unique laser wavelength. The measuring modules are positioned so that the entire surface of the foot is digitized. This prevents unwanted overlapping between adjacent light patterns. The calibration procedure for each measuring module and for the entire system is based on measurements of the surface of a reference object. The system parameters are determined using an iterative optimization algorithm. The precision of the system is better than ±0.3 mm. The system is capable of measuring objects in motion. The results of the shape of a foot rising on its toes are given as an example.

Acoustic in situ monitoring of excimer laser ablation of different ceramics

Abstract Measurements of excimer laser generated acoustic transients were performed both on the back surface of various ceramic samples and in the surrounding gas. Several techniques of acoustic signal analysis were used to study the correlation between acoustic signal parameters and the ablation rate. The technique based on the defined ablation parameter E s is proved to be suitable for monitoring the excimer laser ablation processes.

Optodynamic energy-conversion efficiency during an Er:YAG-laser-pulse delivery into a liquid through different fiber-tip geometries.

When an erbium-laser pulse is directed into water through a small-diameter fiber tip (FT), the absorption of the laser energy superheats the water and its boiling induces a vapor bubble. We present the influence of different FT geometries and pulse parameters on the vapor-bubble dynamics. In our investigation, we use a free-running erbium: yttrium aluminum garnet (Er:YAG) (λ=2.94 μm) laser that was designed for laser dentistry. Its pulse is directed into the water through FTs with a flat and conical geometry. Our results show that in the case of the conical FT, a spherical bubble is induced, while a channel-like bubble develops for the flat FT. The ratio between the mechanical energy of the liquid medium and the pulse energy, which we call the optodynamic energy-conversion efficiency, is examined using shadow photography. The results indicate that this efficiency is significantly larger when a conical FT is used and it increases with increasing pulse energy and decreasing pulse duration. The spherical bubbles are compared with the Rayleigh model in order to present the influence of the pulse duration on the dynamics of the bubbles expansion.

Optical measurements of the laser-induced ultrasonic waves on moving objects

We performed a single-shot, contactless measurement of ultrasonic waves on a laser-propelled rod with a homodyne quadrature laser interferometer (HQLI) during the entire duration of its motion. This is the first such experimental demonstration of the laser-induced motion of an elastic body where the most important mechanisms that reveal the nature of its motion are presented and explained. Furthermore, these measurements quantitatively demonstrate that the HQLI is an appropriate tool for monitoring high-amplitude (1.3 microm) and high-frequency (200 MHz) ultrasonic waves on moving objects. The applicability of the HQLI can also be extended to measure other optodynamic and high-frequency transient phenomena with a constant sensitivity and a resolution below 1 nm.

High-speed two-frame shadowgraphy for velocity measurements of laser-induced plasma and shock-wave evolution

We describe a high-speed, two-frame shadowgraph method for the two-dimensional visualization of an expanding laser-induced plasma and shock wave in two time instances. The developed experimental method uses a 30 ps, green-laser, polarized pulse for the direct and delayed illumination separated by a variable time delay in the range from 300 ps to 30 ns. Since the exposed images of a single event are captured with two CCD cameras, the established method enables velocity measurements of the fast laser-induced phenomena within the nanosecond excitation-laser pulse as well as at later times-when the excitation-laser radiation has already ended.