Ladislav Grad

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.

Laser pulse shape influence on optically induced dynamic processes

Abstract In this paper, the influence of laser pulse shape on laser drilling process and on optoacoustic effect is studied. Results for two laser applications: laser drilling in aluminium and laser drilling in hard dental tissue are presented. Observed changes in the interaction processes are compared to the optoacoustic (OA) signals. Results confirm that laser pulse shape might have a significant influence on the interaction processes and can be used to increase machining efficiencies. With laser pulse shape, one can effectively induce desired dynamics of the melted material. However, if the process is cumulative in its nature like in the case of drilling in dentin, then the laser pulse shape is less important. Optoacoustic signals are found to depend on the laser pulse shape. However, usually used signal parameters for on line monitoring are not linearly correlated with the drilling efficiency. At least more complex signal analysis is needed. Presented method of signal analysis enables determination of the onset time of changes in the interaction mechanism and determination of energy threshold in the case of hard dental tissue drilling.

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.

Optodynamic studies of Er:YAG laser interaction with wood

Measurements of laser drilling rate of different woods were performed. Using an Er:YAG laser, very long and narrow holes with depth-to-diameter ratio above 100 were achieved. This indicates the occurrence of laser beam trapping. The process has been investigated as a typical optodynamic process. Optoacoustic waves were detected by microphone and analysed to monitor the hole depth produced by consecutive laser pulse exposure of the same spot. The results may have relevance to Er:YAG laser applications in the wood industry.

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.

Optodynamic studies of Er:YAG laser induced microexplosions in dentin

Er:YAG laser drilling of dental tissue has been investigated as the typical optodynamic process. Optoacoustic signals were detected by microphone and analyzed within the frame of linear systems theory to monitor laser induced microexplosions. Statistical distribution of the optodynamic efficiency was performed in terms of partial weights of optoacoustic response function. The results indicate that the laser spikes shorter than 2 μs are more efficient in ablative mechanism.

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.

Optodynamic monitoring of laser tattoo removal

The goal of this research is to use the information contained in the mechanisms occurring during the laser tattoo removal process. We simultaneously employed a laser-beam deflection probe (LBDP) to measure the shock wave and a camera to detect the plasma radiation, both originating from a high-intensity laser-pulse interaction with a tattoo. The experiments were performed in vitro (skin phantoms), ex vivo (marking tattoos on pig skin), and in vivo (professional and amateur decorative tattoos). The LBDP signal includes the information about the energy released during the interaction and indicates textural changes in the skin, which are specific for different skin and tattoo conditions. Using both sensors, we evaluated a measurement of threshold for skin damage and studied the effect of multiple pulses. In vivo results show that a prepulse reduces the interaction strength and that a single strong pulse produces better removal results.

Energy conversion efficiency during optical breakdown

The efficiency of optical to acoustical energy conversion during laser-induced optical breakdown has been examined. A point-explosion model has been studied to determine the value of laser-induced shock wave energy. The influence of incoming laser-pulse energy on conversion efficiency has been studied for several absorber materials.

Optoacoustic monitoring of multiple pulse ablation of Al2O3 ceramic

Optoacoustic signals were used to study the multiple pulse excimer laser ablation of Al2O3 ceramic. Simultaneous measurements of the laser generated acoustic transients were performed both on the opposite side of the irradiated surface and in the surrounding gas. Removal rate changes can be monitored using the analysis of the evolution of the optoacoustic waves amplitudes. Similar evolution phenomena was found for different laser fluences obtained by variation of laser energy and laser beam spot sizes.