이중 파장 조사법을 이용한 레이저 유도 기계적 효과 제어에 관한 연구

Abstract

In this thesis, we propose a dual-wavelength irradiation method that can be applied to control laser-induced thermoelastic effects and demonstrate its feasibility through a simulation study. Recently, many studies have focused on evoking tactile sensations using laser-induced thermoelastic effects. Specifically, laser pulses directly irradiated on human skin can generate displacement and stress waves through laser-induced thermoelastic effects that propagate into the skin and activate sensory receptors located near the skin's surface. A facile method to control the strength of these tactile sensations is to regulate the laser pulse energy. In particular, recent studies have demonstrated that the perception probability of tactile sensations increases in direct relation to increasing laser pulse energy. However, as laser pulse energy increases, skin temperature also rises resulting in unwanted sensations via thermal effects. This relationship imposes limitations on the parameters of this method and its applicability. The dual-wavelength irradiation method proposed in this thesis, which involves two laser pulses with different wavelengths that simultaneously irradiate human skin, addresses the problem of the facile method. Here, the laser-induced thermoelastic effects can be controlled by varying the ratio of pulse energy values from the two laser pulses. Doing so allows researchers and clinicians to maintain a constant skin temperature in patients. In Chapter 2, we describe the thermoelastic wave equation governing the laser-induced thermoelastic effects, as well as its simulation parameters. Through a preliminary simulation, we showed that both the temperature and displacement of the skin caused by the laser-induced thermoelastic effects rises when laser pulse energy is increased. And then, we outline the selection of two wavelengths, 532 nm and 1064 nm, and the corresponding pulse energy values of each wavelength component. These parameters can limit a maximal temperature increase to 1 °C. In Chapter 3, We present results of subsequent simulations to demonstrate the feasibility of the dual-wavelength irradiation method. The magnitude of the skin displacement caused by laser-induced thermoelastic effects increased in direct relation to the percentage contribution of a 1064-nm laser pulse to temperature increase in skin. In doing so, we found that the change in magnitude of the skin displacement strongly depended on the penetration depth of the laser pulse. Penetration depth of the laser pulse was also influenced by varying the laser beam diameter. A possible implementation of the proposed dual-wavelength irradiation method is explored using a frequency-doubled Nd:YAG laser system equipped with an external second-harmonic generator, in which two laser pulses of 532-nm and 1064-nm are available simultaneously. The ratio between the two laser pulses can be controlled by adjusting the conversion efficiency of the second-harmonic generator. Experimental verification of the proposed implementation needs to be pursued in the near future. We believe that the results presented in this thesis can broaden the applicability of the laser stimulation technique based on laser-induced thermoelastic effects.