7 May 1996 Influence of acoustic diffraction on laser-induced stress wave effects in absorbing media
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Abstract
Short pulses of laser radiation are capable of producing strong stress transients in absorbing materials. If the stress waves contain negative components, material can be fractured or even ablated. This study investigates the propagation of thermoelastic stress waves and in particular the occurrence of tensile stresses due to acoustic diffraction in front of a submerged optical fiber. Pulses with a duration of 6 ns from an optical parametric oscillator (OPO) were transmitted through optical quartz fibers into aqueous dye solution or gelatin. The absorption of the samples could be widely varied by tuning the wavelength of the OPO. Stress wave propagation and tensile stress induced cavitation were observed by use of a time gated video camera. Depending on the absorption of the sample, tensile stress induced cavitation could be observed either outside (at high absorption) or inside the laser irradiated volume (at low absorption) in front a submerged fiber tip, the latter at temperatures calculated to be below the boiling point. The experimental findings together with theoretical calculations demonstrate that the limited size of the stress wave source, leading to diffraction of the wave is the reason for the occurrence of tensile stress waves. This was further supported by the results obtained from the comparison of cavitation bubble formation at a rigid boundary (fiber submerged) and at a free boundary (fiber outside the liquid). In medical applications where fiber-transmitted laser pulses are applied, tensile stress can weaken the mechanical strength of the tissue. This may facilitate the onset of tissue ablation but might also cause unwanted side effects, which can occur even far outside the optically heated zone.
© (1996) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Guenther Paltauf, Martin Frenz, Heinz Schmidt-Kloiber, "Influence of acoustic diffraction on laser-induced stress wave effects in absorbing media", Proc. SPIE 2681, Laser-Tissue Interaction VII, (7 May 1996); doi: 10.1117/12.239592; https://doi.org/10.1117/12.239592
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