Optical studies of pulsed-laser fragmentation of biliary calculi

Nonlinear absorption: intraocular microsurgery and laser lithotripsy

“…The contributions of thermionic emission and cascade ionization to plasma formation vary depending on α, F and the laser pulse duration τ . With µs pulses and absorption coefficients typical for intracorporeal stones, thermionic emission may even dominate during a major part of the laser pulse, and cascade ionization starts to play an important role only after a high electron density has been created by heating of the target (Teng et al 1987a). The threshold for plasma formation is therefore determined by the radiant energy incident on the stone rather than the intensity.…”

“…Since plasma formation inside the fibre depends on the intensity of the laser pulse, it can be markedly reduced if, at the same energy, the pulse duration is prolonged . A longer pulse duration does not compromise plasma formation on the linear absorbing stone, because that depends on the radiant energy applied (Teng et al 1987a). Laser pulses with a duration of about 1 µs can thus efficiently fragment stones while causing little damage to the fibre.…”

“…Plasma is formed when enough free electrons are produced by heating of the stone to support cascade ionization. Near threshold, this occurs only at the end of the laser pulse, but well above threshold it occurs further at the beginning of the pulse (Teng et al 1987a). This leads to an extended energy range in which the coupling of light energy into the plasma and thus the transformation into mechanical energy grows with increasing pulse energy (Rink et al 1995, Rudhardt 1995.…”

“…The sequence of events is plasma formation, emission of a pressure transient, and generation of a cavitation bubble, as in intraocular microsurgery (Teng et al 1987a, Lo et al 1990, Ihler 1992, Rink et al 1995, Rudhart 1995. The details of the process are, however, quite different.…”

Laser-induced acoustic wave generation/propagation/interaction in water in various internal channels

“…and gall stone fragmentation [6]. The laser energy interaction with the surface of an absorbing liquid or a transparent liquid in contact with an absorbing solid boundary induces rapid heating, thermoelastic expansion or explosive phase change and finally emission of a strong ultrasonic wave or a shock wave, depending on the applied laser energy [7,8].…”