RESUMO
A LiNbO(3) optical parametric oscillator (OPO) pumped at 930nm shows a wide phase-matching curve. Each pulse produced by the OPO has a very broad natural linewidth, from 1480 to 1800 nm for the signal and from 1950 to 2550 nm for the idler. The emission wavelength is controlled thanks to an electrically tunable Fabry-Perot interferometer inserted into the OPO cavity. The signal wavelength is electrically tuned in the range 1450-1850nm without crystal rotation.
RESUMO
A transient model of degenerate four-wave mixing in saturable amplifiers is developed for calculation of the phase-conjugate reflectivity in the weak-probe-beam limit. Our model takes into account the laser amplification of all the interacting beams together with the wave-mixing process between the two counterpropagating pump waves. Experimental investigations are made in a flash-lamp-pumped Nd:YAG amplifier with nanosecond pulses at 1.06 microm. A reflectivity of 22% is achieved when all the waves interfere within the laser medium.
RESUMO
A tunable IR laser source is described. A Q-switched Nd:YAG laser at 1.064 µm is used to pump two optical parametric oscillators (OPO's) in series. The first OPO uses a LiNbO(3) crystal and works in the vicinity of degeneracy. Its signal beam (adjusted to 1.82 µm) is used to pump the second OPO, which uses a AgGaSe(2) crystal and covers the range 2.6-6 µm. The conversion efficiency between the energy produced by this OPO in the signal and idler beams (λ(s) = 2.6 µm and λ(i) = 6 µm, respectively) and the input energy at 1.064 µm is 1.75%.
RESUMO
The aim of this study was to evaluate the thermal diffusion of a pulsed laser beam in atheroma and to obtain in vitro vaporisation of the plaque without causing arterial wall lesions. A computerised mathematical model integrated 4 parameters: reflectivity, thermal conduction, the absorption factor and coefficient of diffusion. The thermal diffusion was shown to be dependent on the time constant and the temperature of vaporisation may be best attained with a short burst (200 ns) with a high peak power (6000 w). The experimentation was performed on fresh debris and segments of epicardial coronary arteries which were exposed to a pulsed laser beam with a frequency of 1000 Hz in bursts of 200 ns at wave lengths of 1060 and 532 nm. The results were evaluated by microscopic examination of transverse sections perpendicular to the lumen of the artery. Effective vaporisation of atheroma was observed with weak mean dissipating powers (0.4 w) about 10 times weaker than with continuous node emission; examination of the underlying arterial wall showed no thermal or mechanical damage.
