Improvement in the measurement of index modulation along an optical fiber grating by movement of the probe spot perpendicularly to the fiber axis.

We demonstrate a large improvement in the efficiency of the method proposed by Krug et al. [Opt. Lett. 20, 1767 (1995)] to measure the amplitude of the refractive-index modulation along a fiber Bragg grating. The basic idea consists of using what to our knowledge is a new modulation scheme for the probe beam that not only allows the user to get a better discrimination of the probe light incoherently scattered by the fiber from that scattered by the grating but also facilitates alignment of the setup.

Photoinscription of Bragg gratings within a germanosilicate fiber subjected to a high static electric field.

The kinetics of Bragg grating growth in germanosilicate fibers subjected to a high static electric field are compared with those obtained without any electric field. The gratings were written by exposure of the fiber core to laser light at 244 or 193 nm. These experiments gave some clues about the mechanisms responsible for both the photosensitivity in germanosilicate fibers and the nonlinear second-order UV-induced susceptibility in silica glasses. The refractive-index modulation proved to be significantly higher in the fibers subjected to an electric field. Furthermore, the change in the fiber's mean effective refractive index as a function of exposure time was not monotonic. This evolution can be explained by the assumption that some electric-field-induced diffusion of electron trapped centers [Ge(1) and Ge(2)] from the fiber core is involved.

Analysis of color-center-related contribution to Bragg grating formation in Ge:SiO 2 fiber based on a local Kramers -Kronig transformation of excess loss spectra.

UV-induced excess losses have been measured at various pulse energy densities and exposure times in germanosilicate optical fiber preform cores. The corresponding refractive-index changes have been determined through a Kramers -Kronig analysis. Because of the nonlinear behavior of the excess losses as a function of both exposure time and fluence per pulse, one should be careful when comparing the refractive-index modulation deduced from such measurements with that obtained from Bragg grating reflectivity. Indeed nonlinear effects such as saturation imply that it is necessary to take into account the local character of the change in absorption to calculate the evolution of the refractive-index modulation accurately as a function of the exposure time. Implications of these results are discussed.

Design of a photowritten cavity neodymium-doped fiber laser tunable around an absorption molecular line.

Experiments have shown that it is possible to realize laser cavities photowritten within rare-earth-doped silicate fibers. Experimental results on a particular application, namely, the detection of gas traces by identification of one of the gas's absorption lines, are presented. Experiments on an absorption line of atmospheric water have been carried out to illustrate this purpose. Two lasers have been designed with particular characteristics (spectral linewidths, slope efficiencies, thresholds). Preliminary experimental results on the detection of an absorption line of atmospheric water have been obtained with fiber lasers and optoacoustic detection.

Ultraviolet-induced permanent Bragg gratings in cerium-doped ZBLAN glasses or optical fibers.

A holographic method was used to write refractive-index gratings in ZBLAN fluoroziroconate glasses or fibers doped by a Ce concentration of 5000 or 10,000 parts-in-10(6) weight. Direct pumping of the (2)F(5/2)-5d transitions of Ce(3+) ions near 245 nm resulted in a change in the refractive index. The photoinduced change partly recovered on a time scale of several hours at room temperature. The remaining change in the refractive index looked stable on a time scale of a month. This permanent change reaches 2 x 10(-5) at 1560 nm.

[Development and evaluation of a laser catheter with tip fiberoptic for coronary angioplasty]. / Mise au point et évaluation d'un cathéter laser avec fibre optique à boule pour angioplastie coronaire.

A fiber-optic tip catheter has been developed for treating coronary and peripheral atherosclerosis percutaneously. The catheter is coupled to an ionised Argon laser emitting at an optical power of 1 to 7 Watts. It is designed to follow a guide wire and to perform tissue ablation by a new concept combining the effects of central vaporisation and peripheral thermal remodelling of the obstructed artery. This mode of function should considerably reduce the risks of false route previously encountered with laser angioplasty. The optical and thermic properties of the catheter prototypes were determined by physical methods. These experiments showed that the temperature of the metallic component of the catheter tip did not exceed 30 degrees C at a continuous power of 6 laser Watts when the flush was functioning. It attained 400 degrees C in the absence of the flush. The performances of the catheter were then tested in a number of in vitro experiments. First of all, the catheter tip was placed perpendicularly to atheromatous cadaver aortae to study the effect of vaporisation with respect to the laser power. The threshold of vaporisation was 1 Watt (irradiance: 1100 Watts/cm2). The catheter was then introduced into plastic tubes obstructed by atheromatous plaques. This experiment demonstrated the mechanical solidity of the catheter and its flexibility in difficult operating conditions: diameter 2mm, 45 degrees angles, irregular calcified plaques. It also showed that the optimal safety-efficacy laser power was 3 laser Watts and that the exposure times varied with respect to the nature of the plaques tested.(ABSTRACT TRUNCATED AT 250 WORDS)

XeCl laser in biliary calculus fragmentation: fluence threshold and ablation products.

The in vitro action of a xenon-chlorine (XeCl) excimer laser on biliary calculi is reported: fluence threshold and rate for ablation process are given. An analysis of gaseous products evolved during irradiation of gallstones, performed through an infrared spectrophotometric technique is also reported. Based on the different results, we discuss the mechanism of destruction.