Efficient Bragg gratings in phosphosilicate and germanosilicate photonic crystal fiber.

We present ArF laser-induced dynamics of Bragg grating (BG) growths in phosphosilicate-doped or germanosilicate-doped core photonic crystal fibers (PCFs). To this end, we have adapted the technique of H2 loading, usually used in conventional fiber, to the case of microstructured fiber, allowing both the concentration of hydrogen in the PCFs to be kept nearly constant for the time of the exposure and the BG spectra to be easily recorded. We compared the characteristics of BG growths in the two types of PCF to those in conventional step-index fibers. We then conducted a study of the thermal stability of the BGs in PCFs through 30 min of isochronal annealing. At the same time we discuss the role played by the microstructuration and the doping with regard to the grating contrast and the Bragg wavelength stability.

Comparing the properties of various sensitization methods in H2-loaded, UV hypersensitized or OH-flooded standard germanosilicate fibers.

The properties of three sensitization processes: UV hypersensitization, OH-flooding and H2-loading have been investigated through Bragg grating (BG) inscription within standard germanosilicate fibers. More specifically, the stability of the sensitization processes and that of the UV-induced index changes have been investigated through isochronal annealing experiments. Moreover, the level of excess loss induced near 1.4 microm by both the sensitization process and the BG inscription has been estimated by means of in fiber absorption spectroscopy. The level of loss proves to be higher in the hypersensitized or OH-flooded fiber than in the H2-loaded counterpart when pulsed 248 nm light was used.

Self induced gratings in ternary SiO2:SnO2:Na2O bulk glasses by UV light seeding.

The diffraction efficiency of gratings written in ternary SnO2:SiO2:Na2O bulk glasses rises dramatically with time after the occultation of the cw 244nm light used to write the thick hologram. This self-induced behavior lasts for several hours and ultimately leads to refractive index changes as high as 3 10-3.

Temperature-induced reversible changes in the spectral characteristics of fiber Bragg gratings.

It is reported that reversible changes in the reflectivity of Bragg gratings can be induced by a change in the temperature of the grating (77 K < T < K). The changes have proved to be greater in highly doped Ge fibers than in standard fibers, whereas they could hardly be detected in hydrogenated fibers. The sign of the change for type I gratings was opposite that for type IIA gratings. The changes are likely due to a temperature-induced increase (or a decrease) in the amplitude of the refractive-index modulation. Possible mechanisms for these changes in modulation are discussed. Interestingly for the purpose of correcting data of isothermal accelerated aging experiments, a numerical relation that accounts for the temperature-induced changes in type I grating reflectivity is given.

Refractive-Index Changes of Standard Telecommunication Fiber through Exposure to Femtosecond Laser Pulses at 810 cm.

We report on permanent refractive-index changes as great as 6 x 10(-3) in standard fibers on irradiation with tightly focused femtosecond (120-fs) near-IR (lambda = 800-nm) pulses. The refractive-index increase was measured through changes in the transmission spectrum of an UV photowritten Bragg-grating-based Fabry-Perot interferometer. We depict the dependence of the index variation on exposure time. The changes in refractive index can be optimized after a few seconds of irradiation. The measured induced excess loss increases during near-IR exposure.

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.

Comparison of isochronal and isothermal decays of bragg gratings written through continuous-wave exposure of an unloaded germanosilicate fiber.

We compare the results of thermally induced isochronal and isothermal decays of fiber Bragg gratings written through cw exposure of an unloaded germanosilicate fiber. We show that isochronal step decays can be used to predict isothermal decays, provided that some corrections are carried out to take into account a reversible change in grating reflectivity induced by the increase of the fiber temperature. The isochronal accelerated-aging method enables one to sample most of the initial distribution of trapped site energies in a fairly short time. Taking advantage of this property of the method, we show that the initial distribution for a weak grating is similar to those for stronger gratings. The consequences of this observation are discussed within the framework of the various reaction pathway model.

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.

[Proximal tubal recanalization by endoluminal laser probe. I. Anatomic results in the female rat]. / Recanalisation tubaire proximale par sonde laser endoluminale. I. Résultats anatomiques chez la rate.

To evaluate the cicatrization of the uterine horn after the recanalization by endoluminal laser probe, 50 white female rats of Wistar's race have been separated into two groups. The group 1 (nb = 10) had for aim to validated the procedure of obstruction. In group 2 (nb = 40), we performed the recanalization six weeks after the obstruction. Histologic samples were obtained 48 hours, 3 weeks and 6 weeks after treatment. In group 1, we found a complete obstruction in 70% of the cases, and a tight stenosis in 30% of the cases. In group 2, at 48 hour's control after the treatment by laser, we found a drill hole with a blackened look without parietal sequel in 80% of the cases. At 6 week's control, we found a normal lumen with a regeneration of the epithelioma in 85% of the cases. For energy deposition under 30 Joules, we found good results in 95% of the cases. Above 30 Joules, we always found bad results (5 cases on 5).

[Proximal tubular recanalization by endoluminal laser. II. Functional results in the female rabbit]. / Recanalisation tubaire proximale par sonde laser endoluminale. II. Résultats fonctionnels chez la lapine.

To evaluate the function of the fallopian tube after a recanalization by endoluminal laser probe, 34 mature female New Zealand white rabbits have been separated into two groups. The group 1 (nb = 12) had for aim to validate the procedure of obstruction. In group 2 (nb = 22), we performed the recanalization six weeks after the obstruction. The female rabbits underwent a fertilization six weeks after the laser treatment. The results have been appreciated by the ratio nidation/ovulation. In group 1, we valided the procedure of obstruction. In group 2 twelve rabbits on thirteen have been fecundated in the treated tube.

[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.