Efficient light coupling from integrated single-mode waveguides to supercollimating photonic crystals on silicon-on-insulator platforms

We propose a practical and efficient solution for the coupling of light from integrated single-mode waveguides to supercollimating planar photonic crystals on conventional silicon-on-insulator platforms. The device consists of a rib waveguide, designed to sustain spatially extended single-modes and matched to a supercollimating photonic crystal, which has been truncated at its boundary to improve impedance matching between the two photonic components. Three-dimensional simulations show transmission efficiencies up to 96 % and reflections below 0.2 % at wavelengths close to 1.55 microns. This approach constitutes a significant step toward the integration of supercollimating structures on photonic chips.Comment: 11 pages, 4 figure

Hydrogen charging in low alloy steels exposed to H 2 S: impact of CO 2 or N 2 in the gas mixture

International audienceThis paper examines the influence of CO 2 / H 2 S ratio on hydrogen charging in low alloy steels. Hydrogen flux through the steel surface is the driving force for sulfide stress cracking (SSC). The impact of H 2 S and pH on SSC severity is extremely well documented, and these two parameters are used in the SSC severity diagram of ISO 15156-2 [1]. However, the CO 2 / H 2 S ratio is not considered in standard SSC testing procedures, even though it has a strong impact on corrosion. Indeed, according to several authors, competitive adsorption between HS-and HCO 3-on the steel surface could modify the charging flux, thus the SSC risks. The goal of the present study was to check if hydrogen charging is influenced by other parameters than the sole pH and P H2S. Experiments consisted in hydrogen permeation measurements through HSLA steel thin membranes. They were performed under cathodic charging in order to avoid corrosion products precipitation. Charging solutions with different H 2 S concentrations with or without CO 2 were used, in order to examine the individual contributions to the charging flux, and hence to evaluate a possible impact on SSC risks

Improving pH Prediction for High Pressure and High Temperature Applications in Oil and Gas Production

International audiencepH prediction represents a crucial step before selecting materials for use in sour oil and gas wells as regards weight loss corrosion and H 2 S cracking. Among the numerous parameters which determine the equilibrium pH, important ones are CO 2 and H 2 S partial pressures (P CO2 and P H2S respectively), the total pressure, the ionic strength and the chemical composition of the solution, and the temperature. Most models used by oil and gas operators present a too narrow range of validity for these parameters, which makes them inappropriate for high pressure and high temperature (HPHT) fields or for CO 2 reinjection. This paper presents modeling improvements which allows extending the prediction validity in temperature and pressure to respectively 200 °C and 1,000 bar of total pressure, and for an ionic strength up to 5 mol.kg-1. These improvements take into account the fugacities in gas phase of CO 2 and H 2 S as determined by the Soreide and Whitson formalism. The influence of water and CH 4 pressure is also taken into account up to several hundred bars. Activity coefficients in the water phase are calculated using the Pitzer model. The consistency of the model is verified by comparison with experimental measurements of pH under high pressure. It is then applied to oil and gas applications at high pressure and high temperature. The impact of the new calculation method is discussed both for pH evaluation and also for H 2 S activity, with strong implications for the evaluation of SSC risks

pH prediction in concentrated aqueous solutions under high pressure of acid gases and high temperature

International audienceAn extended model for pH prediction in oil and gas environments has been developed. Accurate pH calculations for high pressure and high temperature applications depends mainly on CO2 and H2S partial pressures, the ionic strength, the chemical composition of the solution, and the temperature. Accounting for the non-ideal behaviors of liquid and gas phases allows pH calculations up to 200 °C, 2000 bar total pressure, and ionic strengths up to 5 mol.L-1. The results are consistent with experimental measurements and with other models reported in the literature

Investigation of the third-order nonlinear optical properties in porphyrin solutions in the picosecond regime

The D4σ-Z-scan technique is used to evaluate the sign and the magnitude of the nonlinear (NL) refractive index and the NL absorption (NLA) coefficient with a laser delivering single pulses in the picosecond regime at 355 nm, 532 nm and 1064 nm. The NL optical response of the zinc porphyrins (Pph) has been found to be significantly enhanced. The study of the NLA and refraction is performed, taking into account the time dependence and the spectral width of the incident laser. The variations of the NL coefficients as a function of the intensity and the concentration are provided and are found to be linear

Efficient light coupling into a photonic crystal waveguide with flatband slow mode

We design an efficient coupler to transmit light from a strip waveguide into the flatband slow mode of a photonic crystal waveguide with ring-shaped holes. The coupler is a section of a photonic crystal waveguide with a higher group velocity, obtained by different ring dimensions. We demonstrate coupling efficiency in excess of 95% over the 8 nm wavelength range where the photonic crystal waveguide exhibits a quasi constant group velocity vg = c/37. An analysis based on the small Fabry-P\'erot resonances in the simulated transmission spectra is introduced and used for studying the effect of the coupler length and for evaluating the coupling efficiency in different parts of the coupler. The mode conversion efficiency within the coupler is more than 99.7% over the wavelength range of interest. The parasitic reflectance in the coupler, which depends on the propagation constant mismatch between the slow mode and the coupler mode, is lower than 0.6% within this wavelength range.Comment: 11 pages, 7 figures, submitted to Photonics and Nanostructures - Fundamentals and Application

Degeneracy analysis for a super cell of a photonic crystal and its application to the creation of band gaps

A method is introduced to analyze the degeneracy properties of the band structure of a photonic crystal making use of the super cells. The band structure associated with a super cell of a photonic crystal has degeneracies at the edge of the Brillouin zone if the photonic crystal has some kind of point group symmetry. Both E-polarization and H-polarization cases have the same degeneracies for a 2-dimensional (2D) photonic crystal. Two theorems are given and proved. These degeneracies can be lifted to create photonic band gaps by changing the transform matrix between the super cell and the smallest unit cell. The existence of the photonic band gaps for many known 2D photonic crystals is explained through the degeneracy analysis.Comment: 19 pages, revtex4, 14 figures, p

Dark field Z-scan microscopic configuration for nonlinear optical measurements: Numerical study

This study deals with numerical simulations to optimize the parameters of the Dark Filed Z-scan (DFZ-scan) in a microscopic configuration for third-order nonlinear (NL) refraction measurements into thin films. The method allows dynamic, transparent, nonlinear phase shifts to be clearly visible. The simulations of such images are obtained for very low-induced refractive indices. Darkfield illumination requires blocking out of the central light which ordinarily passes through and around (surrounding) the NL specimen. A table to approximate circular aperture stop size versus magnification will be given depending on the focusing lens into the tested material

The effect of discharge chamber geometry on the ignition of low-pressure rf capacitive discharges

This paper reports measured and calculated breakdown curves in several gases of rf capacitive discharges excited at 13.56 MHz in chambers of three different geometries: parallel plates surrounded by a dielectric cylinder (“symmetric parallel plate”), parallel plates surrounded by a grounded metallic cylinder (“asymmetric parallel plate”), and parallel plates inside a much larger grounded metallic chamber (“large chamber”). The breakdown curves for the symmetric chamber have a multivalued section at low pressure. For the asymmetric chamber the breakdown curves are shifted to lower pressures and rf voltages, but the multivalued feature is still present. At higher pressures the breakdown voltages are much lower than for the symmetric geometry. For the large chamber geometry the multivalued behavior is not observed. The breakdown curves were also calculated using a numerical model based on fluid equations, giving results that are in satisfactory agreement with the measurements