Elena López-Lago

Characterization of arbitrarily polarized ultrashort laser pulses by cross-phase modulation

We propose a technique for time resolution of the polarization state of ultrashort light pulses that also provides the overall time-varying phase of the pulse. This method is based on a spectral polarimetric analysis of the pulse after propagation through a Kerr medium. The feasibility of this method is demonstrated both numerically and experimentally.

Guided-mode propagation through an ion-exchanged graded-index boundary

Abstract Transmission of a guided mode through a graded refractive-index boundary generated beneath the mask edge in an ion-exchanged planar waveguide is considered by means of the overlap integral method and a precise electromagnetic scattering model. It is shown that, with optimized post-baking, virtually lossless transmission can be obtained for a wide range of ion-exchange parameters. Moreover, the optimum post-baking time can be found by the overlap integral method. The phase delay distortion caused by the graded boundaries is shown to be appreciable, but it can be compensated by modifying the mask profile. The magnitude of the required compensation can be evaluated by the effective index method.

Mesostructure and physical properties of aqueous mixtures of the ionic liquid 1-ethyl-3-methyl imidazolium octyl sulfate doped with divalent sulfate salts in the liquid and the mesomorphic states

This paper extends the study of the induced temperature change in the mesostructure and in the physical properties occurring in aqueous mixtures of the ionic liquid 1-ethyl-3-methyl imidazolium octyl-sulfate [EMIm][OSO4]. For some compositions, these mixtures undergo a phase transition between the liquid (isotropic in the mesoscale) and the mesomorphic state (lyotropic liquid crystalline) at about room temperature. The behavior of mixtures doped with a divalent metal sulfate was investigated in order to observe their applicability as electrolytes. Calcium sulfate salt is almost insoluble even in the 20 wt% water mixture. The magnesium salt, in contrast, can be dissolved up to concentrations of 730 ppm in the same mixture and it has a profound impact on its properties. Six aqueous mixtures (with water content from 10 wt% to 33 wt%) of [EMIm][OSO4] were saturated with magnesium sulfate salt, producing the ternary mixture [EMIm][OSO4] + H2O + MgSO4. Viscosity, density and ionic conductivity for these samples were measured from 10 °C to 90 °C. In addition, SAXS, FTIR, diffussion NMR and Raman spectroscopy of the most interesting samples have been performed, and structural data indicate a transition between a hexagonal lyotropic liquid crystalline phase below and an isotropic solution phase above room temperature. The octyl sulfate anions of the cylindrical micelles in the hexagonal phase are coordinated with water molecules through H-bonds (about four per sulfate anion), while the [EMIm] cations seem to be poorly coordinated and so free to move. Inorganic salt addition reinforces that network, increasing the phase transition temperature.

Modeling and interference-microscopy characterization of graded index transitions in ion-exchange waveguide integrated elements : transitions between annealed surfaces

We employ interference microscopy to characterize in a direct and local way the graded index transitions at the boundaries of integrated optical elements fabricated by multistep ion-exchange processes, a task not yet possible by other technologies. A particular fabrication method based on purely thermal ion-exchange, which allows local single-mode propagation, is investigated. The refractive-index distribution is modeled on the basis of the linear theory of diffusion and adiabatic transitions of the refractive-index profile. Finally, its validity is analyzed by microinterferometry.

Measurement of the polarization dynamics of ultrashort pulses by using nonlinear phase modulation and channelled spectroscopic polarimetry

Recently we presented a vectorial pulse characterization method based on the spectral polarimetric analysis of femtosecond laser pulses before and after propagation through a Kerr medium. In this work we present an improvement of the method. We propose to carry out the spectral polarimetric analysis by using the technique named spectroscopic polarimetry with a channelled spectrum which has been developed by Oka and Kato (1999 Opt. Lett. 24 1475). This combination supposes an important improvement of the data acquisition process both in velocity and accuracy.

Interferometric characterization method of lateral ion exchange for refractive waveguide elements

A model of lateral diffusion in the ion-exchange fabrication of integrated optical elements, based on two regions with different effective index in the same substrate, is demonstrated. A nondestructive method based on interference microscopy is used to characterize the corresponding graded-index transition regions between the outer and the inner zones of these waveguide elements. An analysis of the effective width of integrated waveguide elements, fabricated by surface ion exchange with annealing, is presented.

White light spectral interferometer for measuring dispersion in the visible-near infrared

We have designed a spectrally resolved interferometer to measure the refractive index of transparent samples over a wide spectral band from 400 to 1550 nm. The measuring device consists of a Michelson interferometer whose output is analyzed by means of three fiber spectrometers. The first one is a homemade prism spectrometer, which obtains the interferogram produced by the sample over 400 to 1050 nm; the second one is a homemade transmission grating spectrometer thought to measure the interferogram in the near infrared spectral band from 950 to 1550 nm; the last one is a commercial Czerny-Turner spectrometer used to make high precision measurements of the displacement between the Michelson mirrors also using white light interferometry. The whole system is illuminated by a white light source with an emission spectrum similar to black body. We have tested the instrument with solid and liquids samples achieving accuracy to the fourth decimal on the refractive index after fitting it to a Cauchy formula

Imidazolium decyl sulfate: a very promising selfmade ionic hydrogel

In this paper, we show, for the first time, the synthesis, structural characterization, phase diagram and physical properties of the ionic liquid, 1-ethyl-3-methyl imidazolium decyl sulfate [EMIm][DSO4]. At 25 °C it is either a crystalline solid or a liquid depending on the thermal history as its melting point is about 33 °C and its point of solidification is about 22 °C. The interest of this new IL lies in its ability to become a rigid hydrogel when mixed with water. As observed in many ILs, the as-prepared IL is hygroscopic and it adsorbs about 14 wt% of water at usual laboratory conditions and up to 27 wt% in a 100% saturated atmosphere. Due to the H-bonds between water and the amphiphilic [DSO4] anions, a lyotropic HI liquid crystalline phase is formed in the hydrated state, which can be observed in micrographs recorded using white polarized light. The moisture adsorption is a completely reversible process; thus, the rigid-gel sample loses all adsorbed water when it is left in a dry atmosphere for a few hours, transitioning to the liquid state. Phase diagrams of the temperature-water concentration is presented and compared with that of the parent compound [EMIm] octyl sulfate, [OSO4]. X-ray diffraction revels that below 15 °C the hydrated compound crystallizes into a P2/m monoclinic structure. The structure of the new compound was confirmed by NMR, FTIR and mass spectroscopy (MS). In addition, the temperature behavior of ionic conductivity was experimentally measured and analyzed for the pure compound and for two samples hydrated with 10 wt% and 39 wt% of water. Viscosity and density were also measured vs. temperature for the pure sample. The as-prepared IL shows great potential for numerous practical applications.

Ultrashort pulse characterisation by using thick birefringent retarders

In this communication we consider the measurement of the polarisation dynamics of arbitrarily polarised ultrashort pulses. It is well-known that a spectral analysis of the polarisation state of an optical pulse can be performed easily with suitable polarisation optics and a spectrometer. However the information which is extracted from this measurement can not be used directly to characterise the polarisation dynamics of the pulse in the temporal domain: the spectral analysis provides only the phase difference between the spectral components of the pulse while we need to know the spectral phase of each Cartesian (or circular) component. In ref. 1 we demonstrated that these phases can be obtained from the spectral analysis of the right-handed and left-handed circular components before and after propagation through a Kerr medium. In this work, we show how this method can be improved by applying the technique developed in ref. 2 to get the experimental data. We applied this procedure to the characterisation of femtosecond laser pulses delivered by a Ti-Sapphire oscillator. The polarisation state of the pulses is modified by means of optical elements. These pulses are injected in a short piece of a single mode silica fiber. This fiber is short enough ( I cm) to avoid undesired dispersive and residual birefringent effects. The light exiting the fiber crosses a pair of thick birefringent retarders made of calcite (dl= 6mm. d2=2 mm) and a polariser. Due to the delay accumulated in the retarders by the Cartesian components of the pulse we obtain four delayed pulses from a single initial pulse. In a spectrometer we obtain the spectral interference between these pulses. Starting from this channelled spectrum and by using the techniqur: which is described in ref. 2, we obtain the spectra of each Cartesian component of the pulse and the phase difference between them. Similar measurement are taken with an absorbing filter placed beside the fiber to avoid nonlinear effects. In this case the output spectrum coincides with the input one. In fig. l a we show the channelled spectrum corresponding to the input pulse and in fig. Ib its Fourier transform. By isolating each peak in this figure we obtain the desired spectral information [21. In fig2 we plot the experimental (circles) and the reconstructed spectral intensities (continuos line) and phases (dotted line) of each Cartesian component. The phase is recovered by applying an iterative algorithm similar to the one that is described in ref. I.

Modelling of thermal annealing of ion-exchanged surface waveguides by an averaging approach

A method to obtain analytical models which allow one to characterize and fundamentally predict the index profiles of annealed ion-exchanged waveguides is presented. It is based on an average of the nonlinear ion-exchange effects in the nonlinear diffusion equation. The method can be applied to different initial conditions. As an example, a comparison with the numerical solution is shown for a Gaussian function as the initial condition, which covers many of the practical cases of interest.