Bertrand Poumellec

Densification involved in the UV-based photosensitivity of silica glasses and optical fibers

A comprehensive survey of photosensitivity in silica glasses and optical fiber is reviewed. Recent work on understanding the mechanisms contributing to germanium or aluminum doped fiber photosensitivity is discussed within the framework of photoelastic densification models.

Experimental evidence of two types of photorefractive effects occuring during photoinscriptions of Bragg gratings within germanosilicate fibres

Abstract Photoinscriptions of Bragg gratings have been carried out within six germanosilicate fibres. Real time measurements of the spectral transmission, of the fwhm bandwidth and of the resonance wavelength of the Bragg gratings permit to estimate the refractive index change photoinduced in the core of the fibre. It has been observed that the refractive index changes were strongly nonlinear as a function of the number of ultraviolet (UV) laser pulses used for writing the gratings. Experimental results show that, at least, two photorefractive effects should be responsible for the complicated dynamical evolution of the grating growth observed within some fibres.

Femtosecond laser irradiation stress induced in pure silica.

We reveal stress fields induced by femtosecond laser irradiation by investigating the topography of surface relaxation of a cleavage of silica plates in which irradiation was performed, varying intensity, laser polarization and displacement of the writing beam. The stress field appears to depend on the writing parameters differently according to the laser intensity. For pulse intensity larger than 0.1 microJ, a first shear stress developed. Above 0.25 microJ, another shear stress appears that is dependent on the direction of writing and coupling with a phase matching condition between the pump wave and the third harmonic.

UV induced densification during Bragg grating inscription in Ge:SiO2 preforms

Abstract The only investigation of a Bragg grating in Ge:SiO 2 in the direct space known to date is a view from an optical microscope. Here, we show surface level modulations associated with the writing of the grating. Our conclusion is that a densification takes place. We estimate the corresponding peak to peak change of the refractive index to be 1.4−1.7 × 10 −4 whether a pulsed or a CW irradiation mode is used. Compared to the refractive index change obtained by optical measurement, this accounts for a non-negligible part (7% for pulsed and 26% for CW irradiation). We discuss the possible application of the observations to fiber and the differences in photo-chemical reactions according to the irradiation mode.

Anatomy of a femtosecond laser processed silica waveguide [Invited]

Waveguides are inscribed through densification of the surrounding region of a damage induced channel created by femtosecond irradiation within silica. Single mode propagation at 1.5 μm is obtained below the damage region whilst at shorter wavelengths guidance is only observed away on either side of the region. The quasi-periodic nanostructure that is induced can explain the mode profile elongation observed with polarised light at 45°. The origin of this guidance area is explored using SEM analysis, which reveals nanoporous regions within laser track structure above and below the densified region where 1.5 μm propagates. Shorter wavelength light is not supported in this area.

The UV-induced refractive index grating in Ge: preforms: additional CW experiments and the macroscopic origin of the change in index

Bragg gratings have been written within MCVD germanosilicate preform slices using CW laser light at 244 nm. Afterwards, scanning light interferometry was used to perform a 3D imaging analysis of the sample surface topography. The results show that the same process (namely densification) pointed out in a previous work in which we used UV pulsed exposure of preform slices is also active here. A simplified model which assumes that the UV-induced periodic corrugations result from both permanent and elastic strains, is presented. This model accounts not only for a large part of the measured refractive index change but also for the localization of the UV-induced birefringence along the grating wavevector. It also gives a satisfactory interpretation of the observations made by Fonjallaz et al, according to whom grating inscription within a germanosilicate fibre increased the axial tensile core stress.

Modification thresholds in femtosecond laser processing of pure silica: review of dependencies on laser parameters [Invited]

This review allows better defining the domains of macroscopic effects produced by the femtosecond laser irradiation in pure silica according to the laser parameters.

Electronic structure and X-ray absorption spectrum of rutile TiO2

The electronic band structure of rutile TiO2 has been calculated using the linear muffin-tin orbital (LMTO) method. The X-ray absorption spectrum has also been obtained from the calculated band structure and self-consistent potential. The dipole as well as the quadrupole contribution to the X-ray absorption spectrum has been calculated. The calculated band structure shows that there is a significant 2p (O)-3d (Ti) mixing in the valence band which, however, becomes weaker in the Ti d-dominated conduction band. The valence band is well separated from the conduction band by a direct energy gap which is almost degenerate with the indirect energy gap. A comparison with experiments suggests that the majority of the observed pre-edge and edge features in the Ti K-edge X-ray absorption spectrum are due to 1s to 4p dipole transitions, a result of the small Ti-Ti (3d-4p) hybridization in the conduction band region. The quadrupole transition contribution is found to be insignificant.

Dependence of the femtosecond laser refractive index change thresholds on the chemical composition of doped-silica glasses

The refractive index changes in doped silica are investigated. We observed that the permanent isotropic index change threshold (T1) is not significantly dependent on the doping. We show that strong birefringence (permanent linear birefringence) exists in doped silica but its threshold (T2) exhibits significant dependence on the used dopants. In our conditions, comparing with silica (0.31 μJ/pulse here), for 1.5 at% Ge-doped silica the T2 threshold is smaller (0.14 ± 0.05 μJ/pulse). For a silica doped with 0.3 at% of fluorine, T2 is close to 1.20 ± 0.05 μJ/pulse. An interpretation is given not only about threshold variation but also about RIC for energies beyond. It is based on the overcoming of relaxation time in the volume interaction.

Thermodynamic properties of titanium dioxide, niobium dioxide and their solid solutions at high temperature

Abstract By means of thermogravimetric measurements, it has been possible to obtain information on the nature of the intrinsic and extrinsic defects of TiO 2 , NbO 2 and solid solutions Nb y Ti 1− y O 2± x Pure TiO 2 is an oxygen-deficient oxide The main defects are oxygen vacancies, doubly ionized V .. o or singly ionized V . o , and interstitial titanium Ti 3 i NbO 2 is a metal-deficient oxide The main defects are neutral niobium vacancies. The solid solutions Nb y Ti 1− y O 2± x may be divided into two groups If y > 0 04, the behavior is analogous to that of NbO 2 ; with the same defects, but the width of the homogeneity range decreases with the titanium content and Nb 0 04 Ti 0 96 O 2 is a stoichiometnc oxide If y 0 995 Nb 0 005 O 2 In the oxygen-deficient domain, the main defects are assumed to be neutral or singly ionized oxygen vacancies In the metal-deficient domain, the main defects are metal vacancies V 4 Ti From these results we have deduced the nature of intrinsic defects in TiO 2 to be Schottky defects: 2 V .. O + V 4 T1 .