Institut de Chimie Moléculaire et des Matériaux d'Orsay


Photosensitivity of silica-based glasses to UV exposure (e.g.: glasses for optical fibers).

with the collaboration of : PhLAM/IRCICA (Université de Lille), IES (Université de Montpellier)

Over the last two decades, UV-induced Δn profiling in SiO2 glasses was widely used for the production of infibre/ waveguide Bragg grating-based (BG) optical devices for the photonics industry. Indeed, photosensitivity allows for the fabrication of an outstanding number of in-fiber Bragg grating-based (BG) optical devices. A BG is a light-induced periodic perturbation of the refractive index of the core (and sometimes of the cladding) of an optical fiber. These devices have found numerous applications in optical fiber sensing, telecommunication and all fiber laser systems. It is the most important (from a practical point of view) photo-induced phenomenon observed each time silica glasses are exposed to convenient low or high UV intensity laser light through one quantum and multi-photon mechanisms respectively.

Accordingly, a great deal of research remains in order to clarify the mechanisms of silica-based glasses–UV laser interaction, especially in the case of two or multi-photon processes, to enlarge the range of applications still further. An increasingly important method of permanent index change writing uses 193 nm from an ArF laser. This method has been proved to induce a specific volume change of glass and thus is successful in tackling the problem of grating writing in Ge-doped fibers without defects sites. Such two-photon excitation is into the band edge of silica through germanium assisted coupling. It has advantages such as higher thermal stability since no hydrogen is needed to obtain high Δn. Furthermore, two-photon grating writing in principle allows grating writing in any medium, such as pure silica glass core of a bimaterial step index fibre with fluorinated cladding and pure silica core of a single material fibre with structured cladding. Grating writing has been demonstrated in a variety of structured fibres including doped (germanosilicate, rare earth, phosphosilicate) photonic crystal fibres and Fresnel fibres.

In fact, depending on the materials, conditions of exposure and conditioning processes (i.e. sensitization process), UV induced index changes may vary from 10−5 up to 10−2. The word material includes factors such as the chemical composition of the glass, the elaboration process (governing the initial concentration of defects and the absorption coefficient at the pump wavelength), the physical form of the glass (fiber, preform plate, waveguide, thin film or bulk) and its thermal and mechanical history. The ‘‘conditions of exposure’’ represents factors such as the characteristics of the pump laser beam (wavelength, pulse duration), exposure time, power density or the temperature of the sample during the exposure.

In the last 15 years, for the purpose of understanding and exploiting the complexity of this multiple-variable dependence, we investigated how factors such as exposure time, laser wavelength, sensitization process, pulse duration, or the chemical composition, can affect the photosensitive response of silica-based glasses, i.e. the number of photons involved in the initial step of absorption, writing efficiency and the thermal stability. .