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Accueil > EN > Research Areas > Photonics > Active fibers > Exotic doping

Exotic doping

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Research Activities :

Bismuth-doped fibres

Since the end of 2005, the team uses MCVD to fabricate aluminosilicate, germanosilicate and phosphosilicates preforms doped with bismuth. The relatively recent interest in this doping (the first report of an IR emission in this kind of glass dates back to 2001) lies in the possibility of obtaining a laser effect in the range 1.1-1.5 µm, depending on the host-matrix composition. Beyond the fact that this spectral range is not accessible to rare-earth ions in silica, the interest in this dopant lies in its light emission bands that are large of several hundreds nanometers. Moreover, the shape of this emission is strongly dependent on the excitation wavelength. It quickly appeared possible to exploit this luminescence in order to achieve fibred lasers. Hence, we were the first ones in 2007 to demonstrate a laser efficiency higher than 20 % in this type of doped fibre. Since then, the work has been pursued, in particular by the study of the optical properties of bismuth when introduced in a pure sol-gel-derived silica matrix and by sudying the impact of the atmosphere on emission/absorption properties of MCVD preforms. This subject is founded since 2013 by ANR (project BOATS).

Left : Attenuation curve of air/silica fibre with bismuth-doped core. Two main absorption bands are evidenced around 800 and 1400 nm. Right : photoluminescence spectra recorded in fibres for different excitation wavelengths. Emission of bismuth centers appears strongly red-shifted as compared to what can be observed in aluminosilicate fibres.
A bismuth-doped optical preform.

Doping fibres with metal nanoparticles

It is well-known that noble metal ou semiconductor nanoparticles (NPs) exhibit high nonlinear optical coefficients, especially in the neighbourhood of their resonance (surface plasmon resonance for metals and exciton for semiconductors). NPs may also be studied in the scope of energy transfer towards active ions in order to enhance amplifier’s or laser’s efficiency. However, these objects, widely studied in thin films or in bulk materials, had never been introduced in the core of an optical fiber in a clearly demonstrated manner. This quasi-absence of realisation is partly ascribable to the difficulty of preserving nanoparticles during the different steps of glass fabrication and fibre drawing . In effect, while melting temperature of metallic gold is close to 1064°C, the glass-working temperature is around 2000°C. In order to attempt to stabilise gold nanoparticles in the core of a fibre, a first stategy is to take advantage of the sol-gel synthesis of silica glass, which limits the impact of high temperature treatments. This approach, developped in Lille since 2002 in the case of monoliths, consists of a chemical way yielding nanoporous glasses that may be impregnated by solutions contening the desired precursors. The use of a gold precursor or of a solution contening gold nanoparticles actually leads to drawable doped monoliths. We have shown that the so-formed nanoparticles were able to resist to several heat-treatments of the glass at high temperature (about 2000°C) and that they could survive until the final fibre geometriy is obtained. The linear and non-linear optical properties of such fibres fabricated using these monoliths are clearly modified by the presence of nanoparticles.

Left : Attenuation curve of air/silica fibre with a core partially doped by gold NPs. Inset : SEM image of the fibre on wich the doped zonehas been symbolised in yellow. Right : Curves illustrating the transmission response as a function of the input intensity in a gold NPs-doped or in an undoped air/silica fibre.

Another option consists in doping the porous gel or the glass with NPs precursors, then to prompt their crystal growth in a localised manner under a laser beam. In this domain, our team has acquired international acknowledgement. In particular in porous sol-gel silica, 2D or 3D gratings of metal nanocristals (Au, Ag) or semiconductors nanocrystals (CdS, PbS) have been generated in different irradiation conditions(pulsed or CW lasers at various wavelengths). Photo-thermal or photo-chemical mechanisms have been identified depending on the employed laser and precursors. This work, falling within the framework of a project supported by ANR POMESCO, has led us to define the best experimental conditions allowing efficient synthesis of stable NPs in silica xerogels. Extrapolation of these techniques to dense glasses, then to optical fibres, is going on, the localised aspect of the NPs growth being particuliarly interesting for the use of optical nonlinearities in pulsed lasers.

Left : Photograph and absorption spectra showing SPR resonance of gold NPs produced in silica xerogel under femtoseconde laser irradiation. Right : Photograph and HRTEM image of CdS NPs produced in the vicinity of a gel surface by visible CW laser irradiation.

Fibres for dosimetry

The field of health is nowadays enquirer of fibred technologies. Beside the biomedical laser or fibroscopy, which are among the most widespread applications of optical fibres, these technologies can lead to the achievement of reliable, precise and innovating measurement systems, in order to offer to practitioners real-time useful tools, more and more efficient during medical interventions. In this context, our team considers developping new microstructured fibres for ionising radiation metrology and imaging (X-rays, gamma,…). The operating mode of such devices would be based on optically stimulated luminescence phenomenon (OSL). These pure silica fibres should play a major role in the future in the area of radiotherapy and of high energy physics, being given their intrinsic advantages (radiation resistance, multi-core fibres, low intrusiveness, etc.). As an example, the conception of fibred dosimeters could allow localised and remote measurements of radiations, which would represent a real progress in cancer treatment by radiotherapy. This research project is conducted in collaboration with the Laboratory Hubert Curien (LaHC) in University of Saint-Etienne, the laboratory of Inorganic Materials (LMI) in University Blaise Pascal of Clermont-Ferrand and with the Centre Oscar Lambret in Lille Hospital centre.

Left : space distribution of emission intensity at 550 nm a the end of a Cu-doped fibre under UV excitation. Right : emission intensity of the 2 sub-bands attributed to Cu+ as a function of exciting power. The linear behaviour can be exploited in a dosimeter.

Keywords :

Bismuth doping, Nanoparticles, Microstructured fibre, Dosimetry