Photosensitivity in tin-doped silica optical fibres

Permanent gratings (refractive index modulation ~3x10 ) were written in tin-doped silica -4 optical fibres with low levels of SnO (~0.15 mol %). In Ge-doped silica, ~10 mol% GeO is 2 2 required to produce comparable photorefractivity under similar conditions. Photosensitivity in tin-doped silica optical fibres G. Brambilla, V. Pruneri, L. Reekie Optoelectronics Research Centre, Southampton University, SO17-1BJ, U.K. Tel: 0044 1703 593954, Fax: 0044 1703 593149 , E-mail: [email protected] Post-fabrication methods (such as hydrogen loading and flame brushing) and co-doping (B, Sn and rare earths) have been used to increase the photosensitivity of silica optical fibres [1] for the production of Bragg gratings. Compared to hydrogen loading and flame brushing the use of Sn keeps the absorption in the third telecom window at 1.5 μm low, is less time consuming and potentially cheaper. Tin also provides better temperature stability of the grating than other techniques, e.g. boron co-doping. So far tin has been used as a codopant to increase the photosensitivity of germanosilicate [2,3] and phosphosilicate [4] optical fibres. To our knowledge pure SnO2 doped silicate fibres have never been investigated. In this work we show that small concentrations (~0.15 mol %) of Sn in the core of a silica fibre provide the means to write permanent refractive index gratings with a high degree of photosensitivity. A comparison with germanosilicate fibres shows that GeO2 concentrations nearly two order of magnitudes higher are needed to produce the same photorefractivity under similar UV irradiation in silica glass. SiO2 doped with only SnO2 could potentially lead to the production of fibres with high enough photosensitivity and at the same time relatively low NA (0.1-0.12), thus compatible with standard telecom fibres. The fibres used in the experiments were produced via MCVD (modified chemical vapour deposition) by depositing a soot rich in SnO2 at 1300 C and then consolidating at 1800 C. The fibre produced had NA ~ 0.097 and cut-off wavelength λc=1.36 μm. The UV source was a KrF excimer laser (wavelength 248 nm) which delivered pulses of about 20 ns duration with a repetition rate of 20 Hz. A phase-mask was used to create the interference pattern for grating writing. Fig. 1 shows the evolution of the refractive index modulation as a function of exposure time The maximum refractive index modulation achieved in these preliminary experiments was ~ 2.8·10-4 after an exposure of 3.7 hours to pulses of 80 mJ/cm2. Measurements at different fluence per pulse (i.e. intensity) show an initial growth rate which increases linearly with intensity, indicating that the photosensitivity in our experiment is based on one-photon process [5]. Similar refractive index changes under the same experimental conditions have been obtained in GeO2:SiO2 optical fibres with ~10% of GeO2 [6,7] and also when a shorter UV writing wavelength (193 nm) was used in a 8 mol % GeO2 fibre [5]. Standard telecom fibres (about 3% mol of GeO2) produced index changes one order of magnitude lower (~2.6·10 -5) [8]. It has to be remarked that also the grating dynamics is different. The growth of the grating in germanosilicate fibres is faster than in our experiment. In fact, as clearly seen from Fig. 1, saturation was not achieved even after very long exposures at a considerable UV fluence per pulse. time (hrs) 0 1 2 3 4 ∆ n m od ( x 10 4 )