Photosensitisation of Phosphosilicate Fibre Bragg Gratings

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Abstract
Photolytic grating writing in phosphorous doped waveguides has recently been possible only with substantial hydrogen loading [1-3]. In [1] this has been demonstrated for phosphosilicate planar waveguides using 193nm whilst for aluminophosphosilicate fibres the writing wavelength was the 248nm KrF line [2]. We have recently demonstrated photosensitivity at 193nm in phosphosilicate fibres [3] fabricated by the flash condensation technique [4]. This technique appears unique in that it has pre-existing phase separation within the fibre [5] which assists in the incorporation of up to 14wt% of rare-earth ions. In addition to showing significantly better performance than at 240nm under similar irradiation conditions the induced photosensitivity was found to be dependent on slow chemical sensitisation with hydrogen prior to irradiation. Fig. 1 is a comparison between the expected diffusion profile for hydrogen loading at 373K and 40atm and the saturated reflectivities obtained for various loading times. The importance of hydrogen loading is accentuated with the observation of only transient gratings (Fig 2) without loading. The decay profile of these gratings cannot be accounted for by a single relaxation process. Instead, we propose that (1) there are several contributions to the index change in which one mechanism promotes the next, and (2) the role of hydrogen is to quench the back reactions. A two step process suggests that memory effects, such as presensitisation, may be observed. In this paper, we use a novel side diffraction technique [6] for measuring the index contrast to demonstrate that these phosphosilicate glass fibres exhibit large photolytic presensitisation.