Preparation of thin films of optical fibre samples for high-resolution X-ray analysis

Abstract

The performance of an optical fibre is very strongly dependent on compositional variations across the fibre diameter. In silica-based fibres the refractive index profile is controlled by the, addition of germanium as a dopant during fibre manufacture [1]. In a typical monomode optical fibre the diameter of this core region would be between 3 and 10/xm. As the spatial resolution of electron-probe X-ray microanalysis (EPMA) of a bulk sample is of the order of /xm, it is clear that such a technique is unsuitable for the analysis of fibre cores. In order to increase the spatial resolution of analysis, a technique has been developed to facilitate the preparation of thin (400 nm) sections of optical fibres. Using such a thin film in a transmission electron microscope (TEM), it has proved possible to achieve a spatial resolution of under 100 nm [2]. Techniques for the preparation of thin sections of bulk glass samples for TEM using mechanical polishing and ion-beam thinning are already well established, as are those for the preparation of transverse sections of carbon fibres using resinembedding and ultramicrotomy [3]. The special difficulty with the preparation of glass optical fibres is that, at around 100/xm in diameter, they are too small to use the established bulk preparation techniques and too large and brittle to consider ultramicrotomy. The following preparation procedure was therefore adopted. Brass sample-holder tubes were manufactured by drilling holes 0.38 mm in diameter axially through 15 mm lengths of 3 mm diameter brass rod. A length of optical fibre was stripped of its protective polymer sheath using varnish remover. After a few seconds immersion it was possible to slide the sheath off lengths in excess of i m. This fibre was then cut into 30 mm sections° A mounting resin mixture was prepared using equal weights of Araldite CY1303 resin and titanium dioxide powder into which the fibre sections were dipped before being threaded through the hole in the brass rod. The size of the hole was chosen to allow seven 125/xm fibres to be inserted into the holder in a hexagonal pattern in order to minimize the amount of resin exposed to the ion beam. Titanium dioxide powder was added in order to reduce the sputtering rate of the resin during ion-beam thinning. Without this filler it was found that the resin was eroded away before the fibre sections achieved electron transparency, allowing the fibres to fall out of the holder. After curing of the resin the specimen was