GB1448080A - Optical fibre transducer - Google Patents

Abstract

1448080 Making optical fibres STANDARD TELEPHONES & CABLES Ltd 10 April 1975 14757/75 Heading C1M [Also in Division G2] A method of making optical fibre transformation sections (e.g. for redistributing the optical energy across the wavefront leaving one component in order to match it for launching into another) includes the steps of (1) forming an optical fibre preform whose core has a longitudinal periodic structure in its refractive index profile; (2) drawing the preform down to reduce its cross-section and (3) parting the drawn preform at intervals along its length to produce optical fibre transformation sections, the two ends of each of which have core crosssections which differ either in size or shape, or in size and shape. The preform may be made by inserting stacking components A, B of higher and lower R.I. respectively into a tube C of lower R.I. (Fig. 1A). The assembly is then drawn using conventional rod-in-tube techniques and the resulting fibre is parted by visual inspection. The components may be prismatic within a rectangular tube. In another embodiment (Fig. 2) short tubes A, B of higher and lower R.I. are stacked around a rod E of higher R.I. and inside a tube C of lower R.I. In a further embodiment (Fig. 3a, not shown) higher R.I. spheres are fitted into a circular tube, the spheres becoming prolate spheroids when drawn. In a modification (Fig. 3c, not shown) spheres are dropped one by one into a tube as it is being collapsed. In another embodiment (Figs. 4a, 4b, not shown) a bundle of fibres is deformed at periodic intervals and is then covered with lower R.I. material. In yet another embodiment (Fig. 5c, not shown) a set of ribs is ruled, etched or pressed into the central region of one major surface of a strip of lower R.I. The resulting indentations are then filled with glass of higher R.I. which is itself covered with a layer of lower R.I. before the assembly is drawn down and cut into lengths. In further embodiments (Figs. 6, 7a, 7b, not shown) the R.I. of selected portions of glass elements is altered by irradiation of ion exchange through a mask. The elements are then assembled into a preform. In the final embodiment (Figs. 8a, 8b, not shown) a clad fibre is made using an inner bushing containing a melt of higher R.I. core glass with an outer bushing of lower R.I. cladding glass. The relative pressure at the two nozzles is cyclically varied to produce the required periodic structure. If the two nozzles are at the same level the external diameter of the fibre will fluctuate but if the inner nozzle is above the outer, only the core diameter varies.