Gradient-index fibres

Optical fibres are convenient for sending light from one place to another. Once the light is coupled into one end of a fibre it arrives at the other, regardless of how the fibre is bent or moved. Optical fibres come in three versions - multimode fibres, gradient-index fibres, and single-mode fibres (Fig. 7.21). 

Fig. 7.21 Light propagation in a multimode fibre, a gradient-index fibre, and a single-mode fibre...

A modified crucible technique allows the preparation of gradient index optical fibres [276,301]. In this technique, a larger distance between the nozzles of inside and outside crucible is used, so that the glass leaving the inner crucible flows through the melt of the outer crucible. During this time, ion diffusion occurs to form a parabolic index profile. An ion diffusion process can also be initiated during an additional thermal treatment step. 

The first advance on stepped index fibres was the graded index fibre. In this design, the refractive index of the fibre varies smoothly, from high at the centre to low at the periphery of the core region (Figure 14.32b). The refractive index gradient... 

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