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The 2.5 mm is the size of the fibre optic connector nose, not the diameter of the ‘star’. The Star diameter is constrained by the diameter of the light conducting section within the fibre itself. Depending on the type of fibre it is usually either 50 microns or 64 microns I think. The actual fibre is thicker at around 120 microns, but the outer jacket does not transmit the light.
I think the fibre size is probably more repeatable than a pinprick in some obscuring media. Also the fibre is quite flexible and lightweight for placement. For another also, the ends of the fibres in the connectors are polished providing an even and scrambled light source from the 64 micron hole. The polarisation and othe phasing oddities that may arise from a close sourced LED are not so predictable. After a few metres travelling down a fibre, all or any of the odd modes are smoothed out.
Cheers.
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