Sunlight, unfortunately, has a huge amount of dispersion, but both single-mode and multi-mode fibre optic require low dispersion light (which must come from a laser or a laser LED). A concentrator, by itself, won't fix the dispersion issue -- but a nice ruby rod with a half-silvered end would (but, if you had THAT, you'd be 80% of the way to a real laser and wouldn't have to mess around with fibre optic cable).