Multi core optical fiber multi core optical fiber

A multi core optical fiber made of quartz glass comprising at least two cores the centers of which are arranged substantially on one line, distances between the centers of the adjacent cores being substantially the same in the longitudinal direction, the first cladding which covers the cores and the second cladding which covers the first cladding and has a larger refractive index than the first one, which fiber has improved mechanical strength and low cross-talk even if a distance between the adjacent cores is short.