Patent ID: 6985660

Claim:
An optical fiber comprising a core region extending along a predetermined axis, and a cladding region surrounding the core region; the cladding region having first to (N+1)-th regions (where N is an integer of 2 or greater) such that the first region surrounds the core region, and the (k+1)-th region surrounds the k-th region (k=1,2, . . . ,N); at least one of the first to (N+1)-th regions including, in a main medium having a predetermined refractive index, a sub-region made of an auxiliary medium having a refractive index different from that of the main medium; letting n[0] be the average refractive index of the core region, and n[k] (k=1, 2, . . . , N+1) be the average refractive index of the k-th region, the optical fiber satisfying the relationship of n[0]>n[1], and n[i]>n[i+1] (∀i=h, h+1, . . . , h+m; where h and m are natural numbers).