Patent ID: 7221839

Claim:
An optical fiber with a lens, which is obtained by fusion-coupling a multi-mode optical fiber to a tip of a single-mode optical fiber, and working a tip of the multi-mode optical fiber, to form a spherical portion at the tip, and in which a radius of curvature of the spherical portion is set at a value larger than a maximum value of a spot size of a beam irradiating at a given distance from the single-mode optical fiber magnified by the multi-mode optical fiber, wherein the multi-mode optical fiber has a core diameter of 50 μm and a clad diameter of 125 μm; and a radius of curvature of the spherical portion is set based on the following correlation with a spot size of a beam incident on the multi-mode optical fiber and a spot size intended at a focal position: R= 69.5 f /(222.7− f ) f=πω 0 2 /√{(ω 0 /ω 0 ′) 2 −1} where ω 0 is spot size of the beam incident on the multi-mode optical fiber, ω 0 ′ is spot size intended at the focal position, and R is radius of curvature of the spherical portion(μm); and f is a focal distance (μm).