Patent ID: 7409122

Claim:
An end face structure of an optical fiber, comprising: a coreless fiber fused to an emitting end face of the optical fiber; and a coating material disposed at least around the coreless fiber, wherein a refractive index of the coating material is higher than a refractive index of the coreless fiber; wherein a length of the coreless fiber is selected so that light that is emitted from the optical fiber and that propagates within the coreless fiber is emitted from the coreless fiber without the light reaching a boundary between the coreless fiber and the coating material; and a spot diameter of reflected light which is incident to a core of the optical fiber and which is fed back to the end face of the optical fiber is a 2 /r 2 times of the core diameter before emitting from the end face, where a is the core diameter of the optical fiber and r is the spot diameter of the emitting end face.