An optical collimator structure comprising an optical fiber assembly of a plurality of unitary single-mode optical fibers each having a core, and a ferrule for supporting the optical fiber assembly inserted partially therein and bonded thereto, wherein the core has a graded index optical fiber bonded to the end face thereof, the graded index optical fiber along with the end portion of the core to which the graded index optical fiber is bonded being inserted in a hole of a capillary provided at an end of the ferrule, and the end face of the graded index optical fiber for emitting or receiving a beam of light has a tilt angle relative to the optical axis of the optical collimator structure.