Due to rapid increase of traffic in optical communications in recent years, data-transmission capacity through a single-core optical fiber presently utilized is approaching the limit. Then, as a means to expand communication capacity furthermore, a multi-core fiber in which plural cores are formed in one fiber has been proposed.
As such a multi-core fiber, for example, there is one having plural core parts within a cladding part and having a flat part perpendicular to the longitudinal direction in one part of the outer circumference of the cladding part (See Patent Document 1).
When the multi-core fiber is utilized as a transmission line and receives/sends transmission signals, each core part of the multi-core fiber needs to be connected to a corresponding core part of another multi-core fiber, to an individual optical fiber and to an individual optical element or the like. For connecting such a multi-core fiber to single-core fibers to receive/send transmission signals, a method has been proposed, in which the multi-core fiber is connected to a bundle fiber having single-core optical fibers arranged at the corresponding positions of the respective core parts of the multi-core fiber (See Patent Document 2). Also, as a method for producing such bundled optical fibers, a method to bundle plural single-core fibers at the predetermined intervals by binding them has been proposed (See Patent Document 3).