Method for producing multicore optical fiber, and multicore optical fiber

A method for producing a multicore optical fiber comprising a common cladding, a plurality of cores having a refractive index higher than a refractive index of the common cladding, and a refractive index changing part having a refractive index different from the refractive index of the common cladding, the method comprising:

• forming a common cladding tube by providing a first glass rod with a plurality of first holes and one or more second holes passing through the first glass rod in an axial direction, the one or more second holes having a diameter different from a diameter of the plurality of first holes;
• subjecting inner surfaces of the plurality of first holes and inner surfaces of the one or more second holes to a gas-phase process;
• and
• inserting a plurality of core rods into the plurality of first holes subjected to the gas-phase process, respectively, inserting a refractive index changing part rod into each of the one or more second holes subjected to the gas-phase process, and performing thermal integration to form a second glass rod,
• wherein the common cladding tube is formed such that the diameter of the plurality of first holes is no more than 4 times the diameter of the one or more second holes.