Method of producing preform for coupled multi-core fiber, method of producing coupled multi-core fiber, and coupled multi-core fiber

A method of producing a coupled multi-core fiber which is provided with a plurality of cores and in which periphery surfaces of adjacent cores among the cores contact with each other, the method comprising:

• preparing a plurality of core glass bodies to be the plurality of cores and a clad glass body, wherein the respective core glass bodies have outer regions having a predetermined thickness from the periphery surfaces and made of silica glass undoped with germanium, the respective outer regions extending along entire circumference of the respective core glass bodies, and the clad glass body is made of silica glass having a refractive index lower than a refractive index of the outer regions of the core glass bodies;
• arranging the plurality of core glass bodies and the clad glass body in such a way that the plurality of core glass bodies are surrounded by the clad glass body, the periphery surfaces of adjacent core glass bodies among the core glass bodies contact with each other;
• and
• drawing the core glass bodies and the clad glass body while collapsing a gap between the core glass bodies and the clad glass body and while preventing generation of bubbles, due to volatilized gas of germanium, between the core glass bodies and the clad glass body.