Image fiber, image fiber preform, and manufacturing processes thereof

A process for preparing an image fiber preform for use in an image fiber, comprising the steps of:

• (a) forming a silica glass based core which contains a dopant for increasing a refractive index of the core;
• (b) subsequently, forming a silica glass based cladding which surrounds the silica glass based core and which has an outer peripheral surface to form an optical fiber rod, the silica glass based cladding being doped uniformly with fluorine which decreases a refractive index of the cladding;
• (c) subsequently, subjecting the optical fiber rod to heat treatment so that the fluorine doped in the outer peripheral surface of the silica glass cladding of the optical fiber rod evaporates;
• (d) subsequently, subjecting the optical fiber rod to melt-drawing operation to form an optical fiber strand;
• (e) subsequently, preparing a bundle of a plurality of the optical fiber strands;
• (f) subsequently, arranging the bundle closely in a quartz tube to form a preform.

A process for preparing an image fiber, comprising the steps of:

• (a) preparing a plurality of optical fiber strands, each fiber strand comprising a core, a cladding part surrounding the core, and a light-shield part surrounding the cladding part, the light-shield part having a refractive index gradually increasing radially outward so that the maximum refractive index thereof is higher than a refractive index of the cladding;
• (b) subsequently, arranging closely the optical fiber strands in a quartz tube to form a preform;
• (c) subsequently, heating the preform from an edge thereof;
• and
• (d) subsequently, subjecting the preform to melt-drawing operation to form an image fiber.

A process for preparing an image fiber preform for use in an image fiber, comprising the steps of:

• (a) forming a silica glass based core which contains a dopant for increasing a refractive index of the core;
• (b) subsequently, forming a silica glass based cladding which surrounds the silica glass based core, the silica glass based cladding containing a second dopant for decreasing a refractive index of the cladding;
• (c) subsequently, forming a light-shield part surrounding the silica glass based cladding to form an optical fiber rod, the light-shield part containing a third dopant, wherein the light-shield part has a non-step type of refractive index distribution which is smaller than the refractive index of the core and larger than the refractive index of the cladding;
• (d) subsequently, subjecting the optical fiber rod to a melt-drawing operation to form an optical fiber strand;
• (e) subsequently, preparing a bundle of a plurality of the optical fiber strands;
• and
• (f) subsequently, arranging the bundle closely in a quartz tube to form a preform.