Method for fabricating an optical fiber, preform for fabricating an optical fiber, optical fiber and apparatus

A method for fabricating a secondary processed optical fiber preform, comprising the steps of:

• inserting a primary optical fiber preform having an outer surface into a thin-walled, elongated overcladding tube having an inner surface, wherein the outer surface and the inner surface define an interior space limited at a first end of the thin-walled, elongated overcladding tube by a conically formed closure;
• holding the primary optical fiber preform in a substantially longitudinally coaxial relationship with the thin-walled, elongated overcladding tube;
• supplying solely overcladding grain into the interior space, wherein the overcladding grain is a homogeneous crystalline silica grain that is selected from the group consisting of pure or doped synthetic silica powder;
• forming an unprocessed secondary preform, wherein said unprocessed secondary preform comprises;
• the primary optical fiber preform, the thin-walled, elongated overcladding tube, the conically formed closure, and overcladding grain,
• limiting the interior space at a second end of the overcladding tube by an adjoiner;
• generating a reduced pressure within the interior space;
• and
• heating at least a portion of the unprocessed secondary preform in order to transform at least the heated portion to a processed secondary preform from which an optical fiber can be drawn.