a method of manufacturing a multi - core optical fiber , the method including assembling together a plurality of substantially identical single - core optical fiber preforms , referred to as &# 34 ; single - core preforms &# 34 ;, each of which includes a core bar surrounded by a layer of optical cladding , so as to form a &# 34 ; multi - core preform &# 34 ; and drawing down the multi - core preform so as to obtain the multi - core optical fiber . the assembly step includes securing the single - core preforms to one another by fusing them over their entire lengths or over portions thereof along their tangential lines of contact , without inserting the multi - core preform into a holding tube . a vacuum is maintained in the preform during the drawing step , the vacuum being formed before or during the drawing step .