An optical fiber, comprising: a core with a first refractive index
(n.sub.1); a silica based outer cladding surrounding the core, the outer
cladding having a refractive index (n), such that the core is
substantially surrounded by a gap situated between the core and the outer
cladding, the gap containing at least one support structure adjacent to
the outer cladding and situated between the outer cladding and the core,
wherein the support structure is either hollow or gas filed and is not
connected to any other support structure situated within the gap.