not to scale the cladding tube is shown in fig6 and 7 at 26 a typical length to diameter ratio is 10 15 the core rod 27 is shown being inserted into the cladding tube the rod at this point is typically already consolidated in an alternative overcladding method cladding soot is deposited on top of a core rod as mentioned above there exist several common options for the composition of the core rod it may be pure silica with a down doped cladding it may have a pure silica center region with a down doped outer core region it may have an up doped e g germania doped center core region surrounded by a pure silica region it may have an up doped center core region surrounded by a down doped outer core region all of these options are well known in the art and require no further exposition here after assembly of the rod 27 and tube 26 the combination is sintered to produce the final preform 28 shown in fig7 with the core 29 indistinguishable from the cladding except for a small refractive index difference typical dimensions of the rod and cladding tube are also well known the diameter of a consolidated cladding tube for a standard multi mode fiber is approximately twice the diameter of the core rod in the case of a preform for a single mode fiber the diameter of.