Single-mode optical fiber is 125 microns in diameter.
     It is covered by a protective polymer to an outer diameter of 250 microns.
     The core is about 8-10 microns in diameter.  It is doped with germanium to increase the index of refraction by about 0.4% (allowing for TIR).
     Light is lost by impurities and scattering.
     To achieve such low absorption, the concentration of iron must be reduced to less than 1 ppb (iron gives window glass its green tint).

     Hydroxyl ion impurities (water) must be less than 10 ppb.
     A large-diameter "rod" of fiber is made using modified chemical vapor deposition (MCVD):

Reacting silicon-containing vapors (e.g. silicon tetrachloride) with oxygen to form small particles of pure silica (done inside of a silica tube).

These particles of silica deposit on the inner surface of the tube, and are sintered to clear glass when heated by an oxyhydrogen torch to around 2000 deg. C.  (because of the chlorine and high temperatures, impurities such as transition metals and water exit the tube as volatile chlorides, increasing the purity of the silica by orders of magnitude).

As the glass is formed in thin multiple layers, the index of refraction may be increased (adding germanium) or decreased (adding fluorine) to create a desired refractive index profile (higher index in the core layers).

Finally, the tube is heated to 2300 deg. C at which point surface tension causes it to collapse to a solid rod of very pure glass, called a preform.  This preform has a doped region at the center, with a higher refractive index core, surrounded by cladding layers.

     Next, the preform is slowly fed into a furnace heated to 2100 deg. C to soften the glass, and a fiber is drawn continuously:

The fiber emerges from the furnace with a defect-free surface, which is preserved by applying a UV-curable urethane acrylate coating, before any jacket layers are added.

Each hair-thin fiber can support a load of more then 15 lb.

More than 1000 km of fiber can be drawn from a single preform, at speeds of around 20 m/s (45 mph).

The resulting optical fiber has the same refractive index profile as the preform, but the diameter has been reduced about 500 times.