The MCVD Preform and the Draw Tower

Work on a heatproof surface with the goggles on and long hair tied back. Have a metal tray of sand ready for offcuts; drawn glass fibre is very fine, very sharp and nearly invisible on a bench. Hold a 6 mm borosilicate rod in both hands and heat a 20 mm zone in the middle with the propane torch, rolling it continuously so it softens evenly. Borosilicate works at around 820 C, glowing a dull orange. When the zone is soft, take it out of the flame and pull steadily apart. Do five pulls at clearly different speeds — slow and deliberate, then progressively faster — and lay each drawn fibre on the bench labelled with roughly how long the pull took. The stopwatch and a marked 500 mm ruler line give you a usable speed. Measure the diameter of each with the caliper at the midpoint of the drawn section. Under 0.2 mm the caliper is at its limit; use the hand lens against a scale instead.

THE DEPOSITION, IN ORDER. A fused-silica tube about 25 mm across turns in a glass lathe. Oxygen carries SiCl4 vapour, and GeCl4 for the core layers, down the bore. An oxy-hydrogen torch traverses the outside at a few centimetres a minute, heating a zone to about 1600 C. The chlorides oxidise, but the particles form in the GAS and thermophoresis drives them onto the cooler wall just downstream of the torch; the next pass sinters that soot to clear glass. Fifty or more passes build the cladding and then the germanium-doped core, which is why a graded index profile is simply a schedule of GeCl4 flow. THE COLLAPSE. Deposition finished, the torch goes to about 2000 C and traverses slowly with the flow reduced. Surface tension pulls the softened tube inward until the bore closes into a solid rod. This step decides whether the core stays round and centred, and it is where germanium evaporates back off the innermost surface — which is why MCVD fibre has a small index dip at the very centre. THE SIBLINGS. Corning's OVD deposits soot on the OUTSIDE of a rotating bait rod, which is withdrawn before the porous body is dried in chlorine and sintered separately. NTT's VAD grows a soot body axially from the end and runs continuously instead of in batches. All three make excellent fibre. MCVD is cheapest to set up and cleanest, because deposition happens inside a sealed tube; OVD and VAD scale to bigger preforms and so to more kilometres per draw. Capital cost against throughput — and all three are still in production. THE COATING IS NOT OPTIONAL. Freshly drawn silica has a tensile strength of several gigapascals, above most steels. That lasts exactly until the surface acquires a flaw, and touching anything gives it one. So the coating die sits inside the tower about a metre below the furnace, and the acrylate is applied and UV-cured while the fibre is still in free flight. Every length is then proof-tested under tension, typically near 0.7 GPa, before spooling. WHAT THE BENCH VERSION CANNOT SHOW. A hand-held torch has no fixed hot zone, so pulling faster grows the softened region and feeds more glass in from the sides. Expect an exponent shallower than -0.5 and a diameter that varies along one pull. A real tower has a defined hot zone, a constant preform feed, a micrometer 500 mm below the neck and a winder servo. The physics is identical; the CONTROL is the whole engineering difference.