"The optical fibre manufacturing process comprises of four major processes - Core preparation, Core rod draw, Clad preparation, Quality and grading
1) Core Preparation : Core is the heart of OF and governs the performance of OFs. It is prepared through a soot deposition process on an alumina rod. Its made with silica and germanium in a fixed ration. Germanium is used to increase the refractive index of core glass and to make it denser than the clad. An aluminium mandrel is taken. This is to deposit the soot of chemicals over it which would give us a soot preform. Through a vapour deposition process in a trailer like structure, soot is deposited over the rod.Chemicals and gases are released from the burners and are deposited on the rod.
a) Core – Soot deposition : After the soot deposition, the soot preform is hung in the cooling cabinet & on cooling the rod is removed from the center.The output is cylindrical mass of whitish soot.
b) Soot Preform Output : Now the soot preform is ready for sintering. Sintering is a process of turning a powdered material into a solid mass by means of heating or pressure. It basically fuses particles together. By this process, the soot is consolidated into a solid mass of glass.
c) Core preform – soot sintering : Output is a shrunk version of soot preform, but transparent. This is the core preform
d) Core preform : The core preform is then soaked at 1050 degrees for 18 hours, to release entrapped gases
2) Core Rod Draw : A draw tower is a building, 6/7 floor tall with drawing or pulling equipment starting from the top floor to the ground floor. It usually has one process on one floor.
a) Draw Process : The core preform is loaded on to the top of the draw tower. It is placed in an induction furnace. With controlled flow of heat and gases, gravity pulls out 7 rods out of each preform in around 3 hours
b) Core Rods: Then core rods go through a series of tests – physical checks for bends or bubbles, diameter checks and refractive index checks. After the quality go ahead, the rods are then prepared for cladding process. They are cleaned and prepared for hanging.
3) Cladding - After the core rods are prepped for cladding process, they are used for soot deposition. The soot process and composition in clad making is different from the core process. Here the soot is Sicl 4 – Silicon tetrachloride plus H2 and O2. It doesn’t have germanium because germanium is known to increase the RI & clad RI should be lesser than the core. Soot deposition happens through OVD or outside vapour deposition process in the same trailer like structure. Chemicals and gases are released from the burners and are deposited on the core rods.
a) Soot deposition for cladding process: Soot preforms are then kept in the cabinets for cooling. The soot preform appears white in color. Through a similar sintering process, the soot preform is converted into a wholesome glass preform. This time we can see the two layers – the core & the clad. It is 99.99% pure.
b) Glass preforms : After this the preform undergoes soaking to release entrapped gases. It happens at 1050 0 for around 18 hours. This process doesn’t change the physical or chemical properties of the preform in any manner. The last step in the process is the coning of the preform. This is done to reduce the diameter of the preform. Since quality of preform defines the performance of the OF, a rigorous process of testing which includes – particle count, optical parameters, physical parameters & cone length testing
c) Completed glass preforms with coning
d) Fibre draw process, Glass Preform to Fibre: Glass preform is loaded on to the draw towers and is passed through an induction furnace. They are coated with silica is put on the edges to avoid burning. The temperature in the furnace is 2200 C and inert gases are injected to melt the glass into fibre. From 100 mm preform, 250 micron fibre is drawn. The fibre strand coming out of the induction furnace is simmering hot. It is then passed through the annealing furnace, which enables gradual cooling to reduce residual stress. 900 C is the stress relieving point of the glass. Hence its first cooled to this temperature and then helium is used to cool it to 70 C. After cooling, the fibre needs to be coated. It has 2 kinds of coating – primary or inner layer which is soft and secondary or outer layer which is hard. First fibre is passed through the primary coating tube and then it goes through 2 gauges for diameter & strength checks. After these checks, the fibre goes through the secondary coating tube and a UV curing to protect it from environmental damage. A second diameter check happens after the curing stage. At the lowest level of the tower, there is a pinch wheel to move the fibre and wrap it in a big spool. Each and every fibre is tested for optical parameters like attenuation and spooled into smaller spools called bobbins. After the physical testing, the bobbins are sent for D2 soaking. D2 is contained in large cylinders and bobbins are placed in these for 16-18 hours to replace the hydrogen ions.
e) D2 soaking: This gives us the final bobbins with standard colour. According to customer requirement, the fibres coloured.
f) Coloring Process: After another, but final check, bobbins are numbered and placed in the Robo storage – an automated system for storage. These bobbins are then packed and dispatched to customers.
4) Quality Testing
a) Mechanical testing lab – Tests the mechanical parameters of OFs – tensile strength with bends, tensile strength in degree of curing and colour matching. Tensile strength standard is >4.8 kg but Sterlite fibres has > 6.5 kg. We also do a long length (20m fibre) tensile strength test along a fibre to test signal strength at various lengths
b) Environmental testing lab – tests fibres under all kinds of temperature and humidity conditions like Temperature ranges between -60 to 85 C, Water immersion and hot water ageing, Dry heat and damp heat, T and RH cycle, Jelly ageing, Thermal shock, Wash spray or insecticides, Extended water ageing tube and Fibre failing conditions."