2.7. FIBER MANUFACTURING 69
Figure 2.21: MCVD process commonly used for fiber fabrication. (After Ref. [87];©c 1985
Academic Press; reprinted with permission.)
Several methods can be used to make the preform. The three commonly used meth-
ods [87]–[89] are modified chemical-vapor deposition (MCVD), outside-vapor deposi-
tion (OVD), and vapor-axial deposition (VAD). Figure 2.21 shows a schematic diagram
of the MCVD process. In this process, successive layers of SiO 2 are deposited on the
inside of a fused silica tube by mixing the vapors of SiCl 4 and O 2 at a temperature
of about 1800◦C. To ensure uniformity, a multiburner torch is moved back and forth
across the tube length using an automatic translation stage. The refractive index of the
cladding layers is controlled by adding fluorine to the tube. When a sufficient cladding
thickness has been deposited, the core is formed by adding the vapors of GeCl 4 or
POCl 3. These vapors react with oxygen to form the dopants GeO 2 and P 2 O 5 :
GeCl 4 +O 2 →GeO 2 +2Cl 2 ,
4POCl 3 +3O 2 →2P 2 O 5 +6Cl 2.The flow rate of GeCl 4 or POCl 3 determines the amount of dopant and the correspond-
ing increase in the refractive index of the core. A triangular-index core can be fabri-
cated simply by varying the flow rate from layer to layer. When all layers forming the
core have been deposited, the torch temperature is raised to collapse the tube into a
solid rod of preform.
The MCVD process is also known as theinner-vapor-deposition method,asthe
core and cladding layers are deposited inside a silica tube. In a related process, known
as theplasma-activated chemical vapor depositionprocess [90], the chemical reaction
is initiated by a microwave plasma. By contrast, in the OVD and VAD processes the
core and cladding layers are deposited on the outside of a rotating mandrel by using the
technique offlame hydrolysis. The mandrel is removed prior to sintering. The porous
soot boule is then placed in a sintering furnace to form a glass boule. The central hole
allows an efficient way of reducing water vapors through dehydration in a controlled
atmosphere of Cl 2 –He mixture, although it results in a central dip in the index profile.
The dip can be minimized by closing the hole during sintering.
The fiber drawing step is essentially the same irrespective of the process used to
make the preform [91]. Figure 2.22 shows the drawing apparatus schematically. The
preform is fed into a furnace in a controlled manner where it is heated to a temperature
of about 2000◦C. The melted preform is drawn into a fiber by using a precision-feed
mechanism. The fiber diameter is monitored optically by diffracting light emitted by
a laser from the fiber. A change in the diameter changes the diffraction pattern, which