Modified Chemical Vapor Deposition (MCVD)
In MCVD a hollow glass tube, approximately 3 feet long and 1 inch in diameter
(1 m long by 2.5 cm diameter), is placed in a horizontal or vertical lathe and spun
rapidly. A computer-controlled mixture of gases is passed through the inside of
the tube. On the outside of the tube, a heat source (oxygen/hydrogen torch) passes
up and down as illustrated in Figure 2-5.
Each pass of the heat source fuses a small amount of the precipitated gas
mixture to the surface of the tube. Most of the gas is vaporized silicon dioxide
(glass), but there are carefully controlled remounts of impurities (dopants) that
cause changes in the index of refraction of the glass. As the torch moves and the
preform spins, a layer of glass is formed inside the hollow preform. The dopant
(mixture of gases) can be changed for each layer so that the index may be varied
across the diameter.
After sufficient layers are built up, the tube is collapsed into a solid glass rod
referred to as a preform. It is now a scale model of the desired fiber, but much
shorter and thicker. The preform is then taken to the drawing tower, where it is
pulled into a length of fiber up to 10 kilometers long.
Outside Vapor Deposition (OVD)
The OVD method utilizes a glass target rod that is placed in a chamber and spun
rapidly on a lathe. A computer-controlled mixture of gases is then passed between
the target rod and the heat source as illustrated in Figure 2-6. On each pass of the
heat source, a small amount of the gas reacts and fuses to the outer surface of the
rod. After enough layers are built up, the target rod is removed and the remaining
soot preform is collapsed into a solid rod. The preform is then taken to the tower
and pulled into fiber.
Vapor Axial Deposition (VAD)
The VAD process utilizes a very short glass target rod suspended by one end. A
computer-controlled mixture of gases is applied between the end of the rod and
the heat source as shown in Figure 2-7. The heat source is slowly backed off as
the preform lengthens due to tile soot buildup caused by gases reacting to the heat
and fusing to the end of the rod. After sufficient length is formed, the target rod
is removed from the end, leaving the soot preform. The preform is then taken to
the drawing tower to be heated and pulled into the required fiber length.