FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

Fig. 2.28
FURNACE—
PREFORM —
n
II
THICKNESS GAGE — - - -
JACKETING UNIT z+
P
DRYING FURNACE —-----r!l II
+
TAKEUP DRUM -
6)
~> ‘
The double-crucible optical fiber drawing
system.
In principle, the double-crucible process has
the advantage that it may be used to draw continuous
fibers of any desired length. Unfortunately, because
the core and cladding glasses must be contained and
heated within the crucibles, it is difficult to maintain
the very high purity levels required to yield the
very low-loss fibers.
A much different procedure for producing extremely
low-losa fibers was developed during the early
and middle 1970’a. Though several variations of the
same approach are being used by manufacturer, they
all are based on the production of glass fiber using a
vapor-phase oxidation (VPO) process.
2.2.2.2 The Inside Vapor-Phase Oxidation (IVPO)
Process
The inside vapor-phase oxidation process
(IVPO) is shown in Fig. 2.29. Vapors of various metal
SiCl 4
J.
GeC14
Q
r
MIXING MANIFOLD
AND FLOW
CONTROLLER
SILICA BAIT TUBE
‘+== BURNER
H2+02TORCH
1
shown in Fig. 2.29. In each case, the halides are introduced
into the mixing manifold by means of a vapor
distillation process. For example, high purity oxygen
may be bubbled through the liquid silicon tetrachloride
(SiC14)”and germanium tetrachloride (GeC14). This process
reduces the level of impurities in the halide
vapors that are fed into the reaction tube. Heat is
applied to the outside of the tube using a movable
hydrogen-oxygen torch. This leads to oxidation of the
metal halides, yielding a precipitate of very fine
glass particles (soot) that builds up on the walls of
the bait tube.
The tube is mounted in a glass-working lathe
and continuously rotated during the oxidation process
so that the precipitate deposits uniformly around the
inner circumference of the tube, as shown in Fig. 2.30.
BAIT TUBE
/
REACTANTS
_ SOOT FORMATION_
(METAL HALIDES+ 02)
SINTERED GLASS
TRAVERSINiG BURNER
EXHAUST
:)
u’
SOOT DEPOSIT
Fig. 2.30 The inside vapor-phase oxidation (IVPO)
process for producing optical fibers.
After P. Schultz, Appl. Opt. Q, 3684 (1979)
The traversing burner not only provides the heat required
to oxidize the various metal halide vapors but
also transforms the porous soot deposit into thin
sintered glass layers that are built up as the burner
slowly traverses back and forth along the length of the
bait tube. By controlling the concentration of the
various reactants fed into the bait tube it ia possible
to build up layers of Si02 glass with any desired level
of doping. These will eventually form the cladding and
the core of fibers that may be drawn from the resulting
glass boule (preform) that is produced in this process.
Several other steps are carried out before
the tube is ready for fiber drawing. These are show
in Fig. 2.31. After the cladding and core glasses are
depoaited, the tube is heated so that under surface
Fig. 2.29
i)He
The vapor-phase oxidation (VPO) process for
producing optical fiber preforms.
halides are mixed with oxygen and helium to desired
highly-controlled concentration levels and fed into a
hollow silica cylinder (bait tube). The chlorides of
silicon and germanium exist as liquids at atmospheric
pressure and room temperature, while those of phosphorus
and boron must be stored under high pressure, as
2-14
SUSSTRATE
TUBE
CLADDING
DEPOSITED
CORE
DEPOSITED
COLLAPSED
PREFORM
Fig. 2.31
c1 Q+ ,- d
I
/
FIBER DRAWING
‘1 ~ÿÿÿÿÿÿÿÿÿÿÿ
@ /; SUBSTRATE REMOVED
DRAWING
gjjj/p”FIBER
1// THIN LAYER DEPOSITED
‘, //
—u
(3”
oc’? FIBER ORAWING
Stages in the processing of preforms in
production of optical fibers.
the