Note: Descriptions are shown in the official language in which they were submitted.
~Q~833
Back~rouncl of the Invention
, . .... _
1. Field of the Inve tlon
The invention is concerned with fibers for use as
transmission lines in communications systems operating in
-the visible or near visible spectra. Such fibers are
generally clad for guiding purposes so that refractive index
decreases in value from the core center to the periphery
either as a step function or as a continuous gradient.
2. Description of the Prior_Art
10"Optical" communications systems, that is systems
operating in the visible or near visible spectra, are now at
an advanced stage of development. In accordance with the
view held by many, commercial use may be expected within a
period of about five years.
A system most likely to find initial, and probably
long term, use utilizes clad glass fibers as the transmission
medium. These fibers, generally having an overall cross-
sectional diameter of about 100 ~m, are generally composed
`~ of at least two sections: core and cladding. The cladding
layer is necessarily of lowered refractive index relative
to the core with typical index variation from core to clad
being in the range from about 0.01 to 0.05. Structures
under study mav be single mode or multimode. The former is
characterized by a sufficiently small core section to
efficiently accommodate only the first order mode. Such
structures may have a core about 1 or 2 ~m. Multimode
lines typically have core sections from 50 ~m to 85 or
90 ~m in diameter.
Multimode structures appear to be of somewhat
greater interest at this time. The greater core section
facilitates splicing and permits more efficient energy
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coupling to source and repeater. Introduction of many modes
into or, alternatively, generation o~ many modes within the
line does give rise to a dispersion limitation which takes
-the form of a smearing due to the differing velocities of
different order modes. Mode dispersion effects have been
minimized by a continuous focusing structure~ This structure ;
takes -the form of a fiber, the index of which is graded
generally exponetially from a high value at the core center.
The fundamental mode which traverses the length of material ~;
is generally confined to the highest index (lowest velocity)
region, while higher order modes as path length increases
spend longer and longer periods in relatively low index `~
(high velocity) regions.
A number of procedures have been utilized for
fabricating clad glass fibers. Most have yielded to
procedures which in some way involve vapor source material.
So, typically, chlorides, hydrides, or other compounds of ~ ~ -
silica, as well~as desired dopants, tailoring the index, are
reacted with oxygen to produce deposits which directly or
ultimately serve as glass source material from which the
fiber is drawn. Dopant materials include compounds of, for
example, boron for lowering index and germanium, titanium,
aluminum, and phosphorus for increasing index. Where the
ultimate product is to be a graded multimode line, index
gradation may be accomplished, for example, by altering the
amount or type of dopant during deposition.
- One procedure utilizing vapor source material is
chemical vapor deposition (CVD). In this procedure, compounds
are passed over a heated surface--e.g., about a rod or within
a tube. Temperatures and rates are adjusted so that
reaction is solely heterogeneous, i.e., occurs a-t the heated
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sur~ace so that the initial material is a continuous glass
layer.
An alternative procedure results in -the introduction
of such precursor materials into a flame produced by ignition
of a gaseous m~xture of, for example, methane and oxygen.
Reaction is, in this instance, homogeneous resulting in
formation of glassy particles within the flame. Combustion
product and glassy particles then form a moving gas stream
which is made incident again on a heated surface, such as
-a rod or tube. Adherent particles sometimes called "soot"
are in subsequent processing flushed, and are sintered and
fused to result in a glassy layer.
The CVD process has advantages including high
purity but suffers from prolonged required deposition
periods. Typically, a suitable preform adequate for
fabrication of a kilometer of fiber may require periods of a
day or longer.
The soot process has the advantage of high speed;
preforms adequate for fabrication of a kilometer of fiber
may be prepared in a few hours or less. Disadvantages,
however, include at leas-t initial introduction of contaminants,
such as solid carbonaceous residue. Since formation takes
place within the combustion environment, hydration is
inevitable; and this gives rise to the well-known water
absorption peaks with their related subharmonics so consequen-
tial in various portions of the infrared spectrum.
--3--
:
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Both procedures are now an established part of the
art. See, for example, U.S. Patent Nos. 3,711,262, 3,737,~92,
and 3,737,293. Modifications in the processes have, to some
extent, increased the speed of the CVD process and reduced
the effects of contamination ~ hydration in the soot process.
Fibers a kilometer or more in length with losses as low as 2 or
3 dB/kilometer in selected regions of the infrared are now
regularly produced in pilot operations.
Summary of the Invention
... . _
According to the invention there is provided
; process for fabrication of a glass fiber optical trans-
mission line, comprising a core section and a cladding, wherein
the cladding has an index of refraction of a value lower than
the maximum index of the core for energy of the wav~length to
be transmitted, comprising introducing a moving stream of a
vapor mixture including at least one compound glass-forming
precursor together with an oxidizing medium into a tube while
heating the tube so as to react the said mixture and produce
a glassy deposit on the inner surface of the tube, characterized
in that heating of tube and contents are by a moving hot zone
- produced by a correspondingly moving heat source external to
the tube in that combustion within the tube is avo:ided and in
that temperature within the hot zone, composition of the
vapor mixture, and rate of introduction of the vapor
mixture are maintained at values such th~t at least a part of
the reaction takes place within the gaseous mixture at a position
. .
spaced from the inner walls of the said tube, thereby producing
a suspension of oxidic reaction product particulate material,
whereby the particulate material while traveling downstream comes
to rest on the inner surface of the tube within a xegion which
extends from a position within the said hot zone, the moving
zone serving the dual functions of: nucleation site for homo-
~L(3S~ 33
geneous reac-tion to produce particulate matter; and consolidation
site for previously produced particulate matter.
The invention provides for fabrication o~ clad glass
fibers by a procedure which combines some of the advantages of
the prior art CVD and soot processes.
With usual hea~ing means there is simultaneous
he-terogeneous reaction so that a glassy layer is produced within
the moving hot zone by reaction at the heated wall surface.
This deposit, which is present under ordinary circu~stances,
is identical to the layer produced in the normal CVD processing.
In accordance with the preferred embodiment, the
tube within which formation is taking place is continuously
rotated about its,own axis. For example, at a speed of
100 rpm, uniformity about the periphery is enhanced. The
surface produced by the molten CVD layer may help to hold
~ 4a -
~05~33 ~
the "soot" particles d~lring fusion.
~eactant materials include chlorides and hydrides,
as well as other compounds which will react with oxygen as ~;
described. ~s in other vapor reaction processes, other
gaseous material may be introduced, for example, to act as
;~ carrier or, in the instance of extxemely combustible matter
such as hydrides, to act as a diluent.
Continuous fusion within the hot zone and the
resultant thickness uniformity of deposit facilitates
formation of graded index structures. As in CVD, gradients ~-
may be produced by varying reactant composition with the -~
ratio of high index-producing dopant increasing, in this
instance, with successive hot zone traversals. Since
reaction conditions for different constituents in ~he
reactant mix are different, it is possible also to produce a
gradient by altering temperature and/or flow rate during
processing.
Typical reaction temperatures maintained at least
at the tube wall are within the range of from 1200 to 1600
degrees C. These temperatures, high relative to CVD, are
responsible for rapidity of preform production. Particularly
at the high temperature end of the range, distortion of the
usually sillca tube is avoided by rotation. Narrow zones,
increased rotation speed, and vertical disposition of the
tube may all contribute to the avoidance of tube distortion.
Preforms adequate for preparation of one or a few
kilometers of fiber may be prepared during deposition
periods of one or a few hours. These preforms are prepared
by conventional processing from the deposited product ~o a
final configuration which, as presently practiced, may be of
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MacChesney-O'~onnor 9-4
1~)83;3
1 rod shape with an internal dlame-ter of frorn 4 to fl mm
2 and a length of 18 inches. In usual process-Lng, the tube
3 which served as the deposition substrate becomes the clad.
4 It may, in accordance with the system, be composed of pure
silica or of silica which has been doped to alter, generally
6 to reduce its index. Variations may include removal of the
7 tube, as well as deposition of additional material on the
8 outer surface. The tube serving as the substrate during
9 deposition may be retained to serve as a clad, may be
removed, or may, during simultaneous deposition, on its
11 outer surface be provided with encompassing layer/s.
12 Brief Description of the Drawing
13 FIG. 1 is a front elevational view of apparatus
14 suitable for practice of the deposi-tion process in
accordance with the invention;
16 FIG. 2 is a front elevational view of apparatus ;
17 alternative to that of FIG. l;
18 FIG. 3 is a front elevational view of a section of
19 tubular material depic-ting observed conditions during
processing; and
21 FIG. 4, on coordinates of insertion loss in units
22 of dB/kilometer and wavelength in nanome-ters, is a plot
23 showing the relationship of those two parameters for a clad
24 multimode fiber produced in accordance wi-th the invention.
Detailed Description -~
26 1. The Drawing
. .
27 FIG. 1 depicts a lathe 1 holding substrate tube 2
28 within which a hot zone is produced by heating means 3 and
29 4 Tube 2 may be rotated, for example, in the direction
shown by arrow 5a by means not shown and hot zone is caused
31 to traverse tube 2 by movement of heating means 3 and
32 support 4 as
- 6 _
- MacChesney-O'Connor 9-~
33
1 schematically depicted by double headed arrow 5b, for
2 example, by a threaded feed member 6. ~ gaseous material is
3 introduced into tube 2 via inlet tube 7 which is, in turn,
4 connected to source material reservoirs 8. Such reservoirs
may include an oxygen inlet 9 connected to means not shown.
6 As depicted, gaseous material ma~ also be introduced by inlets
7 10 and 11 by means not shown and through in]et 12 from
8 reservoir 13. Reservoirs 14 and 15 contain normally li~uid
9 reactant material which is introduced into tube 2 by means
of carrier gas introduced through inlets 10 and 11 with the
be;~a
11 arrangement ~ ~ch that the carrier gas is bubbled through
12 such liquids 16 and 17. Exiting material is exhausted -through
13 outlet 18. Not shown is the arrangement of mixing valves
14 and shut off valves which may be utilized to meter ~lows and
to make other necessary adjustments in composition. The
16 apparatus of FIG. 1 is generally horizontally disposed. ~ `~
17 The apparatus of FIG. 2 is, in its operational
18 characteristic, quite similar to that o~ FIG. 1. Vertical
19 disposition of the substrate tube, however, lends stability
., .
to the portion of the tube within the hot zone and may
21 permit attainment of higher temperature or o~ longer hot
22 zones in the traversal direction without objectionable
23 dis-tortion. Apparatus depicted includes tube 20 which may .
24 optionally be provided with rotation means not shown. This
tube ~s secured to the apparatus by means o~ chucks 21 and
26 22 and a traversing hot zone is produced within tube 20
27 by means of a ring burner 23 which is caused to constantly
28 traverse tube 20 in the direction depicted by double headed
29 arrow 24 by moving means 25. Gaseous material, for example,
~rom source such as 8 o~ FIG. 1 is introduced via inlet
7 ~ -
~50833
tube 26 and e~iting material leaves via exhaust 27.
FIG. 3 is a front elevational view o~ a section of
a substrate tube 30 as observed during deposition. Depicted
is a heating means 31 producing a hot zone 32 which is
traversing tube 30 in the direc-tion shown by arrow 33 by
means not shown. Gaseous material is introduced at the :Left
end of tube 30 and flows in the broken section of the Figure
in the direction shown by arrow 34. For the processing
conditions, which with respect to traversal direction and
hot zone temperature are those of Example l, two regions are
clearly observable. Zone 35 downstream of hot zone 32 is
filled with a moving powdery suspension of particulate
oxidic material, while region 36, devoid of such particulate
matter, defines the region within which fusion of deposited
material is occurring.
FIG. 4 is a plot for measured loss in uni~s of
dB/kilometer as measured on 713 meters of fiber prepared in
accordance with an Example herein. Abscissa units are
wavelength in nanometers. It is seen that loss is at a
minimum of about 2 dB/kilometer for the wavelength range of
about 1060 to 1100 nm (the limiting value on the plot). The
peak at about 950 nm, as well as those at 880 and 730 nm,
are characteristic sub-harmonics of the fundamental water
absorption.
2. Processing Re~uirements -
a. Reaction Temperature
Superficially, the inventive technique resembles
conventional chemical vapor deposition. However, whereas
CVD conditions are so arranged that deposition is solely the
result of heterogeneous formation at a heated substrate
surface, procedures of this invention rely upon significaht
.
-8-
,
``~ ` 1(3154C~833
homogeneous reaction. In generalj 50 percent or more of
reaction product is produced in a position removed from
substrate surface and results in the formation of solid~
oxidic particles of the desired glass composition. These
particles are similar to -those produced during the "soot"
process.
Homogeneous reaction is the result of sufficient
rate of reactant introduction and sufficient reaction
temperature. Such conditlons may be achieved simply by
increasing one or both parameters until homogeneous reaction
i9 visually observed. To optimize the process from the ~ ;~
standpoint of reaction, high temperatures are utilized. For `~
the usual silica based systems which comprise the preferred
embodiment, temperatures at least at the substrate wall are
; generally maintained at a minimum oE 1200 degrees C at the
moving position corresponding with the hot 20ne. Maximum
a ` : :~
tempe ~ ures are ultimately limited by significant wall
distortion. For horizontally disposed apparatus, ~uch as
~ that shown in FIG. 1, in which a hot zone of the length of
~ 20 approximately 2 cm moves at the!rate of about 45 cm/min
within a tube rotated at the rate of about 100 rpm~ a
temperature of 1600 degrees C may be produced without ~;
harmful tube distortion. Decreasing the length of the hot
, . ~
zone, increasing the rate of rotation, increasing reactant
flow rate, vertical disposition of the tube, are all factors
.,:: .
which may permit use of higher maximum temperatures without ;~
variation in tube geometry. Indicated temperatures are
those measured by means of an optical pyrometer focused at
the outer tube surface. It has been estimated that for
:
typical conditions the thermal gradient across the tube may
be as high as 300 degrees C.
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33
b. Flow Rates
This parameter, like temperature, is dependen-t upon
other processing conditions. Again, a ~inimum accep-table
rate for those purposes may be determined by visual observation.
~ighest flow rates are ~or those materials which by virtue
of combustibility, high vapor pressure, etc., are diluted
to a signiEicant extent by inert material. Examples are the
hydrides where dilution frequently is as high as 99.5 volume
percent based on the total reactant content may necessitate
a linear flow rate of at least 1 m~ter per second. Chlorides, ;
which do not present this problem, need not be diluted to
avoid combustion. Inert material, such as nitrogen or helium,
is introduced solely for transfer purposes and need b~ present
only in amount typically of up to 10 percent by volume.
Flow rates are, as indicated, temperature dependent, with
the required homogeneous reaction taking place at acceptable
rate only by an increase flow of about 50 percent for each
hundred degree increase in reaction temperature.
c. Reactants
Examples were carried out using chlorides and
hydrides. Other gaseous materials of sufficient vapor
pressure under processing conditions preferably which react
with oxygen or oxygen bearing material to produce the required
oxidic glass may be subs'cituted. In a typical system, the
substrate tube is silica-~generally undoped. Where this
-~ tube is of ordinary purity, first reactant introduced may be
such as to result in the formation of a flrst layer of
undoped silica or doped with an oxide such as B2O3 which
serves to lower the refractive index, which acts as a part
of the clad and presents a barrier to diffusing impurlty
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~50~333
from the tube. It may be considered that, under these
- circumstances, the substrate tube ultimately serves as a
mechanical support rather than as an optical cladding.
Subsequent to formation of this firs-t barrier layer or ~ ~
absent such procedure, where the tube is of sufficlent ~-
purity, reactant materials of such nature as to result in
the desired index-increased core are introduced. In a
chloride system, these may take the form of a mixture of
SiC14 together with, for example GeC14, and oxygen.
Chlorides of other index increasing materials, such as ~`~
phosphorus, titanium and aluminum may be substituted for
GeC14 or admixed. BC13 may also be included perhaps to
facilitate glass formation because of lowered fusion
temperature; or because of refractiva index l~wering, the
initial mixture may be altered during successi~e hot zone
traversals so as to increase index (by increasing GeC14 or ~
other index-lncreasing dopant precursor or by decreasing ~ ;
BC13)
Since the usual vapor phase glass precursor
compounds are not oxidic, oxygen or a suitable oxygen
bearing compound is generally included to form the ultimate
oxidic glass. A satisfactory procedure, followed in
exemplary procedures, takes the form of an oxygen stream
bubbled through reservoirs of li~uid phase glass forming
compounds. In one procedure, for example, oxygen streams
were bubbled through silicon tetrachloride, and through
germanium tetrachloride. These streams were then combined
with vapor phase boron trichloride and additional oxygen,
the resultant mixture being introduced into the reaction
chamber.
Relative amounts of glass forming ingred~ents are
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MacChesney-olconnor 9_1l
~V~JI~ ,
1 dependent upon a variety o~ factors, such as vapor
2 pressure, temperature, flow rate, desired index, etc.
3 The appended examples indicate suitable amounts f'or producing
L~ the noted indices under -the noted conditions. Variants
are known to those f`amiliar wi-th glass forming procedures.
6 A variety of diluent ma-terials may be utilized for ;~
7 any of' the noted reasons so, for example, argcn, nitrogen,
8 helium, etc., may serve to maintain desired f'low rates to ;
9 prevent precombustion, etc. Oxygen bearing compounds which ; ;-
may replace oxygen in whole or in part include N20, NO, and
2'
12 In general, concentration of 3d-transition metal
13 impurities in the gas stream is kept below 10 percent,
14 although furtAer reduction in loss accompanies reduction of'
those impurities down to the part per billion range. Such
16 levels are readiIy available from commercial sources or by
17 purification by means similar to those taught by H.C.
18 Theuerer, U.S. Patent No. 3~07],444. As compared with the
19 usual soot process, the inventive procedure is carried out
in a controlled environment without direct exposure to
21 combustion products. This inherently results in avoidance
22 of inclusion of particula-te combustion products. Where
23 desired, h~dra-tion resulting from combustion in the soot
24 process may be minimized. This is a particularly significant
advantage for operation in several portions of the infrared
26 spectrum which suffers from sub-harmonics of' the f`undamental
27 H O absorption. Water vapor may, therefore, be a
28 particularly significant impurity and, for many purposes,
29 should be kept to a level below a few ppm by ~olume.
3. General Procedure
31 The procedure described is that which was followed
~ : .
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- 12 -
~ ',.
~ .. . . . . . .. . . .
~l)S~ 3
in Examples 1 through 4O Deposition was carried out within
a 12 I.D. by 14 O.D. mm silica tube. The tube was placed on
a glass lathe within which it was rotated a-t 100 rpm~
Before in-troduction of reactants, it was flushed with a
continuous stream of oxygen while -traversing with an
oxyhydrogen burner sufficient to bring the wall temperature
to 1400 degrees C. The purpose was to remove any volatile
impurities on the inside wall of the tube.
Following a period of 5 minutes, a mixture of
lC oxygen, SiC14, and BC13 replaced the oxygen flow. The
composition of approxima'cely 10 percent ~iC14, 3 percent
BC13, remainder oxygen, maintaining temperature at 1400
degrees C within the moving hok zone as measured at the
wall. In this particular example, the zone was moved at a
speed of approximately 45 cm/min in the forward direction
(direction of gas flow) and was rapidly returned to its ~`
initial position (approximately 30 sec. elapsed time to the
beginning of the slow traversal).
Formation of flaky material within the tube, at a
position spaced from the wall generally downstream of the
hot zone, was visually observed. It was deduced and
verified that homogeneous reaction was largely within the
zone with particulates being carried downstream by the
moving gas. Deposition was continued for approximately
twenty minutes following which flow of chloride reactants
was discontinued. Oxygen flow was continued for several
passes of the hot zone.
The procedure to this point results in deposition
of a layer serving as cladding. Core material was next
deposited by introduction of SiC14 and GeC14. These
reactants, too, were introduced with an oxygen carrier, as
-13-
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~S0~33 ~:
before. With the temperature of the hot zone increased
somewhat to about 1450 degrees C, deposition was continued -
for about one hour.
In this particular example, tube collapse was
in:itiated with reactants still flowing simply by reducing
the rate of traverse of the hot zone. This resulted in a
; temperature increase which ultimately attained a level of
about 1900 degrees C to produce nearly complete collapse.
Reactant flow was then stopped with final collapse producing
a finished preform consisting of a GeO2-SiO2 core with a
borosilicate cladding supported, in turn, by a silica layer.
It will be recognized by those skilled in the art of fiber
drawing, that the tube, without first being collapsed, can
also be drawn into acceptable fiber. ~he resulting preform
was then drawn to result in a fiber having an overall
diameter of approximately 100 ~m with a core area defined
. . , , ~ ~
as the region within the borosilicate layer having a ~ -
diameter of approximately 37 ~m. The length of fiber drawn ~
:
was approximately 0.7 km. The method described in some
detail in N.S. Kapany, Fiber Optics Principles a d Applications
(Academic Press, New York) (1967) pages 110-117, involved ~ ~-
;. ~. ~;;.
the local heating of an end of the preform which was affixed
to the fiber, which was, in turn, drawn at a constant velocity
of approximately 60 meters/min by winding on a 30 cm diameter ;
mandrel rotating at 60 rpm.
~; The above description is in exemplary terms and is
usefully read in conjunction with the appended examples.
,.~; .
The inventive process departs from conventional CVD as
described--i.e., in that reactant introduction rate and -;~
temperature are such as to result in homogeneous reaction to -;~
produce oxidic particles within the space enclosed, but
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~acChesney-O'Connor 9-4
333
1 separated from the walls of a tube. Thls, when combined
2 wlth a moving hot zone~ results ln rapld preparation of a
3 hlgh quality preform as descrlbed. The movlng hot zone i5
4 responsible for (1) homogeneous reactlon; (2) to a large
extent, the adherence of oxldlc particles -to the wall; and
6 (3) fuslon of the deposlted particles and CVD-produced layer
7 into a unitary, homogeneous glassy layer. In general, it is
8 deslrable to maintaln the hot zone as short as posslble
9 depending upon constancy of traversal speed to result in
uniform layer-production. Motlon of the hot zone should be
11 such -that every portion of -the tube ls heated to the zone
12 temperature for the same perlod of time. I'hls is easily
13 accompllshed by passlng the heating means through a
14 traversal dlstance whlch extends beyond the tube at both
ends. Experimentally, hot zones of the order of 2 cm length
16 (defining the heated region extending 4 cm on either side of
17 the peak) have resulted in uniform coating under all
18 experimental condltlons. Whlle, ln prlnciple, heating the
19 entlre tube may result ln uniformity of depositlon
approachlng that attained by use of a moving zone, very high
21 flow rates are required to avoid inhomogeneity and
22 differing thickness of deposit along the length of the tube.
23 4. Examples
~ 24 The following examples, utlllzing chlorlde or
i 25 hydrlde reactants, are set forth. The selection was made
26 wlth a vlew to demonstrating a wlde varlety of composltlons
27 and dlfferent types of optical wavegulde preforms for whlch
28 the procedure can be used.
29 The tube of commerclal grade fused quartz was
3 flrst cleaned by lmmerslon ln hydrofluoric acid-~nitric acid
31 solution for three minutes and was rinsed with delonlzed
33
water for a period of one hour. Tubing was cut into 18"
lengths, and such sections were utilized in each of the
examples. The substrate tube was provided with appropriate
input and exhaust sections, and was heated with a moving
oxyhydrogen torch producing a hot zone which traversed the
tube in from one to eight minutes. In each instance,
flushing was by oxygen at a flow rate of between 100 and
500 cm3/min corresponding with a linear rate of 4.5 meter/mins,
and this flushing was continued for several traversals of
the zone.
Example 1
The fused quartz tube used in this example was
12 mm I.D. x 14 mm O.D. Initial deposition was of a
cladding material, Sio2-B203, by introduction of 41 cm3/min.
SiC14, 12.5 cm3/min BC13, both carried by oxygen such that
the total oxygen flow was 250 cc/min. Sixteen passes of the ~;
., ~ . .
hot zone were made at a temperature of 1430 degrees C. Core
material was next deposited by flows of 32 cc/min SiC14,
48 cc/min GeC14, and oxygen 650 cc/min. This was continued
for 68 minutes and temperatures of the hot zone were
maintained at 1460 degrees C. Remaining steps, including
partial collapse with flowing gas and final collapse under ~
no flow conditions, were as specified under Section 3. The ;~;
fiber that resulted from this procedure had a core of
approximately 40 ~m with an overall diameter of approximately
100 ~m. Its length was 723 meters and optical attenuation
was 2 dB/km at 1060-1100 nm.
Example 2 ;~
A fused quartz tube 6 mm I.D. x 8 mm O.D. was
30 cleaned as described and positioned in a glass lathe. Flows
of diluted (1 percent by volume in N2) silane, germane,
~ -16-
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- 1C~5~)833
diborane, and oxyyen were passed through the tube as
follows:
SiH4 1,000 cc/min.
Ge~4 150 cc/min.
B2~l6 50 cc/min.
Deposition commenced by heating the tube locally using an
oxyhydrogen flame which was traversed along the length of
the tube. The complete cycle took 3.7 minutes, and the
highest temperature attained was 1400 degrees C. After
175 minutes, the gas flows were stopped and the tube
collapsed in one additional pass, made at a much slower ~ -~
rate. Temperatures achieved here were in the vicinity oE
1750-1900 degrees C. The preform was removed to a pulling
apparatus and drawn to a fiber whose diameter was 100 microns
overall. This consisted of a core whose composition was
SiO2-GeO2-B2O3 of approximately 25 microns diameter. The
cladding had the composition of SiO . The index difference
produced by the core was 0.007.
Example 3
A clean fused silica tube 6 mm I.D. x 8 mm O.D.
was positioned in a glass lathe as previously described.
Flows of diluted (3.05 percent by volumn in M2) silane,
diborane, and oxygen were passed through the tube as follows: -
SiH4 295 cc/min. ~-;
.' :
B2H6 49 cc/min.
2 900 cc/min.
~; Deposition commenced by heating the tube locally
using an oxyhydrogen torch which traversed along the tube at
a rate of 0.10 cm/sec as the tube rotated at 100-120 rpm.
The torch was adjusted so as to produce a temperature
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--- MacChesney-O'connor 9-4
~Lal5~8~3
1 locally o~ 1375-1450 degrees ~. When the torch had moved
2 to the end of the -tube, it ~as returned at 0.15 cm/sec with
3 the SiHl~ and B2H6 flows stopped~ This procedure continued for
4 three hours. At this time the B H6 flow was s-topped and
~ust SiH4 and 2 continued. At the same time, the torch was
6 ad~usted to produce temperatures of 1600-1650 degrees C3 ?
7 other conditions remaining the same as previously.
8 Depositing the pure sio2 layer continued for 1.5 hours.
g ~t this time, silane flow was stopped and just 0
flow continued at 600 cc/min. Temperatures were varied
11 during the next two passes to 1650-1700 degrees C. Now the
12 oxygen was stopped, the traverse slowed to 0.05 cm/sec, and
13 the temperature raised to 1850-1890 degrees C to bring about
14 complete collapse of the tube.
This procedure produced a preform having a core of
16 pure SiO2, a cladding layer of B203-SiO2~ and an outer
17 jacket of commercial grade SiO . The fiber drawn from this
18 preform had a core o~ 30 ~ m, cladding thickness of 15~ m
19 and an outer jacket of 20~ m, with an index difference of
0.007 percent and losses of 3 dB/km at 1.06~m wavelength.
21 Example 4
22 For optical communications employing multimode
23 optical fibers it is desirable to more nearly equalize the
24 group velocities of propagating modes. This result is
25 expscted if the index of the core is gradually increased ; ~.
26 from the cladding toward the interior of the core. To
27 accomplish this a 8 mm I.D. x 10 mm 0 D. fused quartz tube
28 was positioned and borosilicate layer intended to serve as
29 a portion of the cladding and as a barrier layer was
30 deposited as in Example 1. Next deposition of the GeO2 -
; 31 B203 - SiO2 core was commenced except that the germania
32 content was
_ 18 -
~ '.
1C~5;~833
gradually increased from zero during the period of
deposition. The conditions used duriny the deposition were
as follows:
Barrier layer SiC14 32 cc/min
BC13 12.5 cc/min
2 250 cc/min
Temp 1740 degrees C
Time 25 min;
Graded Index portion of the core
SiC14 33 cc/min
BC13 12.5 7.5 cc/min
17 equal increments at
2 min intervals
GeCl~ 0 - 35 cc/min
17 equal increments at
2 min intervals
-; 2 460-830 cc/min
17 equal increments at
2 min intervals ~-
Temp 1470 degrees C ~
Constant Index portion of core ~-
SiC14 32 cc/min
BC13 7-5 cc/min
GeC14 35 cc/mln
~; 2 830 cc/min
Temp 1470 degrees C
Time 53 min. ;
At the conclusion of the deposition, the tube was
collapsed to yield a solid preform which was then pulled to
.
~ yield an optical fiber. When the mode dispersion of this
. . .
fiber was measured, it behaved in a manner expected oE a
3Q graded index. This behavior can be expressed by relation
;~ (Bell System Technical Journal 52, pp. 1566 (1973))
= nO[l-2~(r/a)~]l/2 where in this instance the value of
~ = 5.
: - 1 9 . '
:' ~
~ .. .. . . . . . .
