Note: Descriptions are shown in the official language in which they were submitted.
``` l~S1457
HIGH BANDWIDTH OPTICAL WAVEGUIDE
Background of the Invention
This invention relates to multimode optical fibers and
; more particularly to multimode optical fibers wherein a plurality
of dopants are radially graded throughout the core in order to
minimize modal dispersion.
- It has been recognized that optical waveguides, the
cores of which have radially graded index profiles, exhibit
significantly reduced pulse dispersion resulting from group
10 velocity differences among modes. This dispersion reducing effect,
which is discussed in the publication by D. Gloge et al., entitled
"Multimode Theory of Graded-Core Fibers" published in the November
1973 issue of the Bell System Technical Journal, pp. 1563-1578,
employs a radially graded, continuous index profile from a maximum
value on-axis to a lower value at the core-cladding interface.
r The index distribution in this type of waveguide is given by the
equation
n(r) = nl [1-2~(r/a)a]l/2 for r< a (1)
where nl is the on-axis refractive index, n2 is the refractive
20 index of the fiber core at radius a, ~ = (nl2-n2 )/2nl2 a is the
core radius, and a is a parameter between 1 and ~.
It was initially thought that the parabolic profile
wherein a is equal to 2 would provide an index gradient that
would minimize dispersion caused by group velocity differences
among the modes. Thereafter, other values of a were derived for
the purpose of improving various optical properties such as lower-
ing attenuation and providing high bandwidths over broad bands of
wavelengths. For example, see U.S. Patents Nos. 3,904,268;
4,006,962; and 4,057,320. The aforementioned Gloge et al. publi-
30 cation also pertains to an attempt to minimize dispersion by
setting a equal to 2-2~.
- 11514S7
. .
TQ make a hi~h bandwidth multimode Qptical waveguide
fibex it is necessary to precisely control the radial index of
xefraction of the core. A common method of forming such fibers is
taught in U.S. Patents Nos. 3,823,995 to Carpenter and 3,711,262
to Keck and Schultz, which patents teach examples of gradient
index optical waveguides as weIl as examples of the formation of
optical waveguides by inside vapor phase oxidation processes. The
inside vapor phase oxidation processes include chemical vapor
deposition, flame hydrolysis and any other processes by which
vaporous material is directed into a heated tube, reacted with
.~
oxygen under the influence of heat and deposited on the inside wall
surface of said tube. The material is deposited within the tube
in successive layers and the tube is then removed from the heat to
leave a fused blank. As will be understood, the central hole may
- be collapsed at the end of the deposition process, the blank may
subsequently be reheated and the hole collapsed, or the hole may
be collapsed during the drawing process. In any event, the blank
or preform is subsequently heated and drawn into an elongated, fine
strand. Inasmuch as the structure of the drawn strand or filament
.: .
reflects the structure of the drawing blank or preform, it is
important that the physical characteristics of the blank be care-
fully controlled.
In order to effect such change of the index of refrac-
tion of a blank or preform being formed by an inside vapor
phase oxidation process, the chemical composition of the
, source materials, which, after reaction, comprise the ultimate
` material deposited on the inside surface of the tube, may be varied.
The vapor mixture is hydrolyzed or oxidized and deposited on the
inside surface of the tube and subsequently fused to form a high
quality and purity glass. At the same time, one or more additional
vapors can be supplied to the tube, each vapor being constituted
.
15~7
-
of a chemical termed a~"dopant" whose ~resence affects the index
of refraction or other characteristics of the glass being formed.
In general, the method of forming optical waveguide
blanks or preforms by the inside vapor phase oxidation process
includes forming a barrier layer on the inside of the support or
substrate tube prior to the deposition of the core glass with the
substrate tube being the cladding. The principal function of the
barrier layer is to minimize interface scattering and absorption
losses by removing the core-cladding interface which would exist
between deposited layers of high purity, low attenuation glasses
and the substrate tube inner surface. The barrier layer is con-
ventionally a borosilicate glass composition since doping silica,
which is generally the base glass, with boron reduces the deposi-
~ tion temperature and thereby minimizes shrinkage of the substrate
`~ tube. Other advantages of doping silica with boron are that it
reduces the refractive index of the glass and it acts as a
barrier to the diffusion of hydroxyl ions, commonly referred to
as water, from the subst~ate tube to the deposited core glass at
the elevated processing and drawing temperatures.
It has been found that the bandwidth of an optical ~-
waveguide filament produced by the inside vapor phase oxidation
- process sometimes falls far short of the predicted theoretical
value. For example, an attempt was made to form low loss high
bandwidth optical waveguide filaments of SiO2 doped with B2O3,
GeO2 and P2O5 in amounts represented by curves 10, 12 and 14 of
` Figure 1. In this Figure, "CL." refers to cladding, "B.L."
refers to barrier layer, and the radii rO, ra, rb and rc refer to
the filament axis, the core radius, the barrier layer radius and
the outside or cladding radius, respectively. As noted in U.S.
Patent No. 4,230,396, "High Bandwidth Optical Waveguides and
Method of Fabrication" issued October 28, 1980, Olshansky et al.,
.
.
~ 1~5~457
such a filament exhibits a combination o~ step-graded index of
~efraction profile which causes pulse spreading of higher order
modes, a factor which lowers bandwidth. One of the causes of the
step-graded profile is the abrupt elimination of B2O3 at core-
barrier layer interface. Because of the profile discontinuity at
the edge of the core, the'average bandwidth of thi's type of prior
art optical filament is only about 240 MHz. Differential mode
delay (DMD) analysis of this type of filament consistently re-
vealed that the lower order modes were concurrent, but that the
higher order modes were increasingly delayed, the highest order
modes showing from 1 to 3 ns/km relative deIay. More than half
of the propagating modes showed relative delay.
The bandwidth values reported herein are as measured
with a "mode scrambler" apparatus of the type described by W. F.
Love in "Digest of Topical Meeting on Optical Fiber Communication"
(Optical Society of America, Washington, D.C., 1979), paper ThG2,
pp. 118-120. It is noted that these values may be considerably
' lower than those obtained without the use of a mode scrambler.
An attempt was made to improve bandwidth by eliminating
the step-graded refractive index feature caused by the abrupt
elimination of B2O3 at the core-barrier layer interface. The
B2O3 concentration was graded from the barrier layer level to
zero at a radius rd as shown by dashed line 16 while maintaining
the concentrations of the other dopants at the values represented
by curves 10, 12 and 14 of Figure 1. In filaments wherein the
' radius rd was 27.2 ~m and the core radius ra was 31.25 ~m, the
average bandwidth was 250 MHz at 900 nm. An examination of the
differential mode delay of representative filaments showed that
grading B2O3 into the core is not without merit. Figure 2, which
is a DMD curve for a fiber wherein rd is 27.2 ~m shows that
about 60% of the modes were relatively concurrent for that
filament. This DMD curve indicates that a refractive index pro-
-- 4 --
.
~is~
file discontinuity is a.dversely affecting the higher order modeseven when the abrupt step is eliminated by ramping the B2O3 from
the barrier layer level to some finite radius within the core.
-Thus in a broad aspect the present invention provides
an optical waveguide filament comprising an outer cladding layer,
a barrier layer disposed on the inside wall surface of said
cladding layer, and
a core of high purity glass having a gradient index of refrac-
tion disposed within said barrier layer and adhered thereto to
form an interface therebetween, said core comprising SiO2 doped
with a sufficient amount of a first oxide to increase the refrac-
tive index of the core to a value greater than that of said outer
cladding layer, the concentration gradient of said first oxide
following a power law gradient between the filament axis and a
radius rd that is less than the radius ra of said core such that
the refractive index is greatest at the filament axis, the con-
centration of said first oxide in said core at radii between ra
and rd increasing from the concentration thereof in said barrier
layer to the concentration thereof at said radius rd at a rate
greater than the concentration of said first oxide would increase
if said power law gradient of said first oxide extended into said
region between radii ra and rd.
In another aspect the invention provides a gradient
index optical waveguide filament having a bandwidth of at least
700 MHz, said filament comprising
an outer cladding layer,
a barrier disposed on the inside wall surface of said cladding
layer, said barrier layer comprising SiO2, GeO2, B2O3 and
P2O5, the concentration of each of said SiO2, GeO2, B2O3, and
P2O5 being substantially uniform throughout the thickness of said
barrier layer, and
-- 5 --
57
a core of high ~uritX glass h~avin~ a gradient index of refrac-
tion disposed within said barrier layer and adhered thereto to
form an interface therebetween, said core consisting essentially
of SiO2, B2O3, P2O5 and GeO2, the quantity of B2O3 decreasing
within said core from the barrier layer level to zero at a radius
. rd which is located between'0.75 and'0.9 times the core radius ra,
: the concentration gradient of P2O5 and GeO2 substantially follow-
': ing a power law curve between said radius rd and the filament axis,
the concentration of P2O5 in said core at radii between ra and rd
10 increasing from the concentration the'reof in said barrier layer
to the concentration thereof at said radius rd at a rate greater
than the concentration of P2O5 would increase if said power law
P2O5 gradient extended into said region between radii ra and rd,
and the concéntration of said GeO2 between radii rd and ra being
constant and equal to or less than the barrier layer level of
GeO
'~ In still a further aspect the present invention pro-
~ vides a method of forming a preform for a high bandwidth optical
j ,~
;' waveguide filament comprising the steps of
~ ,:
~ 20 providing a glass bait tube,
`~ forming a barrier layer on the inside wall surface of said tube,
depositing n layers of core glass on the surface of said barrier
layer, said core glass comprising a high purity base glass and at
least one dopant, the concentration of said dopant changing
' between said barrier layer and the inner surface of said core
; glass in such a manner that the refractive index of said core
glass continually increases between said barrier layer and the
inner surface of said core glass, the change in concentration of
said dopant with each layer in the final (1 - x)n layers being
such that the dopant concentration in the corresponding inner core
region of the resultant filament follows a power law curve with
-- 6 --
~ ~15~45'7
respect to core radius, and the change in concentration of said
dopant in the first xn layers being such that the dopant concen-
.- tration in the corresponding outer core'region of the resultant
filament has a greater change with respect to radius than it
: would have if said power law governed the dopant concentration
'~ in said outer core region, wherein x is a number between 0.2 and
0.43.
. In another broad aspect the invention provides an opti-
cal waveguide filament comprising
an outer cladding layer, .
a barrier layer disposed on the inside wall surface of said
cladding layer, and
. a core of high purity glass having a gradient index of re-fraction disposed within said barrier layer and adhered thereto
~ to form an interface therebetween, said core comprising a high
~ purity glass doped with a sufficient amount of a first oxide to
;' increase the refractive index of the core to a value greater
than that of said outer cladding layer, the concentration
gradient of said first oxide following a power law gradient
. ' 20 between the filament axis and a radius rd that is between
0 75 and 0.9 times the core radius ra, such that the refractive
index is greatest at the filament axis, the concentration of said
first oxide in said core at radii between ra and rd varying from
' the concentration thereof in said barrier layer to the concentra-
tion thereof at said radius rd at a rate greater than the concen-
~' tration of said first oxide would vary if said power law gradient
of said first oxide extended into said region between radii ra
and rd.
Brief Descriptioh of th'e''Inventi'on
:` 30 The present invention pertains to a high bandwidth
gradient index optical filament comprising an outer cladding layer,
- a barrier layer and a core. The barrier layer, which has an index
-- 7 --
.. . .. ... .. . . . .. . .. . .. .... .. . . . .. .. . . . . . . ... ..... .. . .. . . . . .
`` ` llSl~S7
of re~raction e~ual to or less than th~t Qf the cladding layer,
is disposed on the inside wall surface of the cladding layer.
The barrier layer comprises SiO2, GeO2, B2O3 and P2O5, the quan-
tity of each of these oxides being substantially uniform through-
out the thickness of the barrier layer. The core, which is formed
of a high purity glass having a gradient index of refraction, is
disposed within the barrier layer and adhered thereto to form an
interface therebetween. The core preferably has an index of
refraction approximately equal to the barrier layer at the core-
barrier layer interface. The core consists essentially of SiO2,
B2O3, P2O5 and GeO2. The quantity of B2O3 decreases within the
core from the barrier layer level to zero at a radius rd which is
between 0.75 and 0.90 times the core radius ra. The concentra-
tion gradient of P2O5 and GeO2 substantially follow a power law
curve for the core between radius rd and the filament axis. The
concentration of P2O5 in the core at radii between ra and rd
increases from the concentration thereof in the barrier layer to
the concentration thereof at radius rd at a rate greater than the
concentration of P2O5 would increase if the power law P2O5
gradient extended into the region between radii ra and rd. The
concentration of GeO2 between radii rd and ra is equal to or less
than the barrier layer level of GeO2.
Brief Description~of the Drawin2s
Figure 1 is a graphical illustration of the dopant con-
centration in a prior art gradient index of refraction profile
optical waveguide.
Figure 2 is a graphical illustration of the principal
mode number versus delay time of an optical waveguide formed in
accordance with the graph of Figure 1.
Figure 3 is an oblique view of an optical waveguide in
accordance with the present invention.
-- 8 --
llS~
Figure 4 is a ~ra~hi~al illustration of the dopant con-
centration levels of an optical ~aveguide filament formed in
accordance with the present invention.
Figure 5 is a schematic illustration of an apparatus
employed in the formation of the optical filament of the present
invention.
Figure 6 is a graphical illustration of the source
material flow rates employed in the formation of an optical wave-
guide filament in accordance with the present invention.
Figures 7 and 8 are graphical illustrations of the
principal mode number versus deIay time for two optical waveguides
formed in accordance with the present invention.
: DETAILED DESCRIP~ION OF IHE INVENTION
.
It is to be noted that the drawings are illustrative and
~` symbolic of the present invention and there is no intention to
indicate scale or relative proportions of the elements shown
therein.
This invention pertains to optical waveguide filaments
' of the type illustrated in Figure 3 wherein core 40 having radius
ra is centered around axis 42. Surrounding core 40 are barrier
`~ layer 44 having radius rb and cladding 46 having radius rc.
In accordance with the present invention, the opical
waveguide filament comprises silica doped with the oxides illus-
trated in Figure 4. The barrier layer contains B2O3, P2O5 and
GeO2 as illustrated by lines 4~, 58 and 64, respectively. This
filament differs from prior art filaments in that, in addition to
the B2O3 decreasing to zero at radius rd as indicated by line 50,
the concentrations of P2O5 and GeO2 within the core are as follows.
The concentration of P2O5 varies according to a power law for the
core between radius rd and axis rO as indicated by curve 52, but
it increases at a rate greater than the power law gradient 52
_ g _
115~4S7
.
between radii ra and rd as indicated b~ cur~e 54. In order to
emphasize the steepness of curVe 54 as compared to power law
curve 52, the power law curve 52 is extended between radii ra and
rd as illustrated by dashed line 56. The concentration of P2O5
in the barrier layer is the same as that at the core-barrier
layer interface as indicated by curve 58. The concentration of
GeO2 between axis rO and radius rd varies according to a power
law for the core as indicated by curve 60. The concentration of
GeO2 between radii ra and rd is preferably equal to the concentra-
tion thereof in the barrier layer in order to avoid abrupt con-
,~ ~
centration changes. However, the GeO2 concentration betweenradii ra and rd may be less than the barrier layer level, and the
- GeO2 may be completely omitted from that region of the waveguide.
~, .
Also, point C on curve 60 may be any level between zero and the
barrier layer level of GeO2. The bandwidth of such a filament is
much greater than that of the prior art filaments described above
in conjunction with Figures 1 and 2 when rd is optimized. Band-
widths of at least 700 MHz at some wavelength for a 1 km length
; of filament have been achieved in filaments wherein radius rd is
between 0.75 and 0.90 ra. Bandwidths greater than 1 GHz have
been achieved by specific embodiments to be described hereinbelow
, wherein radius rd is 0.82 ra.
To design a specific optical waveguide filament embody-
ing the features of the present invention, the following well
known factors should be considered. The axial composition is
chosen to satisfy numerical aperture, attenuation and dispersion
requirements. The barrier layer composition is chosen to meet
sinterability and refractive index requirements. As taught in the
aforementioned Olshansky et al. patent, the refractive index of
the barrier layer should be equal to or less than that of the
tubular starting member which forms the filament cladding. The
-- 10 --
115~4~7
tubular ~tarting member has a high $ilicA content and may consist
of pure SiO2 or ~iO2 doped with B2O3 or the like.
The ~hapes of curves 52 and 60 are conventionally
determined by the alpha profile shape seIected for the particular
waveguide. As shown in Figure l, these curves would have COII-
ventionally terminated at the core-barrier layer interface at
`~ oxide concentration levels determined by the barrier layer compo-
sition. The optical waveguide filament of the present invention
; differs from the prior art in that, although the power law curves
, 10 52 and 60 are determined by the alpha profile shape selected for
i~ ~ the waveguide, these curves are interrupted at the radius rd.
-` More specifically, the concentration Cp of P2O5 in weight percent
,~ at radius r between the filament axls and radius rd is described
by the equation
Cp = P + (H-P) [l-(r/ra)a] (2)
,
where H is the predetermined axial concentration of P2O5 and P is
J, the value which the concentration of P2O5 would achieve if the
curve 52 were extended to radius ra. It is noted that the value
~' P is greater than the concentration of P2O5 in the barrier layer.
The value of Cp at radius rd is G as shown in Figure 4.
The concentration CG of GeO2 in weight percent at radius
r between the filament axis and radius rd is described by the
equation
' CG = R + (D-R) [l-(r/ra)a] (3)
where D is the predetermined axial concentration of GeO2 and R is
the value which the concentration of GeO2 would achieve if the
curve 60 were extended to radius ra. It is noted that the value
R is less than the concentration of GeO2 in the barrier layer.
The value of the GeO2 concentration curve 60 at radius rd is C as
shown in Figure 4. The value C can be any GeO2 concentration
equal to or less than the barrier layer concentration. To minimize
llS~457
discontinuities, both line 62 and point C are preferably equal
to the barrier layer concentration.
For equations 2 and 3 the term is the refractive
index gradient determining term appearing in equation 1. Curve
54 represents an increase in the P2Q5 concentration between radii
ra and rd which follows a gradient steeper than the power law
gradient of curve 52. A slight grading between the slope of
curve 54 and that of curve 52 in the region of point G is pre-
ferred to avoid possible discontinuities in the refractive index
profiles; however, the diffusion of P2O5 tends to smooth out the
concentration profile in this region. Curve 50 is graded from
the barrier layer level of B2O3 to zero. Curves 50 and 54 may
be linear, or they may curve slightly so that their second deriva-
tives are negative or positive, ie., either of these curves may
have a slight bow downward or upward as viewed in Figure 4. As
illustrated in Figure 4, curves 5~ and 54 preferably follow a
core power law profile shape wherein the exponent is about 2.
The balance of the filament composition consists essentially of
si2
The optical waveguide filament of the present invention
is formed by depositing the barrier layer on the inner surface of
a bait tube and thereafter depositing the core material on the
surface of the barrier layer. Generally, both the barrier layer
and the core material are formed by depositing a plurality of
layers of oxides. Any one of the many well known methods of
forming glassy deposits on the inner surface of a bait tube may
be employed. For examples of such deposition processes, see U.S.
Patents Nos. 3,823,995 and 3,711,262, and the publications:
MacChesney et al., Appl'ied Phys'ic's'Let't'e'rs, Vol. 23, No. 6,
30 September 15, 1973, p. 340, MacChesney et al., Proceedings of the
IEEE, Vol. 62, No. g, September 1974, page 1278, and Jaeger et al.,
1~5~457
Bulletin of the American Cer~mi-c $ociety, Vol. 55, No. 4, April
1976, page 455. In such pxocesses, glass layers are deposited
on the inner surface of the glass bait tube to yield a glass
preform which is ultimately drawn into a filament.
Referring to Figure 5, there is a schematic illustra-
tion of an apparatus for forming the barrier layer and core
material on the inner surface of bait tube 70. Heating means 72
is caused to move relative to tube 70 as indicated by arrow 74.
Tube 70 rotates about its axis as indicated by arrow 76. Reac-
tants flow into and through tube 70 as indicated by arrow 78.Each traverse of heating means 72 in the direction of arrow 78
causes a layer of uniform composition to be deposited. A pre-
determined number of passes of the heating means along the tube
is required to form the barrier layer and the core portion of the
preform. As used herein, the phrase "pass fraction" means the
number of a particular pass of the heating means during the
formation of the core portion divided by the total required number
of passes required to form the core portion, i.e., that portion
of the filament between radius ra and the filament axis rO. If
50 passes of the heating means were required to form the core,
the pass fraction of the tenth pass would be 0.2, and that of the
fiftieth pass would be 1Ø The radius r is related to the pass
fraction x by the equation
r = ra ~ (4)
where ra is the core radius. Thus, the previously mentioned range
of rd as being between 0.75 ra and 0.90 ra would correspond to a
pass fraction between 0.2 and 0.43.
The bait tube may be formed of pure SiO2 or SiO2 doped
with one or more oxides including B2O3. The reactants necessary
to form the oxide layers in bait tube 70 include oxygen and com-
pounds containing the elements necessary to form the desired
- 13 -
-~`" 115~7
oxides. The oxides ~iQ2, B2O3~ P2Q5 and GeQ2 can be deposited
by emplo~ving the reactants SiC14, BC13, ~OC13 and GeC14, respec-
tively. This list of reactants is intended to be exemplary, it
being well known that other reactants can be employed to form
the listed oxides.
Numerous reactant deIivery systems known in the prior
art are suitable for deIivering reactants 78 to tube 70. Reference
is made in this regard to the teachings of U.S. Patents Nos.
3,826,560, 4,148,621 and 4,173.305 and U.S. Patent 4,212,663,
"Reactants Delivery System for Optical Waveguide Manufacturing"
filed January 26, 1978 by M. Aslami, issued July 15, 1980.
The relative flow rates of the reactants required for
forming the filament represented by the composition diagram of
Figure 4 are shown in Figure 6, wherein "Flow Rate" is plotted on
a log scale. The same letters that are used in Figure 4 to repre-
sent concentratlon values are primed in Figure 6 to represent the
corresponding flow rates necessary to provide those concentration
values.
The flow rates of all reactants are held constant to
form the barrier layer.
To form the outer region of the core material between
the barrier layer and the core break-point located at pass frac-
tion xd corresponding to radius rd, the flow rates are controlled
as follows. The flow rate of GeC14 is either terminated after
formation of the barrier layer, or it is maintained at some
constant value up to the flow rate employed to form the barrier
layer. The flow rate of BC13 is decreased from level Jl to zero
flow at pass fraction Xd. The flow rate of POC13 is increased
from level F' to level G'.
The inner portion of the core material is formed in the
following manner. The flow rate FG of GeC14 is increased from
- 14 -
" llS~4S~
point C' t~ point D' according to the e~uation
FG = R' + (D'-R') [l-tl-x) ~ ] (5)
where xd < x < 1Ø Flow rate R' is a value below the barrier
layer value of GeC14. Flow rate C', which is the value of F~
when x = Xd, is a value of GeC14 within the range of zero through
the barrier layer level of GeC14. At the same time, the flow
rate Fp of POC13 is increased from level G' to level H' according
to the equation
Fp = P + (H'-P') [l~ x)a/2] (6)
where xd < x < 1Ø Flow rate P' is a value above the barrier
layer flow rate of POC13. Flow rate G' is the value of Fp when
X = Xd.
The flow rate of SiC14 may remain constant throughout
the entire deposition process.
The following is a typical example of the formation of
an optical waveguide filament in accordance with the present
invention. A silica substrate tube containing about 4 wt. percent
B2O3 and having an outside diameter of 25 millimeters and a wall
thickness of about 1.3 millimeters is mounted in a deposition
lathe well known in the art. The constituent reactants SiC14,
BC13, GeC14, and POC13 are delivered to the substrate tube by a
chemical vapor deposition system well known to one familiar with
the art; specifically, reference is made in this regard to the
teachings of the above mentioned Aslami U.S. Patents Nos.
3,826,560, 4,217,663, 4,148,621 and 4,173,305. Illustrative
parameters of the process of this example have been mathematically
calculated and estimated to be as set out in Table I following.
115~45~
TABLE ~
~eactants
Flow Rates in g/min
~iC14 GeC14 POC13 BC13
Barrier Layer 0.83 0.041 0.009 0.026
First Core Pass 0.83 0 0.009 0.026
Pass Fraction 0.33 0.83 0.02 0.059 0
Pass Fraction 1.0 0.83 0.243 0.110 0
For the example of Table I the oxygen provided for re-
action was about 1000 sccm. The forming temperature was about
1880C during the deposition of the barrier layer, and it then
decreased linearly to 1720C at pass fraction x = 1. The trans-
verse rate of layer application was about 15 cm/min for both the
barrier layer and core; the barrier layer being formed by 10
passes or layers while the core was formed by 70 passes. As used
herein the term "core pass" means the number of the particular
pass being made by the burner during the formation of the core.
The flow rate of BC13 was decreased linearly from the barrier
layer value at point J' of Figure 6 to zero on the 23rd core pass
20 (pass fraction equals 0.33). The flow rate of GeC14 remained at
zero until the 23rd pass, after which it was increased to its
maximum value in accordance with equation 5. The flow rate of
POC13 was increased linearly from the barrier layer level at point
F' to 0.059 g/min on the 23rd core pass. The flow rate of POC13
was thereafter increased to its maximum value in accordance with
equation 6. In the determination of the flow rates of GeC14 and
POC13 in accordance with equations 5 and 6, the term "~" was set
equal to 1.975 and the term "x" was varied betweer. 0.33 and 1.0
depending upon the burner pass number.
The resultant preform was then collapsed under pressure
in accordance with the teachings of U.S.Patent No. 4,154,591.
During collapse, the preform temperature was increased from
- 16 -
` -
115~4~7
2000C to 250QQC during five burner passés, while the preform was
rotated at a speed of ~0 rpm. The resultant solid preform was
then mounted in a drawing apparatus as well known in the art, the
end thereof heated to a temperature of about 1900C and drawn
into an optical waveguide filament having the cross-sectional
profile as illustrated in Figure 4, except that segment 62 was
zero, and a DMD curve as illustrated by Figure 7, measured at
799 nm. The resulting optical waveguide had an outside diameter
of about 125 micrometers,a core diameter of about 60 micrometers,
and a barrier layer thickness of about 5 micrometers. The silica
cladding containing 4 wt. % B2O3. The barrier layer contained
6 wt. ~ B2O3, 1 wt. ~ GeO2 and 2 wt. % P2O5 with the balance being
silica. At radius rd, which was 24.2 micrometers, the concentra-
tion of GeO2 was O.S wt. % and that of P2O5 was about 8 wt.%.
The axial composition of the core was about 8 wt. % GeO2, about
16 wt. % P2O5 and about 76 wt. % SiO2.
Another optical waveguide referred to as filament No. 2
was made in accordance with the previous example except that the
value of ~ was set at 1.98 instead of 1.975, all other parameters
remaining unchanged. The DMD curve for the second filament,
measured at 799 nm, is illustrated in Figure 8. The cross-
sectional concentration profile of this filament was similar to
that described above for filament No. 1.
Filament No. 3 was made in accordance with the foregoing
description for making filament No. 2 except that the flow of
.GeC14 was maintained at the barrier layer level of 0.041 g/min
during the formation of the outer region of the core, and point
c' of segment C'D' of Figure 6 was also 0.041 g/min. Thus, the
resultant optical waveguide filament had a cross-sectioned profile
as illustrated in Figure 4.
- 17
liS~ 7
Table II ~ets forth some of the phy~ical and optical
charactexistics of the filaments of ~xamples 1, 2 and 3.
TABLE II
~ilament No.
1 2 3
a - value 1.975 1.98 1.98
core diameter 59 ~m 60.5 ~m 64.0 ~m
radius rd 24.2 ~m 24.8 ~m 26.2 ~m
NA (100 s) 0.208 0.209 0.207
10 Attenuation (dB/km)
820 nm 2.9 3.7 2.5
900 nm 2.0 2.8 1.9
1060 nm 1.6 2.3 1.0
Bandwidth (MHz)
900 nm 630 1390 1024
1300 nm 1060 700 1082
Length (km) 1.12 1.12 1.16
Note that the DMD curves for filaments 1 and 2 (Figures
7 and 8) are virtually straight. The tilt to the DMD curve of
Figure 7 indicates that filament No. 1 exhibits high bandwidths
at wavelengths greater than the DMD measurement wavelength of
799 nm. Indeed, Table II shows that filament No. 1 exhibits a
higher bandwidth at 1300 nm than at 900 nm. It is noted that a
perfectly horizontal DMD trace would manifest a filament whose
bandwidth is optimized at 799 nm, the wavelength of measurement.
The DMD trace of filament No. 2 (Figure 8) is more nearly hori-
zontal, thus indicating that the bandwidth thereof is optimized
near 799 nm. Table II shows that the bandwidth of filament No. 2
is 1390 MHz at 900 nm. It can thus be seen that the bandwidth
can be optimized at a desired wavelength by controlling the index
gradient factor . The greater than one GHz bandwidths which were
obtained for filaments made in accordance with the present inven-
tion are almost twice as great as the bandwidths exhibited by
filaments having oxide concentrations of the type represented by
Figure 1.
- 17a -
