CA1151457A - High bandwidth optical waveguide - Google Patents

Abstract

Abstract of the Disclosure An optical waveguide filament comprising a cladding layer, a core and a barrier layer disposed between the core and cladding. The barrier layer comprises silica doped with B2O3, P2O5 and GeO2. The core consists of an inner region and an outer region, the boundary between which is referred to as the core break-point. In the outer region of the core, the concentration of B2O3 de-creases linearly from its barrier layer level to zero at the core break-point, the P2O5 increases at a rapid rate from the barrier layer level to a first concentration level at the core break-point and the GeO2 remains at a constant value between zero and the barrier level. In the outer region of the core, the GeO2 increases from some level up to the barrier layer level to a greater value at the filament axis in a power law fashion. The P2O5 also increases in a power law fashion from the value thereof at the core break-point to a higher value at the filament axis, the increase in concentration of P2O5 in the outer core region increasing at a rate greater than the concentration of P2O5 would increase if the power law P2O5 gradient extended into the outer core region.

Description

``` 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~.

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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-.

~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 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 ~ ~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 .. . .. ... .. . . . .. . .. . .. .... .. . . . .. .. . . . . . . ... ..... .. . .. . . . . .

`` ` 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.

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
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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 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 -~`" 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 " 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 ` -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.

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.

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.

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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 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 refraction 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 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 between the filament axis and a radius rd that is less that the radius ra of said core 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 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.

2. A filament in accordance with claim 1 wherein said first oxide is P2O5.

3. A filament in accordance with claim 2 wherein said core also contains GeO2, the concentration of said GeO2 substantially following a power law curve between radius rd and the filament axis.

4. A filament in accordance with claim 3 wherein said barrier layer comprises 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.

5. A filament in accordance with claim 4 wherein the concentration of said GeO2 between radii rd and ra is constant and equal to or less than the barrier layer level of GeO2.

6. A filament in accordance with claim 5 wherein said core further comprises B2O3, the quantity of B2O3 decreasing within said core from the barrier layer level to zero at said radius rd.

7. A filament in accordance with claim 1, 2 or 6 wherein said radius rd is located between 0.75 and 0.9 times said core radius ra.

8. A filament in accordance with claim 1, 2 or 6 wherein the index of refraction of said barrier layer is equal to or less than that of said cladding layer.