HIGH BANDWIDTH FIBER AND METHOD OF FORMING THE SAME

Abstract: The data carrying capacity of an optical fiber (38) drawn from a glass preform (22) is enhanced and optimized by effecting refractive index changes by preform rotation during drawing.

Description: HIGH BANDWIDTH FIBER AND METHOD OF FORMING THE SAME This invention relates to a high, bandwidth optical fiber of high data-carrying capacity and a method for forming the same. Optical fibers employed for lightwave communi¬ cation are well-known in the art. The many advantages afforded by the use of light waves for signal transmission purposes over electrical waveforms of energy are also well- known and have led to increasing substitution of optical systems for electrical systems. As background for the provided invention the following will be noted. It is well-known that light entering an optical fiber does not propagate randomly as a continuum but rather is channeled into discrete modes having unique wave patterns. If the optical fiber has an index of refraction which varies radially from a maximum at the core center to a minimum at the cladding interface, in accordance with an exponent α defining a roughly parabolic slope of the gra¬ dient, the bandwidth or information carrying capacity thereof is greatly increased. The latter fiber is known as a graded index fiber. In graded index fibers, incident light rays apparently do not propagate in modes following sharp zig-zag paths but define substantially sinusoidal paths for pro- pagating along the fiber length; oblique rays or modes in such fibers similarly follow helical paths. In any event, graded index fibers serve to minimize the effects of modal dispersion in light waves traversing the fiber. The degree to which dispersion is minimized, with resulting bandwidth enhancement, is dependent upon the index of refraction profile possessed by the specific fiber. In general, graded index fibers have an intrinsic superiority over step index fibers in information carrying capacity, . _* ' 200 times the capacity, as the gradient of the index change has a constant effect of refocusing the light, i_.e_. , reducing the dispersion. Marcuse in his paper Multimode Fiber With Z- Dependent α - Value appearing in Vol. 18, No. 13 of Applied Optics, July 1, 1979 discloses the dependence of the rms pulse width response of a graded index multimode fiber on the index of refraction profile of such fiber. Also, this paper discloses an extremely pronounced minimum rms pulse width, response at a specific α value. This paper further discloses that the optimum pulse width response is achiev¬ able for fibers with nonoptimum α values, if the fiber α varies slowly along the fiber length and deviates on an average by equal amounts to either side of its (constant) optimum value. Weirholt in Vol. 15, No. 23 of Electronics Letters, November 8, 1979, proposes a high bandwidth fiber geometry with a radially varying α profile for achieving an almost flat rms spectral reponse over a spectral region of several hundred nanometers. Formation of such model is suggested by rigorous control the formation of the α profile through appropriate doping procedures. Weirholt suggests an overcompensated or lower α in the central fiber region and an undercompensated or higher α in the peripheral region. In neither Marcuse nor Weirholt is there sugges¬ tion of α optimization by a simple twisting technique during fiber drawdown. In accordance with the invention herein provided, the data carrying capacity of a graded index fiber is increased by preform rotation in the course of fiber draw- ing. As a result of such rotation the density of the drawn fiber core is progressively reduced radially of the fiber with an attendant substantially precise lowering in re¬ fractive index. Thus, by controlled preform rotation, the index gradient exponent α of a graded index fiber may be altered to approach or attain the optimum α, thereby en¬ hancing the refocusing effect of the fiber and hence its bandwidth for greater data carrying capacity. It is be¬ lieved that the shearing action effected by relative twist¬ ing action between the preform and fiber in the plastic neck-down preform region stretches the inter-atom bonds such as silicon-oxygen bonds, which then are frozen in during the rapid fiber cooling phase of drawing, thereby progressively lowering the refractive index radially of the drawn fiber. As this invention provides fibers with rotation induced lowering of the refractive index, the method dis¬ closed must of necessity lower the α of graded index pre¬ forms, and therefore is useful with preforms which have an α higher than the final desired α. Thus, preform index of refraction gradients may be lowered to an optimum during fiber drawing by the rotation-induced lowering of the drawn fiber refractive index as will hereinafter be apparent. The prior art has previously expended considerable effort in endeavoring to effect light mode velocity equaliza¬ tion by causing mode conversion, as in Miller et al. 3,687,51 As a result, the light energy of the various modes present spend roughly equal amounts of time in different modal con¬ figurations and arrive at the fiber end more nearly after the same average time. The Miller et al. patent deals with a step index fiber formed with geometrical irregularities by moving or "jiggling" a preform in the course of fiber drawing for accomplishing the desired mode conversion OMPI results. Miller et al. do not describe a change in core refractive index. Marcatili U.S. patent 3,666,348 discloses preform control during fiber drawing to reduce unguided mode for- mation and light loss in a step index fiber. Kaiser U.S. patent 3,969,016 discloses mode con¬ version by heated roller engagement with drawn fibers, relying on fiber axis shifts in the step index fibers processed. Other patent art dealing with mode conversion in step index fibers comprises Presby U.S. patent 3,912,478 in which heat is employed for imparting geometrical changes for mode-conversion purposes. Marcuse U.S. patent 3,909,110 discloses the build- ing of core index luctuations into the preform by employing modified doping procedures. Olshansky U.S. patent 4,163,601 discloses the formation of perturbations in graded index fibers for mode conversion purposes by employing noncircular draw wheels. In the foregoing, geometrical changes are imparted to the light waveguides for mode conversion purposes. There is no suggestion of the use of rotation to lower the re¬ fractive index of a fiber for purposes of attaining a desired change in the α or index of refraction profile. It is an object of this invention, therefore, to provide a high bandwidth optical fiber which is formed by superimposing an index of refraction profile on the profile of the preform so as to obtain a more desired final index. It is another object of this invention to provide high bandwidths in optical fibers by controlled drawing of the same from a preform. It is another object of this invention to provide a method of precisely lowering the index of refraction profile of a fiber drawn from a preform so as to approach a desired index by rotating the preform while drawing such fiber therefrom. It is a further object of this invention to pro¬ vide optical fibers which are tailored to efficiently conduct light of specific wavelengths. The above and other objects of this invention will become more apparent from the following detailed discussion when read in the light of the drawing and appended claims. In one embodiment of the provided invention a graded index glass preform having an index of refraction profile higher than a desired optimum is heated and rotated during drawing of an optical fiber therefrom. The preform rotatio generates a shearing action at the neck down region whereat the formed fiber is drawn from the preform. The shearing action results in a lowering of the glass density which progressively decreases from the fiber center radially outwardly in direct proportion to the increased shearing action occurring during drawing. A concomitant with the lowering of the glass density is a lowering of the index of refraction. As a result of the simultaneous fiber-drawing and preform rota- tion, a straight-line, rotation-induced lowering of the index of refraction occurs from the center of the fiber radially outwardly. By controlled preform rotation, a glass fiber may be formed with a desired, substantially precise index of refraction profile whereby the fiber may be tai- lored for optimum data-carrying capacity as will hereinafter be explained in greater detail. For a more complete understanding of this inven¬ tion, reference will now be made to the drawing wherein: Fig. 1 is a graph from the aforementioned prior article of Marcuse illustrating minimum pulse width response obtainable as a function of a specific index of refraction profile in a graded index fiber; Fig. 2 is a graph showing the straightline re¬ lationship between the reduction in index of refraction and radial distance in a fiber subjected to the rotational process employed in drawing a fiber from a preform in accor- " K Tfr OMPI T A TIC© dance with the teachings of this invention; Fig. 3 is a graph schematically illustrating in solid line a typical relationship between the index of re¬ fraction and radial distance from the fiber center in a conventional graded index fiber; the radial reduction in index of refraction of Fig. 2 is graphically subtracted from the solid line curve resulting in the dotted line curve illustrative of a modified α resulting in a fiber after the invention of this application has been carried out; Fig. 4 schematically illustrates apparatus which may be employed for purposes of rotating a preform in the course of fiber draw down when forming fibers made in accordance with this invention; and Fig. 5 is a schematic representation of a chart which may be employed in the course of carrying out the method hereinafter disclosed for purposes of forming fibers made in accordance with this invention. The aim of this invention is to increase the data carrying capacity of optical fibers by increasing the fiber bandwidth. Such bandwidth increase may be effected in both step index fibers and graded index fibers. In step index fibers the light modes progress in "steps" comprising reflections within the fiber core with the angles of reflection being equal to or greater than " the fiber critical angle. In step index fibers, bandwidth increase is attained by causing the constituent light modes to couple with each other so that their respective velocity- distance products average out. As a result the higher- angle, zig-zag light modes couple with each other so that their respective velocity-distance products average out. The higher angle zig-zag light modes and the lower, more axial modes result in light pulses with reduced dispersion characteristics, thereby allowing more information to be conveyed by such fiber. Although mode coupling increases the bandwidth in step index fibers, power losses are also increased as certain modes are coupled so as to radiate out of the fiber. Many prior art endeavors seek to effect mode coupling by geometric alteration of the fiber core so as to result in surface perturbations as exemplified by prior art patents above discussed. Graded index fibers are adapted to minimize modal dispersion while allowing multimode propagation in a fiber core. The index distribution is as the name implies "gra¬ ded" with a core center of highest refractive index surroun- ded by cylindrical segments having indicies of refraction which progressively decrease in relation to the radial distance from the center of the fiber. The net result is that the concentric fiber sections function similarly to • a series of light-bending lenses. They bend angularly pro- pagated light modes toward the fiber axis. Thus a higher order light mode propagating at an angle relative to the axis of a graded index fiber is continually bent toward the fiber axis by the index profile of lens-like structure as the light mode proceeds along the fiber length. Such higher order mode still travels a greater distance than the cen¬ tral, more axial modes. However, as most of the path of the higher order made comprises fiber material of lower index material its rate of passage is faster than if propagating on the fiber central zones of higher inde . The degree to which modal dispersion is controlled and bandwidth enhanced in graded index fibers is dependent upon preciseness of formation of the index of refraction profile of the fiber. The slope of this profile or gradient of the.index of refraction, defined by the aforenoted exponent α, is critical in the obtaining of optimum high bandwidth under normal circumstances. Referring to Fig. 1 of the drawing, which is reproduced from the aforenoted Marcuse article, solid line curve 10 shows index of refraction gradient value (α) plotted versus rms pulse width responses, from which it will be noted that the optimum α or index of refraction gradient OMPI . _ for the chosen parameters is 1.967, whereat minimum pulse width response is obtained. If α is changed by 1.3% from the optimum, the pulse width response doubles; a 17% change in α increases the pulse width by 1000%. Thus, pulse spreading and attendant light dispersion and signal attenu¬ ation are very sensitive to the index of refraction gradient. Curves 12, 13 and 14 in Fig. 1 depict the near optimum impulse responses which Marcuse states in his article are obtainable with a multimode fiber having gradually varying alternating values of α which deviate evenly on either side of the optimum value of α as long as the average value of α, averaged along the length of the . fiber, is close to the optimum value for a constant α, i.. _., the constant value providing the lowest pulse width. Thus, Marcuse has shown in his article that appropriate fluctuations of α to either side of the optimum along the fiber length can result in almost optimum state transmission conditions. In the present state of the art relating to fiber optics, the limiting factor in long distance light signal transmission is bandwidth as will be noted from the follow¬ ing. The best fibers now available operate at a 1.3μ wavelength where the fiber attenuation is approximately 1 dB/km and the dynamic range for an adequate signal-to-noise ratio is 55 dB (0 dB m to -55 dB m) . On the basis of loss alone, the fiber run between repeaters could be 55 km at 1 dB/km. However, the highest data rate possible over this distance would be 400M.Hz.km/55 km which is a 7.3M Hz bandwidth. This is a very low data rate, far below the 44.3M Hz consistent with the telephone rates currently required. However, the same fiber but with a 7G Hz.km bandwidth gives 127M Ez over the 55 km run, well in excess of the current requirements. The purpose of this invention is to facilitate attaining desired α profiles bandwidths in optical fibers than now practicable. This is accomplished in the illus¬ trated embodiment by the relatively simple expedient of rotation of the preform during fiber drawing to give a desired α profile along the fiber length. Glass fiber preforms employed in formin graded index fiber comprise extremely expensive starting boules from which the fibers are drawn. It is difficult and expensive to form such preforms with a precise index of re raction gradient for drawing fibers with a corresponding precise gradient by conventional processes. The present invention comprises a method for altering an imprecise actual gradient of the initial preform and forming therefrom a fiber having substantially the precise gradient needed for optimum light transmission. The refractive index profile in a standard graded index preform is given by: where n, - highest core index n~ = lowest clad index a = core radius r = radial distance from core center, n = refractive index at r. Equation 1 may be rewritten as by substituting for Δ and rearranging. OM?I Figure 3 of the drawing illustrates schematically the refractive, index decrease of the equations with in¬ creasing radial distance from the fiber central axis in the solid line curve. Thus, as the fiber radial location progresses from the fiber center toward "a," the index of refraction decreases from the maximum n, at the fiber center to the minimum n_ at the cladding interface. The foregoing equation states that the progression of change of the index n is a function of α, the gradient exponent. For minimum pulse dispersion α must hold closely to a specific value. One purpose of this invention is to demonstrate that a beneficial modification of a nonoptimized α can be brought about by superimposing a linear index gradient/radius function, the latter gradient resulting from preform rotation and- its concomitant shear-induced structure modification, upon the "built in" or actual gradient profile otherwise obtained in formation of the fiber. The rotational profile or index of refraction modification provided by this invention may be described by the equation: where n. = preform starting index of refraction - = greates reduction in index of refraction t nr = refractive index modification at any radius r Figure 2 of the drawings illustrates such linear function comprising index of refraction reduction increase with fiber radial distance. This reduction is subtracted from the solid line curve in Fig. 3. The resulting dotted line curve defines the index of refraction gradient of a fiber made pursuant to this invention. The combined effects of the standard and rota¬ tional profiles may be expressed by the following equation in which equations la and 2 are combined to give: In (§) The equivalent form of Equation.la, for compariso m (f> to which 3 reduces when n. = n_ , ι_.e_., no rotational index changes are induced. The superimposing of a linear function (of Fig. 2) on a roughly parabolic function (solid line curve of Fig. 3) results in a slightly varying, or hybrid, parabolic function Ldotted line curve of Fig. 3) whose exponent changes slightl with radius. Two examples of the effect of rotational gradient upon the standard compositional gradient will demonstrate this point. In these examples, an optimum α of 1.964 will be sought, and the starting extreme refractive indices of n χ = 1.465 of the glass core and n 2 = 1.450 for the fiber cladding are assumed, and a drawing speed of 20 meters per minute a typical drawing temperature i.e., about 2050°C. "W^ OMPI fc V/IPO " are also assumed as conditions present. EXAMPLE 1 In Example 1 the data of which is set forth below in Table 1, starting α of formation of 2.00Q is assumed in a graded index preform from which a fiber is drawn. It is desired that the fiber drawn therefrom be modified to have a rotation-induced change in α to 1.964 for optimum effici¬ ency in a specific light conveying application. Table 1 lists typical n or index of refraction values in column b and column c for such ideal and starting α values respectively. The index values of these two columns are listed as a function of the core radius ratio C^I, see column a. Column a indicates the radial location or the percentage of radial distance from the preform or fiber center at which the values of columns b, c, d, e and f are tabulated. Tabulated in column d of the table is the difference between the values, percentagewise, for the two gradients 2.000 and 1.964 at the radial locations of column a. Table 1 lists in columns e and f typical refrac¬ tive index exponent α* values expected from index of re¬ fraction decreases occasioned by two different preform rotations in the course of fiber drawing. As the index reduction values of column e are less than those of column f, the speed of rotation and resulting shearing action would be greater in drawing the fiber of column f than in drawing the fiber of column e, if both fibers are drawn from pre¬ forms having an original α of 2.000. In accordance with this invention each of the α' values of columns e and f may be obtained by employing equation 3 above set forth. In each of the tabulated values of columns e and f, the α' is not a single value across the fiber profile inasmuch as the shearing action resulting in reduced index of refraction is most pronounced at the fiber core outer periphery and progressively decreases as the fiber central longitudinal axis is approached. However, it is readily apparent from the tabulated data that even with the slight changes i gradient, the average α would be much closer to the optimum α than the starting value of 2.0Q0. It can be seen from column (e) that a maximum fiber core index depression of 0.00017 CO.012%) results in average α of 1.980, while a depression of 0.00032.CO.022%1 would provide an ideal α average of 1.964 as seen in column f. It will be appreciated that the bandwidth optimum need not necessarily coincide with the average optimum. This is so as the radial α drift comprising the superimposed, rotation- induced α depression extends radially from the fiber axis in a nonuniform manner. (a) (b) (σ) (d) (e) (f) S S α = 1.964 α = 2.000 α 2.000 α = 2.000 & (IDEAL) (STARTING) Δn r % (n χ - n 3 ) - 0.00017 (n 1 -n 3 )=0.00032 <!> n. n α' α' 0.1 1.464838 1.464851 0.0009 1. ,959 1. .929 0.2 1.464366 1.464403 0.0025 1. ,973 1. .956 0.3 1.463597 1.463656 0.0040 1. ,978 1. .960 0.4 1.462530 1.462610 0.0055 1. .981 1. .966 0.5 1.461169 1.461264 0.0065 1. ,984 1. .969 0.6 1.459518 1.459618 0.0069 1. ,985 1. .973 0.7 1.457574 1.457669 0.0065 1. ,986 1. .975 0.8 1.455349 1.455418 0.0047 1. ,987 1, ,976 0.9 1.452816 1.452862 0.0032 1. ,988 1. .977 1.0 1.450000 1.450000 0.0000 1.980 1.964 EXAMPLE 2 A starting preform α of 2.2Q0 is assumed, with the desired α value of 1.964 being targeted. Table 2 sets out the ideal and starting radial profiles, and the results of different rotational index depressions, as in Table 1. <!> n r n r «' α' ' 0.1 1.464838 1.464906 0.0046 1.813 1.766 0.2 1.464367 1.464507 0.0137 1.918 1.873 0.3 1.463597 1.463944 0.0237 1.973 1.930 0.4 1.462530 1.463011 0.0329 2.006 1.967 H σ. 0.5 1.461169 1.461749 0.0397 2.028 1.993 0.6 1.459518 1.460141 0.0427 2.045 2.012 0.7 1.457574 1.458175 0.0412 2.057 2.027 0.8 1.453400 1.458837 0.0301 2.067 2.039 0.9 1.452816 1.453116 0.0206 2.076 2.049 1.0 1.450000 1.450000 0.0000 As in Table 1, two rotational index depression values are used. In Table 2 a maximum index change of 0.0016 CO.110%) is seen to reduce the average α from 2.200 to 1.998, while a refractive index change of 0.0020 CO.138%) 5 achieves an average α of 1.962, quite close to the desired 1.964. Naturally, there is a bandwidth-reduction penalty in the greater range of α values present across the fiber core extending from 2.200 down to 1.964, compared with the less drastic α reduction from 2.200 to 1.998. It is obvious that 0 this penalty is minor when taken in reference to the start¬ ing α deviation from optimum present in the original preform The theoretical benefit of improving the α value from 2.2 to the optimum value is one of increasing the bandwidth from, say, 200 MH Z.km to 8GHZ.km; a 40 fold 5 improvement in data carrying capacity. The improvement gained from the smaller shift of an starting value of 2.0 is naturally less, being on the order of a four fold im¬ provement from 2GHz.km to 8GHz.km. The slight α variation about the mean induced Q through rotation limits the gains to some bandwidth below the optimum. Nonetheless, the gains are appreciable and simply achieved. Other benefits provided by the preform rotational treatment during fiber drawdown is an observed approximately 5 15% increase in fiber breaking strength due to silicon- oxygen bond elongation and resulting increase in ionicity, and a slight but tangible increase in numerical aperture. In the above two examples, the numerical aperture is in¬ creased from 0.209 (half acceptance angle of 12.07°) to Q 0.211 (12.20°) and 0.223 C12.86°) respectively for the rotational derived α values of 1,964 and 1,962. The larger angles allow more light modes to enter the fiber for signal conveying purposes. The absolute inability to obtain the desired α value at all radial positions simultaneously suggest the beneficial adoption of α value variation along the f ber OMP length. This is achieved by slightly varying the rotation speed during the drawing procedure in a cyclical fashion, thereby creating an α range of values disposed on either side of the optimum. The foregoing examples relate to graded index fibers. However, the foregoing technique of forming outer fiber zones of reduced density and index of refraction as the result of a compound shearing action caused by preform rotation during fiber drawing may also be employed for purposes of mode coupling in stepped index fibers. Periodic rotation of preforms during glass fiber draw down has resulted in step index fibers having reduced mode dispersion. Experimental fibers formed by sporadically rotating a preform at 1000 rpm on cycles of 2 seconds on and 3 seconds off and a drawing rate of 20 meters per minute had markedly lower numerical apertures and a modal distribution pattern devoid of axial modes. Such processed fibers indicate that the guided modes have been restricted to narrow windows in the fiber section which result in reduced mode dispersion and increased bandwidth. Rotating the preform about an eccentric axis will give rise to exaggerated surface perturbations on the fiber periphery if the same were desired for mode coupling pur¬ poses in the drawing of fiber from a step index preform. The high bandwidth graded index fibers of this invention, however, as above noted, neither require nor benefit from surface irregularities. The provided density reduction resulting from rapid axial rotation of the preform and simultaneous axial reduction in the drawn fiber is thus seen to impress a modified gradient on the "actual" gradient already present in the preform. The provided method of altering the index of refraction gradient may be employed to impose a desired gradient on fiber drawn from a preform having a gradient which is higher than the gradient desired. As the provided method can only lower the index gradient, preforms may be manufactured with an α value higher than the optimum desired in the fibers to be formed therefrom. Using the method herein provided, preforms of specific profile may have fiber drawn therefrom which is tailored for a specific use and possesses any of a variety of optimum α or index profiles. Thus, fibers may be tailored to provide minimum signal disperson by being designed for the optimum α in accordance with the light wavelengths to pass therethrough, as the longer the wavelength the lower the necessary α for optimum transmission. Thus, whereas an α of 1.967 may be desired for a light wavelength of .82μ, an α of 1.821 might be desired for a wavelength of 1.32u. It will be noted from Fig. 4, that a motor 20 for rotating preform 22 may be precisely controlled by con¬ troller 24 to impart a precise speed of rotation to preform 22 during the fiber drawing process. The preform 22 is secured in chuck 25 attached to support rod 26. Rod 26 has a bevel gear or worm section 28 for engaging a mating gear or worm 30 disposed on shaft 32 driven by motor 20. Preform 22 is heated in furnace 34. A neck down region 36 is illustrated in Fig. 4 whereat the twisting, shearing action effected by rotating the preform relative to the axially moving but nonrotating fiber 38 is effected. The desired speed of preform rotation may be arrived at after first determining the index profile of preform 22. A chart specific to a particular α such as that illustrated in Fig. 5 is then consulted, the latter chart is specific to a preform having an α of 2.2 and assumes rpm carried out at a typical drawing speed .£. , 20M/min. , at typical drawing temperatures, e_-g_. , 2050°C. Fibers drawn from such preform may have their profiles reduced to an α of as low as 1.5 by employing the line 40 depicting the straight-line relationship between α and speed of rotation of the preform. Thus assuming an α of 1.97 in desired for the drawn fiber, the chart of Fig. 5 indicates that the * TRE OMPI - 0 preform is rotated at 400 rpm to provide such reduced α, the chart of Fig. 5 is based on the constant axial draw speed, increased draw speed tending to uniformly reduce the fiber density across the fiber width. Inasmuch as light of a specific wavelength effects minimum pulse dispersion when propagating through a graded index fiber of a specific α or index profile, the provided invention enables fibers to be formed and tailored to pro¬ vide minimum loss and optimum efficiency in conveying light of a specific wavelength. The graded index preform on which a rotational profile is to be superimposed may be continuously rotated as in the manner above discussed in connection with the apparatu of Fig. 4 for effecting the lowering of the elevated α in the preform to a desired constant α in the drawn fiber. As an alternative process, the graded index preform may be inter¬ mittently rotated to effect a preform speed of rotation which will effect a lowering of the α in the respective length of the fiber to a value which is less than the desired <* to the same degree that the actual preform α of formation exceeds the desired α. As a result, the light modes tra¬ versing such fiber will encounter optimum or near optimum average transmission conditions as noted above in the discussion of the curves of Fig. 1. Fibers made from rotating preforms in accordance with this invention may also be formed by utilizing a combination of these alternate methods. The preform speeds of rotation employed to effect desired reduction in the index of refraction and impart desired index gradients in the drawn fibers may be readily ascertained empirically by those skilled in the art. It is thus seen that a high bandwidth, light- conveying fiber has been provided, as well as a method for forming the same. The method for forming such fibers is both, simple and flexible enabling fibers of desired index profiles to be readily formed by a simple step of preform rotation. In an obvious process modification the drawn - ϋRE OMPI S VΛPO fiber may be rotated relative to the preform to obtain the desired index reduction, the desired result being the shearing action in the neck down region. The foregoing has made apparent a number of modifications which may be made in the disclosed invention without departing from the ambit thereof. This invention is to be limited, therefore, only by the scope of the appended claims.

Claims: 1. An optical fiber comprising a glass fiber drawn from a preform and having a radial refractive index gradient in at least a portion of the length thereof; said gradient comprising the index of refraction profile of the preform modified by the index of refraction profile imposed thereon by effecting relative rotational motion between said fiber and the preform from which formed in the course of drawing said fiber. 2. The optical fiber of claim 1 in which said fiber is a modified step index fiber formed from a step index preform. 3. The optical fiber of claim 1 in which said fiber is a modified graded index fiber formed from a graded index preform. 4. The optical fiber of claim 1 in which the imposed profile is imposed at intervals along the fiber length. 5. An optical fiber having a high bandwidth comprising an optical fiber drawn from a graded index pre¬ form and having an index of refraction gradient imposed on the preformed gradient by effecting relative twisting action between said fiber and the preform from which drawn in the course of drawing said fiber. 6. The optical fiber of claim 5 in which said fiber is a modified graded index fiber formed from a graded index preform. 7. The optical fiber of claim 6 in which, said fiber has a modified parabolic index of refraction pro¬ file across the width thereof comprising a substantially parabolic profile of the preform upon which said imposed profile is superimposed. 8. The optical fiber of claim 1 or 5 in which the imposed gradient is substantially uniform over the fiber length. 9. An optical fiber drawn from a preform and having radial index of refraction gradients which differ in various portions along the length of the fiber; said gra¬ dients defining a desired average gradient; one of said gradients being formed by rotating said preform relative to said fiber in the course of drawing said fiber so as to impose an index of refraction profile linearly decreasing from the fiber center outwardly. 10. The fiber of claim 9 in which said one refractive index profile is imposed in a gradual manner. 11. The optical fiber of claim 2 in which said gradient is formed in accordance with the formula: I in which n = index of refraction imposed by twisting n.' = the starting index of refraction 3 = the lowest induced index of refraction r = radial position in the core a = core radius -W& X& y OMPI 12. In a method for forming an optical fiber from a preform the steps comprising drawing a fiber from said preform while heated, and twisting said fiber relative to the preform in the course of drawing said fiber from said preform so as to impose decreases of the refractive index of said fiber. 13. The method of claim 12 in which such relative rotation imposes progressive decreases of the refractive index of said fiber radially outward from the center of the fiber. 14. The method of claim 12 in which said relative twisting is imposed at intervals in the course of drawing said fiber from said preform. 15. The method of claim 12 in which said relative twisting is continuous in the course of drawing said fiber. 16. The method of claim 12 in which said relative preform twisting is continuous in the course of drawing first alternate continuous lengths of said fiber and perio¬ dic in the course of drawing second alternate continuous lengths of said fiber. 17. The method of claim 12, 13, 14, 15 or 16 in which said optical fiber is drawn from a preform having a uniform index of refraction across the width thereof. 18. The method of claim 12, 13, 14, 15 or 16 in which said optical fiber is drawn from a preform having a graded index across the width thereof. "SJ E OMPI r 19.. A method for modifying the index of refrac¬ tion profile of a graded index optical fiber preform to a desired, lower index of refraction profile in an optical fiber formed therefrom, comprising the steps of deter- mining the index of refraction profile in the preform, determining the degree to which the preform index of re¬ fraction must be reduced to obtain the desired index of refraction profile in the optical fiber to be drawn there¬ from; reducing the index of refraction by effecting relativ twisting between the drawn fiber and the preform in the preform neck down region at a speed of rotation which will effect a reduction in the preform index of refraction to substantially the desired lower index of refraction profile. 20. A method for modifying a preform index of refraction gradient value in an optical fiber drawn there¬ from for purposes of attaining a lower gradient value in the fiber than in the preform, comprising twisting the preform relative to the drawn fiber in the neck down region of the preform in the course of the fiber draw down so as to reduce the original index of refraction gradient value of the preform, and controlling the speed of the preform rotation relative to the fiber for purposes of controlling the shearing action in the neck down region whereby a desired lowering of the index of refraction gradient value of the preform is effected in the resulting fiber index of re¬ fraction. 21. A method for lowering the index of refraction of a preform in the optical fiber drawn therefrom comprising effecting relative rotation between such fiber and preform in the preform neck down region during the fiber draw down. "gUREA~7> OMPI 22. The method of claim 21 in which, said relative rotation is carried out continuously during fiber draw down. 23. The method of claim 21 in which said relative rotation is carried out at intervals during fiber draw down. 24. The method of claims 11, 17 and 18 in which the drawn fiber is a step index fiber and is subjected to rapid cooling following drawing from the preform whereafter such fiber is subjected to surface heat treatment to anneal out the surface effects of rotation, whereafter the surface of the fiber has substantially the same index as the start¬ ing preform thereby satisfying those mode mixing and minimizin high order mode loss criteria for high bandwidth step index fibers.

Inventors: HODGE MALCOLM HELLYER, HODGE, MALCOLM, HELLYER
Applicants: TRW INC, TRW INC.
Application year: 1981
Main classification: C03B37
IPC codes: C03B37, G02B6