Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/028—Optical fibres with cladding with or without a coating with core or cladding having graded refractive index
  • G02B6/0288—Multimode fibre, e.g. graded index core for compensating modal dispersion
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
  • C03B37/01—Manufacture of glass fibres or filaments
  • C03B37/02—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor
  • C03B37/025—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor from reheated softened tubes, rods, fibres or filaments, e.g. drawing fibres from preforms
  • C03B37/027—Fibres composed of different sorts of glass, e.g. glass optical fibres
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2203/00—Fibre product details, e.g. structure, shape
  • C03B2203/02—External structure or shape details
  • C03B2203/06—Axial perturbations, e.g. twist, by torsion, undulating, crimped
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2203/00—Fibre product details, e.g. structure, shape
  • C03B2203/10—Internal structure or shape details
  • C03B2203/18—Axial perturbations, e.g. in refractive index or composition
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2203/00—Fibre product details, e.g. structure, shape
  • C03B2203/10—Internal structure or shape details
  • C03B2203/18—Axial perturbations, e.g. in refractive index or composition
  • C03B2203/20—Axial perturbations, e.g. in refractive index or composition helical
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2203/00—Fibre product details, e.g. structure, shape
  • C03B2203/10—Internal structure or shape details
  • C03B2203/22—Radial profile of refractive index, composition or softening point
  • C03B2203/26—Parabolic or graded index [GRIN] core profile
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2203/00—Fibre product details, e.g. structure, shape
  • C03B2203/36—Dispersion modified fibres, e.g. wavelength or polarisation shifted, flattened or compensating fibres (DSF, DFF, DCF)
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2205/00—Fibre drawing or extruding details
  • C03B2205/06—Rotating the fibre fibre about its longitudinal axis
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/02214—Optical fibres with cladding with or without a coating tailored to obtain the desired dispersion, e.g. dispersion shifted, dispersion flattened
  • G02B6/02285—Characterised by the polarisation mode dispersion [PMD] properties, e.g. for minimising PMD
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
  • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
  • G02B6/03622—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 2 layers only
  • G02B6/03627—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 2 layers only arranged - +

Definitions

• 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.
• 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 ⁇ defining a roughly parabolic slope of the gra ⁇ 130, the bandwidth or information carrying capacity thereof is greatly increased.
• the latter fiber is known as a graded index fiber.
• 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.
• graded index fibers serve to minimize the effects of modal dispersion in light waves traversing the fiber.
• 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.
• the data carrying capacity of a graded index fiber is increased by preform rotation in the course of fiber draw- ing.
• the density of the drawn fiber core is progressively reduced radially of the fiber with an attendant substantially precise lowering in re ⁇ fractive index.
• the index gradient exponent ⁇ of a graded index fiber may be altered to approach or attain the optimum ⁇ , thereby en ⁇ hancing the refocusing effect of the fiber and hence its bandwidth for greater data carrying capacity.
• 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.
• 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 ⁇ 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.
• 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.
• a straight-line, rotation-induced lowering of the index of refraction occurs from the center of the fiber radially outwardly.
• 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.
• 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-
• 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 ⁇ 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.
• 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.
• 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.
• bandwidth increase is attained by causing the constituent light modes to couple with each other so that their respective velocity- distance products average out.
• 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.
• 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 concentric fiber sections function similarly to • a series of light-bending lenses. They bend angularly pro- pagated light modes toward the fiber axis.
• 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.
• 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 ⁇ , is critical in the obtaining of optimum high bandwidth under normal circumstances.
• solid line curve 10 shows index of refraction gradient value ( ⁇ ) plotted versus rms pulse width responses, from which it will be noted that the optimum ⁇ or index of refraction gradient
• OMPI . _ for the chosen parameters is 1.967, whereat minimum pulse width response is obtained. If ⁇ is changed by 1.3% from the optimum, the pulse width response doubles; a 17% change in ⁇ 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 ⁇ which deviate evenly on either side of the optimum value of ⁇ as long as the average value of ⁇ , averaged along the length of the . fiber, is close to the optimum value for a constant ⁇ , i.. _., the constant value providing the lowest pulse width.
• ⁇ i.. _.
• the best fibers now available operate at a 1.3 ⁇ 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) .
• the fiber run between repeaters could be 55 km at 1 dB/km.
• 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.
• 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 ⁇ 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 ⁇ 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:
• Equation 1 may be rewritten as
• 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.
• 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.
• Example 1 the data of which is set forth below in Table 1, starting ⁇ 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 ⁇ 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 ⁇ 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 ⁇ * values expected from index of re ⁇ fraction decreases occasioned by two different preform rotations in the course of fiber drawing.
• 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 ⁇ of 2.000.
• each of the ⁇ ' values of columns e and f may be obtained by employing equation 3 above set forth.
• the ⁇ ' 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.
• the average ⁇ would be much closer to the optimum ⁇ than the starting value of 2.0Q0.
• the theoretical benefit of improving the ⁇ 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 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 ⁇ values of 1,964 and 1,962.
• the larger angles allow more light modes to enter the fiber for signal conveying purposes.
• OMP length This is achieved by slightly varying the rotation speed during the drawing procedure in a cyclical fashion, thereby creating an ⁇ range of values disposed on either side of the optimum.
• the foregoing examples relate to graded index fibers.
• 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.
• step index fibers Periodic rotation of preforms during glass fiber draw down has resulted in step index fibers having reduced mode dispersion.
• 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.
• preforms may be manufactured with an ⁇ value higher than the optimum desired in the fibers to be formed therefrom.
• preforms of specific profile may have fiber drawn therefrom which is tailored for a specific use and possesses any of a variety of optimum ⁇ or index profiles.
• fibers may be tailored to provide minimum signal disperson by being designed for the optimum ⁇ in accordance with the light wavelengths to pass therethrough, as the longer the wavelength the lower the necessary ⁇ for optimum transmission.
• an ⁇ of 1.967 may be desired for a light wavelength of .82 ⁇
• an ⁇ of 1.821 might be desired for a wavelength of 1.32u.
• 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.
• FIG. 5 A chart specific to a particular ⁇ such as that illustrated in Fig. 5 is then consulted, the latter chart is specific to a preform having an ⁇ 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 ⁇ of as low as 1.5 by employing the line 40 depicting the straight-line relationship between ⁇ and speed of rotation of the preform. Thus assuming an ⁇ of 1.97 in desired for the drawn fiber, the chart of Fig. 5 indicates that the
• 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.
• 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 ⁇ in the preform to a desired constant ⁇ in the drawn fiber.
• the graded index preform may be inter ⁇ mittently rotated to effect a preform speed of rotation which will effect a lowering of the ⁇ in the respective length of the fiber to a value which is less than the desired ⁇ * to the same degree that the actual preform ⁇ of formation exceeds the desired ⁇ .
• 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.
• OMPI S V ⁇ PO 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.

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• 1981
  • 1981-09-04 EP EP19810902537 patent/EP0087411A1/de not_active Withdrawn
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