FORM 2
The Patent Act 1970
(39 of 1970)
&
The Patent Rules, 2005

PROVISIONAL SPECIFICATION
(SEE SECTION 10 AND RULE 13)

TITLE OF THE INVENTION

“AN OPTICAL FIBER WITH REDUCED POLARIZATION MODE DISPERSION AND A METHOD OF FABRICATING AN OPTICAL FIBER WITH REDUCED POLARIZATION MODE”

APPLICANTS:

Name : Sterlite Technologies Ltd.

Nationality : Indian (An Indian Company)

Address : E1/E2/E3, MIDC, Waluj, Aurangabad.

The following specification describes the invention.

FIELD OF INVENTION
[001] Embodiments in general relate to optical fiber, and more particularly to optical fiber which is spun to reduce Polarization Mode Dispersion (PMD).

BACKGROUND OF INVENTION
[002] Optical fibers are inherently versatile as a transmission medium for all forms of information, such as voice, video and data. The primary object of telecommunication industry is to transmit large amount of information over longer distances in shorter period of time. In the recent years, certain improvements have taken place to improve the light carrying ability of the optical fibers.
[003] The light carrying ability of the optical fibers for communication is determined by attenuation loss or optical loss. The optical losses, that is, attenuation losses in the optical waveguide fiber are caused by many factors including absorption loss, scattering of light, and PMD, which in turn are caused by imperfect geometry of the optical fiber, stress and structural defects in the optical fiber.
[004] Conventionally, single mode optical waveguide fiber commonly used in communication systems is not purely single mode. Rather, two modes, with perpendicular polarizations, exist in the single mode optical waveguide fiber. These two polarizations form an orthogonal basis set. Any configuration of light that propagates through a single mode fiber can be represented by a linear superposition of these two modes.
[005] It is well understood that, in an optical waveguide fiber having perfect circular symmetry in both geometry and internal applied stress, the two polarization modes propagate with the same group velocity, wherein there is no time delay difference after traveling the same distance in the optical waveguide fiber. However, an optical fiber with imperfect circular symmetry, imperfections such as geometric deformation and stress asymmetry makes the two polarization modes to propagate with different velocities, wherein the velocities are a function of propagation constants. The difference between the propagation constants is termed as birefringence. The differential time delay between the two polarization modes is called PMD.
[006] The PMD is known to limit the high bit rate transmission in the communication system. A method of reducing PMD in single mode optical fibers involves imparting spin in the optical fiber while drawing the optical fiber from the optical fiber preform. The spin causes the internal geometric asymmetries and/or stress asymmetries of the optical fiber to rotate about the optical fiber''s axis along the optical fiber’s axis.
[007] Imparting spin in the optical fiber while drawing the optical fiber from the optical fiber preform is disclosed in US patent 5,298,047 (US’047) and US patent 5,418,881 (US’881). Both US’047 and US’881 disclose imparting spin in the optical fiber alternatively in clockwise and counterclockwise, in a substantially sinusoidal manner. However, it has been observed that the reduction in PMD achieved by imparting pure sinusoidal spin in the optical fiber is not sufficient, and hence improved reduction in PMD in optical fiber is desired.
[008] Attempts have been made to overcome the disadvantages of imparting pure sinusoidal spin in the optical fiber by imparting sinusoidal spin which is amplitude and/or frequency modulated. Such a method of imparting spin has been disclosed in US patent 5,943,466 and US patent 6,859,596. However, it is observed that if the optical fiber is spun with the spin functions as disclosed in the above method, the optical fiber during the drawing stage vibrates. The vibration leads to breakage of the optical fiber during drawing stage or may lead to uneven application of coating(s) being applied. Such vibration is observed to be more prominent at high optical fiber drawing speeds of the order of about 15 m/s or greater. It is further observed that, spinning the optical fiber with the spin functions as disclosed above leads to increased breaks in the proof testing process, which is done after or during the optical fiber drawing process. This increase in breaks of the optical fiber during the proof testing process has been found to be related to the vibration of fiber during the drawing stage. Additionally, this vibration of the optical fiber, as in Aeolian motion, may lead to production of resonant frequencies, which in turn are responsible for generation of defects or voids or bubbles in the optical fiber being drawn, which leads to breakage of optical waveguide fiber at the proof testing stage.
[009] Yet another method of reducing PMD in optical fiber is disclosed in US patent 7,310,974. In this method, spin function used to spin the optical waveguide fiber is a combination of four discrete spin rates which repeat itself in a periodic fashion after a certain length of optical fiber drawn and the spin rate of each spin function varies in a trapezoidal manner along the drawn length of the fiber.
[0010] It is observed that spinning the optical fiber in accordance with the prior art method disclosed above reduces the PMD in the optical fiber to a sufficiently low value, wherein a large fraction of the optical fiber has PMD reduced under a certain value. However, remaining fraction of the optical fibers deviate away from the mean value. For example, if the optical fibers is desired to have PMD under 0.15 ps/(km)1/2, about 60 to 70 % of fiber lengths will fall under this value, but remaining length has a PMD value greater than 0.15 ps/(km)1/2. Hence, a method that enable reduction of PMD under 0.15 ps/(km)1/2 of greater fraction of optical fiber length is desired.

OBJECT OF INVENTION
[0011] An object is to provide a method of spinning optical fiber to reduce Polarization Mode Dispersion (PMD).
[0012] Another object is to provide optical fiber with reduced PMD.
[0013] Still another object is to enable drawing of optical fiber at greater speed.
[0014] Yet another object is to enable drawing optical fiber at speed in the range of 15 meters/second to 25 meters/second.
[0015] Another object is to provide spin functions such that optical fiber can be drawn at speed in the range of 15 meters/second to 25 meters/second.
[0016] Yet another object is to provide a method of spinning the optical fiber such that the spinning of the optical fiber during the drawing stage does not have undesirable effect on the quality of the optical fiber being drawn.
[0017] Still another object is to reduce the optical fiber breakage during the drawing stage.
[0018] Still another object is to reduce any undesirable effect on the coating application while spinning the optical fiber.
[0019] Still another object is to reduce breakage of the optical fiber in proof testing stage.
[0020] Still another object is to provide a method of spinning the optical fiber such that the spinning of the optical fiber during the drawing stage does not result in vibrations in the optical fiber being drawn.
[0021] Still another object is to provide a method of spinning the optical fiber, such that PMD of the optical fiber is reduced below 0.15 ps/(km)1/2 for more than 95 % of the fiber population.
[0022] The other objects and advantages of the present invention will be apparent from the following description when read in conjunction with the accompanying drawings which are incorporated for illustration of preferred embodiments of the present invention and are not intended to limit the scope thereof.
STATEMENT OF INVENTION
[0023] Accordingly the invention provides an optical fiber having reduced Polarization Mode Dispersion. The optical fiber comprises spin imparted in the optical fiber along the length of the optical fiber about the axis of the optical fiber. The spin is imparted alternatively in clockwise and anticlockwise direction, wherein maximum number of spins imparted in the optical fiber in successive clockwise direction and anticlockwise direction are not equal, whereby, at least the spins imparted in the optical fiber in successive clockwise direction and anticlockwise direction are asymmetric.
[0024] There is also provided a method of fabrication an optical fiber having reduced Polarization Mode Dispersion. The method comprises imparting spin in the optical fiber along the length of the optical fiber about the axis of the optical fiber, wherein the spin is imparted alternatively in clockwise and anticlockwise direction, wherein maximum number of spins imparted in the optical fiber in successive clockwise direction and anticlockwise direction are not equal, whereby, at least the spins imparted in the optical fiber in successive clockwise direction and anticlockwise direction are asymmetric.
[0025] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

BRIEF DESCRIPTION OF FIGURES
[0026] This invention is illustrated in the accompanying drawings, through out which like reference letters indicate corresponding parts in the various figures. The embodiments herein will be better understood from the following description with reference to the drawings, in which:
[0027] FIG. 1 illustrates a spinning function, in accordance with embodiments as disclosed herein;
[0028] FIG. 2 illustrates a spinning function, in accordance with embodiments as disclosed herein;
[0029] FIG. 3 illustrates a spinning function, in accordance with embodiments as disclosed herein; and
[0030] FIG. 4 illustrates a spinning function, in accordance with embodiments as disclosed herein.

DETAILED DESCRIPTION OF INVENTION
[0031] The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
[0032] The embodiments herein achieve an optical fiber with reduced Polarization Mode Dispersion (PMD) by imparting spin in the optical fiber while drawing the optical fiber from the optical fiber preform. Referring now to the drawings, and more particularly to FIGS. 1 through 4, where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments.
[0033] FIG. 1 is a graph illustrates a spinning function in accordance with an embodiment. The spin function is number of spins imparted (Y axis) on the optical fiber as a function of distance (meters) (X axis). The FIG. 1 illustrates a trapezoidal spin function. The spin function comprises a first swivel length S1, followed by a first constant spin region F1, which in turn is followed by a second swivel length S2, followed by a zero spin length K, followed by a third swivel length S3, which is followed by a second constant spin region F2, which in turn is followed by a fourth swivel length S4.
[0034] The length of the optical fiber indicated by first swivel length S1, first constant spin region F1 and second swivel length S2 has spin imparted on it in the clockwise direction. The numbers of spins imparted in the clockwise direction over the first swivel length S1 gradually increase as indicated in the graph. Subsequently, after reaching the end of the first swivel length S1, when the number of spins imparted per meter has reached L1, constant spin at the rate of L1 is imparted on the optical fiber over the length of the optical fiber indicated by first constant spin region F1. Thereafter, at the end of the first constant spin region F1 and the beginning of the second swivel region S2, the number of spins imparted in the clockwise direction over the second swivel length S2 is gradually reduced as indicated in the graph to reach a value of zero spins per meter at the end of the second swivel length S2. Thereafter, no spin is imparted over a length of the optical fiber indicated by zero spin length K. Subsequent to which spin is imparted on the optical fiber in the anticlockwise direction.
[0035] The length of the optical fiber indicated by third swivel length S3, second constant spin region F2 and fourth swivel length S4 has spin imparted on it in the anticlockwise direction. The numbers of spins imparted in the anticlockwise direction over the third swivel length S3 gradually increase as indicated in the graph. Subsequently, after reaching the end of the third swivel length S3, when the number of spins imparted per meter has reached L2, constant spin at the rate of L2 is imparted on the optical fiber over the length of the optical fiber indicated by second constant spin region F2. Thereafter, at the end of the second constant spin region F2 and the beginning of the fourth swivel region S4, the number of spins imparted in the anticlockwise direction over the fourth swivel length S4 is gradually reduced as indicated in the graph, to reach a value of zero spins per meter at the end of the fourth swivel length S4. Thereafter, no spin is imparted over a length of the optical fiber indicated by zero spin length K.
[0036] The number of spins imparted L1 in the clockwise direction over the first constant spin region is not equal to the number of spins imparted L2 in the anticlockwise direction over the second constant spin region.
[0037] In an embodiment, the number of spins imparted L1 in the clockwise direction over the first constant spin region is not equal to the number of spins imparted L2 in the anticlockwise direction over the second constant spin region, at least for the successive spins imparted in the clockwise and anticlockwise direction.
[0038] The spins are imparted alternatively in clockwise and anticlockwise direction in the optical fiber.
[0039] In an embodiment, L1 is kept constant for each of the clockwise spin imparting cycles.
[0040] In another embodiment, L1 is varied for each of the clockwise spin imparting cycles.
[0041] In an embodiment, L1 for some of the clockwise spin imparting cycles are equal.
[0042] In an embodiment, L2 is kept constant for each of the anticlockwise spin imparting cycles.
[0043] In another embodiment, L2 is varied for each of the anticlockwise spin imparting cycles.
[0044] In an embodiment, L2 for some of the anticlockwise spin imparting cycles are equal.
[0045] In an embodiment, zero spin length K is kept constant throughout the length of the optical fiber.
[0046] In an embodiment, zero spin length K is varied throughout the length of the optical fiber.
[0047] In an embodiment, zero spin length K in some of the spin imparting cycles are equal.
[0048] In an embodiment, as illustrated by the graph in FIG. 2, zero spin length K is equal to zero.
[0049] In an embodiment, throughout the length of the optical fiber, zero spin length K is equal to zero.
[0050] In another embodiment, zero spin length K is equal to zero for some of the spin imparting cycles among the multiple spin imparting cycles that the optical fiber has undergone.
[0051] In an embodiment, as illustrated in FIG. 3, first constant spin region F1 and second constant spin region F2 are equal to zero.
[0052] In an embodiment, first constant spin region F1 is equal to zero, whereas second constant spin region F2 is not equal to zero.
[0053] In an embodiment, first constant spin region F1 is equal not to zero, whereas second constant spin region F2 is equal to zero.
[0054] In an embodiment, first constant spin region F1 and second constant spin region F2 are equal to zero throughout the entire length of the optical fiber.
[0055] In an embodiment, first constant spin region F1 and second constant spin region F2 are equal to zero only at portions of the entire length of the optical fiber.
[0056] In an embodiment, as illustrated in FIG. 4, first constant spin region F1, second constant spin region F2, and zero spin length K are equal to zero.
[0057] In an embodiment, first constant spin region F1, second constant spin region F2, and zero spin length K are equal to zero throughout the entire length of the optical fiber.
[0058] In an embodiment, first constant spin region F1, second constant spin region F2, and zero spin length K are equal to zero only at portions of the entire length of the optical fiber.
[0059] In an embodiment, method of making optical fiber comprises, heating an optical fiber preform to a draw temperature. Thereafter providing a spin function and drawing the optical fiber from the optical fiber preform, while simultaneously applying the spin function to create a relative spin between the optical fiber and the preform, wherein at least the spins imparted in the optical fiber in successive clockwise direction and anticlockwise direction are asymmetric.
[0060] The various actions in foregoing methods may be performed in the order presented, in a different order or simultaneously. Further, in some embodiments, some actions listed in may be omitted.
[0061] It is an advantage that by spinning the optical fiber in accordance with the spin function of the above embodiments, the optical fiber can be drawn at high speed in the range of 15 m/s to 25 m/s.
[0062] It another advantage that using the spin functions of the embodiments, the optical waveguide fiber can be drawn at high speed, particularly in the range of 15 m/s to 25 m/s, wherein the optical fiber spun has reduced polarization mode dispersion.
[0063] It is observed that by having L1 ? L2, the spinning of the optical fiber during the drawing stage does not have any undesirable effect on the quality of the optical waveguide fiber being drawn.
[0064] It is observed that having L1 ? L2, the vibrations in the optical fiber during drawing are minimized.
[0065] It has been additionally observed that spinning the optical fiber during the drawing stage does not result in vibrations in the optical waveguide fiber being drawn, thereby reducing any undesirable effect on the coating application.
[0066] Further, it is observed that spinning the optical waveguide fiber in accordance with the embodiments during the drawing stage does not create defects or voids or bubbles due the spinning of the optical waveguide fiber, thereby reducing the breakage of the optical waveguide fiber in the proof testing stage.
[0067] It is also observed that using the spin function as disclosed in various embodiments, the PMD of the optical waveguide fiber is reduced below 0.15 ps/(km)1/2 for more than 95 % of the fiber population.
[0068] The embodiments disclosed herein can be implemented through at least one software program running on at least one hardware device.
[0069] It is to be noted that though the embodiments are explained with the help of the trapezoidal spin function, additional ways for practicing the methods of the present invention will be evident to persons skilled in the art from the disclosure herein, that is, instead of using the trapezoidal spin function, a sinusoidal spin function with spins rates differing in the clockwise and anticlockwise directions is also possible. Similar logic may be applied to any other spin functions, such as triangular, square, rectangular etc. Such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments.
[0070] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.


ABSTRACT
An optical fiber having reduced Polarization Mode Dispersion is provided. The optical fiber comprises spin imparted in the optical fiber along the length of the optical fiber about the axis of the optical fiber. The spin is imparted alternatively in clockwise and anticlockwise direction, wherein maximum number of spins imparted in the optical fiber in successive clockwise direction and anticlockwise direction are not equal, whereby, at least the spins imparted in the optical fiber in successive clockwise direction and anticlockwise direction are asymmetric.

Dated 28th June 2009.

Mr. Nishant Kewalramani
Patent Agent