Title of Invention

METHOD FOR PRODUCING TWISTED OPTICAL FIBER WITH REDUCED POLARIZATION MODE DISPERSION

Abstract

A method for drawing an optical fiber having reduced polarization mode dispersion (PMD) comprising the steps of: a) drawing an optical fiber from an optical fiber preform whose core ovality is at max about 3 %; b) coating the said optical fiber with acrylic polymer; c) twisting the said coated optical fiber with a pair of spinning wheels; d) the step of spinning the fiber is characterized by that (i) the spin function used to spin the fiber is a combination of four discrete spin rates which repeat itself in a periodic fashion after a certain length of fiber drawn; and (ii) the spin rate of each spin function varies in a trapezoidal manner along the drawn length of the fiber; wherein the combination of four discrete spin functions repeats after a specific length in the range varying between about 800-1200 meters

Full Text

GRANTED 30/5/2005 FORM 2 THE PATENTS ACT 1970 [39 OF 1970] COMPLETE SPECIFICATION [See Section 10] "METHOD FOR PRODUCING TWISTED OPTICAL FIBER WITH REDUCED POLARIZATION MODE DISPERSION" STERLITE OPTICAL TECHNOLOGIES LIMITED, of E-2, MIDC, Waluj, Aurangabad 431136, Maharastra, India, The following specification particularly describes the nature of the invention and the manner in which it is to be performed:- ORIGINAL 38/MUMNP/2003 13/1/2003 TITLE OF THE INVENTION Method for Producing Twisted Optical Fiber with Reduced Polarization Mode Dispersion. 5 Abstract Disclosed is a method for producing low Polarization Mode Dispersion (PMD) optical fiber from preform having core ovality at max about 3 % by using combination of discrete spin rates. This combination of spin rates repeats itself after a fixed length of fiber during drawing, 10 thereby effectively reducing the PMD of cabled fiber. BACKGROUND OF INVENTION: Field of invention The present invention relates to a method for drawing an optical fiber having reduced polarization mode dispersion (PMD). The present 15 invention also relates to the single mode optical fiber made in accordance with the presently disclosed method. Single mode optical fiber commonly used in communication systems is not purely single mode. Rather, two modes, with perpendicular polarizations, exist in single mode fiber. These two 20 polarizations form an orthogonal basis set. Accordingly, any configuration of light that propagates through a single mode fiber can be represented by a linear superposition of these two modes. If the fiber has perfect circular symmetry in both geometry and internal applied stress, the two polarization modes propagate with the 25 same group velocity. They have no time delay difference after traveling the same distance in the fiber. But a practical fiber does not have perfect circular symmetry. Imperfections such as geometric deformation and stress asymmetry makes the two polarization modes to propagate with different velocities (a function of propagation constant). The difference 30 between the propagation constants is termed as birefringence. The differential time delay between the two polarization modes is called PMD (polarization mode dispersion). This limits the high bit rate transmission in the communication system. Description of prior art: According to prior art of spinning the fiber mentioned in US patent 5 No. 5,418,881; the fiber is subjected to a sinusoidal spin function, which varies with the spatial distance of the fiber drawn. However, this patent does not teach about the discrete spin rates and a trapezoidal wave function of the spin rate with the spatial distance of the fiber. In another published PCT application bearing No.WO/97/26221, 10 the authors have identified the disadvantage of above-cited US Patent No. 5,418,881. The major disadvantage identified is being in the difference of beat lengths along the different lengths of the fiber. A particular combination of spin amplitude and spin frequency will lower the PMD effectively for a particular beat length. However, this published PCT 15 application WO/97/26221 does not teach the manner for spinning the fiber, particularly after coating and twisting the fiber so as to achieve effective lowering of the PMD of the fiber, particularly cabled fiber. The author of this published PCT application has not visualized that the use of combination of discrete spin rates of different values, which repeats 20 itself in a periodic fashion can effectively lower the PMD of the cabled fiber. The present invention now makes a disclosure of a method by which it is possible to achieve the reduced PMD in optical fiber as low as less than about 0.1 ps/Vkm and the post cabling PMD to be as low as 25 less than about 0.3 ps/Vkm when the spinning is performed on a fiber, particularly on a drawn, coated and twisted fiber by employing the spin function, which is combination of four discrete spin rates which repeat itself in a periodic fashion after a certain length of the fiber drawn and the spin rate of each spin function varies in a trapezoidal manner along the drawn length of fiber. The fibers drawn from performs in accordance with the method of the present invention have ovality at max about 3 %. SUMMARY OF THE INVENTION: Accordingly, the main object of the present invention is to provide 5 a method for effectively reducing PMD in the fiber after drawing and in post cabling stage. The method uses an experimental technique of determining, the discrete spin functions. These spin functions are made to vary in a periodic fashion with the length of fiber to achieve the PMD of the fiber after cabling to be as low as less than about 0.3 ps/Vkm. Due 10 to mechanical stresses generated during cabling the PMD value of fiber increases after cabling. The present invention describes a method in which a discrete spin function is used to effectively reduce the PMD of the fiber in post cabling stage. Accordingly, the present invention relates to a method for drawing 15 an optical fiber having effectively reduced polarization mode dispersion (PMD) as low as less than about 0.1 ps/Vkm and the post cabling PMD as low as less than about 0.3 ps/Vkm. In accordance with the preferred embodiment of the present invention the spinning is performed on a fiber, particularly on a drawn, 20 coated and twisted fiber by employing the spin function, which is a combination of four discrete spin rates which repeat itself in a periodic fashion after a certain length of the fiber drawn and the spin rate of each such spin function varies in a trapezoidal manner along the drawn length of fiber. 25 The fibers drawn from performs in accordance with the method of the present invention have ovality at max about 3 %. The effective lowering of PMD is achieved in accordance with the present invention by providing spin functions characterized by : (a) The maximum spin rate is less than about 15 spins/meter 30 (b) The spin function is a trapezoidal spin function. (c) A defined change sequence of discrete spin rates. (d) The combination of discrete spin rates which repeats itself after a specific spatial distance. (e) Draw speed range varies from about 10 to about 23 5 meters/sec The other objects and the preferred embodiments of the present invention will become more apparent from the following description when read in combination with the accompanying drawings which are not intended to limit the scope of the present invention. 10 BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS: Fig 1 shows a schematic diagram of the fiber draw apparatus in accordance with the preferred embodiment of the present invention. Fig 2 shows a schematic diagram of a spinning/ guiding wheel in accordance with the preferred embodiment of the present invention. 15 Fig 3 shows a schematic diagram of spinning wheel swing mechanism in accordance with the preferred embodiment of the present invention. Fig 4 shows a schematic diagram of the spinning wheels in the displaced position in accordance with the preferred embodiment of the 20 present invention. Fig 5 shows a trapezoidal spin function and the different recipe parameters used to define it in accordance with the preferred embodiment of the present invention. Fig 6 shows a schematic diagram of an optical communication link 25 in accordance with the preferred embodiment of the present invention. DETAIL DESCRIPTION OF THE PREFERRED INVENTION: The present invention relates to a method of effectively reducing PMD in the fiber post drawn and post cabling stage by variably spinning the fiber during the drawing process. The process of spinning is done 30 with the help of spinning wheels, which imparts a twist to the coated fiber. This twist gets impressed in the fiber near the hot zone. Now referring to the accompanying drawings, Fig 1 shows the schematic diagram of the draw tower where the preform is drawn to fiber. An optical fiber preform 300 is fed into a drawing furnace 310 for softening by heating. An optical fiber 320 is drawn form one end of the 5 softened optical fiber preform 300. The drawn optical fiber 320 is passed through a coating unit 340 via a diameter monitor 330, to be coated with a polymer coating by a coating unit 340. Then the optical fiber 320 is preferably sequentially passed through a coating concentricity monitor 350, a coating resin setting unit 360 e.g. UV lamp, and a coating 10 diameter monitor 370. Subsequently, the optical fiber 320 is passed through a Fiber Spinning Unit (FSU) 410. Although any fiber spinning unit may be used in the presently disclosed invention. However, in accordance with the preferred embodiment of the present invention, the fiber spinning unit 15 comprises of four wheels one servomotor to control the spinning of the wheels. Out of four wheels two are guiding wheels and two are spinning wheels. These two guiding wheels are on the upper side of the unit and they guide the fiber during normal run. According to this invention, the lower pair of wheels are spinning wheels, these wheels directly touch the 10 fiber during draw. Fig 2 shows a schematic diagram of a spinning wheels/guiding wheels. According to the preferred embodiment of this invention, the diameter of the wheel is of the order of about 80 mm, the width of the wheel on which the fiber spins is about 12 mm, and a flange is curved out of the wheel as shown in Fig 2. Further, the depth and 25 thickness of the flange are about 5.66 mm and about 2.8 mm respectively. In accordance with this invention, during drawing, the gap between the guiding wheels is maintained at about 3 mm. This gap is maintained to prevent the slip of the fiber on the spinning wheels. 30 Further, the distance between the spinning wheels is kept at about 0.3 mm. The height difference between the centers of guiding and spinning wheels is maintained at about 50 mm. According to this invention, during spinning of fiber; the compression force is maintained in the range varying from about 0.7 to about 1.2 N. This force can be adjusted in the conventional manner. However, according to this invention it is 5 adjusted with the spring attached to the wheels. The clockwise and anticlockwise motion of the spinning wheel is driven by means of a tilting plate 710. The tilting plate 710 is a circular plate whose thickness varies along the circumference. The tilting plate is supported on a non-moving base 720. The spinning wheel is connected to a contact roller 730 with a 10 rod 740. The contact roller moves on the circumference of the tilting plate 710. There is a spring 750 that connects the rod 740 to the base 720. The spring force given by the spring 750 provides the flexible movement of the rod about an axis at the contact roller. During the rotation of the tilting plate the contact roller 730 moves up and down 15 with respect to the horizontal reference plane XX'. This motion in turn gives an up and down motion to the rod 740. Thus the spinning wheel gets a swing motion. The servomotor is coupled with the drive, which receives a feedback from the PLC attached with the system. All Process parameters to rotate the wheels like clockwise angle and counter 20 clockwise angle for different ranges of length can be set on user friendly MMI. This FSU is mounted on a table. The horizontal and vertical alignment of the system can be done. In the normal run the alignment of the FSU is important, any misalignment may generate vibration in the fiber resulting in wrong reading in the Bubble detector mounted below 25 FSU or bad coating application. In accordance with the present invention, the fiber then passes through a flaw detector 440. The optical fiber 320 after passing through the flaw detector 440 runs through the capstan 450. The optical fiber 320 then is passed onto a take up spool with a set of guide rollers. 30 The spin rate of the presently disclosed method results in effectively reduced PMD value which is as low as less than about 0.3 ps/Vkm at post cabling stage. The spin function of the presently disclosed method also results in draw speed ranging between about 10 to about 23 meters/sec. The different parameters used in the recipe of the presently 5 disclosed method are shown in Fig 5. The spin function in accordance with the present invention has a trapezoidal shape with the following parameters: a) Spin per meter clockwise [1/m] It is constant spin rate in the clockwise direction. 10 b) Spin per meter counter clockwise [1/m] It is constant spin rate in counter clockwise direction. c) Spin length clockwise [m] Spatial length of fiber subjected to clockwise spinning. d) Spin length counter clockwise [m] 15 Spatial length of fiber subjected to counter clockwise spinning. e) Swivel Length Clockwise [m] Spatial length of fiber spun during transition of spinning wheel (rotating in clockwise direction) from current position 20 to set position. f) Swivel Length Counter Clockwise [m] Spatial length of fiber spun during transition of spinning wheel (rotating in anticlockwise direction) from current position to set position. 25 g) Straight length [m]. Spatial length of fiber not subjected to any twist by the spinning wheel. The amplitude of rotation is directly proportional to the spin rate and is given by the relation (1) : 30 θ = K (spin rate) (1) where 9 [Fig. 4] is the amplitude of swing motion and K is a constant. This implies that when the spin rate increases the angle of maximum rotation also increases and vice versa. 5 According to the present invention, the speed of the angular motion of the spinning wheel is dependent on the line speed, swivel length and the spin rate. The relation between them is as shown below by the relation (2) : Angular speed = K'*(spin rate)*(drawing speed)/(swivel length) (2) 10 where K' is a constant. In accordance with the present invention the presently disclosed method for drawing an optical fiber having reducing polarization mode dispersion (PMD) comprises the steps of : a) drawing an optical fiber from an optical fiber preform whose 15 core ovality is at max about 3 %; b) coating the optical fiber with acrylic polymer; c) twisting the coated optical fiber with a pair of spinning wheels; d) the step of spinning the fiber is characterized by that: 20 (i) the spin function used to spin the fiber is a combination of four discrete spin rates which repeat itself in a periodic fashion after a certain length of fiber drawn; and (ii) the spin rate of each spin function varies in a 25 trapezoidal manner along the drawn length of the fiber. According to the present invention, the combination of four discrete spin functions repeats after a specific length in the range varying between about 800-1200 meters. The cabled fiber PMD achieved has a 30 minimum of about 99.0 % compliance with about 0.3-ps/Vkm. The fiber drawing speed varies from about 10 to about 23 meters/sec. The coated optical fiber diameter is in the range varying from about 240 to about 250 micron. The gap between the two guiding wheels is at least about 0.3 mm. The gap between the two spinning wheels is at max about 0.35 mm. The difference between gap of spinning wheels and coated fiber diameter is in the range varying from about 30 to about 100 micron for achieving a compression force in the range varying from about 0.7 to about 1.2 N. The distance between the centers of the guiding wheels and the spinning wheels is at least about 45 mm. The distance between the spinning roller and capstan is less than about 35 cm for efficient damping of vibration of the fiber. In accordance with the present invention, the single mode optical fiber made in accordance with the presently disclosed method is also provided. The single mode optical fiber is used in the optical communication system consisting of a transmitter and receiver. The experimental study was carried out in 13 stages with different spin functions to achieve the effectively reduced PMD of the fiber. The fiber drawn with different spin functions were cabled. The PMD of the cabled fiber was measured and these values were used as feedback to modify the spin function. The first spin function and the cable fiber results are as follows: Table 1(a) Spin function for Stage No 1 Clockwise Counter clockwise Spins per meter 8 8 Spin length 8 8 Swivel length 2 2 Straight length 1 Table # 1(b) PMD of Cabled fiber of Stage No 1 Preform core ovality SPECS (0.3) Eight different single spin functions were experimented to achieve 5 highest percentage of cabled fiber meeting the about 0.3-ps/Vkm specification of PMD. Table 2(a) shows the spin function with which the inventors were able to achieve the best results in the cabled fiber PMD using single spin function. 10 Table: 2(a) Clockwise Counter clockwise Spins per meter 10 10 Spin length 16 16 Swivel length 1 1 Straight length 1 Table: 2(b) Cabled fiber PMD results. Preform core ovality SPECS (0.3) 15 As stated hereinabove, the PMD results the percentage compliance to the desired PMD value of about 0.3-ps/Vkm was low, the inventors experimented on a combination of four different spin rates to achieve the effectively reduced PMD of the fiber. These spin rates were repeated in a definite sequence to effectively lower the PMD of the fiber. The inventors merged the best of single spin rate functions. It was observed on experimenting five different combinations of different spin functions that the combination which resulted in best 5 compliance of cabled fiber PMD ( No. 3(a). It was also observed that this spin function works well for a drawing speed ranging between about 10 to about 23 meters/sec. Table No: 3(a) Recipe # 1 Recipe # 2 Recipe # 3 Recipe # 4 Spins/meter CW 10' 7 12 9 Spins/meter CCW 10 7 12 9 Spin length CW 16 9 8 5 Spin length CCW 16 9 8 5 Swivel length CW 1 1 4 3 Swivel length CCW 1 1 4 3 Straight length 1 0 1 0 Repeat Gain 7 11 8 11 Meters/cycle 37 22 33 22 M/Repeat cycle 259 242 264 242 10 The structure of the recipe is designed such that recipe # 1 is continued for the times mentioned in repeat gain and then switch to recipe 2. This process continues till recipe # 4 is completed. The complete recipe finishes within a length of about 1000 meters of fiber drawn. The above recipe is independent of drawing speed and has shown good 15 compliance with cabled fiber PMD. The results of the cabled fiber spun with the above recipe are given in Table 3(b). Table No 3(b) Preform core ovality SPECS (0.3) 5 The present invention, therefore, provides a fiber spinning method, which uses a combination of four different spin rates, which repeats itself after a length of about 1000 meters. This method ensures about 99.1 % compliance of cabled fiber PMD. The results mentioned in above Table No 3(b) shows a better 10 compliance of PMD for preforms having core ovality less than about 2 %. This cannot be practiced on a production scale, as it would lead to increase in preforms rejection. Hence, in accordance with the present invention the preforms were qualified in accordance with the specification of less than or equal to about 3 % core ovality. 15 The present invention has been described and illustrated with the help of accompanying drawings which are not intended to limit the scope of the present invention. It is obvious to the persons skilled in the art that it is possible to modify the disclosed method without deviating from the scope of the present invention. Accordingly, such modifications are 20 intended to be included in the scope of the present invention. The present invention is restricted with the following claims. WE CLAIM: 1 A method for drawing an optical fiber having reduced polarization mode dispersion (PMD) comprising the steps of: a) drawing an optical fiber from an optical fiber preform whose core ovality is at max about 3 %; b) coating the said optical fiber with acrylic polymer; c) twisting the said coated optical fiber with a pair of spinning wheels; d) the step of spinning the fiber is characterized by that (i) the spin function used to spin the fiber is a combination of four discrete spin rates which repeat itself in a periodic fashion after a certain length of fiber drawn; and (ii) the spin rate of each spin function varies in a trapezoidal manner along the drawn length of the fiber; wherein the combination of four discrete spin functions repeats after a specific length in the range varying between about 800-1200 meters. 2. A method as claimed in claim 1, wherein the cabled fiber PMD achieved has a minimum of about 99.0 % compliance with about 0.3-ps/ √km. 3. A method as claimed in claim 1, wherein the fiber drawing speed varies from 10 to 23 meters/sec. 4. A method as claimed in claim 1, wherein the coated optical fiber diameter is in the range varying from 240 to 250 micron.' 5. A method as claimed in claim 1, wherein the gap between the two guiding wheels is at least about 0.3 mm. 6. A method as claimed in claim 1, wherein the gap between the two spinning wheels is at max about 0.35 mm. 7. A method as claimed in claim 1, wherein the difference between gap of spinning wheels and coated fiber diameter is in the range varying from 30 to 100 micron for achieving a compression force in the range varying from 0.7 to 1.2 N. 8. A method as claimed in claim 1, wherein the distance between the centers of the guiding wheels and the spinning wheels is at least about 45 mm. 9. A method as claimed in claim 1, wherein the distance between the spinning roller and capstan is less than about 35 cm for efficient damping of vibration of the fiber. 10. The single mode optical fiber made in accordance with the method of any of the preceding claims has polarization mode dispersion less than 0.3 ps/Vkm. 11. A method for drawing an optical fiber having reducing polarization mode dispersion (PMD) substantially as hereinbefore described with reference to the accompanying drawings. Dated this 13th day of January, 2003. [RANJNA MEHTA-DUTT] OF REMFRY & SAGAR ATTORNEYS FOR APPLICANTS

Documents:

38-mum-2003-cancelled pages(30-05-2005).pdf

38-mum-2003-claims(granted)- (30-05-2005).pdf

38-mum-2003-correspondence(31-05-2005).pdf

38-mum-2003-correspondence(ipo)-(01-11-2006).pdf

38-mum-2003-drawing(30-05-2005).pdf

38-mum-2003-form 1(12-01-2004).pdf

38-mum-2003-form 1(13-01-2003).pdf

38-mum-2003-form 1(27-01-2003).pdf

38-mum-2003-form 13(12-01-2004).pdf

38-mum-2003-form 19(18-03-2004).pdf

38-mum-2003-form 2(granted)-(30-05-2005).pdf

38-mum-2003-form 3(12-01-2004).pdf

38-mum-2003-form 3(13-01-2003).pdf

38-mum-2003-form 3(30-05-2005).pdf

38-mum-2003-form 5(12-01-2004).pdf

38-mum-2003-petition under rule 137(30-05-2005).pdf

38-mum-2003-petition under rule 138(30-05-2005).pdf

38-mum-2003-power of authority(05-05-2003).pdf

38-mum-2003-power of authority(12-01-2004).pdf

38-mum-2003-power of authority(30-05-2005).pdf

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