MXPA00002944A - Draw constant downfeed process - Google Patents

Abstract

Optical fiber (14) is drawn from a preform (10) that is fed to a furnace at a constant downfeed rate. The optical fiber (14) is drawn by a tractor (20) at a rate of at least 10 meters per second and the tractor (20) is allowed to vary the draw speed of the fiber (14) based on the fiber diameter as measured by a diameter monitor (15) during the draw processing in order to maintain a relatively constant fiber diameter. Maintaining the preform downfeed rate constant even at high draw rates in excess of 20 meters per second does not adversely effect the draw process and is believed to reduce or eliminate oscillations in the draw control loop that can cause variations in the core shape during fiber formation, resulting in reduced PMD and improved MFD uniformity.

Description

CONSTANT FEEDING PROCEDURE DURING STRETCHING FIELD OF THE INVENTION The present invention relates to optical waveguide fibers, and more particularly, to methods for stretching an optical fiber from an optical fiber preform wherein the fiber shows a more uniform diameter (MFD) and polarization mode dispersion. reduced (PMD).

BACKGROUND OF THE INVENTION In the manufacture of optical fiber, a glass core preform is made which typically comprises SiO 2, whose axial portion is inclined with a compound such as GeO 2 to increase the refractive index. When a fiber is stretched from the glass preform, the inclined region provides the light transmission portion or core of the fiber. The process described above is well known in the art and will not be described in more detail. To obtain the optical fiber, the glass preform is supplied in a stretching furnace heated to a melting temperature, and a small amount of glass, with a drag fiber, falls from the root of the preform. The fiber is supplied to a tractor and guide assembly that stretches the fiber from the preform and the fiber is spun on a reel. As the fiber is stretched from the preform, the preform is supplied in the furnace, and the fiber diameter is monitored closely. Fiber diameter control is usually achieved by varying certain operating parameters in the stretch tower. Typically, a fiber diameter measuring device is located just below the furnace outside the fiber diameter measurement. The measured diameter is compared with a nominal diameter value and a signal is generated to increase the speed of the tractor (thus reducing the diameter of the fiber), or reduce the speed of the tractor (thus increasing the diameter of the fiber ). In the 70s and half of the 80s, the preforms from which the fiber was stretched were relatively small. The speeds of stretching did not exceed about 8 or 9 meters per second. Because the size of the preform and the stretching speeds were used, the diameter of the fiber was controlled by varying the speed of the tractor while maintaining the temperature of the furnace and the speed of delivery of the preform relatively constant. During the mid-1980s a new procedural control strategy was developed and introduced as a result of increasing stretch rates. Specifically, as the stretch speeds reached 10 meters / second, those experts in the art they abandoned the use of constant feed rates. More specifically, it was believed that in order to achieve adequate control at high stretch speeds, for example approach speeds and in excess of 10 meters / second, it was necessary to restore a waterfall or two-level procedure control strategy where , in response to an error signal indicating that the actual or measured fiber diameter was not equal to the desired diameter, there would be a change in the drawing speed and a change in the down feed speed of the preform in the stretch oven . For example, if the fiber diameter measured was greater than the desired fiber diameter, then the control system would increase the speed of the tractor and, at the same time, reduce the speed at which the preform was supplied in the stretching oven. Said control philosophy reflected the belief that when the operation of a fiber stretching process had a velocity greater than 8-9 meters / second, it was necessary to vary the feed rate of the preform when the stretch rate varied to maintain a more constant fiber diameter. Although said two-level control procedure results in an essentially constant fiber diameter, it was found that other harmful effects occur as a result of said operation. It is believed that said oscillations in the stretch control ring, specifically oscillations in the down feed rate of the preform, can cause variations in the shape of the core during the formation of the fiber. The foregoing may be particularly important at the root of the preform from which the fiber is stretched. It is believed that oscillations of the root of the preform in the furnace can affect the shape of the core as the root of the preform is formed, and it is believed that this causes a poor PMD and a non-uniform MFD. For the optical fiber to be used in telecommunications applications, the PMD should be as small as possible, and the MFD should be kept as uniform as possible. Several solutions have been proposed to solve some of the problems mentioned above. For example, the patent applications of E.U.A. commonly assigned and co-pending Nos. 08 / 858,836 and 08 / 784,574, and PCT Application No. PCT / US97 / 02541 describe various methods and apparatuses for imparting a turn to the fiber as it is stretched to reduce PMD. The rotation of the optical fiber as it is stretched causes the internal geometry and / or fiber tension asymmetries to rotate about the axis of the fibers along the axis; however, the rotation of the fiber does not solve the underlying problems in the glass that cause the PMD, nor does the turn completely eliminate the PMD or solve the problem of the uniformity of MFD. In view of the disadvantages of the art, it would be desirable to provide a method to maintain or increase the uniformity of MFD while at the same time reducing the PMD. There is an explicit need for the above when the stretched optical fiber is at high stretch speeds, ie, greater than 10 meters / second, which can to contribute with the oscillation of feeding descendant increased in the root, increasing in this way the PMD in the fiber.

BRIEF DESCRIPTION OF THE INVENTION Accordingly, the present invention relates to a method for high-speed stretching of the optical fiber that solves one or more of the problems due to the limitations and disadvantages of the related prior art. The main advantage of the present invention is the provision of a method for controlling the diameter of a stretched optical fiber while reducing the PMD in the fiber and maintaining the MDF uniform when the fiber is stretched at a high speed. The method comprises the fiber drawn at a high speed while maintaining the constant down feed rate of the preform. It is believed that the constant down feed rate prevents oscillation of the root of the preform in the furnace which causes the variability in the shape of a core during the formation of the fiber. It is believed that these variations contribute to a poor PMD and MFD in the final fiber. To achieve said and other advantages in accordance with the purpose of the invention, as is amply described, the invention is a method for reducing the polarization mode dispersion in the stretched optical fiber comprising the steps of supplying an optical fiber performa of a predetermined size in an oven at a feed rate predetermined descent, by stretching an optical fiber from the optical fiber preform at a stretch speed of at least 10 meters per second, and varying the stretch speed to maintain a substantially constant fiber diameter while maintaining the predetermined steady feed rate . Preferably, the drawing speed is greater than 14 meters per second and more preferably, greater than 20 meters per second. In a preferred embodiment, the down feed rate is constant for a first stretch rate range or scale and is then changed to a constant down feed rate for a second stretch rate range or scale. As the stretch speed varies in each zone, the down feed rate remains constant within each zone. In addition, the down feed speed may be different for each zone. The method can also include the step of reducing the down feed speed as the stretch speed changes from one area to another having a higher speed of stretch speeds, or increasing the feed rate downward as the stretch speed changes from one to the other, having a lower scale of stretch speeds. The invention may also include the step of spinning the fiber as it is stretched to further reduce the PMD.

According to another embodiment of the invention, there is provided a method for stretching the optical fiber from an optical fiber preform, comprising the steps of supplying the optical fiber preform of a predetermined size in a stretch oven at a feed rate. constant descent and stretching the optical fiber from the fiber optic preform at a stretch speed of at least 10 meters per second. The method further comprises the steps of measuring the stretched fiber diameter and generating a signal representative of the measured diameter and comparing the generated signal with a nominal fiber diameter. A second signal representative of the comparison difference is generated and used to vary the stretch speed to adjust the stretched fiber diameters. The method also includes the step of locating the stretch speed to determine if it is within a zone of predetermined speeds and changing the feed rate down to another predetermined speed if the localized stretch speed is outside the zone. The down feed rate is constant for a first zone or scale of stretch speeds and is then changed to a different constant down feed rate for a second zone or scale of stretch speeds. Preferably, the down feed rate is kept constant within each zone and as the drawing speed is varied among the plurality of zones, the down feed rate is changed accordingly. The method of according to said embodiment may include the additional step of spinning the optical fiber as it is stretched. It is understood that the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide additional explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram and a fiber stretching apparatus.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for reducing polarization mode dispersion in the stretched optical fiber wherein an optical fiber preform of predetermined size is supplied in an oven at a predetermined down feed rate. Preferably, the down feed rate is kept constant throughout the stretching process in order to minimize the oscillation of the preform root in the furnace in order to maintain the uniformity of the MFD and reduce the PMD in the fiber optic stretched. Figure 1 illustrates a well-known fiber optic stretching system, generally designated by the reference number 1. The preform is arranged vertically in a cushion 11 of a stretching oven. The preform 10 includes a handle (not shown) that is attached to a fastening device (not shown) in a known manner. The clamping device is part of the supply control of the preform 22, which controls the speed at which the preform 10 is supplied in the oven. The heating element 12 provides heat to at least the lower portion of the preform 10. The temperature of the heating element 12 is controlled by the temperature controller 49 in a known manner. After a well-known starting procedure is employed, the preform supply control 22 supplies the preform in the oven. As the preform 10 is supplied in the oven, the final portion of the preform 10, commonly referred to as the root, is fused and the fiber 14 is stretched from the root portion 13 of the preform 10 by the tractor 20. After moving away from the damper 11, fiber 14 passes through the diameter monitor 15 which produces a signal that is used in a feedback control loop to regulate the speed of the tractor 20 and the supply control of the preform 22, as well as regulating the temperature in the oven through the temperature controller 49. After the diameter monitor 15, the fiber 14 passes through a cooling tube 17 and a coater 18 by which a curable protective coating is applied to fiber 14. The coated fiber can also pass through a coating cure apparatus and additional coaters (not shown) are desired. The feed control feedback control of the preform 22, control of tractor 21 and temperature controller 49 can be implemented by known control algorithms. The tractor control 21 is provided with an input for the control algorithm 48 which is part of the stretch control computer 47. Due to the demand for optical fiber, it is advantageous to operate the tractor 20 at a speed of at least 10. meters per second. Preferably, the tractor 20 produces a drawing speed greater than 14 meters / second, and more preferably greater than 20 meters per second. The present invention relates to a method for reducing the polarization mode dispersion in the stretched optical fiber comprising the step of supplying a glass proforma and the stretching of an optical fiber at a speed greater than 10 meters / second. The size of the preform 10 can be measured by weight or by its diameter. The down feed rate of the preform 10 is selected based on the size of the preform 10. Preferably, the down feed rate, once selected, remains constant during the fiber stretch process. Alternatively, the down feed rate may remain constant within the predetermined zone or the scale of stretch speeds. There may be any number of stretch velocity zones and the scale of stretch speeds within each zone may also vary. However, each zone has a down feed rate associated with it and the down feed rate remains constant within the given zone.

If the drawing speed, which is controlled by the tractor control 21, is increased or reduced outside of the specific stretch rate zone, a signal is sent from the control algorithm 48 to the preform feed control 22 for change the feed speed down to the proper down feed speed for the particular area of the stretch speed. The control algorithm 48 is set so that the speed of the tractor changes from one area to another, the down feed speed changes in small increments until the predetermined down feed speed is reached. The above allows the down feed speed to be adjusted back to the original speed quickly if the tractor speed suddenly returned to the original zone. According to another aspect of the invention, the method can comprise the additional steps of locating the stretch rate to determine if it is within a predetermined velocity zone and the feed rate decreases if the localized stretch speed is found. outside the area. In this embodiment, a stretch speed sensor (not shown) continuously monitors the stretch speed in a stretch control computer 47. If the stretch speed changes from one zone to another, the control algorithm 48 sends a signal to the feed control 22 of the preform to increase or decrease the down feed speed at the predetermined constant speed associated with the stretch rate zone. The present inventive method also includes the step of varying the drawing speed in response to the measured fiber diameter to maintain a constant fiber diameter while maintaining the predetermined predetermined down feed rate. In order to maintain a constant fiber diameter, the fiber 14 is constantly monitored by the diameter monitor 15. The diameter monitor 15 produces a signal representative of the fiber diameter measured. Said signal is sent to the stretch control computer 47. In the stretch computer 47, the measured signal is compared to a diameter value of predetermined nominal fiber. A second signal is generated based on any difference between the measured fiber diameter value. The second signal is sent to the tractor control 21 and the tractor speed changes to maintain a constant fiber diameter. Said procedure is carried out hundreds of times per minute and the down feed speed remains constant during the stretching process during all the scales of the tractor speed. It can also be advantageous to spin the fiber as it is stretched. It has been shown that the rotation in the fiber also reduces the PMD. Various methods and apparatuses have been developed to impart the spin on a fiber as it is stretched. Reference is made to the patent applications of E.U.A. Commonly assigned and co-pending Nos. 08/858, 836 and 08 / 784,574 and PCT application No. PCT / US97 / 02541; and patent of E.U.A. No. 5,298,047, for a more detailed understanding of the methods and apparatus used for the spinning fiber, each of which is incorporated by reference. The advantages associated with the invention are numerous. In the prior art stretching systems, the fiber diameter is controlled by the speed of the tractor. The control loop involves the two-step procedure control in stretching. If the speed of the tractor varies, the down feed speed responds to a variation in tractor speed. Although it is not intended to be limited by any theory or explanation as to why the current invention works, it is believed that it in turn produces an oscillation of root 13 in the furnace. It is believed that such oscillation of the root portion of the preform 10 in the furnace causes the variability in the core shape of the stretched optical fiber in that variations in the core shape lead to a larger PMD and to the inequality in the MFD. , which adversely affect the performance of the fiber. The present invention helps reduce the oscillation of the preform by providing a constant down feed rate during the stretching process. Contrary to the known two-step control approach for stretching the fiber at high speeds, the control algorithm 48 is set to maintain the down feed speed of the preform constant even as the speed of the tractor is changed to maintain the diameter of the fiber . It is believed that said Control mechanism reduces or perhaps eliminates oscillations in the stretch control ring that can cause variations in the shape of the core during fiber formation, and results in reduced PMD and improves the uniformity of the MFD.

EXAMPLES The invention will be further described by the following examples, which are intended to be exemplary of the invention.

EXAMPLE 1 An optical fiber was produced without winding using a stretching system similar to that illustrated in Figure 1. The speed of the tractor could vary up to a maximum of 19 meters per second to maintain a constant fiber diameter, while the down feed speed was kept constant at around 2.75 millimeters per minute. The resulting fiber was tested for PMD and uniformity of MFD. The results compared to a stretch of fiber under a standard procedure (eg, variable feed rate) are shown in Table 1 below: TABLE 1 As the results indicate, the PMD was significantly reduced and the MFD was improved in the stretching process according to the present invention compared to a standard procedure.

EXAMPLE 2 A fiber was stretched using an apparatus similar to that shown in Figure 1. The fiber was also rotated during the stretching process. The down feed rate was established in accordance with the zone mode of the present invention as described above to achieve a nominal drawing speed of 15.5 meters per second. The stretched fiber was tested and the PMD results and the uniformity of MFD were compared to a fiber stretch using a standard stretching procedure. Several different operations were taken and the results are shown in Table 2 below.

TABLE 2 As shown in Table 2, there is a significant reduction in PMD in the stretched fibers according to the present invention when compared to the fibers drawn according to the standard procedure. The uniformity of the MFD was also significantly improved. It will be apparent to those skilled in the art that various modifications and variations may be made in the method of the present invention which, however, fall within the scope of the appended claims and their equivalents.

Claims (16)

NOVELTY OF THE INVENTION CLAIMS

1. A method for reducing polarization mode dispersion in stretched optical fiber comprising the steps of: supplying an optical fiber preform of a predetermined size in an oven at a predetermined drawing speed; stretching an optical fiber of the optical fiber preform at a stretch speed of at least 10 meters per second; and varying the drawing speed to maintain a substantially constant fiber diameter while maintaining the predetermined down feed rate constant.

2 - The method for reducing the polarization mode dispersion in the stretched optical fiber according to claim 1, further characterized in that the drawing speed is greater than 14 meters per second.

3. The method for reducing the polarization mode dispersion in the stretched optical fiber according to claim 1, further characterized in that the stretching speed varies between about 14 and 20 meters per second.

4. The method for reducing the polarization mode dispersion in the stretched optical fiber according to claim 1, characterized further because the speed of stretching varies between about 14 and 20 meters per second.

5. The method for reducing the polarization mode dispersion in stretched optical fiber according to claim 4, further characterized in that it comprises the step of defining at least one stretch velocity zone.

6. The method for reducing the polarization mode dispersion in stretched optical fiber according to claim 5, further characterized in that the stretching speed varies in each zone, the down feed speed remains constant within each zone.

7. The method for reducing the polarization mode dispersion in stretched optical fiber according to claim 6, further characterized in that the down feed speed is different for each zone.

8. The method for reducing the polarization mode dispersion in stretched optical fiber according to claim 7, further characterized in that while the stretching speed changes from one area to another having a larger scale of stretching speeds, the down feed speed.

9. The method for reducing the polarization mode dispersion in stretched optical fiber according to claim 7, further characterized in that while the stretch speed changes from a zone to a another having a smaller scale of stretch speeds, the down feed speed is increased.

10. The method for reducing the polarization mode dispersion in stretched optical fiber according to claim 1, further characterized in that the fiber is spun as it is stretched.

11. The method for stretching the optical fiber of a fiber optic preform comprising the steps of: supplying the optical fiber preform of a predetermined size in a stretch oven at a down feed speed; stretching the optical fiber of the fiber optic preform at a stretch speed of at least 10 meters per second; measure the diameter of the stretched fiber and generate a signal representative of the diameter measured; comparing the generated signal with the nominal fiber diameter value and generating a second signal representative of the difference of the comparison; locate the stretch speed to determine if it is within a predetermined velocity zone; varying the stretching speed based on the second signal to adjust the diameter of the drawn fiber; and vary the down feed speed if the localized stretch speed is outside the zone.

12. The optical fiber stretching method according to claim 11, further characterized in that it comprises the additional step of spinning the optical fiber as it is stretched.

13. - The optical fiber stretching method according to claim 11, further characterized in that the drawing speed comprises a plurality of zones, each zone comprising a predetermined scale of stretching speeds.

14. The optical fiber stretching method according to claim 13, further characterized in that while the drawing speed varies among the plurality of zones, the downstream feed is changed.

15. The fiber optic stretch method according to claim 14, further characterized in that the down feed speed is maintained substantially constant in each zone.

16. A method for reducing the polarization mode dispersion in the stretched optical fiber comprising the steps of: supplying an optical fiber preform of a predetermined diameter in an oven at a constant down feed rate; stretching an optical fiber of the optical fiber preform at a stretch speed of at least 10 meters per second; and varying the stretching speed while maintaining a substantially constant fiber diameter.