CN1271334A - Constant speed down-feed drawing process - Google Patents

Abstract

The preform (10) is fed into the furnace at a constant downfeed rate, and an optical fiber (14) is then drawn from the preform. The puller (20) draws the optical fiber (14) at a speed of at least 10 m/s and allows the puller (20) to vary the draw speed of the optical fiber (14) during drawing based on the fiber diameter measured by the diameter monitor (15) so as to maintain the fiber diameter relatively constant. Maintaining the preform downfeed rate constant at higher draw speeds in excess of 20 m/sec does not adversely affect the drawing process and it is believed that vibrations in the draw control loop that can cause variations in the core shape during fiber formation can be reduced or eliminated, thereby reducing PMD and improving MFD uniformity.

Description

Constant speed down-feed drawing process

Field of Invention

The present invention relates to optical fibers, and more particularly to a method of drawing optical fibers from optical fiber preforms, which results in optical fibers exhibiting a more uniform mode field diameter (MFD) and reduced polarization mode dispersion (PMD).

Background of the Invention

In the optical fiber manufacturing process, a glass core preform is to be made. The preform generally includes SiO ₂ , and its axis is doped with compounds such as GeO ₂ to increase the refractive index. When the fiber is drawn from the glass preform, the doped region will form the light transmitting portion or core.

The above processes are well known in the art and will not be described in detail here. To obtain optical fiber, a glass preform, or blank, is fed into a drawing furnace heated to melting temperature, and a small drop of glass is shed from the root of the blank along with the trailing fiber. The fiber is fed into a puller and capstan assembly that pulls the fiber from the blank and winds the fiber onto a spool.

When drawing an optical fiber from a blank, the blank is fed into a drawing furnace and the fiber diameter is closely monitored. The diameter of the fiber is generally controlled by changing certain operating parameters of the draw tower. Typically, a fiber diameter measuring device is installed directly below the furnace exit to measure the fiber diameter. Compares the measured diameter with the nominal diameter value and generates a signal to increase the pulling speed (thus reducing the fiber diameter) or decrease the pulling speed (thus increasing the fiber diameter)

During the 1970s and throughout the mid-1980s, the blanks from which optical fibers were drawn were relatively small. The wire drawing speed does not exceed about 8 or 9 meters per second. Because of the blank size and the drawing speed used, the fiber diameter can be controlled by varying the draw speed while keeping the furnace temperature and blank feed rate relatively constant.

In the mid-1980s, a new method of process control was developed as a result of maximizing the drawing speed. Specifically, when the drawing speed reaches 10 m/s, those skilled in the art will give up using a constant feeding speed. More specifically, it is believed that to achieve adequate control at higher wire drawing speeds (i.e., speeds up to and exceeding 10 m/s), a cascaded or two-stage process control approach is necessary, allowing control of the actual or measured In response to the error signal that the fiber diameter is not equal to the desired diameter, the drawing speed and the speed at which the blank is fed down into the drawing furnace are varied. For example, if the measured fiber diameter is greater than the desired fiber diameter, the control system will increase the pull-off speed while reducing the speed at which the blank is fed into the drawing furnace. This control perspective reflects the idea that when operating a fiber drawing process at speeds greater than 8-9 m/s, it is necessary to vary the blank feed rate while varying the drawing speed in order to maintain a relatively constant fiber diameter.

Although this two-stage control process results in a substantially constant fiber diameter, it has been found that this mode of operation has other detrimental effects. It is believed that oscillations in the drawing control loop, and specifically, in the speed of the blank feed, change the shape of the fiber core during fiber shaping. This is especially noticeable at the root of the blank where the optical fiber is pulled down. It is believed that when the core is formed at the root of the blank, the vibration of the root of the blank in the furnace can affect the shape of the core and cause poor PMD and non-uniform MFD.

For fibers used in telecommunication applications, the PMD should be as small as possible and the MFD should be as uniform as possible. Several solutions have been proposed to solve the above problems. For example, co-assigned and co-pending US Patent Application Serial No. 08/858,836 and PCT Application PCT/US97/02541, commonly assigned to the assignee of the present invention, disclose various methods and apparatus for rotating optical fibers as they are drawn to reduce PMD. The practice of spinning the fiber during drawing causes geometric and/or stress asymmetries inside the fiber to rotate around the fiber axis along the length of the axis; however, spinning the fiber does not solve the fundamental problem of PMD in glass, nor does spinning completely eliminate PMD, Or solve the uniformity problem of MFD.

In view of these shortcomings in the art, it is desirable to provide a method that can maintain or improve MFD uniformity while reducing PMD. This approach is clearly required when drawing optical fibers at higher drawing speeds (ie, greater than 10 m/s). Because a higher drawing speed will increase the down-feeding oscillation of the root, thereby increasing the PMD of the optical fiber.

Contents of the invention

Accordingly, the present invention seeks to provide a high speed optical fiber drawing method that alleviates one or more of the problems due to limitations and disadvantages of the related prior art. The main advantage of the present invention is that it provides a method for controlling the diameter of the drawn fiber while reducing the PMD of the fiber and keeping the MFD uniform when drawing the fiber at high speed. The method includes drawing an optical fiber at a high speed while maintaining a constant blank feeding speed. It is believed that the constant downfeed speed avoids vibration of the blank root in the furnace, which can cause changes in the shape of the core during fiber shaping. A change in the shape of the core will degrade the PMD and MFD of the finished fiber.

To obtain these and other advantages, and in accordance with the objects of the present invention, as embodied and broadly described, the present invention is a method for reducing polarization mode dispersion in drawn optical fiber, the method comprising the steps of: feeding an optical fiber preform having a predetermined size into a furnace; drawing an optical fiber from the optical fiber preform at a drawing speed of at least 10 m/s; and varying the drawing speed so as to maintain a substantially constant fiber diameter while maintaining a predetermined downfeed The speed does not change. Preferably, the drawing speed is greater than 14 m/s, more preferably greater than 20 m/s.

In a preferred embodiment, the downfeed speed is constant for a first zone or range of drawing speeds, then for a second zone or second range of drawing speeds, the downfeed speed is changed to another different constant downfeed speed. While the drawing speed is varied in each zone, the downfeed speed remains constant in each zone. In addition, the delivery speed can be different for each zone. The method may also include the step of reducing the downfeed speed when the wire speed is changed from one zone to another zone with a higher wire speed; or when the wire speed is changed from one zone to another with a lower wire speed In the zone, the downloading speed increases. The present invention may also include the step of spinning the fiber while drawing, which further reduces PMD.

According to another embodiment of the present invention, there is provided a method for drawing an optical fiber from an optical fiber preform, the method comprising the following steps: feeding an optical fiber preform having a predetermined size into the drawing at a constant down-feeding speed In a furnace; the optical fiber is drawn from the optical fiber preform at a drawing speed of at least 10 m/s. The method also includes the steps of: measuring the diameter of the drawn fiber and generating a signal indicative of the measured diameter; and comparing the generated signal to a nominal fiber diameter. A second signal indicative of the difference in comparison is generated and used to vary the draw speed, thereby adjusting the draw fiber diameter. The method also includes the steps of: sensing the drawing speed to determine whether it is within a predetermined speed zone; and if the sensed drawing speed is outside said zone, changing the downfeed speed to another predetermined speed zone. speed. The downfeed speed is constant for a first zone or range of drawing speeds and then becomes another constant downfeed speed for a second zone or range of drawing speeds. Preferably, the downfeed speed remains constant within each zone, and when the drawing speed is varied between zones, the downfeed speed is varied accordingly. The method according to this embodiment may also include the step of rotating the optical fiber while drawing.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

Overview of drawings

Figure 1 is a schematic diagram of an optical fiber drawing device.

A detailed description

The present invention aims to provide a method for reducing polarization mode dispersion of drawn optical fiber. The method feeds an optical fiber preform having a predetermined size into a furnace at a predetermined downfeed speed. Preferably, the downfeed speed is kept constant throughout the drawing process to minimize the vibration of the preform root in the furnace, thereby maintaining a uniform MFD and reducing the PMD of the drawn fiber.

FIG. 1 shows a well-known fiber drawing system, generally indicated by reference numeral 1 . The preform 10 is vertically placed in the muffle furnace 11 of the drawing furnace. The preform rod 10 includes a handle (not shown) which is connected to support means (not shown) in known manner. The support device is part of the preform feed drive 22 which controls the speed at which the preform 10 is fed into the furnace. The heating unit 12 provides heat to at least the bottom of the preform rod 10 . A temperature controller 49 controls the temperature of the heating unit 12 in a known manner. The preform feed drive 22 feeds the preform 10 into the furnace using a well-known start-up procedure. As the preform 10 is fed into the furnace, the end (commonly referred to as the root) of the preform 10 is melted, and the puller 20 pulls the optical fiber 14 from the root 13 of the preform 10 .

After the fiber 14 exits the muffle furnace 11, it passes through a diameter monitor 15, which generates a signal that is used by a feedback control loop to regulate the speed of the puller 20 and preform feed drive 22, and through the temperature controller 49 to regulate the furnace temperature. After passing the diameter monitor 15 , the fiber 14 passes through a cooling tube 17 and a coater 18 which applies a curable protective coating to the fiber 14 . Coated fibers may also pass through coating curing equipment and, if desired, through additional coaters (not shown). Feedback control of the preform feed drive 22, puller drive 21 and temperature controller 49 can be accomplished using known control algorithms. An input is provided to the puller drive 21 from a control algorithm 48 which is part of the drawing control computer 47 . For fiber optic requirements, it is beneficial to run tractor 20 at a speed of at least 10 m/s. The tractor preferably produces a drawing speed of greater than 10 m/s, and most preferably produces a drawing speed of greater than 20 m/s.

The present invention aims to provide a method for reducing the polarization mode dispersion of drawn optical fiber, which method includes the steps of feeding into a glass preform and pulling out the optical fiber at a speed greater than 10 m/s. The size of the preform 10 can be measured by weight or its diameter. The feeding speed of the preform 10 is selected according to the size of the preform 10 . Preferably, once selected, the downfeed speed remains constant throughout the fiber drawing process. Another method is that the feeding speed remains constant within a predetermined drawing speed zone or range. There may be any number of wire speed zones, and the range of wire speeds within each zone may vary. However, each zone has a predetermined downfeed speed associated with it, and the downfeed speed remains constant within each given zone.

If the draw rate controlled by puller drive 21 is increased or decreased beyond a particular zone of draw speed, then control algorithm 48 signals preform feed drive 22 to change the downfeed speed to be appropriate for that particular draw speed zone. download speed. The control algorithm 48 is set up so that as the pull speed changes from one zone to another, the downfeed speed is changed in small increments until a predetermined downfeed speed is reached. This will allow the drop speed to be quickly adjusted back to the original speed if the pull speed should suddenly return to the original zone.

According to another aspect of the present invention, the method may also include the steps of sensing the wire drawing speed, judging whether it is within a predetermined speed zone, and changing the feeding speed if the sensed wire drawing speed is outside the speed zone. speed. In this embodiment, a wire drawing speed sensor (not shown) continuously monitors the wire drawing speed in the wire drawing control computer 47 . If the draw speed changes from one zone to another, the control algorithm signals the preform feed drive 22 to increase or decrease the downfeed speed to a predetermined constant speed associated with that draw speed zone.

The method of the present invention also includes the step of varying the drawing speed in response to the measured fiber diameter to maintain the fiber diameter substantially constant while maintaining the predetermined downfeed speed. The fiber 14 is often monitored with a diameter monitor 15 in order to maintain a constant fiber diameter. Diameter monitor 15 generates a signal indicative of the measured fiber diameter. This signal is sent to the drawing control computer 47. In the drawing computer 47, the measured signal is compared to a predetermined nominal fiber diameter value. A second signal is generated based on the difference between the measured fiber diameter value and the nominal value. A second signal is sent to the puller driver 21 and the puller speed is varied to keep the fiber diameter constant. This process is performed several hundred times per minute, and the downfeed speed remains constant during the entire range of draw speeds throughout the drawing process.

It is also beneficial to spin the fiber while drawing. Spun fibers have been shown to further reduce PMD. Various methods and devices have been developed for rotating optical fibers as they are drawn. For a more detailed understanding of methods and apparatus for spinning optical fibers, reference is made to commonly assigned and co-pending U.S. Patent Application Nos. 08/858,836 and 08/784,574, PCT Application PCT/US97/ 02541 and US Patent No. 5,298,047, the contents of all of which are incorporated herein by reference.

The advantages of the invention are numerous. In prior art drawing systems, the fiber diameter is controlled by the drawing speed. The control loop implements two-step process control for wire drawing. If the pull speed changes, the drop speed responds to the change in pull speed. While not wishing to be bound by any theory or explanation as to why the invention works, it is believed that this causes the root 13 to vibrate in the furnace. Vibration of the preform 10 root in the furnace can change the core shape of the drawn fiber, and the change in core shape can lead to higher PMD and MFD non-uniformity, both of which are detrimental to fiber performance.

The present invention reduces the vibration of the preform by providing a constant downfeed speed during the wire drawing process. Contrary to the well-known two-step control method for drawing fiber at high speeds, the present invention establishes a control algorithm to maintain a constant preform feed-down speed and thus fiber diameter even as the draw speed changes. This control mechanism can reduce or possibly eliminate various vibrations in the drawing control loop that cause changes in the core shape during fiber shaping, and reduce PMD and improve MFD uniformity.

Example

The invention is further described below by way of examples which are intended to illustrate the invention.

example 1

Fabricate non-rotating fibers with a drawing system similar to that shown in Figure 1. The pull-up speed was allowed to vary up to 19 m/s in order to keep the fiber diameter constant while the feed-down speed remained constant at about 2.75 mm/min. The PMD and MFD uniformity of the obtained fiber were tested. Table 1 below shows the results of comparison with conventional process (variable downfeed speed) draw drawn optical fiber.

Table 1 The delivery speed remains unchanged Percent reduction in PMD 71% Improve the percentage of MFD uniformity 83%

As shown by the results, compared with the general process, in the drawing process of the present invention, the PMD is obviously reduced, and the MFD is increased.

Example 2

Fiber was drawn using equipment similar to that shown in Figure 1. The fiber is also rotated during the drawing process. As mentioned above, according to the embodiment of the present invention with respect to the speed zone, the downfeed speed is set so as to achieve a nominal wire drawing speed of 15.5 m/s. Test drawn fibers to compare their PMD and MFD uniformity results to fibers drawn by typical drawing processes. Several different experiments were performed and the results are shown in Table 2 below.

Table 2 The delivery speed remains unchanged Percent reduction in PMD 80% Improve the percentage of MFD uniformity 76% As shown in Table 2, compared with the optical fiber drawn by the general process, the PMD of the optical fiber drawn according to the present invention is significantly reduced. MFD uniformity is also significantly improved.

Various modifications and changes to the method of the present invention will be apparent to those skilled in the art without departing from the scope of the appended claims and their equivalents.

Claims (16)

1. A method for reducing polarization mode dispersion in drawn fiber, characterized in that, comprising the following steps: feeding an optical fiber preform with a predetermined size into the furnace at a predetermined delivery speed; Draw the optical fiber from the optical fiber preform at a drawing speed of at least 10 m/s; The drawing speed is varied so as to keep the fiber diameter substantially constant while maintaining the predetermined downfeed speed. 2. The method for reducing polarization mode dispersion of drawn optical fiber according to claim 1, wherein the drawing speed is greater than 14 m/s. 3. The method for reducing polarization mode dispersion in drawn optical fiber according to claim 1, wherein the drawing speed is varied between about 14 m/s and 20 m/s. 4. The method for reducing polarization mode dispersion in drawn optical fiber of claim 1, wherein the drawing speed is varied between about 14 m/s and 20 m/s. 5. The method for reducing polarization mode dispersion of drawn optical fiber according to claim 4, further comprising the step of determining at least one drawing speed region. 6. The method for reducing polarization mode dispersion of drawn optical fiber according to claim 5, wherein when the drawing speed is changed in each zone, the downfeed speed is kept constant in each zone. 7. The method for reducing polarization mode dispersion in drawn optical fiber according to claim 6, wherein the downfeed speed is different for each zone. 8. The method for reducing the polarization mode dispersion of drawn optical fiber as claimed in claim 7, characterized in that, when the drawing speed is changed from one zone to another zone having a higher drawing speed range, the downfeed speed is reduced . 9. A method for reducing polarization mode dispersion in drawn optical fiber as claimed in claim 7, wherein the downfeed speed is increased as the drawing speed is changed from one zone to another zone having a lower drawing speed range . 10. The method for reducing polarization mode dispersion of drawn optical fiber according to claim 1, wherein the optical fiber is rotated during drawing. 11. A method for drawing an optical fiber from an optical fiber preform, comprising the steps of: Send an optical fiber preform with a predetermined size into the drawing furnace at a feeding speed; Draw the optical fiber from the optical fiber preform at a drawing speed of at least 10 m/s; measuring the diameter of the drawn fiber and generating a signal indicative of the measured diameter; comparing the generated signal to a nominal fiber diameter value, and generating a second signal representing a difference in the comparison; Sensing the drawing speed in order to judge whether it is within the predetermined speed zone; changing the drawing speed according to the second signal, so as to adjust the diameter of the drawing fiber; and If the sensed drawing speed is outside the zone, then the downfeed speed is changed. 12. The optical fiber drawing method according to claim 11, further comprising the step of rotating the optical fiber during drawing. 13. The optical fiber drawing method according to claim 11, wherein the drawing speed includes a plurality of zones, and each zone includes a predetermined drawing speed range. 14. The optical fiber drawing method of claim 13, wherein the downfeed speed is varied as the drawing speed is varied between the plurality of zones. 15. The optical fiber drawing method of claim 14, wherein the downfeed speed remains substantially constant in each zone. 16. A method for reducing polarization mode dispersion in drawn fiber, comprising the steps of: feeding an optical fiber preform with a predetermined diameter into the furnace at a constant feeding speed; Draw the optical fiber from the optical fiber preform at a drawing speed of at least 10 m/s; The drawing speed was varied so as to keep the fiber diameter substantially constant.